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Title: Voyage to Jupiter
Author: Samz, Jane, Morrison, David
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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    [Illustration: A special color reconstruction of the eruption of the
    volcano Loki on the Jovian satellite Io. The picture was taken by
    Voyager 1 from a range of about half a million kilometers.

                                                             NASA SP-439

    [Illustration: Voyage To Jupiter]

                           Voyage To Jupiter

                             David Morrison
                             and Jane Samz

    [Illustration: NASA]

          Scientific and Technical Information Branch    1980
             National Aeronautics and Space Administration
                                                          Washington, DC
  For sale by the Superintendent of Documents
  U.S. Government Printing Office, Washington, D.C. 20402
  Library of Congress Catalog Card Number 80-600126


Few missions of planetary exploration have provided such rewards of
insight and surprise as the Voyager flybys of Jupiter. Those who were
fortunate enough to be with the science teams during those weeks will
long remember the experience; it was like being in the crow’s nest of a
ship during landfall and passage through an archipelago of strange
islands. We had known that Jupiter would be remarkable, for man had been
studying it for centuries, but we were far from prepared for the torrent
of new information that the Voyagers poured back to Earth.

Some of the spirit of excitement and connection is captured in this
volume. Its senior author was a member of the Imaging Team. It is not
common that a person can both “do science” at the leading edge and also
present so vivid an inside picture of a remarkable moment in the history
of space exploration.

  April 30, 1980
                                                         Thomas A. Mutch
                                                 Associate Administrator
                                                 Office of Space Science


The two Voyager encounters with Jupiter were periods unparalleled in
degree and diversity of discovery. We had, of course, expected a number
of discoveries because we had never before been able to study in detail
the atmospheric motions on a planet that is a giant spinning sphere of
hydrogen and helium, nor had we ever observed planet-sized objects such
as the Jovian satellites Ganymede and Callisto, which are half
water-ice. We had never been so close to a Moon-sized satellite such as
Io, which was known to be dispersing sodium throughout its Jovian
neighborhood and was thought to be generating a one-million-ampere
electrical current that in some way results in billions of watts of
radio emission from Jupiter.

The closer Voyager came to Jupiter the more apparent it became that the
scientific richness of the Jovian system was going to greatly exceed
even our most optimistic expectations. The growing realization among
Voyager scientists of the wealth of discovery is apparent in their
comments, discussions, and reports as recounted by the authors in their
descriptions of the two encounters.

Although many of the discoveries occurred in the few weeks around each
encounter, they were, of course, the result of more than those few weeks
of effort. In fact, planning started a decade earlier, and the Voyager
team of engineers and scientists had been designing, building, and
planning for the encounters for seven years. The Pioneer spacecraft made
the first reconnaissance of Jupiter in 1973-1974, providing key
scientific results on which Voyager could build, and discoveries from
continuing ground-based observations suggested specific Voyager studies.
Voyager is itself just the second phase of exploration of the Jovian
system. It will be followed by the Galileo program, which will directly
probe Jupiter’s atmosphere and provide long-term observations of the
Jovian system from an orbiting spacecraft. In the meantime, the Voyager
spacecraft will continue their journey to Saturn, and possibly Uranus
and Neptune, planets even more remote from Earth and about which we know
even less than we knew of Jupiter before 1979.

As is clearly illustrated in this recounting of the voyage to Jupiter,
scientific endeavors are human endeavors; just as Galileo could not have
foreseen the advancement in our knowledge initiated by his discoveries
of the four Jovian moons in 1610, neither can we fully comprehend the
scientific heritage that our exploration of space is providing future

  April 1980
                                                         Edward C. Stone
                                               Voyager Project Scientist


The authors are grateful to the many members of the Voyager Project team
who not only made this historic mission of exploration possible but also
took time from their busy schedules to offer us assistance, information,
and encouragement in the preparation of this book. Among many too
numerous to name individually, we particularly thank E. C. Stone, A. L.
Lane, C. H. Stembridge, R. A. Mills, E. Montoya, M. A. Mitz, and B. A.
Smith, L. Soderblom, and their colleagues on the Voyager Imaging Team.
We are grateful to F. E. Bristow, D. L. Bane, L. J. Pieri and especially
J. J. Van der Woude for their assistance in obtaining optimum versions
of the photographs printed here. C. B. Pilcher and I. de Pater kindly
made available their groundbased pictures of the Jovian magnetosphere.
Many colleagues have read and provided helpful comments on parts of the
manuscript, among them G. A. Briggs, S. A. Collins, S. Cruikshank, J.
Doughty, A. L. Guin, A. L. Lane, R. A. Mills, E. Montoya, E. C. Stone,
J. L. Ward, and especially C. R. Chapman.


  Foreword                                                              v
  Introduction                                                        vii
  Acknowledgment                                                       ix
  Chapter 1. The Jovian System                                          1
  Chapter 2. Pioneers to Jupiter                                       11
  Chapter 3. The Voyager Mission                                       23
  Chapter 4. Science and Scientists                                    33
  Chapter 5. The Voyage to Jupiter—Getting There                       47
  Chapter 6. The First Encounter                                       63
  Chapter 7. The Second Encounter: More Surprises From the “Land” of
          the Giant                                                    93
  Chapter 8. Jupiter—King of the Planets                              117
  Chapter 9. Four New Worlds                                          139
  Chapter 10. Return to Jupiter                                       169
  Appendix A. Pictorial Maps of the Galilean Satellites               177
  Appendix B. Voyager Science Teams                                   195
  Appendix C. Voyager Management Team                                 197
  Additional Reading                                                  199

    [Illustration: Jupiter is the largest planet in the solar system—a
    gaseous world as large as 1300 Earths, marked by alternating bands
    of colored clouds and a dazzling complexity of storm systems. The
    Voyager mission gave us our first close look at this spectacular
    planet. [P-21085]]

                               CHAPTER 1
                           THE JOVIAN SYSTEM


In the Sun’s necklace of planets, one gem far outshines the rest:
Jupiter. Larger than all the other planets and satellites combined,
Jupiter is a true giant. If intelligent beings exist on planets circling
nearby stars, it is probable that Jupiter is the only member of our
planetary system they can detect. They can see the Sun wobble in its
motion with a twelve-year period as Jupiter circles it, pulling first
one way, then the other with the powerful tug of its gravity. If
astronomers on some distant worlds put telescopes in orbit above their
atmospheres, they might even be able to detect the sunlight reflected
from Jupiter. But all the other planets—including tiny inconspicuous
Earth—would be hopelessly lost in the glare of our star, the Sun.

Jupiter is outstanding among planets not only for its size, but also for
its system of orbiting bodies. With fifteen known satellites, and
probably several more too small to have been detected, it forms a sort
of miniature solar system. If we could understand how the Jovian system
formed and evolved, we could unlock vital clues to the beginning and
ultimate fate of the entire solar system.

Ancient peoples all over the world recognized Jupiter as one of the
brightest wandering lights in their skies. Only Venus is brighter, but
Venus, always a morning or evening star, never rules over the dark
midnight skies as Jupiter often does. In Greek and Roman mythology the
planet was identified with the most powerful of the gods and lord of the
heavens—the Greek Zeus; the Roman Jupiter.

As befits the king of the heavens, the planet Jupiter moves at a slow
and stately pace. Twelve years are required for Jupiter to complete one
orbit around the Sun. For about six months of each year, Jupiter shines
down on us from the night sky, more brightly and steadily than any star.
During the late 1970s it was a winter object, but in 1980 it will
dominate the spring skies, becoming a summer “star” about 1982.

                           Early Discoveries

Even seen through a small telescope or pair of binoculars, Jupiter looks
like a real world, displaying a faintly banded disk quite unlike the
tiny, brilliant image of a star. It also reveals the brightest members
of its satellite family as starlike points spread out along a straight
line extended east-west through the planet. There are four of these
planet-sized moons; with their orbits seen edge-on from Earth, they seem
to move constantly back and forth, changing their configuration hourly.

In January 1610, in his first attempt to apply the newly invented
telescope to astronomy, Galileo discovered the four large satellites of
Jupiter. He correctly interpreted their motion as being that of objects
circling Jupiter—establishing the first clear proof of celestial motion
around a center other than the Earth. The discovery of these satellites
played an important role in supporting the Copernican revolution that
formed the basis for modern astronomy.

A few decades later the satellites of Jupiter were used to make the
first measurement of the speed of light. Observers following their
motions had learned that the satellite clock seemed to run slow when
Jupiter was far from Earth and to speed up when the two planets were
closer together. In 1675 the Danish astronomer Ole Roemer explained that
this change was due to the finite velocity of light: The satellites only
seemed to run slow at large distances because the light coming from them
took longer to reach Earth. Knowing the dimensions of the orbits of
Earth and Jupiter and the amount of the delay (about fifteen minutes),
Roemer was able to calculate one of the most fundamental constants of
the physical universe—the speed of light (about 300 000 kilometers per

    [Illustration: Galileo’s notes summarizing his first observations of
    the Jovian satellites Io, Europa, Ganymede, and Callisto in January
    1610 were made on a piece of scratch paper containing the draft of a
    letter presenting a telescope to the Doge in Venice. These
    observations were the result of Galileo’s first attempt to apply the
    telescope to astronomical research.]

The four great moons of Jupiter are called the Galilean satellites after
their discoverer. Their individual names—Io, Europa, Ganymede, and
Callisto—were proposed by Simon Marius, a contemporary and rival of
Galileo. (Marius claimed to have discovered the satellites a few weeks
before Galileo did, but modern scholars tend to discredit his claim.)
Io, Europa, Ganymede, and Callisto are names of lovers of the god
Jupiter in Greco-Roman mythology. Since Jupiter was not at all shy about
taking lovers, there are enough such names for the other eleven Jovian
satellites, as well as for those yet to be discovered.

In the century following Galileo’s death, improvements in telescopes
made it possible to measure the size of Jupiter and to note that it
bulged at the equator. The equatorial diameter is known today to be 142
800 kilometers, while from pole to pole Jupiter measures only 133 500
kilometers. For comparison, the diameter of the Earth is 12 900
kilometers, only about one-tenth as great, and the flattening of Earth
is also much smaller (less than one percent). By measuring the orbits of
the satellites and applying the laws of planetary motion discovered by
Johannes Kepler and Isaac Newton, astronomers were also able to
determine the total mass of Jupiter—about 2 × 10²⁴ tons, or 318 times
the mass of the Earth.

Once the size and mass are known, it is possible to calculate another
fundamental property of a planet—its density. The density, which is the
mass divided by the volume, provides important clues to the composition
and interior structure of a planetary body. The density of Earth, a body
composed primarily of rocky and metallic materials, is 5.6 times the
density of water. The mass of Jupiter is 318 times that of Earth; its
volume is 1317 times that of Earth. Thus Jupiter’s density is
substantially lower than Earth’s, amounting to 1.34 times the density of
water. From this low density, it was evident long ago that Jupiter was
not just a big brother of Earth and the other rocky planets in the inner
solar system. Rather, Jupiter is the prototype of the giant, gas-rich
planets Jupiter, Saturn, Uranus, and Neptune. These giant planets must,
from their low density, have a composition fundamentally different from
that of Mercury, Venus, Earth, Moon, and Mars.

                     Jupiter Through the Telescope

Jupiter is a beautiful sight seen with the naked eye on a clear night,
but only through a telescope does it begin to reveal its magnificence.
The most prominent features are alternating light and dark bands,
running parallel to the equator and subtly shaded in tones of blue,
yellow, brown, and orange. However, these bands are not the planet’s
only conspicuous markings. In 1664 the English astronomer Robert Hooke
first reported seeing a large oval spot on Jupiter, and additional spots
were noted as telescopes improved. As the planet rotates on its axis,
such spots are carried across the disk and can be used to measure
Jupiter’s speed of rotation. The giant planet spins so fast that a
Jovian day is less than half as long as a day on Earth, averaging just
under ten hours.

During the nineteenth century, observers using increasingly
sophisticated telescopes were able to see more complex detail in the
band structure, with wisps, streaks, and festoons that varied in
intensity and color from year to year. Furthermore, observations
revealed the remarkable fact that not all parts of the planet rotate
with the same period; near the equator the apparent length of a Jovian
day is several minutes shorter than the average day at higher latitudes.
It is thus apparent that Jupiter’s surface is not solid, and astronomers
came to realize that they were looking at a turbulent kaleidoscope of
shifting clouds.

Although the face of Jupiter is always changing, some spots and other
cloud features survive for years at a time, much longer than do the
largest storms on Earth. The record for longevity goes to the Great Red
Spot. This gigantic red oval, larger than the planet Earth, was first
seen more than three centuries ago. From decade to decade it has changed
in size and color, and for nearly fifty years in the late eighteenth
century no sightings were reported, but since about 1840 the Great Red
Spot has been the most prominent feature on the disk of Jupiter.

    [Illustration: This ground-based photograph of Jupiter showing the
    Great Red Spot in the southern hemisphere was taken with the
    Catalina Observatory’s 61-inch telescope in December 1966.]

It was not until the twentieth century that the composition of the
atmosphere of Jupiter could be measured. In 1905 spectra of the planet
revealed the presence of gases that absorb strongly at red and infrared
wavelengths; thirty years later these were identified as ammonia and
methane. These two poisonous gases are the simplest chemical compounds
of hydrogen combined with nitrogen and carbon, respectively. In the
atmosphere of Earth they are not stable, because oxygen, which is highly
active chemically, destroys them. The existence of methane and ammonia
on Jupiter demonstrated that free oxygen could not be present and that
the atmosphere was dominated by hydrogen—a reducing, rather than
oxidizing, condition. Subsequently, hydrogen was identified
spectroscopically. Although much more abundant than methane or ammonia,
hydrogen is harder to detect.

In the 1940s and 1950s the German-American astronomer Rupert Wildt used
all the available data to derive a picture of Jupiter that is still
generally accepted. He noted that both the low total density and the
observed presence of hydrogen-rich compounds in the atmosphere were
consistent with a bulk composition similar to that of the Sun and stars.
This “cosmic composition” is dominated by the two simplest elements,
hydrogen and helium, which together make up nearly 99 percent of all the
material in the universe. Wildt hypothesized that the giant planets,
because of their large size, had succeeded in retaining this primordial
composition, whereas the hydrogen and helium had escaped from the
smaller inner planets. He also used his knowledge of the properties of
hydrogen and helium to calculate what the interior structure of Jupiter
might be like, concluding that the planet is mostly liquid or gas. Wildt
suggested that there probably was a core of solid material deep in the
interior, but that much of Jupiter is fluid—extremely viscous and
compressed deep below the visible atmosphere, but still not solid. The
atmosphere seen from above is just the thin, topmost layer of an ocean
of gases thousands of kilometers thick.

                 Recent Earth-Based Studies of Jupiter

In the past, a great deal of planetary research was basically
descriptive, consisting of visual observations and photography.
Beginning in the 1960s, a new generation of planetary scientists began
to apply the techniques of modern astrophysics and geophysics to the
study of the solar system. Inspired in part by the developing space
programs of the United States and the Soviet Union, scientists began to
ask more quantitative questions: What are the surfaces and atmospheres
made of? What are the temperatures and wind speeds? Exactly what
quantities of different elements and isotopes are present? And how can
these new data be used to infer the origin and evolution of the planets?

    [Illustration: The major features of Jupiter are shown in schematic
    form. The planet is a banded disk of turbulent clouds; all its
    stripes are parallel to the bulging equator. Large dusky gray
    regions surround each pole. Darker gray or brown stripes called
    belts intermingle with lighter, yellow-white stripes called zones.
    Many of the belts and zones are permanent features that have been
    named. One feature of particular note is the Great Red Spot, an
    enigmatic oval larger than the planet Earth, which was first seen
    more than three centuries ago. During the years the spot has changed
    in size and color, and it escaped detection entirely for nearly
    fifty years in the 1700s. However, since the mid-nineteenth century
    the Great Red Spot has been the most prominent feature on the face
    of Jupiter. [2935]]

  North polar region
  North north north temperate belt
  North north temperate zone
  North north temperate belt
  North temperate zone
  North temperate belt
  North tropical zone
  North equatorial belt
  Equatorial zone equatorial band
  South equatorial belt
    N. component
    S. component
  South tropical zone
    Great Red Spot
  South temperate belt
  South temperate zone
  South south temperate belt
  South south temperate zone
  South polar region

Wildt had already suggested the basic gases in the atmosphere of
Jupiter: primarily hydrogen and helium, with much smaller quantities of
ammonia and methane. Undetected but possibly also present were nitrogen,
neon, argon, and water vapor. The abundance of helium was particularly a
problem; although it was presumably the second-ranking gas after
hydrogen, it has no spectral features in visible light and its presence
remained only a hypothesis, unconfirmed by observation.

Although the presence of a gas can usually be inferred from
spectroscopy, solids or liquids cannot normally be detected in this way.
Thus the composition of Jupiter’s clouds could not be determined
directly. However, the presence of ammonia gas provided an important
clue. At the temperatures expected in the upper atmosphere of the
planet, ammonia gas must freeze to form tiny crystals of ammonia ice,
just as water vapor in the Earth’s atmosphere freezes to form cirrus
clouds. Most investigators agreed that the high clouds covering much of
Jupiter must be ammonia cirrus. But ammonia crystals are white, so the
presence of this material provides no explanation for the many colors
seen on Jupiter. Additional materials must be present—perhaps colored
organic compounds, produced in small amounts by the action of sunlight
on the atmosphere.

Because Jupiter is five times farther from the Sun than is the Earth, a
given area on Jupiter receives only about four percent as much solar
heating as does a comparable area on Earth. Thus Jupiter is colder than
Earth; even though it may be warm deep below its blanket of clouds,
Jupiter presents a frigid face.

    [Illustration: These blue filter photographs of Jupiter were taken
    at Mauna Kea Observatory, Hawaii. They show changes on Jupiter’s
    surface between 1973 and 1978, with the dates of the observations.]

    [Illustration: July 25, 1973]

    [Illustration: October 5, 1974]

    [Illustration: October 2, 1975]

    [Illustration: November 20, 1976.]

    [Illustration: January 28, 1978.]

    [Illustration: December 19, 1978.]

The development of a new science, infrared astronomy, in the 1960s made
it possible to measure these low temperatures directly. In the case of a
cloudy planet like Jupiter, the infrared emission evident at various
wavelengths originates at different depths in the atmosphere. It is a
general property of any mixed, convecting atmosphere that the
temperature varies with depth; the rate of variation depends only on the
composition of the atmosphere, the gravity of the planet, and the
presence or absence of condensible materials to form clouds. On Jupiter
it is about 1.9° C warmer for each kilometer of descent through the
atmosphere. Thus, although the ammonia clouds are very cold, a little
above -173° C, if one goes deep enough one can reach temperatures that
are quite comfortable. With a variation of 1.9° C per kilometer,
terrestrial “room temperature” would be reached about 100 kilometers
below the clouds.

To measure the total energy radiated by a planet, it is necessary to
utilize infrared radiation at wavelengths more than one hundred times
longer than the wavelengths of visible light. Even when detectors were
developed that could measure such radiation, it was impossible to
observe celestial sources such as Jupiter because of the opacity of the
terrestrial atmosphere. Even a tiny amount of water vapor in our own
atmosphere can block our view of long-wave infrared. To make the
required measurements, it is necessary to carry a telescope to very high
altitudes, above all but a fraction of a percent of the offending water

In the late 1960s a Lear-Jet airplane was equipped with a telescope and
made available by NASA to astronomers to carry out long-wave infrared
observations from above 99 percent of the terrestrial water vapor. In
1969 Frank Low of the University of Arizona and his colleagues used this
system to make a remarkable discovery: Jupiter was radiating more heat
than it received from the Sun! Repeated observations demonstrated that
between two and three times as much energy emanated from the planet as
was absorbed. Thus Jupiter must have an internal heat source; in effect,
it shines by its own power as well as by reflected sunlight. Theoretical
studies suggest that the heat is primordial, the remnant of an
incandescent proto-Jupiter that formed four and one-half billion years

    [Illustration: Images of Jupiter in visible light (below) and
    five-micrometer infrared light show the planet’s characteristic
    belts and zones. The infrared image reveals areas that emit large
    amounts of thermal energy. The source of the energy is thought to be
    breaks in the Jovian cloud cover, which allow investigators a
    glimpse of the deep regions of the atmosphere. One of the mysteries
    of Jupiter concerns its heat balance: The planet appears to radiate
    more heat than it receives from the Sun. [P-20957]]

    [Illustration: Jupiter in visible light.]

At the same time that the internal heat source on Jupiter was being
revealed with long-wave airborne infrared telescopes, a new discovery
was being made from short-wave infrared observations. The clouds of
Jupiter are too cold to emit any detectable thermal radiation at a
wavelength of 5 micrometers (about ten times the wavelength of green
light). Nevertheless, images of Jupiter at 5 micrometers revealed a few
small spots where large amounts of thermal energy were being emitted.
The sources of the energy appeared to be holes or breaks in the clouds,
where it was possible to see deeper into hotter regions. The discovery
of these hot spots opened the possibility of probing deep regions of the
Jovian atmosphere that had previously been beyond the reach of direct

                        The Jovian Magnetosphere

The discovery of planetary magnetospheres began in 1959 when the first
U.S. Explorer satellite detected the radiation belts around the Earth.
Named for James Van Allen of the University of Iowa, whose
geiger-counter instrument aboard Explorer 1 first measured them, these
belts are regions in which charged atomic particles—primarily electrons
and protons—are trapped by the magnetic field of the Earth. They are one
manifestation of the terrestrial magnetosphere—a large, dynamic region
around the Earth in which the magnetic field of our planet interacts
with streams of charged particles emanating from the Sun.

At almost the same time that the terrestrial magnetosphere was being
discovered by artificial satellites, astronomers were finding evidence
to suggest similar phenomena around Jupiter. Radio astronomy is a branch
of science that measures radiation from celestial bodies at radio
frequencies, which correspond to wavelengths much longer than those of
visible or infrared light. All planets emit weak thermal radio
radiation, but in the late 1950s investigators found that Jupiter was a
much stronger long-wave radio source than would be expected for a planet
with its temperature. This radiation bore the signature of higher-energy
processes. Physicists had seen similar emissions produced in synchrotron
electron accelerators, huge machines in which electrons are whirled
around at nearly the speed of light so that they can be used for
experiments in nuclear physics. The Russian theorist I. S. Shklovsky
identified the Jovian radio radiation as also resulting from the
synchrotron process, due to spiraling electrons trapped in the planet’s
magnetic field. From the intensity and spectrum of the observed
synchrotron radiation, it was clear that both the magnetic field of the
planet and the energy of charged particles in its Van Allen belts were
much greater than was the case for Earth.

Using radio telescopes of high sensitivity, astronomers determined the
approximate strength and orientation of the magnetic field of Jupiter.
Although they were able to measure synchrotron radiation only from the
innermost parts of the Jovian magnetosphere, they could infer that the
total volume occupied by the magnetosphere was enormous. If our eyes
were sensitive to magnetospheric emissions, Jupiter would look more than
twice the diameter of the full moon in the sky.

All four Galilean satellites orbit within the magnetosphere of Jupiter;
in contrast, our Moon lies well outside the terrestrial magnetosphere.
Striking evidence of the interaction of the satellites and the
magnetosphere was provided when it was found that the innermost large
satellite—Io—actually affects the bursts of radio static produced by
Jupiter. Only when Io is at certain places in its orbit are these strong
bursts detected. Theorists suggested that electric currents flowing
between the satellite and the planet might be responsible for this

                         The Jovian Satellites

For nearly three centuries after their discovery in 1610, the only known
moons of Jupiter were the four large Galilean satellites. In 1892 E. E.
Barnard, an American astronomer, found a much smaller fifth satellite
orbiting very close to the planet, and between 1904 and 1974 eight
additional satellites were found far outside the orbits of the Galilean
satellites. The outer satellites are quite faint and presumably no more
than a few tens of kilometers in diameter, and all have orbits that are
much less regular than those of the five inner satellites. Four of them
revolve in a retrograde direction, opposite to that of the inner
satellites and Jupiter itself.

In 1975 the International Astronomical Union assumed the responsibility
for assigning names to the non-Galilean satellites of Jupiter. Following
tradition, they named the inner satellite Amalthea for the she-goat that
suckled the young god Jupiter. The outer eight were named for lovers of
Jupiter: Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and
Sinope. For the non-Galilean satellites, the “e” ending is reserved for
satellites with retrograde orbits; those with normal orbits have names
that end in “a.”

Because they are so large, the Galilean satellites have attracted the
most attention from astronomers. More than fifty years ago large
telescopes were used to estimate their sizes, and a careful series of
measurements of their light variation showed that all four always keep
the same face pointed toward Jupiter, just as our Moon always turns the
same face toward Earth. Also, the subtle gravitational perturbations
they exert on each other were used to determine the approximate mass of

    [Illustration: The pattern of the Galilean satellites changes from
    hour to hour, as seen from Earth. Viewed edge-on, the nearly
    circular orbits produce an apparent back and forth motion with
    respect to Jupiter. These images recreate the kinds of observations
    first made by Galileo in 1610.]

Callisto, the outermost Galilean satellite, is larger than the planet
Mercury. It also has the lowest reflectivity, or albedo, of the four,
suggesting that its surface may be composed of some rather dark,
colorless rock. Callisto takes just over two weeks to orbit once around

Ganymede, which requires only seven days for one orbit, is the largest
satellite in the Jovian system, being only slightly smaller than the
planet Mars. Its albedo is much higher than that of Callisto, or of the
rocky planets such as Mercury, Mars, or the Moon. In 1971 astronomers
first measured the infrared spectrum of reflected sunlight from Ganymede
and found the characteristic absorptions of water ice, indicating that
this satellite is partially covered with highly reflective snow or ice.

Europa, which is slightly smaller than the Moon, circles Jupiter in half
the time required by Ganymede. Its surface reflects about sixty percent
of the incident sunlight, and the infrared spectrum shows prominent
absorptions due to water ice; Europa appears to be almost entirely
covered with ice. However, its color in the visible and ultraviolet part
of the spectrum is not that of ice, so some other material must also be

Io, innermost of the Galilean satellites, is the same size as our Moon.
It orbits the planet in 42 hours, half the period of Europa. Like
Europa, it has a very high reflectivity, but, unlike Europa, it has no
spectral absorptions indicative of water ice. Before Voyager,
identification of the surface material on Io presented a major problem
to planetary astronomers.

When the sizes and masses of these satellites were measured, astronomers
could calculate their densities. The inner two, Io and Europa, both have
densities about three times that of water—nearly the same as the density
of the Moon, or of rocks in the crust of the Earth. Callisto and
Ganymede have densities only half as large, far too low to be consistent
with a rocky composition. The most plausible alternative to rock is a
composition that includes ice as a major component. Calculations showed
that if these satellites were composed of rock and ice, approximately
equal quantities of each were required to account for the measured
density. Thus the two outer Galilean satellites were thought to
represent a new kind of solar system object, as large as one of the
terrestrial planets, but composed in large part of ice.

In 1973 the attention of astronomers was dramatically drawn to Io when
Robert Brown of Harvard University detected the faint yellow glow of
sodium from the region of space surrounding it. It seemed that this
satellite had an atmosphere, composed of the metal sodium! Continued
observations showed, however, that this was not an atmosphere in the
usual sense of the word. The gas atoms were not bound gravitationally to
Io, but continuously escaped from it to form a gigantic cloud enveloping
the orbit of the satellite. Fraser Fanale and Dennis Matson of the
Caltech Jet Propulsion Laboratory suggested that bombardment of Io by
high-energy particles from the Jovian Van Allen belts was knocking off
atoms of sodium by a process called sputtering, releasing these atoms
and allowing them to expand outward to form the observed sodium cloud.
No one anticipated then that powerful volcanic eruptions on Io might
also be contributing to this remarkable gas cloud.

    [Illustration: The Galilean satellites in orbit around Jupiter,
    along with the outer satellites, constitute a miniature solar
    system. Here they are shown relative to the size of Mercury and that
    of the Moon. The portrayal of their internal and external
    composition is based on theoretical models that preceded the Voyager
    flybys. [PC-17054AC]]

    [Illustration: This image of Io’s extended sodium cloud was taken
    February 19, 1977, at the Jet Propulsion Laboratory’s Table Mountain
    Observatory. A picture of Jupiter, drawings of the orbital geometry,
    and Io’s disk (the small circle on the left) are included for
    perspective. The sodium cloud image has been processed for removal
    of sky background, instrumental effects, and the like. This
    photograph demonstrates that the cloud is highly elongated and that
    more sodium precedes Io in its orbit than trails it. [P-20047]]

This picture of the satellites was developed just as the first space
probe reached the Jovian system. In the next chapter we describe the
Pioneer program by which scientists reached out across nearly a million
kilometers of space to explore Jupiter, its magnetosphere, and its
system of satellites.

    [Illustration: Pioneer 10 was launched on March 2, 1972, at 8:49
    p.m. from Cape Canaveral, Florida. A powerful Atlas-Centaur rocket
    served as the launch vehicle, which propelled the space probe to its
    goal nearly a billion kilometers away. The beauty of the night
    launch was enhanced by the rumbling thunder and flashing lightning
    of a nearby storm.]

                               CHAPTER 2
                          PIONEERS TO JUPITER

                     Reaching for the Outer Planets

Since the beginning of the Space Age, scientists had dreamed of sending
probes to Jupiter and its family of satellites. Initially, robot
spacecraft were limited to studying the Earth and its Moon. In 1962,
however, the first true interplanetary explorer, Mariner 2, succeeded in
escaping the Earth-Moon system and crossing 100 million kilometers of
space to encounter Venus, studying Earth’s sister planet at close range
using half a dozen scientific instruments. By the mid 1960s a U.S.
planetary spacecraft had also flown to Mars, there had been a second
flyby of Venus, and an ambitious program was under way for two more
flybys of Mars in 1969, followed by a Mars orbiter in 1971. Based on
this success with the inner planets, NASA scientists and engineers began
to plan seriously to meet the challenge of the outer solar system.

Not only Jupiter, but Saturn and even Uranus and Neptune, were
considered as possible targets. However, the distances between the outer
planets are so vast that many years of flight would be required for a
spacecraft to reach them, even using the most powerful rocket boosters
then contemplated. If a cautious exploration program were followed,
investigating one planet at a time before designing the next mission, it
would be well into the twenty-first century before even a first
reconnaissance of the solar system could be achieved. A way to bridge
the space between planets in a more efficient, economical manner was

In the late 1960s celestial mechanicians—scientists who study the
motions of planets and spacecraft—began to solve problems posed by the
immensity of the outer solar system. If a spacecraft is aimed to fly
close to a planet in just the right way, it can be accelerated by the
gravity of the planet to higher speeds than could ever be obtained by
direct launch from Earth. If a second, more distant planet is in the
correct alignment, the gravity boost given by the first encounter can
speed the craft on to the second. Jupiter, with its huge size and strong
gravitational pull, could be used as the fulcrum for a series of
missions to Saturn, Uranus, Neptune, and even distant Pluto. In
addition, the early 1980s would offer an exceptional opportunity, one
repeated only about once every two centuries. At that time, all four
giant planets would be in approximate alignment, so that gravity-assist
maneuvers could be done sequentially. A single spacecraft, after being
boosted from Jupiter to Saturn, could use the acceleration of Saturn to
continue to Uranus, and in turn could be accelerated all the way out to
Neptune. Such an ambitious, multiplanet mission was named the Grand

The first essential step in the Grand Tour was a flyby of Jupiter.
However, this planet is ten times farther away from Earth than Venus or
Mars. In addition, there were two potentially lethal hazards that had
not been faced before in interplanetary flights: the asteroid belt and
the Jovian magnetosphere.

The first danger was presented by the many thousands of asteroids that
occupy a belt between the orbits of Mars and Jupiter. The largest
asteroid, Ceres, was discovered in 1801 and was initially thought to be
the “missing planet” sometimes hypothesized as lying between Jupiter and
Mars. However, Ceres is only 1000 kilometers in diameter, too small to
deserve the title of planet. Hundreds more of these minor planets were
discovered during the nineteenth century, and by the 1960s more than
3000 had well-determined orbits. Most were only a few tens of kilometers
in diameter, and astronomers estimated that 50 000 existed that were 1
kilometer or more in diameter. Any spacecraft to Jupiter would have to
cross this congested region of space.

Even 50 000 minor bodies spread through the volume of space occupied by
the asteroid belt would present little direct danger, although a chance
collision with an uncatalogued object was always possible. Much more
serious was the possibility that these larger objects were accompanied
by large amounts of debris, from the size of boulders down to
microscopic dust, that were undetectable from Earth. Collisions with
pebble-sized stones could easily destroy a spacecraft. The only way to
evaluate this danger was to go there and find out how much small debris
was present.

A second danger was posed by Jupiter itself. In order to use the gravity
boost of Jupiter to speed on to another planet, a spacecraft would have
to fly rather close to the giant. But this would mean passing right
through the regions of energetic charged particles surrounding the
planet. Some estimates of the number and energy of these particles
indicated that the delicate electronic brains of a spacecraft would be
damaged before it could penetrate this region. Again, only by going
there could the danger be evaluated properly.

                      The Pioneer Jupiter Mission

In 1969 the U.S. Congress approved the Pioneer Jupiter Mission to
provide a reconnaissance of interplanetary space between Earth and
Jupiter and a first close look at the giant planet itself. The Project
was assigned by NASA to the Ames Research Center in Mountain View,
California. The primary objectives were defined by NASA:

  Explore the interplanetary medium beyond the orbit of Mars.

  Investigate the nature of the asteroid belt, assessing possible
  hazards to missions to the outer planets.

  Explore the environment of Jupiter, including its inner magnetosphere.

The Pioneer spacecraft was designed for economy and reliability, based
on previous experience at Ames with Pioneers 6 through 9, all of which
had proven themselves by years of successful measurement of the
interplanetary medium near the Earth. Unlike the Mariner class of
spacecraft being used to investigate Venus and Mars, the Pioneer craft
rotated continuously around an axis pointed toward the Earth. This
spinning design was extremely stable, like the wheels of a fast-moving
bicycle, and required less elaborate guidance than a nonspinning craft.
In addition, the spin provided an ideal base for measurements of
energetic particles and magnetic fields in space, since the motion of
the spacecraft itself swept the viewing direction around the sky and
allowed data to be acquired rapidly from many different directions. The
only major disadvantage of a spinning spacecraft is that it does not
allow a stabilized platform on which to mount cameras or other
instruments that require exact pointing. Thus the spacecraft design was
optimized for measurements of particles and fields in interplanetary
space and in the Jovian magnetosphere, but had limited capability for
observations of the planet and its satellites.

As finally assembled, the Pioneer Jupiter spacecraft had a mass of 258
kilograms. One hundred forty watts of electrical power at Jupiter were
supplied by four radioisotope thermoelectric generators (RTGs), which
turned heat from the radioactive decay of plutonium into electricity.
The launch vehicle for the flight to Jupiter was an Atlas-Centaur
rocket, equipped with an additional solid-propellant third stage. This
powerful rocket could accelerate the spacecraft to a speed of 51 500
kilometers per hour, sufficient to escape the Earth and make the
billion-kilometer trip to Jupiter in just over two years. The specific
scientific investigations to be carried out on Pioneer were selected
competitively in 1969 from proposals submitted by scientists from U.S.
universities, industry, and NASA laboratories, and also from abroad.
Eleven separate instruments would be flown, in addition to two
experiments that would make use of the spacecraft itself.

Three complete Pioneer spacecraft, with their payloads of 25 kilograms
of scientific instruments, were built: one as a test vehicle and two for
launch to Jupiter. One of these—the test vehicle—is now on display at
the National Air and Space Museum in Washington. The first opportunity
to launch—the opening of the “launch window”—was on February 27, 1972.
However, it was not until shortly after dark on March 2 that all systems
were ready, and Pioneer 10 began its historic trip to Jupiter.

Pioneer 10 was the first human artifact launched with sufficient energy
to escape the solar system entirely. Fittingly, the craft carried a
message designed for any possible alien astronauts who might, in the
distant future, find the derelict Pioneer in the vastness of
interstellar space. A small plaque fastened to the spacecraft told the
time and planet from which it had been launched, and carried a symbolic
greeting from humanity to the cosmos.

    [Illustration: Two identical Pioneer spacecraft were designed and
    fabricated by TRW Systems Group at their Redondo Beach, California
    facility. Each weighed only about 260 kilograms, yet carried eleven
    highly sophisticated instruments capable of operating unattended for
    many years in space. Data systems on board controlled the
    instrumentation, received and processed commands, and transmitted
    information across the vast distance to Earth.]

The second Pioneer was to wait more than a year before launch. By
following far behind Pioneer 10, its trajectory—in particular how deeply
it penetrated the radiation belts during Jupiter flyby—could be
modified, depending on the fate of the first spacecraft. At dusk on
April 5, 1973, Pioneer 11 blasted from the launch pad at Cape Canaveral
and followed its predecessor on the long, lonely journey into the outer
solar system.

                           Flight to Jupiter

Within a few hours of launch, each Pioneer spacecraft shed the shroud
that had protected it and unfurled booms supporting the science
instruments and the RTG power generators. After each craft had been
carefully tracked and precise orbits calculated, small onboard rockets
were commanded to fire to correct its trajectory for exactly the desired
flyby at Jupiter. Pioneer 10 was targeted to fly by the planet at a
minimum distance of 3 Jupiter radii (R_J) from the center, or 2 R_J
(about 140 000 kilometers) above the clouds. This close passage, inside
the orbit of Io, allowed the craft to pass behind both Io and Jupiter as
seen from Earth, so that its radio beam could probe both the planet and
its innermost large satellite. Pioneer 11 was intended to fly even
closer to Jupiter, but the exact targeting options were held open until
after the Pioneer 10 encounter.

On Pioneer 10, all instruments appeared to be working well as the craft
passed the orbit of Mars in June 1972, just 97 days after launch. At
this point, as it headed into unexplored space, it truly became a
pioneer. In mid-July it began to enter the asteroid belt, and scientists
and engineers anxiously watched for signs of increasing particulate

Pioneer 10 carried two instruments designed to measure small particles
in space. One, with an effective area of about 0.6 square meters,
measured the direct impact of dust grains as small as one-billionth of a
gram. The other looked for larger, more distant grains by measuring
sunlight reflected from them. To the surprise of many, there was little
increase in the rate of dust impacts recorded as the craft penetrated
more and more deeply into the belt. At about 400 million kilometers from
the Sun, near the middle of the belt, there appeared to be an increase
in the number of larger particles detected optically, but not to a level
that posed any hazard. In February 1973 the spacecraft emerged unscathed
from the asteroid belt, having demonstrated that the much-feared
concentration of small debris in the belt did not exist. The pathway was
open to the outer solar system!

On November 26, 1973, the long-awaited encounter with Jupiter began. On
that date, at a distance of 6.4 million kilometers from the planet,
instruments on board Pioneer 10 detected a sudden change in the
interplanetary medium as the spacecraft crossed the point—the bow
shock—at which the magnetic presence of Jupiter first becomes evident.
At the bow shock, the energetic particles of the solar wind are suddenly
slowed as they approach Jupiter. At noon the next day, Pioneer 10
entered the Jovian magnetosphere at a distance of 96 R_J from the

As the spacecraft hurtled inward toward regions of increasing magnetic
field strength and charged plasma particles, the instruments designed to
look at Jupiter began to play their role. A simple line-scan camera that
could build up an image from many individual brightness scans (like a
newspaper picture transmitted by wire) obtained its first pictures of
the planet, and ultraviolet and infrared photometers prepared to observe
it also. By December 2, when the spacecraft had crossed the orbit of
Callisto, the outermost of the large Galilean satellites, the line-scan
images were nearly equal in quality to the best telescopic photos taken
previously, and as each hour passed they improved in resolution. Near
closest approach, Pioneer 10 transmitted partial frames of Jupiter that
represented a threefold improvement over any Earth-based pictures ever

    [Illustration: The asteroid belt is a region between Mars and
    Jupiter that is populated by thousands of minor planets, most only a
    few kilometers in diameter. Before 1970, some theorists suggested
    that large quantities of abrasive dust might damage spacecraft
    passing through the asteroid belt. Pioneer 10 proved that this
    danger was not present, thus opening the way to the outer solar

  Earth at encounter
  Earth at launch
  Jupiter at encounter
  Jupiter at launch
  Asteroid belt

Tension increased as the spacecraft plunged deeper into the radiation
belts of Jupiter. Would it survive the blast of x-rays and gamma-rays
induced in every part of the craft by the electrons and ions trapped by
the magnetic field of Jupiter? Several of the instruments measuring the
charged particles climbed to full scale and saturated. Others neared
their limits but, as anxious scientists watched the data being sent
back, the levels flattened off. Meanwhile, the spacecraft itself began
to feel the effects of the radiation, and occasional spurious commands
were generated. Several planned high-resolution images of Jupiter and
its satellites were lost because of these false signals. But again the
system stabilized, and no more problems occurred as, just past noon on
December 3, 1973, Pioneer 10 reached its closest point to Jupiter, 130
000 kilometers above the Jovian cloud tops. Pioneer had passed its most
demanding test with flying colors, and at a news conference at Ames,
NASA Planetary Program Director Robert Kraemer pronounced the mission
“100 percent successful.” He added, “We sent Pioneer off to tweak a
dragon’s tail, and it did that and more. It gave it a really good yank,
and it managed to survive.” The Project Science Chief pronounced it “the
most exciting day of my life,” and most of the hundreds of scientists
and engineers who participated in the encounter probably agreed with

Pioneer 11 continued to follow steadily, emerging from the asteroid belt
in March 1974. Based on the performance and findings of Pioneer 10, it
was decided to send Pioneer 11 still closer to Jupiter, but on a more
inclined trajectory. On April 19 thrusters on the spacecraft fired to
move the Pioneer 11 aimpoint just 34 000 kilometers above the clouds of

In using the Pioneer 10 data to assess the hazard to Pioneer 11,
scientists had to consider three aspects of the charged particle
environment. First was the energy distribution of the particles: The
most energetic presented the most danger. Second was the flux, the rate
at which particles struck the craft. Third was the total radiation dose.
One can make an analogy with a boxing match. The energy distribution
tells you how hard the blows of your opponent are. The flux is a measure
of how many times a minute he hits you, and the total dose measures how
many blows land. The spacecraft reacts just like a boxer; the crucial
question is how much total dose it absorbs. Enough radiation blows, and
the system is knocked out. The trajectory chosen for Pioneer 11 resulted
in higher flux, since the craft probed more deeply into Jupiter’s inner
magnetosphere than had Pioneer 10. But by moving at a high angle across
the equatorial regions where the flux is highest, the total dose could
be kept below that experienced by Pioneer 10.

Pioneer 11 entered the Jovian magnetosphere on November 26, 1974, just a
year after its predecessor. Closest approach took place on December 2.
As with Pioneer 10, the radiation dose taxed the spacecraft to its
limit. Again spurious commands were issued, this time affecting the
infrared radiometer more than the imaging system. The craft flew at high
latitude over the north polar region of Jupiter, an area never seen from
Earth, and returned several excellent high-resolution pictures. Once
more the little Pioneers had succeeded against the odds in opening the
way to the giant planets.

    [Illustration: Pioneer 10 and 11 encounters with Jupiter are shown
    as viewed from the celestial North Pole. Pioneer 10 swung around the
    giant planet in the counterclockwise direction, while Pioneer 11
    followed a clockwise approach. In this view, Jupiter rotates

  Pioneer 11
  Pioneer 10

Following their encounters with Jupiter, both Pioneer spacecraft
returned to their normal routine of measuring the interplanetary medium.
Pioneer 10 had gained speed from the gravity field of Jupiter and became
the first craft to achieve the velocity needed to escape from the solar
system. Pioneer 11, however, had used the pull of Jupiter to bend its
trajectory inward, aiming it across the solar system toward Saturn.
Following the successes at Jupiter, NASA announced that Pioneer 11 would
be targeted for a close flyby of Saturn five years later, which was
successfully carried out in September 1979. In early 1980, far beyond
their design lifetimes, both spacecraft were still performing

    [Illustration: In this view of Jupiter, the Great Red Spot is
    prominent and the shadow of Io traverses the planetary disk. The
    gross morphology of the belts and zones, with structures showing
    turbulence and convective cells in the middle latitudes, is clearly
    seen. The small white spots surrounded by dark rings, seen mainly in
    the southern hemisphere, indicate regions of intense vertical
    convective activity, somewhat similar to cumulonimbus or

                            Jupiter Results

The scientific results of the Pioneer flybys of Jupiter were many and
varied. As is always the case, some old questions were answered and new
problems were raised by the spacecraft data. Highlights of these results
are summarized below.

                        Photographs of Jupiter.

The line-scan imaging systems of Pioneer returned some remarkable
pictures of the planet during the two encounters, showing individual
features as small as 500 kilometers across. In addition, the Pioneers
were able to look at Jupiter from angles never observable from Earth.

One of the discoveries made from these pictures was the great variety of
cloud structures near the boundaries between the light zones and dark
belts. Many individual cloud patterns suggested rising and falling air.
The convoluted swirls evident in these regions appeared to be the result
of dynamic motions; unfortunately, with only the few “snapshots”
obtained during the hours of the flyby, the actual motions of these
clouds could not be followed.

                           Thermal Emission.

In the year the Pioneer Project was begun, astronomers on Earth had
measured that Jupiter emitted more heat than it absorbed from the Sun.
From the Earth these measurements could be made only of the sunlit part
of the planet; neither the night side nor the poles could be seen. One
of the main objectives of the Pioneer flybys was to determine the heat
budget accurately from temperature measurements at many points on both
the sunlit and the night sides.

The Pioneer data confirmed the presence of a heat source in Jupiter and
supplied a quantitative estimate of its magnitude. The global effective
temperature was found to be -148° C, to a precision of ±3 degrees. This
temperature implies that Jupiter radiates 1.9 times as much heat as it
receives from the Sun. The corresponding internal heat source is 10¹⁷
watts. Surprisingly, the poles were as warm as the equator; apparently,
the atmosphere is very efficient at transferring solar heat absorbed
near the equator up to high latitudes, or perhaps the internal component
of the heat comes preferentially from the polar regions.

                       Helium in the Atmosphere.

The Pioneer infrared experiment made the first measurement of the amount
of helium on Jupiter. The ratio of the number of helium atoms to the
number of hydrogen atoms was found to be He/H₂ = 0.14 ± 0.08. This is
consistent with the known solar ratio of He/H₂ = 0.11. Measurements of
helium in the upper atmosphere were also made by the ultraviolet

    [Illustration: One of the best Pioneer images of Jupiter was
    obtained at a range of 545 000 kilometers by Pioneer 11. Structure
    within the Great Red Spot and the surrounding belts and zones can be
    seen. There was much less turbulent cloud activity round the spot at
    the time of the Pioneer flybys than was seen five years later by the
    Voyager cameras.]

    [Illustration: Pioneer 10 confirmed theoretical models of Jupiter
    that suggest the planet is nearly all liquid, with a very small core
    and an extremely deep atmosphere. The liquid interior seethes with
    internal heat energy, which is transferred from deep within the
    planet to its outer regions. The temperature at the center may be 30
    000 K. Since the temperature at the cloud tops is around -123° C,
    there is a large range of temperatures within the planet.]

      Distance (km)
  Visible clouds
    Hydrogen gas
    Cloud tops
    Ammonia crystals
    Ammonium hydrosulfide crystals
    Ice crystals
    Water droplets
        -123° C
  Transparent atmosphere
  Hydrogen/Helium gas
      70 000
  Transition zone
        1980° C
      60 000
  Liquid hydrogen
      50 000
        11 000° C
  Transition zone
  3 million atmospheres pressure
      40 000
  Liquid metallic hydrogen
      30 000
      20 000
  Possible “sea” of helium
      10 000
        30 000 K
  Possible solid core

               Project Scientist: J. H. Wolfe, NASA Ames

      Investigation       Principal Investigator       Primary Objectives

  Magnetic fields         E. J. Smith, JPL        Measurement of the magnetic
                                                    field of Jupiter and
                                                    determination of the
                                                    structure of the
  Magnetic fields         N. F. Ness, NASA        Measurement of the magnetic
    (Pioneer 11 only)       Goddard                 field of Jupiter and
                                                    determination of the
                                                    structure of the
  Plasma analyzer         J. H. Wolfe, NASA Ames  Measurement of low-energy
                                                    electrons and ions,
                                                    determination of the
                                                    structure of the
  Charged particle        J. A. Simpson, U.       Determination of the
    composition             Chicago                 number, energy, and
                                                    composition of energetic
                                                    charged particles in the
                                                    Jovian magnetosphere.
  Cosmic ray energy       F. B. McDonald, NASA    Measurement of number and
    spectra                 Goddard                 energy of very high
                                                    energy charged particles
                                                    in space.
  Jovian charged          J. A. Van Allen, U.     Measurement of number and
    particles               Iowa                    energy distribution of
                                                    energetic charged
                                                    particles and
                                                    determination of
                                                    magnetospheric structure.
  Jovian trapped          R. Walker Fillius, UC   Measurement of number and
    radiation               San Diego               energy distribution of
                                                    energetic charged
                                                    particles and
                                                    determination of
                                                    magnetospheric structure.
  Asteroid-meteoroid      R. K. Soberman,         Observation of solid
    astronomy               General Electric        particles (dust and
                                                    larger) in the vicinity
                                                    of the spacecraft.
  Meteoroid detection     W. H. Kinard, NASA      Detection of very small
                            Langley                 solid particles that
                                                    strike the spacecraft.
  Celestial mechanics     J. D. Anderson, JPL     Measurement of the masses
                                                    of Jupiter and the
                                                    Galilean satellites with
                                                    high precision.
  Ultraviolet photometry  D. L. Judge, U.         Measurement of ultraviolet
                            Southern California     emissions of the Jovian
                                                    atmosphere and from
                                                    circumsatellite gas
  Imaging                 T. Gehrels, U. Arizona  Reconnaissance imaging of
    photopolarimetry                                Jupiter and its
                                                    satellites; study of
                                                    atmospheric dynamics.
  Jovian infrared         G. Münch, Caltech       Measurement of Jovian
    thermal structure                               temperature and heat
                                                    budget; determination of
                                                    helium to hydrogen ratio.
  S-Band occultation      A. J. Kliore, JPL       Probes of structure of
                                                    Jovian atmosphere and

                         Atmospheric Structure.

Several Pioneer investigations yielded information on the variation of
atmospheric temperature and pressure in the regions above the ammonia
clouds. Near the equator, at a level where the atmospheric pressure is
the same as that on the surface of the Earth (1 bar), the temperature is
-108° C. About 150 kilometers higher, where the pressure drops to 0.1
bar, is the minimum atmospheric temperature of about -165° C. Above this
point the temperature rises again, reaching about -123° C near a
pressure level of 0.03 bar. Presumably this temperature rise is due to
absorption of sunlight by a thin haze of dust particles in the upper
atmosphere of Jupiter.

                          Internal Structure.

The measurements of the amount of helium, of the gravitational field,
and of the size of the internal heat source on Jupiter greatly clarified
scientists’ understanding of the deep interior of the planet.
Calculations showed that the core of Jupiter must be so hot that
hydrogen cannot become solid, but must remain a fluid throughout the
interior. Even at great depths, therefore, Jupiter does not have a solid
surface. The theory that the Great Red Spot was the result of
interactions with a surface feature below the clouds thus became
untenable. Whatever its exact nature, the Red Spot must be a strictly
atmospheric phenomenon.

                            Magnetic Field.

Pioneer data showed that the magnetic field of Jupiter has a dipolar
nature, like that of the Earth, but 2000 times stronger. The calculated
surface fields measured about 4 gauss, compared to a field of about 0.5
gauss on the Earth. The axis of the magnetic field was tilted 11 degrees
with respect to the rotation axis, and it was offset by about 10 000
kilometers (0.1 R_J) from the center of the planet.

    [Illustration: Pioneers 10 and 11 did not obtain very detailed
    pictures of the satellites of Jupiter. The best view was of
    Ganymede, which showed a surface of contrasting light and dark spots
    of unknown nature.]

    [Illustration: A less detailed image of Europa clearly reveals the
    illuminated crescent but supplies little information about the
    surface of this ice-covered satellite.]

                         Satellite Atmospheres.

Two experiments yielded exciting new information on possible atmospheres
of the Galilean satellites. First was an occultation, in which the
Pioneer 10 spacecraft was targeted to pass behind Io as seen from Earth.
At the moments just before the spacecraft disappeared, and just after
reemergence from behind the satellite, the radio signal was influenced
by a thin layer of ionized gas, in which the electrons had been stripped
from the atoms by absorbed sunlight or by other processes. The
ionosphere thus discovered had a peak density of about 60 000 electrons
per cubic centimeter. In addition, a very extended far-ultraviolet glow,
probably due to atomic hydrogen, was found near the orbit of Io by the
ultraviolet photometer.

                 Masses of Jupiter and Its Satellites.

Precise radio tracking of the Pioneers as they coasted past Jupiter and
its satellites revealed that Jupiter is about one percent heavier than
had been anticipated, and several satellites were found to have masses
that differ by more than ten percent from values determined previously.
These improvements in knowledge of the masses were required to achieve
the close satellite flybys being planned for later missions.

    [Illustration: The Pioneer spacecraft carried this plaque on the
    journey beyond the solar system, bearing data that tell where and
    when the human species lived and that convey details of our
    biological form. When Pioneer 10 flew by Jupiter it acquired
    sufficient kinetic energy to carry it completely out of the solar
    system. Some time between one and ten billion years from now, the
    probe may pass through the planetary system of a remote stellar
    neighbor, one of whose planets may have evolved intelligent life. If
    the spacecraft is detected and then inspected, Pioneer’s message
    will reach across the eons to communicate its greeting.]

                        The Inner Magnetosphere.

A great deal of the scientific emphasis of the Pioneer missions was
directed at characterizing the particles and fields in the inner
magnetosphere, the region in which charged particles are trapped in
stable orbits. The Pioneers found that it extended to about 25 R_J, well
beyond the orbit of Callisto. Within this region, instruments on the
spacecraft recorded the numbers and energy of electrons, protons, and
ions. The electrons reached a maximum concentration near 3 R_J, and
their numbers remained almost constant from there in toward the planet.
The maximum concentration of protons observed by Pioneer 10 was at 3.4
R_J, a little inside the orbit of Io. Pioneer 11 penetrated deeper and
found another maximum, about twenty times higher, at 1.9 R_J; at this
distance, 10 million energetic protons hit each square centimeter of the
spacecraft every second. It was believed that the gap between these two
peaks was due to tiny Amalthea, the innermost satellite, which orbits at
2.5 R_J. Apparently this satellite sweeps up the particles as it circles
Jupiter. Another large dip in the proton distribution was attributed to
sweeping by Io, with smaller effects seen near the orbits of Europa and
Ganymede. There was an additional small effect at 1.8 R_J, later found
by Voyager to be due to Jupiter’s ring and its fourteenth satellite.

                        The Outer Magnetosphere.

From about 25 R_J outward to its boundary near 100 R_J, the Jovian
magnetosphere is a complex and dynamic place. Beyond about 60 R_J, both
Pioneers found the boundary to be highly unstable, apparently blown in
and out by variations in the pressure of the solar wind, which consists
of charged particles flowing outward from the Sun. In this region
concentrations of Jovian particles are sometimes seen that rival the
inner magnetosphere in intensity. Between 60 R_J and 25 R_J, a region
sometimes called the middle magnetosphere, the particle motions are more
ordered, and for the most part electrons and protons are carried along
with the planet’s rotation by its magnetic field. Near the equatorial
plane, the flow of these particles produces an electric current circling
the planet, and this current in turn generates its own magnetic field,
which approaches in strength that of Jupiter itself. Occasionally the
outer magnetosphere collapses down to about 60 R_J, and energetic
particles are squirted from the middle region into space; these bursts
of Jovian particles can sometimes be detected as far away as Earth.

At the same time that Pioneer scientists were analyzing their results
and developing new concepts of the Jupiter system, a new team of
investigators had been selected for the next mission to Jupiter. From
about 1975 on, attention shifted from Pioneer to its successor—Voyager.

    [Illustration: The Voyager spacecraft are among the most
    sophisticated, automatic, and independent robots ever sent to
    explore the planets. Each craft has a mass of one ton and is
    dominated by the 3.7-meter-diameter white antenna used for radio
    communication with Earth. Here Voyager undergoes final tests in a
    space simulator chamber. [373-7162AC]]

                               CHAPTER 3
                          THE VOYAGER MISSION

                           Genesis of Voyager

Voyager had its origin in the Outer Planets Grand Tour, a plan to send
spacecraft to all the planets of the outer solar system. In 1969, the
same year in which the Pioneer Project received Congressional approval,
NASA began to design the Grand Tour. At the same time, the Space Science
Board of the National Academy of Sciences completed a study called “The
Outer Solar System,” chaired by James Van Allen of the University of
Iowa, which recommended that the United States undertake an exploration

  1. To conduct exploratory investigations of the appearance, size,
  mass, magnetic properties, and dynamics of each of the outer planets
  and their major satellites;

  2. To determine the chemical and isotopic composition of the
  atmospheres of the outer planets;

  3. To determine whether biologically important organic substances
  exist in these atmospheres and to characterize the lower atmospheric
  environments in terms of biologically significant parameters;

  4. To describe the motions of the atmospheres of the major planets and
  to characterize their temperature-density-composition structure;

  5. To make a detailed study for each of the outer planets of the
  external magnetic field and respective particle population, associated
  radio emissions, and magnetospheric particle-wave interactions;

  6. To determine the mode of interaction of the solar wind with the
  outer planets, including the interaction of the satellites with the
  planets’ magnetospheres;

  7. To investigate the properties of the solar wind and the
  interplanetary magnetic field at great distances from the Sun at both
  low and high solar latitudes, and to search for the outer boundary of
  the solar wind flow;

  8. To attempt to obtain the composition, energy spectra, and fluxes of
  cosmic rays in interstellar space, free of the modulating effects of
  the solar wind.

The report also noted that “exceptionally favorable astronomical
opportunities occur in the late 1970s for multiplanet missions,” and
that “professional resources for full utilization of the
outer-solar-system mission opportunities in the 1970s and 1980s are
amply available within the scientific community, and there is a
widespread eagerness to participate in such missions.”

An additional Academy study, chaired by Francis S. Johnson of the
University of Texas at Dallas and published in 1971, was even more
specific: “An extensive study of the outer solar system is recognized by
us to be one of the major objectives of space science in this decade.
This endeavor is made particularly exciting by the rare opportunity to
explore several planets and satellites in one mission using long-lived
spacecraft and existing propulsion systems. We recommend that
[Mariner-class] spacecraft be developed and used in Grand Tour missions
for the exploration of the outer planets in a series of four launches in
the late 1970s.”

Thus the stage was set to initiate the Outer Planets Grand Tours. NASA’s
timetable called for dual launches to Jupiter, Saturn, and Pluto in 1976
and 1977, and dual launches to Jupiter, Uranus, and Neptune in 1979, at
a total cost over the decade of the 1970s of about $750 million.

A necessary step was to obtain from the scientific community the best
possible set of instruments to fly on the spacecraft. Following its
initial internal studies, NASA turned for its detailed scientific
planning to an open competition in which any scientist or scientific
organization was invited to propose an investigation. In October 1970
NASA issued an “Invitation for Participation in the Mission Development
for Grand Tour Missions to the Outer Solar System,” and a year later it
had selected about a dozen teams of scientists to formulate specific
objectives for these missions. At the same time, an advanced spacecraft
engineering design was carried out by the Caltech Jet Propulsion
Laboratory (JPL), and studies were also supported by industrial
contractors. In fiscal year 1972, plans called for an appropriation by
Congress of $30 million to fund these developments, leading toward a
first launch in 1976.

Even as the scientific and technical problems of the Grand Tour were
being solved, however, political and budgetary difficulties intervened.
The Grand Tour was an ambitious and expensive concept, designed in the
enthusiasm of the Apollo years. In the altered national climate that
followed the first manned lunar landings, the United States began to
pull back from major commitments in space. The later Apollo landings
were canceled, and in fiscal year 1972 only $10 million of the $30
million needed to complete Grand Tour designs was appropriated. It
suddenly became necessary to restructure the exploration of the outer
planets to conform to more modest space budgets.

    [Illustration: The original plan for the Outer Planets Grand Tour
    envisaged dual launches to Jupiter, Saturn, and Pluto in the
    mid-1970s, and dual launches to Jupiter, Uranus, and Neptune in
    1979. However, political and budgetary constraints altered the plan,
    and the Voyager mission to Jupiter and Saturn, with an optional
    encounter with Uranus, was formulated to replace it. Here the
    original Grand Tour trajectories from Earth to the outer planets are
    shown. [P-10612AC]]

                        Redesign of the Mission

The new mission concept that replaced the Grand Tour dropped the
objectives of exploring the outer three planets—Uranus, Neptune, and
Pluto. In this way the lifetime of the mission was greatly shortened,
placing less stringent demands on the reliability of the millions of
components that go into a spacecraft. Limiting the mission to Jupiter
and Saturn also relieved problems associated with spacecraft power, and
with communicating effectively over distances of more than 2 billion
kilometers. The total cost of the new mission was estimated at $250
million, only a third of that previously planned for the Grand Tour.
Because it was based on the proven Mariner spacecraft design, the new
mission was initially named Mariner Jupiter Saturn, or MJS; in 1977 the
name was changed to Voyager. In January 1972 the President’s proposed
fiscal year 1973 budget included $10 million specifically designated for
Voyager; after authorization and appropriation by Congress, the official
beginning of Voyager was set for July 1, 1972.

With approval of the new mission apparently assured in the Congress,
NASA issued an “Announcement of Flight Opportunity” to select the
scientific instruments to be carried on Voyager. Seventy-seven proposals
were received; 31 from groups of scientists with designs for
instruments, and 46 from individuals desiring to participate in
NASA-formed teams. Of these 77 proposals, 24 were from NASA
laboratories, 48 were from scientists in various U.S. universities and
industry, and 5 were from foreign sources. After extensive review, 28
proposals were accepted: 9 for instruments and 19 for individual
participation. The newly selected Principal Investigators and Team
Leaders met for the first time at JPL just before Christmas, 1972. To
coordinate all the science activity of the Voyager mission, NASA and JPL
selected Edward Stone of Caltech, a distinguished expert on
magnetospheric physics, to serve as Project Scientist.

The team assembled in 1972 by JPL and its industrial contractors
included more than a thousand highly trained engineers, scientists, and
technical managers who assumed responsibility for the awesome task of
building the most sophisticated unmanned spacecraft ever designed and
launching it across the farthest reaches of the solar system. At the
head of the organization was the Project Manager, Harris (Bud)
Schurmeier. Later, Schurmeier was succeeded by John Casani, Robert
Parks, and Ray Heacock. This team had only four years to turn the paper
concepts into hardware, ready to deliver to Kennedy Space Center for
launch in the summer of 1977.

    [Illustration: Project Scientist Edward C. Stone]

    [Illustration: Project Manager Harris (Bud) Schurmeier]

                       The Objectives of Voyager

Voyager is one of the most ambitious planetary space missions ever
undertaken. Voyager 1, which encountered Jupiter on March 5, 1979, was
to investigate Jupiter, its large satellites Io, Ganymede, and Callisto,
and tiny Amalthea; Saturn, its rings, and several of its
satellites—including Titan, the largest satellite in the solar system.
Voyager 2, which arrived at Jupiter on July 9, 1979, was to examine
Jupiter, Europa, Ganymede, Callisto, and Saturn and several of its
satellites, after which it was to be hurled on toward an encounter with
the Uranian system in 1986. Both spacecraft were also designed to study
the interplanetary medium and its interactions with the solar wind.

Scientific objectives as well as the orbital positions of the planets
and satellites influenced the choice of spacecraft trajectories, which
were designed to provide close flybys of all four of Jupiter’s Galilean
satellites and six of Saturn’s satellites—featuring a very close
approach to Titan—and occultations of the Sun and Earth by Jupiter,
Saturn, Titan, and the rings of Saturn. However, all these objectives
could not be accomplished by a single spacecraft. No single path through
the Jupiter system, for instance, can provide close flybys of all four
Galilean satellites. Specifically, a trajectory for Voyager 1 that
included a close encounter with Io precluded a close encounter with
Europa and did not allow the spacecraft the option of being targeted
from Saturn to Uranus without having to travel through the rings of
Saturn. On the other hand, a trajectory that would send Voyager 2 on to
Uranus precluded not only a close encounter with Io, but also a close
encounter with Titan and with Saturn’s rings. The alignment of the
planets and their satellites was such that encounter with Jupiter on or
before April 4, 1979 was necessary for optimum investigation of Io, but
encounter with Jupiter after June 15, 1979 was mandatory for a
trajectory that would maintain the option to go on to Uranus. The latter
trajectory also allowed Voyager 2 to maintain a healthier distance from
Jupiter, avoiding the full dose of radiation that would be experienced
by Voyager 1.

Although the two Voyager trajectories do have certain fundamental
differences, they also have a great deal in common: Both spacecraft were
designed to study the interplanetary medium, and both would investigate
Jupiter and Saturn and have close flybys with several of their major
satellites. Also, if it were decided not to send Voyager 2 on to Uranus,
this spacecraft could be retargeted to a Saturn trajectory similar to
that of Voyager 1, providing a close flyby of Titan and a closer look at
the rings. The flight paths of Voyager 1 and Voyager 2 complement each
other—allowing the planets and some of the satellites to be viewed from
a number of angles and over a longer period of time than would be
possible with only one spaceprobe—yet the trajectories were also
designed to be more or less capable of duplicating each other’s
scientific investigations. This redundancy helps to ensure, so far as is
possible, the success of the mission.

The redundancy built into the mission is evident, not only in the design
of the trajectories, but in the spacecraft themselves. Voyagers 1 and 2
are identical spacecraft. In addition, many crucial elements are
duplicated on each: For example, the computer command subsystem (CCS),
the flight data subsystem (FDS), and the attitude and articulation
control subsystem (AACS)—which function as onboard control systems—each
have multiple reprogrammable digital computers. In addition, the CCS,
which decodes commands from ground control and can instruct other
subsystems from its own memory, contains duplicates for all its
functional units. The AACS, which controls the spacecraft’s
stabilization and orientation, has duplicate star trackers and Sun
sensors. The communications system contains two radio receivers and four
transmitters, two each to transmit both S-band (frequency of about 2295
megahertz) and X-band (frequency of about 8418 megahertz).

    [Illustration: Project Manager John Casani]

                             The Spacecraft

The Voyager spacecraft are more sophisticated, more automatic, and more
independent than were the Pioneers. This independence is important
because the giant planets are so far away that the correction of a
malfunction by engineers on Earth would take hours to perform. Even at
“nearby” Jupiter, radio signals take about forty minutes to travel in
one direction between the spacecraft and Earth. Saturn is about twice as
far away as Jupiter, and Uranus is twice as far as Saturn, slowing
communication even more.

    [Illustration: About the same size and weight of a subcompact car,
    the Voyager spacecraft carry instruments for eleven science
    investigations of the outer planets and their satellites. Power to
    operate the spacecraft is provided by three radioisotope
    thermoelectric generators mounted on one boom; other booms hold the
    science scan platform and the dual magnetometers.]

  High-gain directional antenna
  Extendable boom
  Planetary radio astronomy and plasma wave antenna
  Radioisotope thermoelectric generators
  Plasma detector
  Cosmic ray detector
  Wide angle TV
  Narrow angle TV
  TV electronics
  Ultraviolet spectrometer
  Infrared interferometer spectrometer and radiometer
  Low energy charged particles
  Electronic compartments
  Science instrument calibration panel and shunt radiator
  Propulsion fuel tank
  Planetary radio astronomy and plasma wave antenna

Perched atop the Centaur stage of the launch rocket, each Voyager
spacecraft has a mass of 2 tons (2066 kilograms), divided about equally
between the spacecraft proper and the propulsion module used for final
acceleration to Jupiter. The Voyager itself, with a mass of 815
kilograms and typical dimensions of about 3 meters, is almost the size
and weight of a subcompact car—but enormously more complex. A better
analogy might be made with a large electronic computer—but no
terrestrial computer was ever asked to supply its own power source and
to operate unattended in the vacuum of space for up to a decade.

Each Voyager spacecraft carries instruments for eleven science
investigations covering visual, infrared, and ultraviolet regions of the
spectrum, and other remote sensing studies of the planets and
satellites; studies of radio emissions, magnetic fields, cosmic rays,
and lower energy particles; plasma (ionized gases) waves and particles;
and studies using the spacecraft radios. Each Voyager has a science boom
that holds high and low resolution TV, photopolarimeter, plasma and
cosmic ray detectors, infrared spectrometer and radiometer, ultraviolet
spectrometer, and low energy charged particle detector; in addition,
each spacecraft has a planetary radio astronomy and plasma wave antenna
and a long boom carrying a low- and a high-field magnetometer.

Communication with Earth is carried out via a high-gain antenna 3.7
meters in diameter, with a smaller low-gain antenna as backup. The large
white dish of the high-gain antenna dominates the appearance of the
spacecraft, setting it off from its predecessors, which were able to use
much smaller antennas to communicate over the more modest distances that
separate the planets of the inner solar system. Although the transmitter
power is only 23 watts—about the power of a refrigerator light bulb—this
system is designed to transmit data over a billion kilometers at the
enormous rate of 115 200 bits per second. Data can also be stored for
later transmission to Earth; an onboard digital tape recorder has a
capacity of about 500 million (5 × 10⁸) bits, sufficient to store nearly
100 Voyager images.

    [Illustration: Project Manager Ray Heacock]

    [Illustration: During the early assembly stage, technicians at
    Caltech’s Jet Propulsion Laboratory equip Voyager’s extendable boom
    with low- and high-field magnetometers that measure the intensity
    and direction of the outer planets’ magnetic fields. [373-7179BC]]

Radioisotope thermoelectric generator (RTGs), rather than solar cells,
provide electricity for the Voyager spacecraft. The RTG use radioactive
plutonium oxide for this purpose. As the plutonium oxide decays, it
gives off heat which is converted to electricity, supplying a total of
about 450 watts to the spacecraft at launch. This power slowly declines
as the plutonium is used up, with less than 400 watts expected at Saturn
flyby five years after launch. Hydrazine fuel is used to make mid-course
corrections in trajectory and to control the spacecraft’s orientation.

Since the Voyagers must fly through the inner magnetosphere of Jupiter,
it was imperative that the hardware systems be able to withstand the
radiation from Jovian charged particles. The electronic microcircuits
that form the heart and brain of the spacecraft and its scientific
instruments are especially susceptible to radiation damage. Three
techniques were used to “harden” components against radiation:

  1. Special design using radiation-resistant materials;

  2. Extensive testing to select those electronic components which come
  out of the manufacturing process with highest reliability; and

  3. Spot shielding of especially sensitive areas with
  radiation-absorbing materials.


The Voyager spacecraft will be the third and fourth human artifacts to
escape entirely from the solar system. Pioneers 10 and 11, which
preceded Voyager in outstripping the gravitational attraction of the
Sun, both carried small metal plaques identifying their time and place
of origin for the benefit of any other spacefarers that might find them
in the distant future. With this example before them, NASA placed a more
ambitious message aboard Voyager 1 and 2—a kind of time capsule,
intended to communicate a story of our world to extraterrestrials.

The Voyager message is carried by a phonograph record—a 12-inch
gold-plated copper disk containing sounds and images selected to portray
the diversity of life and culture on Earth. The contents of the record
were selected for NASA by a committee chaired by Carl Sagan of Cornell
University. Dr. Sagan and his associates assembled 115 images and a
variety of natural sounds, such as those made by surf, wind and thunder,
birds, whales, and other animals. To this they added musical selections
from different cultures and eras, and spoken greetings from Earthpeople
in sixty languages, and printed messages from President Carter and U.N.
Secretary General Waldheim.

Each record is encased in a protective aluminum jacket, together with a
cartridge and needle. Instructions, in symbolic language, explain the
origin of the spacecraft and indicate how the record is to be played.
The 115 images are encoded in analog form. The remainder of the record
is in audio, designed to be played at 16⅔ revolutions per second. It
contains the spoken greetings, beginning with Akkadian, which was spoken
in Sumer about six thousand years ago, and ending with Wu, a modern
Chinese dialect. Following the section on the sounds of Earth, there is
an eclectic 90-minute selection of music, including both Eastern and
Western classics and a variety of ethnic music.

Once the Voyager spacecraft leave the solar system (by 1990, both will
be beyond the orbit of Pluto), they will find themselves in empty space.
It will be forty thousand years before they come within a light year of
a star, called AC + 79 3888, and millions of years before either might
make a close approach to any other planetary system. As Carl Sagan has
noted, “The spacecraft will be encountered and the record played only if
there are advanced spacefaring civilizations in interstellar space. But
the launching of this bottle into the cosmic ocean says something very
hopeful about life on this planet.”

                      LANGUAGES ON VOYAGER RECORD

  Ila (Zambia)
  Amoy (Min dialect)


  Planets (music)
  Mud pots
  Crickets, frogs
  Wild dog
  Footsteps and heartbeats
  Dogs, domestic
  Herding sheep
  Blacksmith shop
  Morse code
  Horse and cart
  Horse and carriage
  Train whistle
  Auto gears
  Lift-off Saturn 5 rocket
  Life signs—EEG, EKG


  Calibration circle
  Solar location map
  Mathematical definitions
  Physical unit definitions
  Solar system parameters
  The Sun
  Solar spectrum
  Egypt, Red Sea, Sinai Peninsula and the Nile
  Chemical definitions
  DNA structure
  DNA structure magnified
  Cells and cell division
  Anatomy (eight)
  Human sex organs
  Diagram of conception
  Fertilized ovum
  Fetus diagram
  Diagram of male and female
  Nursing mother
  Father and daughter (Malaysia)
  Group of children
  Diagram of family ages
  Family portrait
  Diagram of continental drift
  Structure of Earth
  Heron Island (Great Barrier Reef of Australia)
  Snake River and Grand Tetons
  Sand dunes
  Monument Valley
  Forest scene with mushrooms
  Fallen leaves
  Tree with daffodils
  Flying insect with flowers
  Diagram of vertebrate evolution
  Seashell (Xancidae)
  School of fish
  Tree toad
  Jane Goodall and chimps
  Sketch of Bushmen
  Bushmen hunters
  Man from Guatemala
  Dancer from Bali
  Andean girls
  Thailand craftsman
  Old man with beard and glasses (Turkey)
  Old man with dog and flowers
  Mountain climber
  Cathy Rigby
  Children with globe
  Cotton harvest
  Grape picker
  Underwater scene with diver and fish
  Fishing boat with nets
  Cooking fish
  Chinese dinner party
  Demonstration of licking, eating and drinking
  Great Wall of China
  House construction (African)
  Construction scene (Amish country)
  House (Africa)
  House (New England)
  Modern house (Cloudcroft, New Mexico)
  House interior with artist and fire
  Taj Mahal
  English city (Oxford)
  UN Building, day
  UN Building, night
  Sydney Opera House
  Artisan with drill
  Factory interior
  X-ray of hand
  Woman with microscope
  Street scene, Asia (Pakistan)
  Rush hour traffic, India
  Modern highway (Ithaca)
  Golden Gate Bridge
  Airplane in flight
  Airport (Toronto)
  Antarctic expedition
  Radio telescope (Westerbork, Netherlands)
  Radio telescope (Arecibo)
  Page of book (Newton, _System of the World_)
  Astronaut in space
  Titan Centaur launch
  Sunset with birds
  String quartet (Quartetto Italiano)
  Violin with music score (Cavatina)

                        MUSIC ON VOYAGER RECORD

  Bach Brandenberg Concerto Number Two, First Movement
  “Kinds of Flowers” Javanese Court Gamelan
  Senegalese Percussion
  Pygmy girls initiation song
  Australian Horn and Totem song
  “El Cascabel” Lorenzo Barcelata
  “Johnny B. Goode” Chuck Berry
  New Guinea Men’s House
  “Depicting the Cranes in Their Nest”
  Bach Partita Number Three for Violin; Gavotte et Rondeaus
  Mozart Magic Flute, Queen of the Night (Aria Number 14)
  Peruvian Pan Pipes
  Melancholy Blues
  Azerbaijan Two Flutes
  Stravinsky, Rite of Spring, Conclusion
  Bach Prelude and Fugue Number One in C Major from the Well Tempered
          Clavier, Book Two
  Beethoven’s Fifth Symphony, First Movement
  Bulgarian Shepherdess Song “Izlel Delyo Hajdutin”
  Navajo Indian Night Chant
  The Fairie Round from Pavans, Galliards, Almains
  Melanesian Pan Pipes
  Peruvian Woman’s Wedding Song
  “Flowing Streams”—Chinese Ch’in music
  “Jaat Kahan Ho”—Indian Raga
  “Dark Was the Night”
  Beethoven String Quartet Number 13 “Cavatina”

    [Illustration: Each Voyager carries a message in the form of a
    12-inch gold-plated phonograph record. The record, together with a
    cartridge and needle, is fastened to the side of the spacecraft in a
    gold-anodized aluminum case that also illustrates how the record is
    to be played. [P-19728]]


The Voyager spacecraft had greater independence from Earth-based
controllers and greater versatility in carrying out complex sequences of
scientific measurements than any of their predecessors. These
capabilities resulted from three interconnected onboard computer
systems: the AACS (attitude and articulation control subsystem); the FDS
(flight data subsystem); and the CCS (computer command system).
Operating from “loads” of instructions transmitted earlier from Earth,
these computers could issue commands to the spacecraft and the science
instruments and react automatically to problems or changes in operating

The complex sequence of scientific observations and the associated
engineering functions were executed by the spacecraft under the control
of an updatable program stored in the CCS by ground command. At
appropriate times, the CCS issued commands to the AACS for movement of
the scan platform or spacecraft maneuvers; to the FDS for changes in
instrument configuration or telemetry rate; or to numerous other
subsystems within the spacecraft for specific actions. The two identical
(redundant) 4096-word memories within the CCS contained both fixed
routines (about 2800 words) and a variable section (about 1290 words)
for changing science sequencing functions. A single 1290-word science
sequence load could easily generate 300 000 discrete commands, thus
providing significantly more sequencing capability than would be
possible through ground commands. A 1290-word sequencing load in the CCS
controlled both the science and engineering functions of the spacecraft
for a period lasting for ¾ day at closest approach and for up to 100
days during cruise.

Each 1290-word program (or load) was built from specific science
measurement units called links. Some links were used repeatedly in a
looping cyclic (like a computer DO loop) to perform the same observation
numerous times; other links that involved special measurement geometry
or critical timing occurred only once. About 175 science links were
defined for the Voyager 1 Jupiter encounter. It took almost two years to
convert the desired science objectives and measurements first into
links, then into a minute-by-minute timeline for the 98-day encounter
period, and finally into the specific computer instructions that could
be loaded into the CCS memory for that portion of the encounter time
represented by a particular load. The total Voyager 1 Jupiter encounter
period used eighteen sequence memory loads, supplemented by about 1000
ground commands to modify the sequences because of changing conditions
or calibration requirements.

For the Voyager 2 encounter, concern about the ailing spacecraft
receiver limited the number of loads that could be transmitted,
particularly while the spacecraft was deep within the Jovian
magnetosphere, where radiation effects caused the receiver frequency to
drift unpredictably. However, a careful redesign of the planned
sequences permitted the accomplishment of very nearly the original set
of observations even with these constraints.

All three approaches were required on the Voyager craft, especially
after Pioneer 10 and 11 demonstrated that the radiation at Jupiter was
even more intense than had been assumed in early design studies.

The steady streams of engineering and scientific data received on Earth
are transmitted from the receiving stations of the Deep Space Network
(DSN) to JPL, where the Voyager control functions are centered. There,
dozens of technicians check and recheck every subsystem to search for
the slightest hint of malfunction. In case of problems, there are thick
notebooks of instructions and racks of precoded computer tapes ready to
be used to correct any apparent malfunction.

Normally Voyager runs itself. Detailed instructions are programmed into
its onboard computers and command systems for dealing with such
potential emergencies as a stuck valve in the fuel system, loss of
orientation in the star trackers, erratic gyroscope functions, failure
of radio communications, or a thousand and one other nightmares. The
instructions for operating the scientific instruments are also stored on
board, with new blocks of commands sent up once every few days to
replace those for tasks already completed. Whether in the calm of cruise
mode or the intense excitement of a planetary encounter, the Voyager
craft is alone in space, continuously sensing and reacting to its
environment, tied by only a tenuous thread of radio communication to the
anxious watchers back on Earth.

    [Illustration: Project Manager Robert Parks]

                       NASA PLANETARY MISSIONS
    Spacecraft       Launch    Destination   Encounter    Type of
                      Date                      Date     Encounter

  Mariner 2           8/26/62  Venus           12/14/62  flyby
  Mariner 4          11/28/64  Mars             7/14/65  flyby
  Mariner 5           6/14/67  Venus           10/19/67  flyby
  Mariner 6           2/25/69  Mars             7/31/69  flyby
  Mariner 7           3/27/69  Mars             8/05/69  flyby
  Mariner 9           5/30/71  Mars            11/13/71  orbiter
  Pioneer 10          3/03/72  Jupiter         12/04/73  flyby
  Pioneer 11          4/06/73  Jupiter         12/03/74  flyby
                               Saturn           9/01/79  flyby
  Mariner 10         11/03/73  Venus            2/05/74  flyby
                               Mercury          3/29/74  flyby
  Viking 1            8/20/75  Mars             6/19/76  orbiter
                                                7/20/76  lander
  Viking 2            9/09/75  Mars             7/07/76  orbiter
                                                9/03/76  lander
  Voyager 1           8/20/77  Jupiter          3/05/79  flyby
  Voyager 2           9/05/77  Jupiter          7/09/79  flyby
  Pioneer Venus       5/20/78  Venus           12/04/78  orbiter
                       8/8/78  Venus           12/09/78  probe

    [Illustration: The Voyager scan platform contains sophisticated
    instruments that gather data for Voyager’s remote sensing
    investigations. Five of the remote-sensing instruments—two TV
    cameras, the infrared spectrometer, the ultraviolet spectrometer,
    and the photopolarimeter—are mounted together on the scan platform,
    which can be pointed to almost any direction in space, allowing
    exact targeting of the observations. [373-7146BC]]

                               CHAPTER 4
                         SCIENCE AND SCIENTISTS


There are many reasons for sending spacecraft to the planets, but in the
final analysis, we send our robot messengers across the vastness of
space for the sake of scientific exploration. Science and exploration
have always gone hand in hand, whether in the transcontinental journey
of Lewis and Clark or the Pacific voyages of Captain James Cook. In this
century, as exploration has become more and more dependent on advances
in technology, the scientific element has attained increased prominence.
The greatest legacy of the NASA Planetary Program is the knowledge it
has provided of the other worlds that share our corner of the universe.

Despite the central role of science in motivating missions to the
planets, specific scientific considerations are not dominant during most
of the development of a mission such as Voyager. The problem of building
a spacecraft and getting it to the outer solar system is too demanding.
Of the ton of mass in a Voyager spacecraft, the scientific instruments
make up only 115 kilograms (about eleven percent). Similarly, the cost
of these instruments amounts to only about 10 percent of the cost of the
spacecraft and launch vehicle. Unless the launch and operation of the
spacecraft are nearly perfect, there can be no scientific return in any
case; even the most sophisticated package of scientific instruments will
not tell much about Jupiter if, following launch, it rests at the bottom
of the Atlantic Ocean. But it is equally true that the ultimate purpose
of the mission is scientific discovery, and NASA makes every effort to
ensure that the very best instruments are flown and that a broad
scientific community is given the opportunity to participate in each

A decade before the 1977 launch, many astronomers and space scientists
began their involvement with the Voyager mission through participation
in study groups convened by NASA and by the National Academy of
Sciences. They came primarily from universities, but also in significant
numbers from NASA laboratories, from industry, and from abroad. In 1971
the Outer Planets Grand Tours mission definition group carried out a
one-year final study of the mission that was to become Voyager. A
competition was held in 1972 to select the Voyager flight instruments
and science teams, and a third review stage followed a year later to
confirm this selection. Out of this process emerged eleven science
investigations, with which more than one hundred scientists were
associated. In this chapter we look at the instruments and the persons
who designed them for this challenging task.

                     Direct and Remote Measurements

The diverse measurements made by Voyager of a planet and its environment
can be divided into two broad categories, usually called direct or _in
situ_ measurements and remote sensing measurements. A direct measurement
involves the analysis of the immediate environment of the spacecraft;
remote measurements can be made by analyzing radiation from distant

The direct measurement instruments on Voyager measure cosmic ray
particles, low energy charged particles, magnetic fields, plasma
particles, and plasma waves. Their activity began immediately after
launch, monitoring the Earth environment and then interplanetary space
until the magnetosphere of Jupiter was reached a few days before the
actual flyby. Long after the flybys of Jupiter and Saturn are completed,
particles and fields data can continue to be acquired by the Voyagers as
they speed outward into previously unexplored regions of space.

The remote sensing investigations are essentially astronomical in
nature, measuring the light reflected from or emitted by the planet and
its satellites. On Voyager, however, these instruments far outstrip
their terrestrial counterparts in capability. Primarily, they derive
their advantage from their proximity to what they observe—at its
closest, Voyager was more than a thousand times nearer to Jupiter than
are Earth-based telescopes; and for the still closer satellite
encounters, Voyager was nearly ten thousand times nearer than
astronomers on Earth. In addition, Voyager provided perspectives, such
as views of the night side of Jupiter, that are impossible from Earth.
Finally, these instruments could exploit the full spectrum of
electromagnetic radiation without concern for the opacity of the
terrestrial atmosphere, which restricts ground-based astronomers to
certain spectral windows and blocks all observation at other

Five of the remote sensing instruments—the two TV cameras, the infrared
spectrometer, the ultraviolet spectrometer, and the photopolarimeter—are
mounted together on a scan platform. This platform can be pointed to
almost any direction in space, allowing exact targeting of the

One remote sensing instrument, the planetary radio astronomy receiver,
is not on the scan platform. It measures long-wave radio emission
without requiring special pointing.

    [Illustration: The fully deployed Voyager spacecraft is capable of a
    wide variety of direct and remote sensing measurements. The
    instruments and their objectives were selected many years before the
    first Jupiter encounter. Because of the exploratory nature of the
    Voyager mission, every effort was made to fly versatile instruments
    that could yield valuable results no matter what the nature of the
    Jovian system. [P-18811AC]]

  Narrow angle TV
  Wide angle TV
  Steerable platform
  Science instrument boom
  Plasma detector
  Cosmic ray detector
  High-gain antenna
  Low-gain antenna
  Extendable magnetometer boom
  Low energy charged particles detector
  Infrared interferometer spectrometer and radiometer
  Ultraviolet spectrometer
  Star trackers
  Radioisotope thermoelectric generators
  Science instrument calibration panel and shunt radiator
  Planetary radio astronomy and plasma wave antenna

    [Illustration: The science instrument boom supports the plasma
    particle detector, the cosmic ray detector, and the low energy
    charged particle detector. These instruments began collecting data
    immediately after launch, monitoring the Earth environment and then
    interplanetary space until the magnetosphere of Jupiter was reached
    a few days before the actual flyby. [353-2992BC]]

A final Voyager investigation did not fit into this pattern of direct
versus remote sensing instruments. In fact, it required no special
instrument at all. This investigation deals with radio science, and it
utilizes the regular communications link between the spacecraft and
Earth to derive the masses of Jupiter and its satellites, to probe the
atmosphere of Jupiter, and to study properties of the interplanetary

    [Illustration: The Voyager Imaging Science Team]


The eyes of Voyager are in its imaging system. Two television cameras,
each with a set of color filters, look at the planets and their
satellites and transmit thousands of detailed pictures to Earth. The
imaging system is probably the most versatile and therefore the most
truly exploratory of the Voyager instruments. No matter what is out
there, the imaging system will let us see it and hence, we hope, begin
to understand its nature.

                    Project Scientist: E. C. Stone, Caltech
     Investigation       Principal Investigator      Primary Objectives at
                             or Team Leader                 Jupiter

  Imaging science       B. A. Smith, U. Arizona   High resolution
                                                  reconnaissance over large
                                                  phase angles; measurement
                                                  of atmospheric dynamics;
                                                  determination of geologic
                                                  structure of satellites;
                                                  search for rings and new
  Infrared radiation    R. A. Hanel, NASA         Determination of
  (IRIS)                Goddard                   atmospheric composition,
                                                  thermal structure, and
                                                  dynamics; satellite surface
                                                  composition and thermal
  Ultraviolet           A. L. Broadfoot, Kitt     Measurement of upper
  spectroscopy          Peak Observatory          atmospheric composition and
                                                  structure; auroral
                                                  processes; distribution of
                                                  ions and neutral atoms in
                                                  the Jovian system.
  Photopolarimetry      C. F. Lillie/C. W.        Measurement of atmospheric
                        Hord, U. Colorado         aerosols; satellite surface
                                                  texture and sodium cloud.
  Planetary radio       J. W. Warwick, U.         Determination of
  astronomy             Colorado                  polarization and spectra of
                                                  radio frequency emissions;
                                                  Io radio modulation
                                                  process; plasma densities.
  Magnetic fields       N. F. Ness, NASA Goddard  Measurement of plasma
                                                  electron densities;
                                                  wave-particle interactions;
                                                  low-frequency wave
  Plasma particles      H. S. Bridge, MIT         Measurement of
                                                  magnetospheric ion and
                                                  electron distribution;
                                                  solar wind interaction with
                                                  Jupiter; ions from
  Plasma waves          F. L. Scarf, TRW          Measurement of plasma
                                                  electron densities;
                                                  wave-particle interactions;
                                                  low-frequency wave
  Low energy charged    S. M. Krimigis, Johns     Measurement of the
  particles             Hopkins U.                distribution, composition,
                                                  and flow of energetic ions
                                                  and electrons;
                                                  particle interactions.
  Cosmic ray particles  R. E. Vogt, Caltech       Measurement of the
                                                  distribution, composition,
                                                  and flow of high energy
                                                  trapped nuclei; energetic
                                                  electron spectra.
  Radio science         V. R. Eshleman,           Measurement of atmospheric
                        Stanford U.               and ionospheric structure,
                                                  constituents, and dynamics;
                                                  satellite masses.

Unlike other Voyager instruments, the imaging system is not the result
of a competition among proposals submitted by groups of scientists. NASA
assigned the development of the cameras directly to JPL, to be
integrated from the beginning with the design of the Voyager spacecraft
and its subsystems. The members of the Voyager Imaging Science Team were
selected, as individuals, on the basis of the scientific studies they
proposed to carry out. Initially, ten members of the Imaging Science
Team were selected, but by the time of the Jupiter encounters, the team
had been expanded to 22 scientists.

    [Illustration: Bradford A. Smith, imaging science Team Leader]

The Team Leader is Bradford A. Smith, a professor in the Department of
Planetary Science at the University of Arizona. Smith was involved in
imaging science on several previous missions, including Mariners 6 and 7
and Viking. He was also active in ground-based photography of Jupiter at
both New Mexico State University and University of Arizona, and he is a
member of the team developing a planetary camera for the Space
Telescope, scheduled for operation in Earth orbit in the mid-1980s.

Originally, the Deputy Team Leader was Geoffrey A. Briggs, a young
British-born physicist from JPL. However, in 1977 Briggs took a position
at NASA Headquarters in Washington and was replaced by geologist
Laurence A. Soderblom of the U.S. Geological Survey in Flagstaff,
Arizona. An energetic and articulate scientist, Soderblom, with his
interest in satellite geology, complemented Smith, whose personal
scientific interests are directed more toward the atmosphere of Jupiter.

The objectives of imaging involved multicolor photography of Jupiter and
its satellites. Both wide- and narrow-angle cameras were needed to
obtain the highest possible resolution while retaining the capability to
study global-scale features on Jupiter and the satellites. In normal
photographic terms, both cameras used telephoto lenses. For the
wide-angle camera, a focal length of 200 millimeters was selected,
giving a field of view of about 3 degrees. This field is similar to that
obtained with a 400-millimeter telephoto lens on a 35-millimeter camera.
The narrow-angle Voyager camera has a focal length of 1500 millimeters
and field of view of 0.4 degrees. The camera optics are combinations of
mirrors and lenses, designed for extreme stability of focus and for
freedom from distortion.

Each camera has a rotating filter wheel that can be used to select the
color of the light that reaches the camera. For the wide-angle camera,
these colors are clear, violet, blue, green, orange, and three special
bands for selective observation in sodium light (589-nanometer
wavelength), and in methane spectral lines at 541 nanometers and 618
nanometers. For the narrow-angle camera, the filters are clear,
ultraviolet, violet, blue, green, and orange. To create a color picture,
the cameras are commanded to take, in rapid succession, pictures of the
same area in blue, green, and orange light. These three pictures can
then be reconstructed on Earth into a “true” color image. Other
combinations of colors are used to investigate particular scientific
problems and to determine the spectrum of sunlight reflected from
features on Jupiter and its satellites.

The detector in the cameras is not photographic film but the surface of
a selenium-sulfur vidicon television tube, 11 millimeters square. Unlike
most commercial TV cameras, these tubes are designed for slow-scan
readout, providing 48 seconds to acquire each picture. The shutter speed
can be varied from a fraction of a second (for Jupiter and Io) to many
minutes (for searches for faint features, such as aurorae on the night
side of Jupiter).

Each picture consists of many numbers, each of which represents the
brightness of a single picture element, or pixel, on the image. There
are 640 000 pixels in each Voyager image, representing a square image of
800 × 800 points. This information content is more than twice that of an
ordinary television picture, which has only 520 lines. For each pixel,
eight binary numbers are required to specify the brightness; the total
information in a single image is thus 8 × 640 000 = 5 120 000 bits. Even
at a transmission of one frame per 48 seconds, the “bit rate” is more
than 100 000 bits per second. For comparison, the bit rate from the
first spacecraft to Mars (Mariner 4) was about 10 bits per second,
requiring a week to transmit 21 pictures, with a total information
content equivalent to a single Voyager picture. Altogether, Voyager took
nearly 20 000 pictures at each Jupiter encounter, representing 10¹¹—a
hundred billion—bits of information.

    [Illustration: Rudolph Hanel, infrared spectrometer Principal

                         Infrared Spectrometer

The infrared investigation on Voyager is based on one of the most
sophisticated instruments ever flown to another planet. In the past,
most infrared instruments on planetary spacecraft measured at only a few
wavelengths, but Voyager carries a true spectrometer, capable of
measuring at nearly 2000 separate wavelengths, covering the spectrum
from 4 to 50 micrometers.

Twelve scientists, led by Principal Investigator Rudolph Hanel of the
NASA Goddard Space Flight Center at Greenbelt, Maryland, proposed this
infrared instrument. Hanel is an acknowledged world leader in infrared
spectroscopy from space. With his co-workers at Goddard, he has
pioneered in adapting the extremely complex art of interferometric
spectroscopy to the rigors of space flight. His spectrometers have made
many studies of the Earth’s atmosphere from meteorological satellites,
and a Hanel interferometer flew successfully to Mars on Mariner 9.

The primary goals of the infrared spectrometer investigation are
directed toward analysis of the composition and structure of the
atmosphere of Jupiter. Among the molecules to be searched for on Jupiter
were hydrogen (H₂), helium (He), methane (CH₄), ammonia (NH₃), phosphine
(PH₃), water (H₂O), carbon monoxide (CO), simple compounds of silicon
and sulfur, and a variety of organic compounds consisting of atoms of
carbon and hydrogen (e.g., C₂H₂, C₂H₄, C₂H₆). In addition to indicating
the abundance of these constituents of the atmosphere, the infrared
spectra also contain information on atmospheric structure, that is, on
the variation of temperature and pressure with altitude. The presence of
clouds or dust layers can also be inferred from the shapes of spectral

In addition to its spectroscopy of Jupiter, the Voyager instrument could
be used as a heat-measuring device to map the temperatures of both the
satellites and the atmosphere of Jupiter. Particularly interesting for
the satellites are measurements of the surface cooling and heating
rates, since these rates reveal the physical compactness of the surface,
easily distinguishing between rock and sand or dust.

The infrared instrument is a Michelson interferometer at the focus of a
gold-plated telescope of 51-centimeter aperture. The spectrum is not
obtained directly, by moving a prism or grating, but indirectly through
the interference effects of light of different wavelengths: hence its
name, an interferometer. The complex interference pattern generated by
the motion of one of the mirrors in the light path is transmitted to
Earth, where computer analysis is required to transform it into a
recognizable spectrum. The entire instrument, which has a mass of 20
kilograms, is called IRIS, for infrared interferometer spectrometer.

The development of the infrared system for Voyager posed many problems.
IRIS was designed to cover the optimum spectral region for studies of
the atmospheres of Jupiter and Saturn. However, when it was decided in
1974 that an extended Voyager mission might also allow a visit to
Uranus, Hanel and his colleagues realized that their instrument had
serious deficiencies for investigation of that colder and more distant
planet, and they proposed that a modified IRIS (MIRIS) be substituted
for the original design. A crash program was authorized to develop MIRIS
in parallel with IRIS, and in early 1977, as launch approached, it
appeared that the improved instrument would be ready. Problems occurred
during testing, however, and for several weeks in June and July the
decision hung in the balance. Unfortunately, there simply was not enough
time to solve all the problems; both Voyagers were launched carrying the
original IRIS, and the final flight qualification of MIRIS came in
October, about six weeks after launch. With no other missions planned
beyond Saturn, no alternate use for MIRIS has been found; it remains “on
the shelf,” one of the rare cases where a technological gamble by NASA
did not pay off.

    [Illustration: Lyle Broadfoot, ultraviolet spectrometer Principal

                        Ultraviolet Spectrometer

A second spectroscopic instrument on the Voyager scan platform examines
short-wave, or ultraviolet, radiation. The Principal Investigator for
the ultraviolet spectrometer (UVS) is A. Lyle Broadfoot of Kitt Peak
National Observatory in Tucson, Arizona. Broadfoot’s expertise lies in
instrumentation for atmospheric research, and he was previously the
Principal Investigator for a similar ultraviolet instrument on the
Mariner 10 mission to Venus and Mercury. Associated with Broadfoot in
the Voyager investigation are fourteen other scientists from the United
States, Canada, and France.

In the upper atmosphere of a planet, the lighter gases tend to diffuse,
rising above their heavier neighbors, and unusual chemical reactions
take place as the action of sunlight breaks some chemical bonds and
stimulates the formation of others. The study of these tenuous regions
of planetary atmospheres is called aeronomy. Many of the exotic chemical
processes that occur can best be studied by examining radiation of very
short wavelength, and it is to these problems that much of the work of
the UVS team is directed.

The observations are also sensitive to special processes in the Jovian
atmosphere resulting from the magnetosphere. At the very top of the
atmosphere, energetic charged particles interacting with the atmospheric
molecules produce ultraviolet aurorae that indicate the location and
nature of the bombarding electrons and ions. Emissions from atoms in the
extended gas clouds around Io and possibly around other satellites were
also expected to be observable.

The ultraviolet instrument is a relatively straightforward optical
spectrometer, adapted for use in space. A diffraction grating disperses
the light into a spectrum, and the other optical elements are
gold-coated mirrors. The field of view is rectangular, about 0.1 degrees
× 0.9 degrees. An array of sensitive electronic detectors provides
simultaneous measurements in 128 wavelength channels over a wavelength
range from 50 to 170 nanometers.


The fourth scan-platform instrument is designed to measure the
brightness and polarization of light with high precision. Unlike the
imaging system, however, it can look at only a single point (one pixel)
at a time; thus it sacrifices spatial capability in favor of precision
of measurement. The prime objectives of this photopolarimeter are
related to study of the clouds of Jupiter.

During the development stage for this instrument, the Principal
Investigator was Charles F. Lillie of the Laboratory for Atmospheric and
Space Physics of the University of Colorado at Boulder. Later, the
Principal Investigator’s role was assumed by Lillie’s colleague, Charles
W. Hord, a physicist with considerable experience in space missions,
primarily with ultraviolet instruments. Four other scientists are
Co-Investigators on the photopolarimeter.

The instrument itself is essentially a small Cassegrain reflecting
telescope of 15-centimeter aperture. Two consecutive filter wheels
select the color and polarization of the light, which is then detected
and measured by a photomultiplier tube. The entire photopolarimeter
weighs just 2.5 kilograms.

In spite of their apparent simplicity, however, the Voyager
photopolarimeters were plagued with troubles almost from the moment of
launch. A succession of mechanical failures on Voyager 1 led to the
sticking first of the polarization wheel and then of the filter wheel.
Even when repeated commands from Earth managed to free the wheels, their
behavior was erratic. Ultimately an electronic failure also took place,
leading to the reluctant decision to shut down the Voyager 1
photopolarimeter. On Voyager 2, similar mechanical problems greatly
restricted the ability of the instrument to carry out its observations.
The photopolarimeter thus was unable to contribute its share to
unraveling the mysteries of Jupiter and its satellites.

    [Illustration: Charles W. Hord, photopolarimeter Principal

    [Illustration: James W. Warwick, planetary radio astronomy Principal

                       Planetary Radio Astronomy

The final Voyager remote sensing instrument is designed to measure radio
emission from Jupiter and Saturn over a wide range of frequencies. These
emissions, which sound like hiss or static if played through an audio
receiver, result from interactions of charged particles in the
magnetospheres and ionospheres of the giant planets. The planetary radio
astronomy (PRA) Principal Investigator is James W. Warwick, an
astronomer in the Department of Astro-Geophysics of the University of
Colorado at Boulder. Eleven colleagues from the United States and France
participate as Co-Investigators. Warwick has been studying Jupiter
longer than any other Voyager Principal Investigator. He has monitored
its radio emissions since the 1960s, and he played a central role in the
discovery that Io influenced these emissions.

Jupiter emits many kinds of radio radiation, ranging from bland thermal
emission at short (centimeter) wavelengths, to synchrotron emission from
energetic electrons at intermediate (decimeter) wavelengths, to erratic,
extremely intense bursts at long (meter and decameter) wavelengths. The
origin of these latter, nonthermal emissions constitutes one of the
major unsolved problems of the Jovian system, and, more generally, of
the physics of plasmas. One of the most important advantages of the
Voyager PRA for studying Jovian radio emission lies in its proximity to
Jupiter and hence its ability to locate the sources of different kinds
of radio bursts.

The PRA instrument consists of an antenna and a radio receiver. The
antenna is made up of two thin metal poles, each 10 meters long,
extended from the spacecraft after launch at an angle of 90 degrees to
each other. These are electrically connected to two receivers of
extremely high sensitivity and broad frequency response: from 1.2
kilohertz to 40.5 megahertz. The PRA can operate in a number of modes,
depending on the measurements desired. At its lowest level of activity,
it monitors intensity in all 198 bands and transmits the data at 266
bits per second. In its highest mode, where searches are made for
variations with very short time scales, the data rate goes up to 108 000
bits per second, essentially the same as that required by imaging. In
fact, the high-rate PRA data actually use an imaging frame as a display
form, and occasionally throughout the mission an unfamiliar looking
“image” was transmitted that was actually a block of PRA data—a portrait
of electrical events in the Jovian atmosphere and magnetosphere that
only Warwick and his colleagues could interpret.

    [Illustration: Norman F. Ness, magnetometer Principal Investigator]

    [Illustration: Voyager’s 13-meter-long magnetometer boom is shown
    fully extended. In space, under zero gravity conditions, the
    triangular epoxy glass mast spirals from its housing and provides a
    rigid support for two magnetometer instruments—one at the end of the
    boom and another at about the midpoint. [260-181]]


The first of the direct sensing instruments to be discussed is the
magnetometer, designed to measure the magnetic fields surrounding the
spacecraft. Such measurements can be interpreted to yield the intrinsic
fields of Jupiter and its satellites and to characterize, in conjunction
with data from particle and plasma instruments, the processes taking
place in the magnetosphere of the planet. The Principal Investigator for
this instrument is Norman F. Ness of the NASA Goddard Space Flight
Center. Ness is an intense, competitive scientist with a great deal of
previous experience in spacecraft magnetometers, primarily on board
Earth satellites. Ness is also the only Voyager Principal Investigator
who has previous experience at Jupiter; he was Principal Investigator on
one of the two magnetometer instruments flown on Pioneer 11. For the
Voyager investigation, Ness is joined by four colleagues from Goddard
and one from Germany.

The magnetometer instrument consists of two systems: a high-field
magnetometer and a low-field magnetometer. Each system contains two
identical three-axis magnetometers that measure the intensity and
direction of the magnetic field. The low-field system requires isolation
from magnetic fields induced by electric circuits in the spacecraft
itself. To achieve this isolation, it is mounted on the largest
component of Voyager—a 13-meter boom, about as long as the width of a
typical city house lot. This boom was coiled tightly in a canister
during launch; later, when the package was opened, it uncurled and
extended automatically.

By using two magnetometers, Ness and his colleagues are able to correct
for the residual artificial magnetic fields that reach even 13 meters
from the main spacecraft. The dynamic range extends from a maximum field
of 20 gauss down to 2 × 10⁻⁸ gauss—a factor of one billion. The fields
can be measured as frequently as 17 times per second. The total mass,
exclusive of the 13-meter boom, is 5.6 kilograms.

    [Illustration: Herbert S. Bridge, plasma particle Principal

                            Plasma Particles

Plasma is the term given to a “gas” of charged particles; the electrons
and protons are separate, yet there are equal numbers of each, producing
a zero net charge. If the velocities of the electrons and protons are
less than about 0.1 percent of the speed of light, they can be measured
by the Voyager plasma instrument; if their energies are higher, they are
measured by one of the other two particle instruments—the low energy
charged particle (LECP) detector or the cosmic ray detector. The plasma
instrument, like the magnetometer, was designed to provide basic data on
the particles and fields environment of Voyager.

The Principal Investigator for the plasma instrument is Herbert S.
Bridge of the Massachusetts Institute of Technology in Cambridge,
Massachusetts. An experienced space physicist, Bridge has flown similar
instruments on many Earth satellites and planetary probes. He also holds
the position of Director of the Laboratory for Space Experiments at MIT.
On the Voyager experiment, he is joined by one German and ten U.S.

The objectives of the plasma investigation are directed toward study of
both the interplanetary medium and the Jovian magnetosphere. At Jupiter,
Bridge expected to determine the plasma populations and processes in the
inner and outer regions of the magnetosphere and in the plasma tail that
extends beyond Jupiter, much as a comet tail is blown outward by the
solar wind. The densities and temperature of electrons were measured,
and their origins determined: Some originate near Jupiter and diffuse
outward through the magnetosphere; others derive from the solar wind.

The plasma instrument, with a mass of 9.9 kilograms, was designed to
view in two directions: one toward the Earth and Sun, primarily to study
the solar wind, and the other sideways, looking toward the direction
plasma would flow if it were caught up in the rotating Jovian magnetic
field. If it is desired to look in other directions, the entire
spacecraft must be tipped, a maneuver that was carried out several times
near Jupiter. The detectors directly sense the flow of electrons,
protons, and alpha particles (helium nucleii, made up of two protons and
two neutrons each). Analysis of the energy spectra can also yield data
on positive ions of higher mass.

    [Illustration: Frederick L. Scarf, plasma wave Principal

                              Plasma Waves

The plasma wave investigation on Voyager was a late addition to the
scientific payload. It was selected to broaden the capability of the
mission to study a wide variety of plasma processes. Because of the
electrically charged nature of the plasma, it responds to energy inputs
in ways that ordinary gas cannot. One of these modes of response yields
plasma waves, which are oscillations in density and electric field that
generally cover the audio range of frequencies. Measurement of such
waves characterizes the density and temperature of the local plasma
surrounding the spacecraft, and it also allows remote sensing detection
of distant events from the plasma waves they produce.

The plasma wave Principal Investigator is physicist Frederick L. Scarf
of the TRW Defense and Space Systems Group of Redondo Beach,
California—he is the only Voyager Principal Investigator to come from
industry. Scarf has been associated with many particles and fields
investigations in the terrestrial magnetosphere, although he is better
known as a theorist than as an experimenter. A member of the Space
Science Board of the National Academy of Sciences, he is familiar in
Washington as an eloquent advocate of space physics—the study of
plasma-physical processes in the space environment.

The plasma wave instrument shares with the planetary radio astronomy
investigation a pair of 10-meter-long antennas. Whereas the PRA uses
these as electric antennas to detect radio radiation, the plasma wave
system uses them to detect directly the oscillations in the plasma near
the spacecraft. Waves are measured over a broad frequency range, from 10
hertz (a bit deeper than the lowest bass note we can hear) to 56
kilohertz (about three times higher than the highest pitch to which the
human ear responds). The instrument electronics have a total mass of
only 1.4 kilograms.

                      Low Energy Charged Particles

Charged particles with energies greater than a few thousand electron
volts are not easily measured by a plasma instrument such as that
designed by Herb Bridge. Instead, these faster moving particles, with
speeds up to a few percent the speed of light, are the subject of a pair
of Voyager instruments called collectively the LECP, or low energy
charged particle instrument. Like the other particles and fields
investigation, the LECP is designed to provide basic data on
plasma-physical processes in the Jovian magnetosphere and the solar
wind, and on their interactions.

The Principal Investigator for the LECP investigation is Stamatios Mike
Krimigis, a Greek-born physicist from Johns Hopkins University. Krimigis
has participated in a number of satellite studies of the terrestrial
magnetosphere, and he now serves as Head of Space Physics and
Instrumentation at the Johns Hopkins Applied Physics Laboratory. He is
joined in this investigation by one German and five U.S.

The LECP instrument consists of two subsystems. The first, called the
low energy magnetospheric particle analyzer, is optimized for
measurement of particles within the Jovian magnetosphere, with high
sensitivity over a broad dynamic range. Measurements of electrons,
protons, and other positive ions can be carried out, determining the
energy and composition of individual particles. The total energy ranges
covered are 10 kiloelectron volts (keV) to 11 million electron volts
(MeV) for electrons and 15 keV to 150 MeV for protons and ions.

    [Illustration: Stamatios Mike Krimigis, low energy charged particle
    Principal Investigator]

The second LECP subsystem is a low energy charged particle telescope,
designed to operate where the density of charged particles is low, such
as in interplanetary space or the outer magnetosphere of Jupiter. For
protons and positive ions, the energy range is from 50 keV to 40 MeV per
nucleon. The energy and species resolution is again sufficient to
determine the composition, both chemical and isotopic, of many ions
encountered. In order to provide directional discrimination even on a
spacecraft of fixed orientation, both LECP subsystems are mounted on a
moving platform that steps through eight positions in a time that can be
commanded to vary from 48 seconds to 48 minutes. The mass of the
instrument and its platform is 6.7 kilograms.

                              Cosmic Rays

The solar system is constantly bombarded by extremely energetic charged
particles. These are called cosmic rays, although they are particles,
not photons—“rays” are only produced when the particles strike
something, such as the molecules of the Earth’s atmosphere, and give up
their energy in a flash of x-rays and gamma-rays. One of the Voyager
instruments is designed to study these galactic cosmic rays,
particularly to look from beyond the orbit of Saturn, where the cosmic
ray particles will be less affected by the solar magnetic field and
solar wind than they are near Earth.

The cosmic ray Principal Investigator is Rochus E. Vogt of the
California Institute of Technology. Vogt has measured cosmic rays from
the ground, from balloons, and from spacecraft for many years. During
1977 and 1978 he served as Chief Scientist at JPL, and then assumed the
job of directing the physics, mathematics, and astronomy programs at
Caltech. Among his six Co-Investigators is Ed Stone, the Voyager Project

Because the cosmic ray instrument was not directed principally toward
measurements of the Jovian system, it is described only briefly. Like
the LECP, it is designed to determine the energy and composition of
individual electrons and positive ions. For electrons, the energy range
is from 3 to 110 MeV, and for ions from 1 to 500 MeV per nucleon; the
corresponding velocities are from about 10 percent to 99 percent of the
speed of light. For the positive ions, composition can be determined for
elements from hydrogen to iron. At Jupiter, this system could be used to
determine the nature of the rare particles accelerated to very high
energies in the Jovian magnetosphere.

                             Radio Science

The final Voyager science investigation is in the field of radio
science. No special instrument was required for this study; rather, NASA
selected members of a Radio Science Team who proposed investigations
that could be carried out using the already existing spacecraft
telecommunication system.

The radio science Team Leader is Von R. Eshleman of the Center for Radio
Astronomy at Stanford University. Eshleman is a radar physicist who has
been interpreting spacecraft radio occultation data since the first such
probe was carried out when Mariner 4 passed behind Mars in 1964. The
Deputy Team Leader is G. Leonard Tyler, a colleague of Eshleman’s at
Stanford. There are five other radio team members, four of them from

The radio science investigations are divided into two groups. The first
deals with the atmosphere of Jupiter. During the Voyager flybys, the
spacecraft passed behind the planet as seen from Earth, and the radio
signal was dimmed by the atmosphere before it was finally extinguished.
During an occultation, the propagation of the radio waves is slowed down
by passage through the neutral atmosphere and is speeded up by passage
through the electrically charged ionosphere. Because of the extreme
stability of the ground-based and spacecraft radio transmitters, it is
possible to measure these shifts in the signal with high precision. The
shifts are proportional to electron density for the ionosphere, and to
gas density for the atmosphere. From a careful study of the interactions
of the transmitted beam with the Jovian atmosphere, Eshleman and his
colleagues can reconstruct a temperature-pressure profile of the
ionosphere and the upper atmosphere of Jupiter. The same approach can be
used to search for tenuous atmospheres on the satellites.

The second area of study is in the field of celestial mechanics. The
frequency stability of the communications system permits measurements of
the speed of the spacecraft, relative to Earth, to a precision of one
part in several million. By careful tracking, gravitational
perturbations on the spacecraft can be detected and used to measure the
gravitational fields, and hence the masses, of Jupiter and its

    [Illustration: Rochus E. Vogt, cosmic ray Principal Investigator]

    [Illustration: Von R. Eshleman, radio science Team Leader]

These scientific instruments and their objectives were selected many
years before the first Jupiter encounter in March 1977. Because Voyager
was an exploratory mission, every effort was made to fly versatile
instruments that could yield valuable results no matter what the nature
of the Jovian system. In addition, the Voyager spacecraft control system
permitted the instruments to receive commands from Earth to adjust their
sensitivities and observing sequences in response to new information. By
the spring of 1977, all the instruments were completed, ready to be
installed in the Voyager spacecraft for testing and launch.

    [Illustration: The first picture to capture crescent Earth and
    crescent Moon in the same frame was taken by Voyager 1, the
    second-launched spacecraft, on September 18, 1977, at a distance of
    12 million kilometers from Earth. On the Earth eastern Asia, the
    western Pacific, and part of the Arctic can be seen. Since the Moon
    is much less reflective than the Earth, JPL image processors
    brightened the lunar image by a factor of three to ensure that both
    Earth and Moon were visible on this print. [P-19891C]]

                               CHAPTER 5


On August 20, 1977, exactly two years after the launch of the Viking
spacecraft to Mars, the first of the Voyagers—actually Voyager 2—was
boosted into space at 10:29 a.m. EDT, less than five minutes after the
launch window opened on the first day of the thirty-day launch period.
Sixteen days later, at 8:56 a.m. EDT on Labor Day, September 5, 1977,
Voyager 1 was hurled into space on a shorter, faster trajectory than its
twin, zipping past the orbit of the Moon only ten hours after launch.
Ultimately, Voyager 1 earned its title by overtaking Voyager 2 as both
spacecraft journeyed through the asteroid belt, to arrive at Jupiter
four months ahead of Voyager 2.

The Voyagers lifted off from Launch Complex 41, Air Force Eastern Test
Range, Kennedy Space Center, Cape Canaveral, Florida, atop the giant
Titan III-E/Centaur rocket. It was the last time such a launch vehicle
was scheduled to be used, as, according to plan, the Space Shuttle would
take over in the 1980s. Thus the launching of the two Voyagers signified
both an end and a beginning: a once-in-a-lifetime opportunity to
explore, in only 8½ years’ time, perhaps fifteen major bodies of the
outer solar system.

But long before even the first Voyager was to make its closest approach
to Jupiter—in fact, even before Voyager 2 was off the launch pad—there
were problems to overcome.

In early August 1977, about three weeks before launch, failures in the
attitude and articulation control subsystem (AACS) and the flight data
subsystem (FDS), two of the spacecraft’s three main computer subsystems,
prevented the VGR77-2 spacecraft, originally scheduled for launch on
August 20, from becoming Voyager 2. Instead, the “spare” spacecraft
VGR77-3 was substituted, becoming Voyager 2 upon launch August 20, and
VGR77-2, after proper repairs, became Voyager 1. Minor problems
continued right up to launch. The low energy charged particle instrument
failed and had to be replaced, and as late as T minus five minutes there
was a halt in the countdown to check on a stuck valve. Unlike a jinxed
dress rehearsal, which is said to “assure” an opening-night success,
Voyager 2’s prelaunch problems were a portent of difficulties to come.

The Voyager 2 launch was witnessed by thousands as the spacecraft
ascended gracefully into the blue Florida sky, accompanied by the
deep-throated rumblings of its rocket, echoing for miles across the
beaches and scrub forests of Cape Canaveral. The Titan-Centaur
performance was nearly flawless, and Voyager 2 quickly achieved an
accurate trajectory toward Jupiter. However, even while the engines were
still firing, the spacecraft began to experience a baffling series of
problems that would absorb the attention of hundreds of persons from
Pasadena to Washington, D.C., for the next several weeks until they were
brought under control.

During the first minutes of flight, there seemed to be two difficulties
with the AACS. The first was a problem with one of the three stabilizing
gyroscopes, but fortunately, the gyroscope began operating normally
without intervention from the ground. The other problem appeared to be
with one of the AACS computers; the spacecraft switched to a backup
computer during the Titan burn, and initial data transmissions were
incomplete. Early analysis seemed to indicate that an event during the
launch itself, rather than a faulty spacecraft computer system, was the
cause of the data loss. At first, on August 23, officials suspected that
perhaps the spacecraft had been bumped by the rocket motor one hour
after liftoff and again about seventeen hours later, when telemetry
signals indicated that the spacecraft had been jolted. However, by the
next day, flight engineers determined that electronic gyrations in the
AACS seemed to have caused the difficulty.

    [Illustration: The Titan-Centaur rocket used to launch Voyager stood
    as tall as a 15-story building and weighed nearly 700 tons. Here the
    rocket waits for launch at Kennedy Space Center, with the Voyager
    spacecraft enclosed in the white protective shroud at the top.

Within an hour after launch, Voyager 2’s science scan platform boom was
to have been fully extended and locked. Instructions to deploy were
given, and the boom moved outward; however, there was no signal to
indicate that the boom was actually locked in place. Efforts to command
the boom to move into the locked position were thwarted by the
spacecraft. The first maneuver designed to try to lock the boom was
aborted by the computer command subsystem (CCS) when the AACS
erroneously indicated that it was in trouble. Three days later another
maneuver was scheduled to reprogram the faulty computer in the AACS, to
align the Sun sensors, and to try to lock the science boom. To provide a
direct check of the boom position, the scan platform was turned so that
the TV cameras could see the spacecraft. Careful measurement of these
pictures verified that the boom was within ½ degree of full deployment,
but still there was no indication that it was locked into place.
Ultimately, it was decided that the sensor to signal actuation of the
lock was at fault, and that the boom itself was almost certainly fully
extended and operational.

Because of the postlaunch problems of Voyager 2, the launch of Voyager 1
was delayed twice—from September 1 to September 3 and then to September
5—in order to inspect Voyager 1’s science boom and to try to prevent a
repetition of Voyager 2’s problems. An extra spring was attached to the
science boom to assure its full extension. Finally, as if to make up for
the troubles of the first launch, Voyager 1’s launch was both “flawless
and accurate.” All launch and postlaunch events went smoothly. The
launch window opened at 8:56 a.m. EDT, and Voyager took off promptly at
8:56:01. The booms and antennas deployed and locked in the first hours
after launch; all instruments scheduled to be on were on and working


The two Voyager spacecraft were carried into space and accelerated
toward Jupiter by the Titan III-E Centaur rocket, the largest launch
vehicle in the NASA arsenal after the retirement of the Saturn rockets
in 1975. The Titan and Centaur vehicles were originally developed
separately and have been used with other rocket stages for many NASA
launches. They were first combined for the two Viking launches to Mars
in 1975, and this powerful four-stage launch vehicle was used again in
1977 for Voyager.

The Titan/Centaur stands nearly 50 meters tall, about the height of a
fifteen-story building. Fully fueled, it weighs nearly 700 tons. At
takeoff, the thrust of the two solid-propellant Stage-0 motors is about
10.7 million newtons. These motors, which burn for 122 seconds, use
powdered aluminum as fuel and ammonium perchlorate as oxidizer.
Together, they have a mass of 500 tons.

The first stage of the liquid propellant core of the Titan rocket
ignites about 112 seconds after takeoff. The propellant is hydrazine as
fuel and nitrogen tetroxide as oxidizer. The first stage is 3 meters in
diameter and 20 meters tall. Fueled, it has a mass of 130 tons. The
motor provides a thrust of 2.5 million newtons for a duration of 146

About 4.3 minutes after takeoff the Titan Stage II liquid propellant
motor begins to fire, and the first stage is separated and falls back
into the Atlantic. The second stage is 3 meters in diameter and more
than 7 meters long, with a fueled mass of 35 tons. The single liquid
fuel motor burns for 210 seconds with a thrust of half a million
newtons. During the second stage burn, the shroud covering the Voyager
spacecraft is jettisoned.

The Centaur and Titan vehicles separate 8 minutes into the flight, and
the Centaur main engine begins its burn. The Centaur is nearly 20 meters
tall and 3 meters in diameter, with a mass of 17 tons. The motors have a
thrust of almost 200 000 newtons, operating on the most powerful
chemical fuels known: liquid oxygen and liquid hydrogen. The Centaur
burns for only 1 minute and 36 seconds as it attains Earth parking
orbit; the engine then shuts down as the vehicle begins a half-hour
coasting period that carries it nearly half way around the Earth. During
this time, careful tracking of the spacecraft supplies the data needed
for Earth-based computers to calculate the proper time to leave parking
orbit and start the long trip toward Jupiter.

About 50 minutes after liftoff, from a position high above the Indian
Ocean, the second burn of the Centaur main engine begins. Six minutes of
additional thrust provides enough energy to break out of Earth’s orbit.
The Voyager then separates from the Centaur for a final boost toward
Jupiter. The solid rocket motor in the spacecraft propulsion module
(acting as final stage of this five-stage launch sequence) fires for 45
seconds at a thrust of 68 000 newtons. Just an hour after liftoff, the
Voyager spacecraft is on its way, coasting on an orbit toward Jupiter at
a speed of more than 10 kilometers per second.

    [Illustration: The Voyager spacecraft is dominated by the large
    3.7-meter-diameter antenna used for communication with Earth. Here
    the spacecraft undergoes final tests before launch. The science
    instrument scan platform is folded against the spacecraft on the
    right; the three cylinders on the left are the RTG power sources.

    [Illustration: Voyager 2 was the first of the spacecraft to be
    launched, on August 20, 1977, propelled into space in a
    Titan/Centaur rocket. [P-19450AC]]

                     The First Year Is the Roughest

During the autumn of 1977 Voyager 2, and to a lesser extent Voyager 1,
continued to plague controllers with erratic actions. Thrusters fired at
inappropriate times, data modes shifted, instrument filter and analyzer
wheels became stuck, and the various computer control systems
occasionally overrode ground commands. Apparently, the spacecraft
hardware was working properly, but the computers on board displayed
certain traits that seemed almost humanly perverse—and perhaps a little
psychotic. In general, these reactions were the result of programming
too much sensitivity into the spacecraft systems, resulting in panic
over-reaction by the onboard computers to minor fluctuations in the
environment. Ultimately, part of the programming had to be rewritten on
Earth and then transmitted to the Voyagers, to calm them down so that
they would ignore minor perturbations, yet still be ready to perform
automatic sequences required to protect the spacecraft from major
threats. Meanwhile, however, more serious problems were developing.

On February 23, 1978, during a series of movements or slews, Voyager 1’s
scan platform slowed and stopped before completing the maneuver. This
failure caused a great deal of concern, since the scan platform houses
the optical instruments that are crucial to the observation of the
Jovian system—the ultraviolet spectrometer, the IRIS, the
photopolarimeter, and the two TV cameras. At JPL, tests were run on a
proof-test model—an exact copy of the Voyager spacecraft—to try to find
out why Voyager 1’s scan platform had become stuck. On March 17, Voyager
1’s scan platform was tested—JPL engineers instructed the platform to
move slowly for a short distance, and Voyager responded as ordered.
Further tests were conducted on March 23. This time the scan platform
was ordered to execute a sequence of four slews, moving away from the
part of the sky where the original failure had occurred and ending with
the position that it would be most useful to leave the platform in—just
in case the platform should become stuck again. On April 4 the scan
platform was commanded to perform a sequence of 38 slews, and fifty more
slews were performed on April 5. All were successful. Yet engineers were
still hesitant to force the platform to move through the region where it
had originally stuck, and extensive discussions were held to determine
if the Jupiter observations could be carried out without risking a
return to the danger area. It was argued, however, that full mobility of
the scan platform really was required, and on May 31 commands were sent
to maneuver the scan platform through the danger region. It moved
normally: The scan platform was operating properly again. After
additional slewing tests were run in mid-June, the scan platform was
pronounced fit for operation. Engineers suspected that the material
caught in the platform gears must have been crushed or moved out of the
way by the continued slewing, allowing the platform to move once more.

    [Illustration: Voyager 1 was launched on September 5, 1977. The
    launch was delayed 5 days to make last-minute adjustments to avoid
    the postlaunch difficulties experienced by Voyager 2. [P-19480AC]]

An even more serious crisis soon endangered the Voyager 2 spacecraft. In
late November 1977, the S-band radio receiver began losing amplifier
power in its high-gain mode, so the solid-state amplifier was switched
to its low-power position. No further problems were noted until April 5,
1978, when Voyager 2’s primary radio receiver suddenly failed, and
shocked engineers discovered that the backup receiver was also faulty.
The trouble was detected after Voyager’s computer command subsystem
directed the spacecraft to switch from the primary radio receiver to the
backup receiver. This command was issued as part of a special protection
sequence: If the primary radio receiver receives no commands from Earth
for seven days, the backup receiver is switched on instead; if the
secondary receiver in turn receives no instructions over a twelve-hour
period, the system reverts to the main receiver. When, on April 5,
Voyager 2’s radio reception was switched from the primary to the
secondary receiver, flight engineers found that they were unable to
communicate with the spacecraft—the secondary receiver’s tracking loop
capacitor was malfunctioning. That meant that the secondary receiver
could not follow a changing signal frequency sent out from Earth. The
frequencies of signals transmitted from Earth are affected by the
Doppler effect—just as the siren on a fire engine seems first to rise in
pitch as the truck approaches, then falls as the truck speeds away, so
the frequency of signals transmitted from Earth fluctuates with the
Earth’s rotation as the Deep Space Network’s radio antennas move toward
or away from the spacecraft. The engineers had to wait until the primary
radio receiver was switched back on before they could communicate with
the spacecraft. Once the primary receiver was on, Voyager 2 began
receiving instructions from Earth, but approximately thirty minutes
later, there was an apparent power surge in the receiver. The fuses
blew. There was no recourse. The main receiver had failed; its loss was
permanent. It remained for the engineers to devise a way to communicate
with the slightly deaf spacecraft.

    [Illustration: Each Voyager spacecraft follows a billion-kilometer
    path to Jupiter. Except for minor thruster firings to achieve small
    trajectory corrections, each Voyager coasts from Earth to Jupiter,
    guided by the gravitational pull of the Sun. At Jupiter, the
    powerful tug of the giant planet deflects the spacecraft and speeds
    them up, imparting an extra kick to send them on their way toward

  Voyager 1
  Voyager 2
    Earth 8/20/77
    Earth 9/1/77
    Mars 8/20/77
    Jupiter 8/20/77
    Jupiter 3/5/79
    Jupiter 7/9/79
    Saturn 8/20/77
    Saturn 11/13/80
    Saturn 8/27/81
    Uranus 8/20/77
    Uranus 1/30/86

Because the switching of the radio receivers was still controlled by the
special protection sequence discussed earlier, flight engineers would
have to wait for seven days—until April 13—before they could attempt
communication with the spacecraft again. During that week special
procedures were established and rehearsed so that commands could be sent
to Voyager in the short time that the backup receiver would be on. On
Thursday, April 13, 1978, the seven days were up and the spacecraft
should have shifted from the dead main receiver to the sick backup
system. There was just a twelve-hour “window” in which to restore
communication. At about 3:30 a.m. PST the Madrid tracking station of the
Deep Space Network sent its first order to the spacecraft, approximately
474 million kilometers away. Almost an hour later, word arrived from
Voyager that the command had been accepted. (One-way light time for a
signal to travel the distance from Earth to Voyager at that time was
almost 27 minutes.) Elated flight controllers went ahead and transmitted
nine hours of commands to the spacecraft.

Voyager 2 was successfully commanded again on April 18 and April 26. The
April 26 commands included a course change maneuver that was executed
properly on May 3. On June 23, Voyager 2 was programmed for a backup
automatic mission at Saturn in the event that the secondary radio
receiver should also fail. These backup mission instructions would
operate all the science experiments, but only a minimum amount of data
would be returned, since the scan platform would only be programmed to
move through three positions rather than thousands as it would in normal
operation. Instructions for a backup minimum automatic encounter at
Jupiter were transmitted to Voyager 2 in two segments, the second of
these on October 12, 1978.

With the backup instructions recorded on board the spacecraft, Voyager
personnel felt their fears partially allayed. If Voyager 2’s secondary
radio receiver failed, the spacecraft would still obtain some science
data at Jupiter and Saturn. But that would mean that there would be no
mission beyond Saturn; our first opportunity to explore Uranus, its
satellites, its newly discovered ring system, and possibly even to get a
look at Neptune, would not come in this century.

Another major concern affecting both Voyager spacecraft was the proper
management of hydrazine fuel reserves. Hydrazine is used by the
thrusters on the Voyagers for stabilization of the spacecraft and for
trajectory correction maneuvers (TCM). Each Voyager was loaded with 105
kilograms of hydrazine budgeted for use on the long flight to Jupiter,
Saturn, and beyond. Because of the excellent performance of the launch
rockets, both Voyagers required less hydrazine than anticipated for
their final boost into proper trajectory toward Jupiter, and at first it
looked as though both spacecraft would have plenty of propellant to

Charles E. Kolhase, Manager of Mission Analysis and Engineering for the
Voyager Project, later explained the situation: “Voyager 1 should have
been launched September 1. Had it been launched on September 1—and I’m
glad it wasn’t—the maneuver to correct the trajectory for a Titan flyby
would have required a change in velocity of 100-110 meters per second—an
enormous maneuver—and we would have had a propellant margin for going on
to Saturn of perhaps 4.5 kilograms. But, by launching on the fifth of
September we increased our margin to 23 kilograms. Fortunately, for
every launch date that went by, that velocity change maneuver was
shrinking at a rate of 10 meters per second per day. Now, a 1 meter per
second change uses about a pound of hydrazine [about 0.5 kilogram]. So
when we launched on the fifth of September, now we suddenly had 40
pounds of hydrazine excess over what we would have had if we had
launched on the first of September. As a result, Voyager 1 is in great
shape as far as hydrazine is concerned.”

                         THE DEEP SPACE NETWORK

A vital component of the Voyager Mission is the communications system
linking the spacecraft with controllers and scientists on Earth. The
ability to communicate with spacecraft over the vast distances to the
outer planets, and particularly to return the enormous amounts of data
collected by sophisticated cameras and spectrometers, depends in large
part on the transmitters and receivers of the Deep Space Network (DSN),
operated for NASA by JPL.

The original network of these receiving stations was established in 1958
to provide round-the-world tracking of the first U.S. satellite,
Explorer 1. By the late 1970s, the DSN had evolved into a system of
large antennas, low-noise receivers, and high-power transmitters at
sites strategically located on three continents. From these sites the
data are forwarded (often using terrestrial communications satellites)
to the mission operations center at JPL.

The three DSN stations are located in the Mohave desert at Goldstone,
California; near Madrid, Spain; and near Canberra, Australia. Each
location is equipped with two 26-meter steerable antennas and a single
giant steerable dish 64 meters in diameter, with approximately the
collecting area of a football field. In addition, each is equipped with
transmitting, receiving, and data handling equipment. The transmitters
in Spain and Australia have 100-kilowatt power, while the 64-meter
antenna at Goldstone has a 400-kilowatt transmitter. Most commands to
Voyager are sent from Goldstone, but all three stations require the
highest quality receivers to permit continuous recording of the data
streams pouring in from the spacecraft.

Since the mid-1960s, the DSN’s standard frequency has been S-band (2295
megahertz). Voyager introduces a new, higher frequency telemetry link at
X-band (8418 megahertz). The X-band signal can carry more information
than S-band with similar power transmitters, but it requires more exact
antenna performance. In addition, the X-band signal is absorbed by
terrestrial clouds and, especially, rain. Fortunately, all three DSN
stations are in dry climates, but during encounters the weather
forecasts on Earth become items of crucial concern if precious data are
not to be lost by storm interference.

As a result of the development of larger antennas and improved
electronics, the DSN command capabilities and telemetry data rates have
increased dramatically over the years. For example, in 1965 Mariner 4
transmitted from Mars at a rate of only 8⅓ bits of information per
second. In 1969, Mariners 6 and 7 transmitted picture data from Mars at
16 200 bits per second. Mariner 10, in 1973, achieved 117 200 bits per
second from Mercury. Voyager operates at a similar rate from Jupiter,
about six times farther away. Many of these improvements in data
transmission result from changes in the DSN rather than in the
spacecraft transmitters.

Problems with hydrazine management developed, however. Voyager 1’s first
trajectory correction maneuver achieved only 80 percent of the required
speed change. Exhaust plumes from the thrusters apparently struck part
of the spacecraft, causing a 20 percent loss in velocity. That being the
case, Voyager might require more fuel than had been expected to complete
the mission. The extra fuel requirements did not threaten Voyager 1
itself, since it held ample fuel to reach Saturn; the concern was for
Voyager 2, where the effective loss of fuel might be enough to
jeopardize the Uranus mission.

    [Illustration: Project members.]

    [Illustration: Project members.]

Because of the plume impingement problem on Voyager 1, Voyager 2’s first
trajectory correction maneuver was adjusted to allow for the possibility
of a 20 percent loss in thrust. The Voyager 2 maneuver was successful,
but controllers felt that additional action was required to conserve
fuel. One way to save was by reducing requirements on control of the
spacecraft orientation. Less control fuel would be needed if the already
miniscule pressure exerted on the spacecraft by the solar wind could be
reduced. Flight engineers at JPL calculated that the pressure would be
reduced if the spacecraft were tipped upside down; however, to
accomplish this, the spacecraft would have to be steered by a new set of
guide stars. By reprogramming the attitude control system it was found
possible to substitute the northern star, Deneb, in the constellation of
Cygnus, for the original reference star, Canopus, in the southern
constellation of Carina. With this change, as well as readjustment of
Voyager 2’s trajectory near Jupiter, inflight consumption of hydrazine
was reduced significantly.

In late August 1978 both Voyagers were reprogrammed to ensure better
science results at Jupiter encounter; for example, the reprogramming
would prevent imaging (TV) photographs from blurring when the tape
recorder was operating. By early November, flight crews had begun
training exercises to rehearse for the Voyager 1 flyby of Jupiter on
March 5, 1979. A near encounter test was performed on December 12-14,
1978: a complete runthrough of Voyager 1’s 39-hour near encounter
period, which would take place March 3-5, 1979. Participants included
the flight team, the Deep Space Network tracking stations, the
scientists, and the spacecraft itself. Results: Voyager and the Voyager
team were all ready for the encounter.

Meanwhile, the spacecraft were busy returning scientific data to Earth.
Technically, the Voyagers were in the cruise phase of the mission—a
period that, for Voyager 2, would last until April 24, 1979, and for
Voyager 1, until January 4, 1979, when each spacecraft would enter its
respective observatory phase.

                          Cruise Phase Science

In the first few days after launch, the spacecrafts’ instruments were
turned on and calibrated; various tests for each instrument would
continue to be performed throughout the cruise phase. This period
presented a great opportunity for the Voyagers to study the
interplanetary magnetic fields, solar flares, and the solar wind. In
addition, ultraviolet and infrared radiation studies of the sky were
performed. In mid-September 1977 the television cameras on Voyager 1
recorded a number of photographs of the Earth and Moon. A photograph
taken September 18 captured both crescent Moon and crescent Earth. It
was the first time the two celestial bodies had ever been photographed

In November both Voyagers crossed the orbit of Mars, entering the
asteroid belt a month later. On December 15, at a distance of about 170
million kilometers from Earth, Voyager 1 finally speeded past its slower
twin. The journey through the asteroid belt was long but uneventful:
Voyager 1 emerged safely in September 1978, and Voyager 2 in October.
Unlike Pioneers 10 and 11, the Voyagers carried no instruments to look
at debris in the asteroid belt.

By April both spacecraft were already halfway to Jupiter and, about two
months later, Voyager 1, still approximately 265 million kilometers from
Jupiter, began returning photographs of the planet that showed
considerable detail, although less than could be obtained with
telescopes on Earth. Both the imaging (TV) and the planetary radio
astronomy instruments began observing Jupiter, and, by October 2, 1978,
officials announced that “the polarization characteristics of Jupiter’s
radio emissions have been defined. In the high frequencies, there is
consistent right-hand circular polarization, while in the low
frequencies, there is a consistent left-hand circular polarization. This
was an unexpected result.” In addition to scientific studies of the
interplanetary medium and a first look at Jupiter, the plasma wave
instrument, which studies waves of charged particles over a range of
frequencies that includes audio frequencies, was able to record the
sound of the spacecraft thrusters firing, as hydrazine fuel is
decomposed and ejected into space. The sound was described as being
“somewhat like a 5-gallon can being hit with a leather-wrapped mallet.”

On December 10, 1978, 83 million kilometers from Jupiter, Voyager 1 took
photographs that surpassed the best photographs ever taken from
ground-based telescopes, and scientists were anxiously awaiting the
start of continuous coverage of the rapidly changing cloud forms. These
pictures, together with data from several ground-based observatories,
were carefully scrutinized by a team of scientists at JPL to select the
final targets in the atmosphere of Jupiter to be studied at high
resolution during the flyby. The observatory phase of Voyager 1’s
journey to Jupiter was about to begin.

    [Illustration: As Voyager 1 approached Jupiter, the resolution of
    the images steadily improved. In October 1978, at a distance of
    about 125 million kilometers, the image was less clear than would be
    obtained with an Earth-based telescope. [P-20790]]

    [Illustration: By December 10, the spacecraft had moved to a
    distance of 85 million kilometers, and the resolution was about 2000
    kilometers, comparable to the best telescopic images. [P-20829C]]

    [Illustration: On January 9, 1979 (c), at a distance of 54 million
    kilometers, the image surpassed all ground-based views and
    approached the resolution of the Pioneer 10 and 11 photos.

    [Illustration: In this, taken January 24 at a distance of 40 million
    kilometers, the resolution exceeded 1000 kilometers. [P-20945C]]

                         The Observatory Phase

The observatory phase, originally scheduled to start on December 15,
1978, eighty days before encounter, was postponed until January 4, 1979,
to provide the flight team a holiday-season break. For the next two
months, Voyager would carry out a long-term scientific study—a “time
history”—of Jupiter, its satellites, and its magnetosphere. On January
6, Voyager 1 began photographing Jupiter every two hours—each time
taking a series of four photographs through different color filters as
part of a long-duration study of large-scale atmospheric processes, so
scientists could study the changing cloud patterns on Jupiter. Even the
first of these pictures showed that the atmosphere was dynamic “with
more convective structure than had previously been thought.”
Particularly striking were the changes in the planet, especially near
the Great Red Spot, that had taken place since the Pioneer flybys in
1973 and 1974. Jupiter was wearing a new face for Voyager.

In mid-January, photos of Jupiter were already being praised for
“showing exceptional details of the planet’s multicolored bands of
clouds.” Still 47 million kilometers from the giant, Voyager 1 had by
now taken more than 500 photographs of Jupiter. Movements of cloud
patterns were becoming more obvious; feathery structures seemed painted
across some of the bands that encircle the planet; swirling features
were huddled near the Great Red Spot. The satellites were also beginning
to look more like worlds, with a few bright spots visible on Ganymede
and dark red poles and a bright equatorial region clearly seen on Io
when it passed once each orbit across the turbulent face of Jupiter.

From January 30 to February 3, Voyager sent back one photograph of
Jupiter every 96 seconds over a 100-hour period. Using a total of three
different color filters, the spacecraft thus produced one color picture
every 4¾ minutes, in order to make a “movie” covering ten Jovian “days.”
To receive these pictures, sent back over the high-rate X-band
transmitter, the Deep Space Network’s 64-meter antennas provided
round-the-clock coverage. Voyager 1 was ready for its far encounter

    [Illustration: The Galilean satellites of Jupiter first began to
    show as tiny worlds, not mere points of light, as the Voyager 1
    observatory phase began. In this view taken January 17, 1979, at a
    range of 47 million kilometers, the differing sizes and surface
    reflectivities (albedos) of Ganymede (right center) and Europa (top
    right) are clearly visible. The view of Jupiter is unusual in that
    the Great Red Spot is not easily visible, but can just be seen at
    the right edge of the planet. Most pictures selected for publication
    include the photogenic Red Spot. [P-20938C]]

                          Far Encounter Phase

By early February Jupiter loomed too large in the narrow-angle camera to
be photographed in one piece; 2 × 2 three-color (violet, orange, green)
sets of pictures were taken for the next two weeks; by February 21,
Jupiter had grown too large even for that, and 3 × 3 sets were scheduled
to begin. When these sets of pictures are pasted together to form a
single picture, the result is called a mosaic.

    [Illustration: By February 1, 1979, Voyager 1 was only 30 million
    kilometers from Jupiter, and the resolution of the imaging system
    corresponded to about 600 kilometers at the cloud tops of the giant
    planet. At this time, a great deal of unexpected complexity became
    apparent around the Red Spot, and movie sequences clearly showed the
    cloud motions, including the apparent six-day rotation of the Red
    Spot. [P-20993C]]

    [Illustration: One of the most spectacular planetary photographs
    ever taken was obtained on February 13 as Voyager 1 continued its
    approach to Jupiter. By this time, at a range of 20 million
    kilometers, Jupiter loomed too large to fit within a single
    narrow-angle imaging frame. Passing in front of the planet are the
    inner two Galilean satellites. Io, on the left, already shows
    brightly colored patterns on its surface, while Europa, on the
    right, is a bland ice-covered world. The scale of all of these
    objects is huge by terrestrial standards; Io and Europa are each the
    size of our Moon, and the Red Spot is larger than the Earth.

While the imaging experiments were in the limelight, the other
scientific instruments had also begun to concentrate on the Jovian
system. The ultraviolet spectrometer had been scanning the region eight
times a day; the infrared spectrometer (IRIS) spent 1½ hours a day
analyzing infrared emissions from various longitudes in Jupiter’s
atmosphere; the planetary radio astronomy and plasma wave instruments
looked for radio bursts from Jupiter and for plasma disturbances in the
region; the photopolarimeter had begun searching for the edge of Io’s
sodium torus; and a watch was begun for the bow shock—the outer boundary
of the Jovian magnetosphere.

On February 10, Voyager 1 crossed the orbit of Sinope, Jupiter’s
outermost satellite. Yet the spacecraft even then had a long way to
go—still 23 million kilometers from Jupiter, but closing in on the
planet at nearly a million kilometers a day. A week later, targeted
photographs of Callisto began to provide coverage of the satellite all
around its orbit; similar photos of Ganymede began on February 25.

Meanwhile excitement was building. As early as February 8 and 9, delight
with the mission and anticipation of the results of the encounter in
March were already evident. Garry E. Hunt, from the Laboratory for
Planetary Atmospheres, University College, London, a member of the
Imaging Team, discussing the appearance of Jupiter’s atmosphere as seen
by Voyager during the previous month, said “It seems to be far more
photogenic now than it did during the Pioneer encounters; I’m more than
delighted by it—it’s an incredible state of affairs. There are
infinitely more details than ever imagined.”

    [Illustration: The Voyager Project was operated from the Jet
    Propulsion Laboratory managed for NASA by the California Institute
    of Technology. Located in the hills above Pasadena, California, JPL
    is the main center for U.S. exploration of the solar system.

    [Illustration: The Voyager TV cameras do not take color pictures
    directly as do commercial cameras. Instead, a color image is
    reconstructed on the ground from three separate monochromatic
    images, obtained through color filters and transmitted separately to
    Earth. There are a number of possible filter combinations, but the
    most nearly “true” color is obtained with originals photographed in
    blue (480 nanometers), green (565 nanometers), and orange (590
    nanometers) light. Before these can be combined, the individual
    frames must be registered, correcting for any change in spacecraft
    position or pointing between exposures. Often, only part of a scene
    is contained in all three original pictures. Shown here is a
    reconstruction of a plume on Jupiter, photographed on March 1, 1979.
    The colors used to print the three separate frames can be seen
    clearly in the nonoverlapping areas. For other pictures in this
    book, the nonoverlapping partial frames are omitted. [P-21192]]

The pictures from Voyager are “clearly spectacular,” said Lonne Lane,
Assistant Project Scientist for Jupiter. “We’re getting even better
results than we had anticipated. We have seen new phenomena in both
optical and radio emissions. We have definitely seen things that are
different—in at least in one case, unanticipated—and are begging for
answers we haven’t got.” There was already, still almost a month from
encounter, a strong feeling of accomplishment among the scientists and
engineers; they had done a difficult task and it has been successful.

By the last week of February 1979, the attention of thousands of
individuals was focused on the activities at JPL. Scientists had arrived
from universities and laboratories around the country and from abroad,
many bringing graduate students or faculty colleagues to assist them.
Engineers and technicians from JPL contractors joined NASA officials as
the Pasadena motels filled up. Special badges were issued and reserved
parking areas set aside for the Voyager influx. Twenty-four hours a day,
lights burned in the flight control rooms, the science offices, the
computer areas, and the photo processing labs. In order to protect those
with critical tasks to perform from the friendly distraction of the new
arrivals, special computer-controlled locks were placed on doors, and
security officers began to patrol the halls. By the end of the month the
press had begun to arrive. Amid accelerating excitement, the Voyager 1
encounter was about to begin.

    [Illustration: The smaller-scale clouds on Jupiter tend to be more
    irregular than the large ovals and plumes. At the lower right, one
    of the three large white ovals clearly shows internal structure,
    with the swirling cloud pattern indicating counterclockwise, or
    anticyclonic, flow. A smaller anticyclonic white feature near the
    center is surrounded by a dark, cloud-free band where one can see to
    greater depths in the atmosphere. This photo was taken March 1 from
    a distance of 4 million kilometers. [P-21183C]]

                               CHAPTER 6
                          THE FIRST ENCOUNTER

                     The Giant Is Full of Surprises

The Voyager 1 encounter took place at 4:42 a.m. PST, March 5, 1979.
About six hours before, while the spacecraft continued to hurtle on
toward Jupiter, overflow crowds had poured out of Beckman Auditorium on
the campus of the California Institute of Technology. There had been a
symposium entitled Jupiter and the Mind of Man. More than one face that
evening had turned to look toward the Pasadena night, for there,
glittering against the fabric of the sky, was the “star” of the show—a
planet so huge that the Earth would be but a blemish on its Great Red
Spot. One might have tried to imagine Voyager 1, large by spacecraft
standards, rapidly closing in on Jupiter—a pesky, investigative
“mosquito” buzzing around the Jovian system.

Meanwhile, at JPL, pictures taken by Voyager 1 were flashing back every
48 seconds, pictures that were already revealing more and more of
Jupiter’s atmosphere and would soon disclose four new worlds—the
Galilean satellites. The encounter was almost at hand.

4:42 a.m.—Voyager 1 had made its closest approach to Jupiter 37 minutes
earlier. Only now were the signals that had been sent out from the
spacecraft at 4:05 a.m. reaching the Earth.

But the moment of closest encounter did not have the same impact a
landing would have had. Although the champagne would flow later for
those who had worked so long and hard on this successful mission, there
was none now. In the press room there were just four coffeepots working
overtime to help keep the press alert. In the science areas, focus had
shifted to the satellite encounters, which would stretch over the next
24 hours; meanwhile, a few persons tried to catch a little sleep at
their desks before the first closeups of Io came in. The instant of
encounter came ... and went ... with no screams, no New Year’s
noisemakers. All the excitement of the mission lay both behind ... and

                         Thursday, February 22.

The encounter activity was kicked off in Washington, D.C., by a press
conference at NASA Headquarters. After introductory statements by NASA
and JPL officials, some of the scientific results from the observatory
and far encounter phases were presented.

Both the plasma wave and the planetary radio astronomy instruments had
detected very-low-frequency radio emission that generally increased in
intensity whenever either the north or south magnetic pole of Jupiter
tipped toward the spacecraft. The plasma wave experiment had also
detected radiation that seemed to come from a region either near or
beyond the orbit of Io—perhaps even as far as the outer magnetosphere.
In addition, Frederick L. Scarf, Principal Investigator for the plasma
wave instrument, released a recording of the ion sound waves created by
5 to 10 kilovolt energy protons traveling upstream from Jupiter.

James Warwick, Principal Investigator of the radio astronomy
investigation, enthusiastically reported the detection of a striking new
low-frequency radiation from Jupiter, at wavelengths of tens of
kilometers. Such radio waves cannot penetrate the ionosphere of the
Earth, and thus had never been detected before. Because of their huge
scale, these bursts probably did not originate at Jupiter itself, but in
magnetospheric regions above the planet—perhaps in association with the
high-density plasma torus associated with Io. Warwick commented that
“only from the magnificent perspective of the Voyager as it approaches
Jupiter have we been able to get this complete picture.”

    [Illustration: The Voyager 1 encounter with Jupiter took place
    during a little more than 48 hours, from the inbound to the outbound
    crossing of the orbit of Callisto. This figure shows the spacecraft
    path as it would be seen from above the north pole of Jupiter.
    Closest approach to Jupiter was 350 000 kilometers. The close flybys
    of Io, Ganymede, and Callisto all took place as the spacecraft was

  Voyager 1 trajectory
    Launch date = 9/5/77
    Jupiter arrival date = 3/5/79
    Satellite closest approach
  Sun occultation
  Earth occultation

Another unexpected result was announced by Lyle Broadfoot, Principal
Investigator for the ultraviolet spectrometer investigation. The
scientists had expected to find very weak ultraviolet emissions on the
sunlit side of Jupiter, caused by sunlight being scattered from hydrogen
and helium in Jupiter’s upper atmosphere. “Instead we are seeing a
spectacular auroral display. There are two features of the emission—the
auroral emission which comes from the planet and a second type of
emission which appears to come from a radiating torus or shell around
the planet at the orbit of Io. The spectral content of these two
radiating sources is distinctly different. What we find is that the
auroral emission from Jupiter’s atmosphere is so strong that it
completely dominates the emission spectrum even on the sunlit side of
the atmosphere.”

“Not since Mariner 4 carried its TV camera to Mars fifteen years ago
have we been less prepared—have we been less certain of what we are
about to see over the next two weeks,” said Bradford Smith, Imaging Team
Leader. He mentioned the time-lapse “rotation movie,” in which the
colorful planet spun through ten full Jupiter days; tiny images of the
satellites passed across Jupiter’s face as though being whipped along by
the rotation of the giant. A week or so earlier, when this film had been
shown for the first time to the full Imaging Team, it provided an
occasion for good-humored rivalry between planet people and satellite
people, with jokes about the satellites getting in the way of the
important studies of Jupiter. For the next few days, the imaging focus
remained on Jupiter; it shifted to Io, Ganymede, and Callisto, as each
was passed in turn after closest approach to the planet.

                         Tuesday, February 27.

(_Range to Jupiter, 7.1 million kilometers_). At a distance of 660
million kilometers from Earth, within 90 Jovian radii (R_J) of Jupiter,
Voyager 1 was prepared to begin the encounter with the planet’s
magnetosphere. On the previous day the spacecraft had crossed the point,
at 100 R_J, at which Pioneers 10 and 11 had found the bow shock, the
first indication of the magnetospheric boundary. The start of Voyager’s
plunge into the Jovian magnetosphere was overdue, and scientists
anxiously watched the data from the particles and fields instruments,
looking for the first indication of disordered magnetic fields and
altered particle densities that would mark the bow shock. Apparently,
higher solar wind pressure, associated with increased solar activity
since 1974, had compressed the magnetosphere, but no one could predict
how strong this compression might be.

     Object      Range to Center at Closest   Best Image Resolution
                    Approach (kilometers)       (km per line pair)

  Jupiter                            349 000                       8
  Amalthea                           417 000                       8
  Io                                  21 000                       1
  Europa                             734 000                      33
  Ganymede                           115 000                       2
  Callisto                           126 000                       2

For the first time since its discovery, Lyle Broadfoot and his UVS
colleagues suggested a probable identification for the unexpected
ultraviolet emission near the orbit of Io. The most likely candidate was
sulfur atoms with two electrons removed (S III), at an inferred
temperature perhaps as high as 200 000 K. An additional indication of
sulfur came from Mike Krimigis, who reported that the low-energy charged
particles instrument had detected bursts of sulfur ions streaming away
from Jupiter that had apparently escaped from the inner magnetosphere.
No explanations were offered, however, for the presence of large amounts
of this element.

At JPL, a press room had been opened in Von Karman Auditorium to
accommodate the hundred or so reporters expected to arrive. To keep all
the interested people informed of Voyager progress, frequent television
reports were beamed over closed-circuit TV throughout JPL. From an
in-lab television studio called the Blue Room, JPL scientist Al Hibbs,
who had played a similar role during the Viking Mission to Mars,
provided hourly reports and interviewed members of the Voyager teams. As
the pressure for constant commentary and instant analysis increased,
Garry Hunt of the Imaging Team was also called on to host activities in
the Blue Room, where his British accent added an additional touch of
class to the operation.

As the encounter progressed, the JPL television reports reached a wider
audience. In the Los Angeles area, KCET Public Television began a
nightly “Jupiter Watch” program, with Dr. Hibbs as host. During the
encounter days, service was extended to interested public television
stations throughout the nation. In this way, tens of thousands of
persons were able to experience the thrill of discovery, seeing the
closeup pictures of Jupiter and its satellites at the same moment as the
scientists at JPL saw them, and listening to the excited and frequently
awestruck commentary as the first tentative interpretations were
attempted. Unfortunately, the commercial television networks did not
make use of this opportunity, and the greatest coverage available to
most of the country was a 90-second commentary on the nightly network

                        Wednesday, February 28.

(_Range to Jupiter, 5.9 million kilometers_). At 6:33 a.m., at a range
of 86 R_J, Voyager 1 finally reached Jupiter’s bow shock. But by 12:28
p.m. the solar wind had pushed the magnetosphere back toward Jupiter,
and Voyager was once more outside, back in the solar wind. Not until
March 2, at a distance of less than 45 R_J, would the spacecraft enter
the magnetosphere for the final time.

At 11 a.m. the first daily briefing to the press was given. “After
nearly two months of atmospheric imaging and perhaps a week or two of
satellite viewing, [we’re] happily bewildered,” said Brad Smith. The
Jovian atmosphere is “where our greatest state of confusion seems to
exist right at the moment, although over the next several days we may
find that some of our smirking geology friends will find themselves in a
similar state. I think, for the most part, we have to say that the
existing atmospheric circulation models have all been shot to hell by
Voyager. Although these models can still explain some of the coarse
zonal flow patterns, they fail entirely in explaining the detailed
behavior that Voyager is now revealing.” It was thought, from Pioneer
results, that Jupiter’s atmosphere showed primarily horizontal or zonal
flow near the equatorial region, but that the zonal flow pattern broke
down at high latitudes. But Voyager found that “zonal flow exists
poleward as far as we can see.”

Smith also showed a time-lapse movie of Jupiter assembled from images
obtained during the month of January. Once each rotation, approximately
every ten hours, a color picture had been taken. Viewed consecutively,
these frames displayed the complex cloud motions on a single hemisphere
of Jupiter, as they would be seen from a fixed point above the equator
of the planet. The film revealed that clouds move around the Great Red
Spot in a period of about six days, at speeds of perhaps 100 meters per
second. The Great Red Spot, as well as many of the smaller spots that
dot the planet, appeared to be rotating anticyclonically. Anticyclonic
motion is characteristic of high-pressure regions, unlike terrestrial
storms. Smith noted that “Jupiter is far more complex in its atmospheric
motions than we had ever imagined. We are seeing a much more complicated
flow of cyclonic and anticyclonic vorticity, circulation. We see
currents which flow along and seem to interact with an obstacle and turn
around and flow back.” There is a Jovian jet stream that is “moving
along at well over 100 meters per second. Several of these curious
little dark features that appear to be small brown spots near Jupiter’s
north temperate region have been seen to overtake one another and gobble
each other up. And then they occasionally spit out a piece here and
there as they move along.”

                           Thursday, March 1.

(_Range to Jupiter, 4.8 million kilometers_). At 5 a.m., at a distance
of 71 R_J, Voyager crossed the bow shock for the third time, catching up
with the contracting magnetosphere of the planet. About noon, at 66 R_J,
the spacecraft finally reached the boundary of the magnetosphere, called
the magnetopause. Herbert Bridge, the plasma instrument Principal
Investigator, noted that the solar wind pressure as monitored by Voyager
2, still between the Sun and Jupiter, had been for several days from two
to five times greater than its level during the Pioneer 10 and 11
encounters. Presumably, this high pressure was the cause of the
compressed state of the magnetosphere. However, in the previous few
hours the solar pressure had dropped, so Bridge anticipated that the
Jovian magnetosphere might soon inflate and expand outward.

    [Illustration: The southern hemisphere of Jupiter presents a
    tremendous diversity of atmospheric structure and motion. The Great
    Red Spot rotates counterclockwise in about six days; above and below
    it high-speed jet streams flow to the right and the left, while a
    complex, dynamic cloud pattern develops in its wake. This picture
    was taken on February 25, when Voyager 1 was 9 million kilometers
    from the planet. [P-21151C]]

Fred Scarf intrigued the press with a tape of the sounds made by
high-energy protons coming upstream from Jupiter. The plasma wave
instrument had recorded the noise of the protons, mixed with the noise
of the spacecraft itself, producing sound effects that sounded somewhat
like a mixture of singing whales, a Nor’easter, and the Daytona 500.

At the press briefing, interest in the imaging results began to shift
from Jupiter toward the satellites. Pictures of each of the four big
Galilean moons revealed bright and dark features as small as about 200
kilometers across. Unfortunately, this resolution is not enough to be
diagnostic—the spots cannot be interpreted in terms of recognizable
geological features, such as mountains or craters. Today one could only
speculate, but tomorrow or the next day the answers would begin to come
in. Deputy Imaging Team Leader Larry Soderblom conveyed his excitement
through a metaphor that would be repeated many times during the next
week: “We’re beginning a stage in this mission which represents, I
think, one of the most exciting points in man’s scientific exploration
of the solar system—in the next few days, we’ll explore four new
worlds,” seeing in a few days’ time what it took us centuries to learn
about other worlds in our solar system. In terms of our experience with
the exploration of Mars, “it is about 1700 AD this morning, tomorrow it
will be about 1800, and it will be about 1976 [the year of Viking] by
Tuesday evening.”

                            Friday, March 2.

(_Range to Jupiter, 3.6 million kilometers_). Early in the morning, a
twelvefold increase in solar wind pressure caused another contraction of
the magnetosphere, which was behaving like a spring, compressing in
response to outside forces. As the magnetopause boundary moved rapidly
inward, it crossed the spacecraft at 59 R_J from the planet. An hour
later the bow shock also flashed past, and Voyager was once more in the
interplanetary medium. By noon reinflation of the magnetosphere began
again; Voyager crossed the bow shock for the fifth and final time at 55
R_J, followed by three more magnetopause crossings, as the
magnetospheric boundary flopped in and out between 45 R_J and 50 R_J.

    [Illustration: A few days before encounter, the Voyager images of
    the larger Galilean satellites, Callisto and Ganymede, were
    beginning to show distinctive surfaces with many bright spots. These
    two pictures were taken on March 5 at a range of 8 million
    kilometers; the resolution is about 100 kilometers. Although
    extremely tantalizing, these images were uninterpretable, because
    the spots could not be associated with any recognizable geological
    features, such as mountains or craters. Like a naked-eye view of the
    Moon, these pictures seemed to reveal more than was actually
    meaningful. [P-21188C and P-21150C]]

    [Illustration: Ganymede]

Some Project officials began to worry about the contracted state of the
magnetosphere. The radiation “hardening” of Voyager was carried out to
protect against the energetic particle fluxes observed by Pioneers 10
and 11. Under the new conditions, would the particles in the inner
magnetosphere be more concentrated and perhaps increase the radiation
levels beyond the design limits? Scientists asked how long the
compression might last and speculated about how much energy might be
pumped into the Jovian radiation belts, but only time could provide the

       Boundary              Day          Distance (R_J)

  S                                2/28                 86
  S                                2/28                 82
  S                                3/01                 72
  M                                3/01                 67
  M                                3/02                 59
  S                                3/02                 58
  S                                3/02                 56
  M                                3/03                 47


  M                                3/13                158
  M                                3/13                163
  M                                3/13                165
  S                                3/16                199
  S                                3/18                227
  S                                3/18                227
  S                                3/19                240
  S                                3/20                256
  S                                3/20                258

Accurate recording of the X-band data being sent from Voyager at 115
thousand bits per second required fairly clear weather at the tracking
sites. The three Deep Space Network (DSN) stations in California, Spain,
and Australia are located at normally dry sites. However, early in the
morning, heavy rain at the Australian site interfered with reception of
the Voyager signal for fourteen minutes. Fortunately, the loss occurred
when the DSN tracking of the spacecraft was being switched from
Goldstone, California, to the Australian station. Mission control was
able to extend Goldstone coverage for several minutes so that only about
three minutes’ worth of data was lost completely.

A more positive announcement was made at the press conference that
morning by Donald Shemansky of the Ultraviolet Spectroscopy Team: the
discovery of a high-energy torus of doubly ionized sulfur (S III)
circling Jupiter in the region of Io’s orbit. “We were surprised out of
our chairs to see a spectacularly bright emission in the 650-1100
angstrom region, immediately implying that we were looking at a plasma
that had to be at a temperature of about 100 000 degrees.” The
scientists estimated that the density of this torus must be at least 500
ions per cubic centimeter, and that the power needed to keep this plasma
at such a high temperature must be in the neighborhood of 500 billion
watts. As Al Hibbs mentioned on “Jupiter Watch” that night, 500 billion
watts of power is the total amount of installed generating capacity of
the United States.

                           Saturday, March 3.

(_Range to Jupiter, 2.5 million kilometers_). Early in the morning the
final crossing of the magnetopause took place at 47 R_J, and Voyager
finally joined the Jovian system. At the end of the day, the rapidly
moving spacecraft crossed the orbit of Callisto, but that satellite
itself was on the far side of the planet. Not until the outbound
crossing of its orbit on March 6 would closeup views of Callisto be

During the preceding night, a severe summer thunderstorm in Australia
again caused a loss of data. A line of storms over the tracking station
blocked the high-rate, X-band science data for three hours and twenty
minutes. Attempts were made to save a part of the data by commanding the
spacecraft to slow its transmission rate, rather like speaking slowly to
a partially deaf listener. But by the time the craft received the signal
and responded, the storms had intensified, and no signal could get
through. Closeups of the Great Red Spot and an extended series of
observations of the glowing sodium cloud around Io were lost.

A black-and-white movie assembled from photos obtained in January and
February was shown at the press conference. The movie was taken from
observatory phase pictures photographed in blue light. From these
pictures, the imaging team “put together this so-called ‘blue movie’,”
said Brad Smith, introducing the film. The film showed changes taking
place in Jupiter’s atmosphere over a period of about seventy Jovian
days. Near the equator there were “bright plumes floating by—at high
speed the plumes seem to wave around, something like a flag waving in
the breeze.” Farther north, along the edge of the north temperate zone,
one could see one of the dark ovals—“a rather fuzzy one would move up,
catch up with the one just ahead of it, get stuck to the outside and
roll around on it for a while, then get ejected a little later.” Dr.
Smith also showed new closeup views of the region around the Red Spot,
showing not only the anticyclonic features but filamentary
“spaghettilike” material which was rotating in a cyclonic direction,
indicating a low-pressure region. “The filamentary material still seems
to be rather a mystery—very difficult to see the details of the motion.”
But the photographs were already showing what seemed to be stream lines
in the white ovals. “In appearance the white ovals seem to resemble the
Red Spot. The stream lines are at least suggestive of divergent flow,
that is, material in each of these spots which is upwelling in these
areas and then moving out tends to go around in a counterclockwise
anticyclonic motion but may, at the same time, be slowly diverging

    [Illustration: At a resolution of about 100 kilometers a planetary
    surface just begins to reveal its personality. Io was photographed
    against the disk of Jupiter on February 26, from a distance of about
    8 million kilometers. Such early pictures whetted the appetites of
    the Voyager scientists at JPL as they anxiously speculated about
    what they would find in closer views of the surface of this
    remarkable satellite. [P-21185 B/W]]

The large white ovals are about forty years old. Dr. Smith explained how
they formed: Between 1939 and 1940, “where those three white spots exist
right now, was a rather bright band similar to the north temperate zone
we see on Jupiter right now. In that time period of a year or so, three
darkish spots formed. A dark cloud spread out at each one of those three
locations and just kept spreading longitudinally until the white
material condensed between them.” In the end, everything was dark except
for the three ovals, which have persisted ever since.

Torrence Johnson of the Imaging Team commented on a photo of Io in which
features resembling circular crater-type structures seemed to be
visible. “Whatever they are, [those circular features] are approaching
the size of the things that we would call basins if they were impact
structures on other planets. We don’t really know whether they’re impact
structures. They have some characteristics that look reminiscent of
impact structures. They could be endogenic—volcanic in origin—or
internally generated in some other way.” Johnson also showed another
photograph of Io, this one taken against black sky, showing a
“strikingly different face” looking, perhaps, like someone’s nightmare,
glaring back at the intruder from Earth. One huge feature—a “bullseye”
or “hoof print” on Io—appeared to be approximately 1000 kilometers long.
No one had ever seen such a feature, and the imaging scientists could
only speculate about its significance.

    [Illustration: Small-scale structures in the jet streams of
    Jupiter’s north tropical zone reveal details of atmospheric
    circulation. The small dark oval near the right edge of the zone may
    offer a glimpse deep into Jupiter’s atmosphere. Between the
    regularly spaced dark ovals near the bottom of the frame are more
    small-scale features that are being studied for their roles in
    Jovian atmospheric activity. The blue-gray regions along the shear
    line between the equatorial zone and the north equatorial belt also
    appear to be windows into the deeper regions of the atmosphere. This
    photo was taken February 19 by Voyager 1 from a distance of 14
    million kilometers. [P-21160C]]

A few days earlier, someone had posted in the Imaging Team area a quote
from a 1975 review paper on the Jovian satellites by David Morrison and
Joseph Burns. The section on Io began, “Io is one of the most intriguing
objects in the solar system.” This statement seemed more and more
appropriate as Voyager images improved. Johnson referred to this day as
equivalent to the “late 1960s” in our study of the Jovian satellites.
“We can see much more clearly than ever before, but still not clearly
enough to provide understanding of what we are seeing.”

                            Sunday, March 4.

(_Range to Jupiter, 1.2 million kilometers_). At 4:37 a.m., the near
encounter phase began: Voyager 1 was almost there! In the press room,
someone taped a fortune cookie message to the Voyager TV monitor: “There
is a prospect of a thrilling time ahead for you.”

Pulled by the powerful gravity of Jupiter, the spacecraft was now on a
curved path through the inner Jovian system. At 2 p.m. PST, it crossed
the orbit of Ganymede, and later in the afternoon it passed within less
than 2 million kilometers of Europa, providing Voyager 1’s closest look
at this satellite. During the afternoon and evening, a number of views
were obtained of Amalthea, the small inner satellite, at a range of less
than 500 000 kilometers. At 8 p.m. the orbit of Europa was crossed, and
increasing attention was drawn to the coming encounter with Io. At about
7 p.m. a full-frame color sequence of Io was received with a resolution
of 16 kilometers. During the night, as Imaging Team members scratched
their heads trying to prepare a press release caption to interpret the
peculiar structures seen, the JPL Image Processing Lab rushed to prepare
a color version for release the next day.

    [Illustration: The Great Red Spot became more and more spectacular
    as Voyager 1 approached, with each day revealing new and intricate
    detail in the clouds. This view was obtained on March 1 at a
    distance of 5 million kilometers; the smallest features that can be
    made out are about 100 kilometers across. To the west of the Great
    Red Spot is a region of great turbulence, and to the south is one of
    the three white ovals. [P-21182C]]

    [Illustration: Large brown ovals in the northern hemisphere of
    Jupiter are apparently regions in which an opening in the upper,
    ammonia clouds reveals darker regions below. This oval, about the
    same length as the diameter of the Earth, was at latitude 15°N.
    Features of this sort are not rare on Jupiter and have an average
    lifetime of one to two years. Above the feature is the pale orange
    north temperate belt, bounded on the south by the high-speed north
    temperate current, with winds of 120 meters per second. The range to
    Jupiter at the time this photograph was obtained on March 2 was 4
    million kilometers, with the smallest resolvable features being 75
    kilometers across. [P-21194C]]

    [Illustration: The best Voyager 1 photos of Europa were obtained on
    March 4 from a distance of about 2 million kilometers. This view of
    the hemisphere centered at about 300° longitude has a resolution of
    about 40 kilometers. Most of the surface is bland and highly
    reflective, being composed almost entirely of water-ice. No craters
    resulting from meteoric impacts can be seen. The most striking
    visual features, which set Europa off from the other satellites, are
    the dark streaks, as much as several thousand kilometers long, that
    cross the surface. Just barely visible to Voyager 1, these streaks
    were dramatically apparent to Voyager 2 in its closer flyby in July.

Once again, tracking station problems interrupted the smooth flow of
data from the spacecraft. Failure at Madrid to reestablish the correct
receiver frequency after a spacecraft maneuver resulted in the loss of
53 minutes of irreplaceable data; an additional eleven minutes were lost
an hour later. As the first problem was being corrected, the tracking
antenna became misaligned, resulting in a noisy data return. Meanwhile,
only a few of the watchers at JPL mourned the lost data, so exciting
were the other new results that kept pouring in.

“For the highlights of this morning Dr. Soderblom will come up and show
some beautiful satellite pictures,” Brad Smith announced at the daily
press briefing. Larry Soderblom’s excitement was hard to contain. “Today
is probably going to turn out to be one of the most memorable days in
our exploration program. For the planetary geologists it’s truly
Christmas Eve. We see tonight the beginning of the exploration of four
new worlds. We’re racing through time and space at an incredible
rate—the rate at which we are learning things is awe-inspiring in
itself.” Callisto was still too far away to see well “but the things
we’re seeing on the closer three satellites have really got us charged
in anticipation.” Ganymede—“What can I say? Loops and swirls and
incredible patches that are difficult now to hazard a guess about.”
Io—an eerie-looking red, orange, and yellow world—“this one we’ve got
all figured out. [Laughter from the press and applause.] It is covered
with thin candy shells of anything from sulfates and sulfur and salts to
all kinds of strange things.”

The low energy charged particle instrument had discovered high-speed
sulfur in the outer Jovian magnetosphere, ten times as far from the
planet as the sulfur torus around Io. The high-speed sulfur, which
whizzed by Voyager 1 at 8000 kilometers per second, was first detected
as the spacecraft crossed the magnetopause and entered the
magnetosphere. Apparently this sulfur had picked up speed as it moved
outward from Io’s torus, but the mechanism for this acceleration was not
known. Roby Vogt reported that the cosmic ray instrument was detecting
two distinct groups of atoms closer in to the planet. One group was
apparently of solar composition (derived from the solar wind), and the
other showed enhanced oxygen as well as sulfur. The plasma instrument
had also begun to measure sulfur in the same form (S III) as that
detected by the UVS in the Io torus. Where, the scientists asked, could
the oxygen and sulfur be coming from? Could Io be the source of these

At one point, as questions from the press became too detailed, asking
for “instant analyses” of the data, Dr. Smith was prompted to make the
following statement: “I want to say something about what’s going on
right now in the imaging area because we get a lot of pointed questions.
We have a remarkably good system that is getting extremely good
photographs. Not only are the cameras working well, but Jupiter and the
satellites are cooperating by showing a lot of truly remarkable detail.
And it may sound unprofessional, but a lot of the people up in the
Imaging Team area are just standing around with their mouths hanging
open watching the pictures come in, and you don’t like to tear yourself
away to go and start looking at numbers on a printout. We will do that,
but in the meantime we’re just caught up in the excitement of what’s
going on.”

While the press briefing was underway, between 11:38 and 11:50 a.m., a
special experiment was being tried on board the Voyager. As the
spacecraft passed through the plane of the equator of Jupiter, it aimed
its narrow-angle camera to a point in space halfway between the cloud
tops and the orbit of Amalthea and took a single, eleven-minute
exposure. The purpose: to search for a possible faint ring around
Jupiter, which, if present, could best be seen by sighting along the
plane of the equator. The image appeared briefly on the TV monitors,
clearly showing something—a strange band of light—streaking across the
center of the frame. Up in the imaging area, analyses began at once to
determine if the strange band were really the sought-for ring, but it
would not be until March 7 that the identification would be confirmed
and the discovery announced.

    [Illustration: The “bullseye” of Io was first photographed at a
    range of 2.8 million kilometers on March 3, and this color version
    was released on March 4. At the time, Imaging Team scientists wrote
    that “The large heart-shaped feature with a dark spot near its
    center could be Io’s equivalent of an impact basin such as Mare
    Orientale on the Moon. Its outer dimensions are about 800 by 1000
    kilometers. Subsequent high-resolution coverage should reveal
    whether the small dark spots are impact craters, or perhaps
    something more exotic such as volcanoes. The reddish color of Io has
    been attributed to sulfur in the salts which are believed by some to
    make up the surface of Io.” It would be another week before this
    feature, later named “Pele” for the Hawaiian volcano goddess, would
    be recognized as an active, erupting volcano. [P-21187C]]

                            Monday, March 5.

(_Encounter Day. Minimum range to Jupiter, 780 000 kilometers; speed of
spacecraft, almost 100 000 kilometers per hour_). Many celebrities,
including the Governor of California, spent the night at JPL to witness
the historic occasion. In Washington, D.C., a special TV monitor was set
up in the White House for the President and his family.

As late Sunday night eased into the early morning of encounter, closeup
images of Jupiter, looking more like abstract art than like planetary
science, flashed across the TV screens, and verbal images far less wild
than the visuals from Jupiter were heard from commentators and from
members of the press. “Are you sure Van Gogh didn’t paint that?” “That’s
not Jupiter; it looks like a closeup of a salad.” “They’re not showing
us Jupiter, that’s some medical school anatomy slide.”

Shortly before closest approach to Jupiter, Voyager began its intensive
observations of Io. Much of this information, taken while the Australian
station was tracking the spacecraft, was recorded on Voyager’s onboard
tape-recorder for playback later that day. But even before the results
of that imaging were known, Larry Soderblom was calling Io “one of the
most spectacular bodies in the solar system.” As more and more vivid
photos of Io appeared on the monitors, members of the Imaging Team in
the Blue Room buzzed with excitement. “This is incredible.” “The element
of surprise is coming up in every one of these frames.” “I knew it would
be wild from what we saw on approach but to anticipate anything like
this would have required some sort of heavenly perspective. I think this
is incredible.”

At 7:35 a.m. Voyager was scheduled to pass through the flux tube of Io,
the region in which tremendous electric currents were calculated to be
flowing back and forth between the satellite and Jupiter. Norm Ness
suggested, after examining magnetometer data, that Voyager skirted the
edge of the flux tube, and that the current in the tube was about one
million amps. As the flux tube results were received, champagne bottles
began to pop in the particles and fields science offices, in celebration
of the successful passage through the inner magnetosphere. Meanwhile, at
7:47 a.m., closest approach to Io occurred, at a range of only 22 000
kilometers. Voyager was 25 000 times closer to this satellite than were
the watchers on Earth.

At 8:14 a.m., while still within 30 000 kilometers of Io, the spacecraft
passed out of sight behind the edge of Jupiter. All scientific data for
the next two hours and six minutes were stored on the onboard tape
recorder for later transmission to Earth. Meanwhile, the radio
communication signal was used to probe the atmosphere of Jupiter,
yielding a profile of electron density in the ionosphere and of the gas
pressure and temperature in the upper atmosphere. While out of sight
from Earth, at 9:07 a.m., Voyager plunged into the shadow of Jupiter. As
the Sun set on the spacecraft, the ultraviolet instrument used the
absorption of sunlight to determine the composition and temperature of
the upper atmosphere. In the darkness, the infrared IRIS measured the
night-side temperatures of the planet, and long-exposure images were
taken to search for aurora, lightning, and fireballs in the Jovian
atmosphere. At 10:20 a.m., Voyager reappeared from behind Jupiter and
radio contact was restored; at 11:24 a.m., it emerged from shadow into
sunlight, speeding on toward encounter with Ganymede.

At 8 a.m. a special press conference was held to mark the successful
Jupiter flyby. Noel Hinners, Associate Administrator for Space Science
and the highest ranking NASA official present, congratulated all those
who had made the Voyager Mission a success. The encounter was the
“culmination of a fantastic amount of dedication and effort. The result
is a spectacular feat of technology and a beginning of a new era of
science for the solar system. Just watching the data come in has been
fantastic. I had a fear that things on the satellites were going to look
like the lunar highlands. Nature wins again. If we’re going to see
exploration of this nature occurring in the 1980s and 1990s we must
continue to expound the results of what we’re finding here, the role of
exploration in the history of our country, what it means to us as a
vigorous national society.”

As time passed, it became apparent that Voyager 1 had been affected by
Jupiter’s radiation environment. The basic timing—the main clock on the
spacecraft—had slowed down. First it slowed by 6.3 seconds, but by March
6 it was found to have slowed a total of eight seconds. In addition, the
two central computers apparently got out of synchronization both with
themselves and with the flight data subsystem. On March 6 it was
reported that the spacecraft cameras were shuttering one frametime (48
seconds) early; this was partly offset by the eight-second spacecraft
“masterclock” slow-down resulting in images (according to our clocks)
being photographed forty seconds early. This timing error resulted in
the camera taking some pictures while the scan platform was moving,
causing some blurred images. A number of the highest resolution images
of Io and Ganymede were seriously degraded by this malfunction.

    [Illustration: When the first color close-up of Io was released,
    Imaging Team Leader Brad Smith said that he had seen “better looking
    pizzas”; hence this view, taken March 4 at a range of about 860 000
    kilometers, became known as “the pizza picture.” The circular
    feature in the center (the piece of pepperoni) was later revealed to
    be the active volcano Prometheus, but at the time of its release
    this lovely but bizarre picture baffled scientists and press alike.

At the regular 11 a.m. press briefing, Brad Smith glowed. “We’re all
recovering from what I would call the most exciting, the most
fascinating, what may ultimately prove to be the most scientifically
rewarding mission in the unmanned space program. The Io pictures this
morning were truly spectacular and the atmosphere up in the imaging area
was punctuated by whoops of joy or amazement or both.” The new color
photo of Io taken the night before was released, showing strange surface
features in tones of yellow, orange, and white. The image defied
description; the Imaging Team used terms like “grotesque,” “diseased,”
“gross,” “bizarre.” Smith introduced the picture with the comment, “Io
looks better than a lot of pizzas I’ve seen.” Larry Soderblom added,
“Well, you may recall [that we] told you yesterday that when we flew by
we’d figure all this out. I hope you didn’t believe it.”

One thing was certain: There were no impact craters on Io. Unless the
satellites of Jupiter had somehow been shielded from the meteoric
impacts that cratered objects such as the Moon, Mars, and Mercury, the
absence of craters must indicate the presence of erosion or of internal
processes that destroy or cover up craters. Io did not look like a dead
planet. Imaging Team member Hal Masursky, looking at the “pizza”
picture, estimated that the surface of Io must be no more than 100
million years old—that is, some agent must have erased impact craters
during the last 100 million years. This interpretation depended on how
often cratering impacts occur on Io. No one could be sure that there had
been any interplanetary debris in the Jovian system to impact the
surfaces of the satellites. Perhaps none of them would be cratered. The
forthcoming flybys of Ganymede and Callisto would soon provide this

    [Illustration: The giant volcanic feature Pele, about 1000
    kilometers across, mystified Voyager scientists when this picture of
    Io was taken on March 5 from a distance of about 400 000 kilometers.
    The brilliant colors, the strange shapes of the surface deposits,
    and the absence of impact craters all testified that Io was unlike
    any world previously encountered in the exploration of the solar
    system. [P-21226C]]

    [Illustration: As Voyager 1 approached Io, the images of the surface
    became more and more spectacular. On the morning of March 5, at a
    range of 130 000 kilometers, this picture was taken centered near
    longitude 320° and latitude 10°S. The width of the picture is about
    1000 kilometers (the distance from the Mexican border to the
    northern edge of California). There are no impact craters,
    signifying a geologically young surface, and the dark center with
    radiating red flows indicates recent volcanic activity of some sort.

The close flyby of Ganymede took place at 6:53 p.m., at a range of 115
000 kilometers. During the preceding four hours, photos revealed a
surface covered with impact craters. Watching these photos and supplying
commentary for the television listeners, David Morrison remarked, “While
I’m delighted to see craters, it’s just the opposite of what I would
have expected. I was telling everyone a few days ago that I thought Io
would have plenty of craters and that Ganymede, because of the ice
surface, simply would not be able to hold large craters over geological
time. So this is fascinating and this is confusing—both what has
happened on Io to erase the craters and why Ganymede’s surface is strong
enough to preserve them.”

Just before closest approach, at 6:35 p.m., the ultraviolet instrument
watched as a bright star passed behind Ganymede and reemerged ten
minutes later. No dimming that could be attributed to an atmosphere was
seen; when the data were analyzed later, scientists set an upper limit
for any gas on this satellite at one-billionth of the atmospheric
pressure at the surface of the Earth.

As encounter day drew to a close, celebrations took place all over JPL.
For many, however, the excitement was tempered by exhaustion. After 48
hours of intense activity, sleep was imperative for some. But the close
approach to Callisto was still to come, as was an examination of the
data already received.

    [Illustration: After its close flyby of Io, Voyager 1 headed for
    Ganymede, the largest of the Galilean satellites. This global view
    of Ganymede, taken on March 4 at a range of 2.6 million kilometers,
    shows features as small as 50 kilometers across. At the time,
    Voyager scientists speculated that the numerous white spots were
    impact craters, surrounded in some cases by icy ejecta blankets
    splashed onto the surrounding surface. However, many narrow white
    streaks, especially those in the lower left quadrant, promised new
    and exciting geological features on this satellite. [P-21207C]]

                           Tuesday, March 6.

Voyager was now receding from Jupiter, accelerated on a new
trajectory—one that would speed it on toward its November 1980 encounter
with Saturn. But first came the encounter with Callisto, with closest
approach at 9:50 a.m., at a range of 126 000 kilometers. The satellite
was littered with craters and there appeared one huge “bullseye” pattern
that might have been the result of an impact. As the photos of Callisto
came in, Garry Hunt described the scene in the imaging area: “The
activity around the monitors now is quite incredible with people caught
breathless by the pictures coming in. Every satellite we’ve seen has
been a different world.” Asked how he felt about the images of Jupiter’s
atmosphere, he replied, “I’m absolutely delighted with what I’ve seen,
and I’m delighted Voyager 2 is not far behind since I’m convinced that
we’ll see yet another face of Jupiter by then. The weather will have
changed by July.”

At the morning press briefing, the wealth of new data began to be
revealed. “If Jupiter had ever posed for Monet, it would probably have
turned out like this,” said Brad Smith as he introduced some enhanced
(exaggerated) color images that had been specially processed to show
more detail in the Great Red Spot region. Indeed, these photographs did
look like paintings with Jupiter displaying some of its best abstract

    [Illustration: As Voyager 1 approached Ganymede on March 5, many
    strange new surface features became visible. In this frame, taken
    from a distance of 250 000 kilometers and showing features as small
    as 5 kilometers across, three distinct types of terrain are seen:
    polygons of old dark surface, extensive areas of lighter, younger
    material, and brilliant white ejecta patterns (probably water-ice)
    around fresh craters. The bright rays in the upper part of the
    picture are 300-500 kilometers long; at the bottom are several
    craters with only faint, muted ejecta patterns. [P-21262C]]

    [Illustration: Voyager 1 found that the surface of Ganymede was
    geologically very complex. This frame, taken March 5 from a range of
    250 000 kilometers, shows a region about 1000 kilometers across
    centered near longitude 0° and latitude 20°S. The surface displays
    many craters, some (probably the younger ones) with bright ray
    systems. Bright grooved bands traverse the surface in various
    directions. One of these bands, running in a north-south direction
    in the lower left of the picture, is offset along a white line that
    may represent a fault. Ganymede is the only Galilean satellite to
    show indications of such lateral offsets in the crust. [P-21266]]

The first picture of Amalthea was shown, revealing an elongated, dark,
reddish object about 265 kilometers long. Smith reported: “There is
actually some structure that one can see—a crater—a couple of bright
features for which we have no interpretation and some other evidence of
cratering. It doesn’t look like much, but after all, Amalthea has never
been seen as anything more than a point of light from the Earth, and, in
fact, there are very few people that have even seen it as a point of
light. I doubt that more than one astronomer out of a hundred has
actually seen Amalthea.”

Larry Soderblom introduced new photographs of the satellites. Io still
showed no craters, even at high resolution. The craters that “should”
have been on Io were on Ganymede and Callisto. Ganymede not only had
craters, it had fault lines as well. “There is transverse motion along
these faults. Things get offset, apparently, for hundreds of kilometers.
So it’s the first time we’ve seen major kinds of transverse motion on
the surface of another planet.” Ganymede, in effect, had shown evidence
of having its own icy version of the San Andreas Fault.

    [Illustration: Amalthea was thought to be the innermost satellite of
    Jupiter until Voyager 2 discovered tiny Adrastea (1979J1). The
    Voyager 1 camera revealed Amalthea as an irregular dark reddish
    object with dimensions of 270 × 160 kilometers. These three images
    have resolutions of (b) 25 kilometers; (c) 13 kilometers; (d) 8
    kilometers. [260-503]]

    [Illustration: Observation geometry for Amalthea]

    [Illustration: (b)]

    [Illustration: (c)]

    [Illustration: (d)]

Although Callisto was heavily covered with craters up to 200 kilometers
in diameter, Dr. Soderblom commented on the absence of larger impact
basins. Perhaps there was one in the huge bullseye feature; however, it
was not a “standard” looking basin like those on the Moon. Callisto was
“extremely smooth and free of any relief. The structure [the bullseye],
if impact caused, shows no relief; the limb does not show any relief;
maybe it’s possible that Callisto cannot support relief.” By the next
day, the geologists on the Imaging Team were becoming more convinced
that the Callisto “bullseye” was the frozen remnant of an enormous
impact into Callisto’s surface. Since Callisto is composed in large part
of water and has an icy crust, the team speculated that any raised
features created by the impact would eventually “slide” back into the
surface, “and the ripple marks from the shock wave caused by the impact
were frozen” into the surface.

    [Illustration: Callisto was the last of the Galilean satellites to
    be studied by Voyager 1. In this photo, taken March 5 from a
    distance of 1.2 million kilometers, with a resolution of about 25
    kilometers, the extensive cratering of the surface began to be
    apparent. Near the upper left edge is the large impact basin
    Valhalla; the numerous light spots are craters 100 kilometers or
    more in diameter. This is the same side of Callisto that was
    photographed at higher resolution during the Voyager 1 flyby of the
    satellite a day later. [P-21284C]]

Lyle Broadfoot announced that the ultraviolet spectrometer had detected
very strong auroral emission in Jupiter’s north and south polar regions.
The aurora seemed to be caused primarily by the excitation of molecular
hydrogen, although some atomic hydrogen was also detected. Auroral
emissions from helium atoms were not detected.

    [Illustration: The largest ancient impact basin on Callisto is
    called Valhalla. The central light area is about 600 kilometers
    across. Surrounding it is a set of concentric low ridges, looking
    like frozen ripple marks, extending about 1500 kilometers from the
    center. This picture was taken by Voyager 1 on March 6 at a range of
    350 000 kilometers. [P-21287C]]

The IRIS infrared measurements required more computer processing than
other Voyager data, and therefore they were not available until a day or
two after the observations were made. However, Rudy Hanel already had
two new results to report. First, the Great Red Spot was about 3° C
cooler than its surroundings, and this cooling extended many kilometers
above the clouds, into the thin upper atmosphere. Second, the thermal
emission from Io was peculiar, with an unexpected shape to the spectrum.
Tentatively, Hanel suggested there might be some hot spots on the
surface of the satellite.

Jupiter has been full of surprises, but the excitement was far from
over. Major discoveries were yet to be made.

    [Illustration: Voyager 1 discovered the rings of Jupiter on March 4
    in a single eleven-minute exposure with the narrow-angle camera.
    Spacecraft motion during the time exposure streaked the picture, as
    can be seen from the hairpin-like images of the stars. (The star
    field was unusually rich, since it happened to include the Beehive
    star cluster in Cancer.) The ring image itself is a multiple
    exposure, with six separate images side by side. The ring does not
    extend out of the right side of the picture, indicating that this
    image captured the outer edge of the ring, about halfway between the
    cloud tops and the orbit of Amalthea. [P-21258]]

    [Illustration: Camera with fields of view]

                          Wednesday, March 7.

At the press briefing, Brad Smith made a spectacular announcement: “This
morning I would like to add yet another important discovery to be
claimed by this outstanding mission—that of a thin flat ring of
particles surrounding Jupiter. Thus Jupiter now joins Saturn and Uranus
to become the third planet of our solar system known to possess a
planetary ring system and leaves Neptune as the only member of the group
of giant planets without a known ring. The discovery of the ring was
unexpected in that the current theory which treats long-term stability
of planetary rings would not predict the existence of such a ring around
Jupiter. The single Voyager camera image which recorded the ring was
planned by the Voyager Imaging Team several years ago, not really with
any great expectation of a positive result, but more for the purpose of
providing a degree of completeness to Voyager’s survey of the entire
Jupiter system. The observation, as planned, involved looking off to the
right of the limb of Jupiter in the planet’s equatorial plane at the
exact moment that the spacecraft would be crossing the equatorial plane.
The image actually was taken at 16 hours and 52 minutes before encounter
from a distance of about 1.2 million kilometers. Exposure time was 11.2
minutes. We weren’t certain of the exact moment that we would cross the
equatorial plane, so we planned to open our shutter and leave it open as
we went through.”

    [Illustration: Since the Voyager 1 ring photograph was taken exactly
    edge-on, it was not possible to determine the width of the ring. In
    this artist’s conception, the ring is drawn as if it were
    ribbon-like, with very little width, quite unlike the broad flat
    rings of Saturn. Voyager 2 later showed this to have been a lucky
    guess. [P-21259]]

The image showed six exposures of the ring, together with streaked
trails of background stars. Smith reported that the thickness of the
ring was less than 30 kilometers, and that it extended to a point 128
000 kilometers from the center of Jupiter, or 57 000 kilometers above
the clouds. In response to this discovery, scientists were eagerly
planning to alter the Voyager 2 encounter sequence to try to obtain
additional information on the ring.

The newspapers carried stories of the ring discovery on Thursday, March
8. One story was seen by two University of Hawaii astronomers, Eric
Becklin and Gareth Wynn-Williams, who were observing at Mauna Kea
Observatory, at an altitude of 4200 meters on the Big Island of Hawaii.
Within two days they had succeeded in detecting the rings by their
reflected sunlight, at an infrared wavelength of 2.2 micrometers,
providing a rapid confirmation of the Voyager discovery.

As the plasma measurements from the Io flyby were analyzed, an
additional clue to the origin of sulfur and oxygen was revealed. Herb
Bridge reported the detection of sulfur dioxide (SO₂), the simplest
molecule composed of these two atoms.

The high-resolution tape-recorded pictures of Io baffled imaging
scientists. A number of features looked like volcanic flows; together
with the absence of impact craters, these features indicated a
geologically active planet. A central point of discussion was the recent
theoretical work on Io by Stanton Peale of the University of California
and two NASA scientists, Pat Cassen and Ray Reynolds. These authors had
just published a paper in the March 2 issue of _Science_ showing how
tidal heating from Jupiter’s gravity could melt the interior of Io. They
wrote that “widespread and recurrent surface volcanism might occur.” It
began to look as if the prediction had been correct.

                           Thursday, March 8.

The last Voyager 1 press briefing was held. Each speaker was allotted
only a few minutes, prompting Larry Soderblom to preface his remarks by
trying to explain how difficult it was to describe four new planets—the
Galilean satellites—in the time allowed. “Torrence [Johnson] was sitting
with me last night, puzzling. He said, ‘You know, Larry, it’s sort of
like imagining we’d flown into the solar system the day before
yesterday, and said, “There’s a thing we’ll call Mercury, and there’s
the Moon, and there’s Earth, and there’s Mars. Now let’s explain them in
ten minutes”.’” There was Callisto, with the highest density of craters
of any Galilean satellite—the oldest of the Galilean surfaces—featuring
a huge “bullseye” that is “the largest single contiguous feature seen so
far in the solar system.” There was Ganymede, cratered, but also overrun
with fault lines that looked, according to one person, like “tire tracks
in the desert,” showing a surface that “had laterally slid—faulted and
sheared and sheared again—twisted and torn apart.” There was Io, the
most bizarre—the one that scientists had thought would be most
lunarlike—showing a surface that had apparently been “cooked and steamed
and fumed out leaving deposits all over the surface much like you might
see around a fumarole at Yellowstone National Park. The fact that these
things exhibit such youth makes it likely that the planet is still
volcanically active.” There was Europa, with huge linear features unlike
those of the other three Galileans—Europa the mystery satellite, waiting
for the probing eyes of Voyager 2 to survey it in early July.

Ed Stone summed it all up: “I think we have had almost a decade’s worth
of discovery in this two-week period, and I think that all of the people
who have been talking to you feel the same saturation of new information
which has occurred. And in fact, we will probably be studying it in
great detail for at least five years.”

Over the next day or so the press packed up and went home. The TV
monitors showed the spacecraft’s parting glance at a crescent Jupiter,
only hinting at the vastness of space Voyager 1 would travel until its
encounter with Saturn in the autumn of 1980. Maybe now there would be a
relative calm that would allow the scientists to begin analyzing that
“decade’s worth of data.” But things were not to be calm just yet.

                           Fire and Brimstone

At about 5 a.m. on March 8, Voyager had taken a historic picture.
Looking back at a crescent Io from a distance of 4.5 million kilometers,
the camera had been used to obtain a long-exposure view for the
spacecraft navigation team—one that showed the satellite against the
field of background stars. During the day, Linda Morabito, an optical
navigation engineer, began to work with this picture on her
computer-controlled image display. She noted what appeared to be a
crescent cloud, extending beyond the edge of Io. But Io has no
atmosphere, so a cloud rising hundreds of kilometers above the surface
did not seem to make sense.

The next day, working with her colleagues, Morabito eliminated all
possibilities for the new feature on Io except the obvious—a cloud. If
it were a cloud, it must be the result of an ongoing volcanic eruption
of incredible violence. The picture was shown to members of the Imaging
Team, who agreed with the identification. But it was Friday and Brad
Smith and Larry Soderblom, along with most of the other team members,
had left for the weekend to try to get some rest. The picture would have
to wait two more days.



Uniform wind speeds for cloud features with widely different size
scales, suggesting that mass motion and not wave motion is being

Rapid formation and spreading of bright cloud material, perhaps the
result of disturbances that trigger convective activity.

A pattern of east-west winds in the polar regions, previously thought to
have been dominated by convective upwelling and downwelling.

Anticyclonic motion of material associated with the Great Red Spot, with
a rotational period of about six days.

Interactions of smaller spots with the Great Red Spot and with each

Auroral emissions in the polar regions, both in the ultraviolet (which
were not present during the 1973 Pioneer encounter) and in the visible.

Cloud-top lightning bolts, similar to terrestrial superbolts, and
meteoritic fireballs.

A temperature inversion layer in the stratosphere and a temperature of
160 K at the level at which the atmospheric pressure is 0.01 bar.

Very strong ultraviolet emission from the disk, indicating a
thermospheric temperature of more than 1000 K.

A hot (1100 K) upper ionosphere on the dayside that was not observed by
Pioneer 10, suggesting there may be large temporal or spatial changes.

An atmospheric composition with volume fraction of helium of 0.11 ±

A substantially colder atmosphere above the Great Red Spot than in the
surrounding regions.

                          Satellites and Rings

At least eight currently active volcanoes on Io, probably the result of
tidal heating of the interior of the satellite, with plumes extending up
to 250 kilometers above the surface.

A large hot spot on Io near the volcano Loki that is about 150 K warmer
than the surrounding surface.

Numerous intersecting, linear features on Europa, possibly due to
crustal cracking.

Two distinct types of terrain, cratered and grooved, on Ganymede,
suggesting that the entire ice-rich crust was once under tension.

An ancient, heavily cratered crust on Callisto, with vestigial rings of
enormous impact basins since erased by flow of the ice-laden crust.

The elliptical shape of Amalthea (270 × 160 kilometers).

A faint ring of material about Jupiter, with an outer edge of 128 000
kilometers from the center of the planet.


An electrical current of more than a million amperes flowing in the
magnetic flux tube linking Jupiter and Io.

Very strong ultraviolet emissions from ionized sulfur and oxygen in the
Io plasma torus, indicating a hot (hundred thousand degree) plasma that
evidently was not present at the time of Pioneer 10 encounter.

Plasma electron densities exceeding 4500 per cubic centimeter in some
regions of the Io plasma torus.

A cold, corotating plasma inside 6 R_J with ions of sulfur, oxygen, and
sulfur dioxide.

High-energy trapped particles inside 6 R_J with significantly enhanced
abundances of oxygen, sodium, and sulfur.

Hot plasma near the magnetopause predominantly composed of protons,
oxygen, and sulfur.

Jovian radio emission at kilometer wavelengths, which may be generated
by plasma oscillations in the Io plasma torus.

Corotating plasma flows unexpectedly far from Jupiter in the dayside
outer magnetosphere.

Evidence suggesting a transition from closed magnetic field lines to a
Jovian magnetotail at about 25 R_J from Jupiter.

Whistler emission interpreted as lightning whistlers from the Jovian

[1]Adapted from a paper by E. C. Stone and A. L. Lane in the Voyager 1
    Thirty-Day Report.

[2]Adapted from a summary prepared by E. C. Stone and A. L. Lane for the
    Voyager 1 Thirty-Day Report.

    [Illustration: The dramatic discovery of active volcanoes on Io was
    made by Linda Morabito and her colleagues from this navigation
    picture, taken March 8 at a range from Io of 4.5 million kilometers.
    On the bright edge, the immense plume of volcanic ash from Pele (P₁)
    rises nearly 300 kilometers above the surface. At the terminator,
    the border between day and night on Io, a second smaller cloud from
    the volcano Loki (P₂) catches the sunlight. These two
    eruptions—captured on this single discovery photograph—are much
    larger than the largest terrestrial volcanic eruption known.
    [P-21306 B/W]]

    [Illustration: Once the existence of giant volcanic eruptions on Io
    was recognized, a reexamination of the Voyager 1 encounter pictures
    revealed many more plumes. These two views of Prometheus (P₃) were
    found by Joseph Veverka and Robert Strom on March 12 when they
    reproduced earlier pictures.]

    [Illustration: The plume is silhouetted against the black space,
    although it is also possible to see dark “feet” where the falling
    material reaches the surface. [P-21295]]

    [Illustration: The complex jets of material are clearly seen as dark
    streaks against the light background of the surface of Io. The plume
    itself rose more than 100 kilometers above Io’s surface. [P-21294]]

Meanwhile, new information about Jupiter was released to the public. A
long-exposure (three minutes and twelve seconds) image of the dark side
of the planet, taken with the wide-angle camera while in the shadow of
the planet, caught Jupiter showing off some Jovian “fireworks.” A long,
broad, white streak across the picture was a visible aurora, the largest
aurora ever seen—almost 29 000 kilometers long. In addition, nineteen
smaller bright splotches, looking insignificant by comparison, were in
reality “superbolts” of lightning. Since huge electrical discharges such
as lightning can, under the right circumstances, power chemical
reactions that form complex organic molecules, the discovery of
lightning on Jupiter could have profound implications. Was
“lightning-inspired” organic synthesis going on in Jupiter’s atmosphere?
No one knew.

Returning to JPL on Sunday night, Brad Smith got his first look at the
Morabito picture of the volcanic cloud. Early Monday morning, other
Imaging Team scientists saw it. As soon as the JPL computers were
operating, Joseph Veverka and Robert Strom began working with the two
interactive TV terminals to look for evidence in other pictures of
ongoing eruptions. Faint clouds or plumes would not show up in normally
processed pictures, but could be brought out easily with the
computer-controlled displays. By midmorning, several additional volcanic
plumes had been found.

    [Illustration: Linda Morabito shows the discovery photo of the
    volcanic eruptions on Io. [P-21718]]

    [Illustration: The dark side of Jupiter revealed many surprising
    phenomena to Voyager 1. A wide-angle view, taken on March 5, led to
    the discovery of a double auroral arc at north-polar latitudes and
    numerous flashes of lightning illuminating the clouds during this
    3-minute, 12-second exposure, taken at a range of half a million
    kilometers. [260-460]]

Meanwhile, on March 11, John Pearl of the IRIS team had independently
drawn the conclusion that volcanic activity must be taking place on Io.
He and Rudy Hanel found evidence of strongly enhanced thermal emission
from parts of the satellite. The most prominent was a source nearly 200°
C hotter than its surroundings. On March 12, Pearl brought his new
results to the Imaging Team, and sure enough, the hot spot was located
near one of the volcanic plumes! A month later, continuing analysis of
IRIS spectra yielded identification of sulfur dioxide (SO₂) gas over
this same erupting volcano. At last a source had been located for the
enigmatic sulfur and oxygen ions in the magnetosphere.

The volcanoes provided a thread with which to weave together the
disparate data on Io. A few months earlier there had been a report of a
sudden brightening of Io in the infrared; now it seemed plausible that
thermal emission from an eruption was the source. The Voyager
ultraviolet experimenters had been worrying over the source of the
intense sulfur emissions they had seen and had been disturbed by the
changes in the gas clouds around Io since the Pioneer 10 and 11 flybys;
now a variable source for these gas clouds was identified. In addition,
the craterfree surface and bizarre features seen in the Voyager images
could be recognized as the product of violent explosive eruptions on Io.
It appeared that Peale, Cassen, and Reynolds had found, in their
theoretical calculations, the key to the most geologically active body
ever encountered in the solar system.

News of the discovery was released to the press on Monday, March 12.
During the next few days, a total of eight gigantic eruptions were
located in the Voyager pictures of Io. Within a few weeks, scientists
all over the world were thinking with renewed energy about this
incredible satellite.

With four new planet-sized satellites now photographed, there was a
sudden requirement for maps and for names to be assigned to the newly
discovered features. The maps were produced from Voyager images at the
Astrogeology Branch of the U.S. Geological Survey at Flagstaff, Arizona.
The names, proposed by a group of scientists headed by Voyager Imaging
Team members Tobias Owen and Hal Masursky, were given official approval
by the International Astronomical Union in August. For a time, a dual
nomenclature persisted for the erupting volcanoes on Io. The eruption
plumes were given numbers, P₁, P₂, etc., while the volcanic features
were given names taken from the mythology of fire and volcano legends.
Thus the “hoof print” of Io was called Pele, for the Hawaiian volcano
goddess, and the 280-kilometer-high plume associated with it was called
P₁. By the time of the Voyager 2 encounter, scientists had prepared maps
on which to plot their new discoveries.

While the Voyager scientists fanned out across the world to share their
findings with colleagues, attention at JPL turned to Voyager 2. In
response to the discoveries of the first encounter, changes were
required in the sequencing of scientific observations for July. Voyager
2, still troubled by a faulty receiver, might require more coddling from
the spacecraft team than had its sister spacecraft, now safely on the
way to Saturn.

    [Illustration: The particles of the rings of Jupiter are stronger
    reflectors of red light than of blue, as can be seen in this view,
    assembled from two images taken in orange and violet light. Since
    the images were registered on the rings, not the planet, the bright
    bands of colors along the edge of Jupiter are an artifact of
    misalignment. [P-21779]]

                               CHAPTER 7

                          Approaching Jupiter

At the beginning of July, the dry summer heat had returned to Pasadena,
and so had the press. The scientists had come days or weeks earlier to
look at data being transmitted from the second Voyager as it approached
Jupiter and its satellites. The mood at JPL seemed quieter than it had
been in March for Voyager 1, although the press room would once again be
deluged with observers on the day of encounter. This would be our second
good, close look at the Jovian system, but it was to be no summer rerun.
Voyager 2 would have a different view of each world, and, in addition,
both Io and Jupiter had undergone changes, as though to ensure that no
one would become bored and fall asleep in front of a TV monitor. In a
sense, this encounter was to be another first look at Jupiter and its
satellites, with a view of each object that was quite different from
what had been seen before.

Changes in Jupiter’s cloud formations became noticeable long before
July. After a gap of six weeks following the first flyby, Voyager 2’s
observatory phase began on April 24, 1979, seventy-six days before its
July 9 encounter with Jupiter. During this time, the spacecraft’s
ultraviolet and fields and particles instruments studied the Jovian
system and its interaction with the solar wind. Between April 24 and May
27, Voyager 2’s imaging system concentrated on the motions in Jupiter’s
atmosphere, creating another approach time-lapse “movie.” From May 27 to
29 photographs were taken in a more rapid sequence, showing the planet
during five 10-hour rotations. From these studies it was apparent that
Garry Hunt’s prediction had been right—the weather _had_ changed by
July. A month before the encounter JPL’s Voyager Bulletin—Mission Status
Report announced that “Jupiter is sporting quite a different face than
it did just four months ago. The bright ‘tongue’ extending upward from
the Red Spot is interacting with a thin, bright cloud above it that has
traveled twice around Jupiter in four months.” The turbulent region west
of the Great Red Spot had begun to break up and separate from the Red
Spot. The white ovals south of the Red Spot had drifted to the east
(about 0.35 degrees a day), while the Red Spot itself had drifted west
(about 0.26 degrees a day). The white zone seen just south of the Red
Spot by Voyager 1 had become very narrow—like a thin white line just
barely outlining the bottom of the spot. The Red Spot had also changed:
It had become a more uniform orange-red, perhaps reverting to the color
seen by Pioneers 10 and 11. The brown spots that had been seen in the
north temperate region at the same longitude as the Red Spot were now on
the other side of the planet. A dark brown spot not present during the
Voyager 1 flyby had developed along the northern edge of the brown
equatorial region on the Red Spot side of the planet. Some of the white
markings that seemed to have protruded into the equatorial region at the
time of the first flyby were missing in the Voyager 2 photographs.

As Voyager 2 entered the far encounter period on May 29, all instruments
on the spacecraft (except for the photopolarimeter) seemed to be in good
shape for encounter. As was the case with Voyager 1, the polarization
wheel on Voyager 2’s photopolarimeter was stuck, so the instrument was
able to obtain only color photometry measurements.

    [Illustration: In early June, as Voyager 2 carried out its
    observatory phase, additional changes in Jupiter’s face began to be
    apparent. These two images, taken from a distance of 24 million
    kilometers, have a resolution of about 500 kilometers.]

    [Illustration: The Great Red Spot and the white oval south of it are
    seen to be followed on the west by regions of chaotic and turbulent
    clouds. This is not the same white oval that was near the Red Spot
    in March; the differential rotation of the planet carried a
    different oval close to the Red Spot during the intervening three
    months. [P-21713C]]

    [Illustration: Io is visible to the right of the planet, and the
    shadow of Ganymede falls on the colored clouds of Jupiter’s
    equatorial belt. [P-21714C]]

    [Illustration: The Voyager 2 trajectory was complementary to that of
    Voyager 1. This time, the satellites were encountered before
    Jupiter, revealing their other hemispheres. As shown in this
    drawing, the spacecraft flew by first Callisto, then Ganymede, then
    Europa. The ten-hour Io volcano watch took place immediately after
    closest approach to Jupiter. [260-533A]]

  Voyager 2 trajectory
    View normal to Jupiter equator
    Sun occultation
    Earth occultation
    Launch date = 8/20/77
    Jupiter arrival date = 7/9/79
    Satellite closest approach

    [Illustration: These two faces of Jupiter were photographed by
    Voyager 2 on May 9 at a distance of 46 million kilometers from the
    planet. Voyager scientists began to detect significant changes in
    the cloud patterns since the Voyager 1 encounter two months earlier.

    [Illustration: Jupiter.]

    [Illustration: Jupiter.]

    [Illustration: The weather is changing over one of the northern
    hemisphere brown ovals in this picture taken July 6. The brown ovals
    are regions in which breaks in the upper layer of ammonia clouds
    reveal darker clouds below. A high, white cloud is seen moving over
    the darker cloud, providing an indication of the structure of the
    cloud layers. Thin white clouds are also seen within the dark cloud.
    At right, blue areas, free of high clouds, are seen. [P-21753C]]

Although Voyager 2’s radio receiver still could not track a
Doppler-shifted radio signal from Earth (the problem is that it “hears a
monotone,” explained Deputy Project Manager Esker K. Davis), the Deep
Space Network engineers had learned to work with the spacecraft,
determining what frequency the spacecraft would listen to at any
particular time. They had discovered that some of the “housekeeping”
telemetry signals from the receiver were sensitive to the match between
the incoming frequency and the receiver frequency. By monitoring these
signals, they could detect a frequency drift in time to correct the
transmission, thus keeping the system in tune in spite of slow changes
in the receiver. The system was slow and demanding but effective; all
the necessary command sequences were successfully loaded into the
computer, and communications during the encounter were entirely

The timing offset experienced by Voyager 1 as a result of Jupiter’s
intense radiation environment was not expected to be a problem on
Voyager 2 for two reasons: Even at closest approach, Voyager 2 would
still be more than twice as far from Jupiter as Voyager 1 had been, and
the Voyager 2 computer was programmed to resynchronize the spacecraft’s
timing systems automatically every hour. In this way, even if the
radiation environment proved to be much higher than anticipated, the
image smear that might occur from a timing offset would be prevented.

    [Illustration: Complex activity in the southern hemisphere of
    Jupiter continued during the Voyager 2 encounter, although changes
    had occurred in the region of the Great Red Spot. A white oval,
    different from the one observed in a similar position at the time of
    the Voyager 1 encounter, was situated south of the Red Spot. The
    region of white clouds extended from east of the Red Spot and around
    its northern boundary, preventing small cloud vortices from circling
    the feature. The disturbed region west of the Red Spot had also
    changed since the equivalent Voyager 1 image. The picture was taken
    on July 3 from a distance of 6 million kilometers. [P-21742C]]

As a result of the discoveries made by Voyager 1, the project scientists
decided to modify some of Voyager 2’s preplanned sequences. As early as
April 1, the painstaking job of constructing new computer commands
began. A ten-hour Io Volcano Watch was added to the spacecraft’s
program, taking advantage of the fact that shortly after closest
approach to Jupiter, the spacecraft would remain within about 1 million
kilometers of Io for a long period, keeping nearly the same face in
view. Provisions were also made to take extensive ultraviolet
measurements of the emission from the glowing torus surrounding Jupiter
near the orbit of Io. Further studies would be made of the dark side of
Jupiter to search for lightning and auroral activity, and there was also
the hope that the plasma wave instrument would be able to detect
lightning whistlers (radio signals created by lightning bolts) as the
Voyager 1 instrument had done. A high priority was given to observations
of the newly discovered ring, which had not been in the original Voyager
2 sequence at all. The spacecraft would cross the ring plane twice,
photographing the ring during both the inbound and the outbound

As was originally planned, Voyager 2 would make its closest approaches
to Callisto, Ganymede, Europa, and Amalthea before encounter with
Jupiter. Because of the difference in the trajectories of the two
spacecraft, Voyager 2 would see the faces of Callisto and Ganymede not
seen by Voyager 1. The most important difference, however, was that the
second Voyager would fly much closer to Europa than Voyager 1 did,
giving scientists their first good look at the mysterious streaks
scratched on the surface of that bright golden world. Voyager 2 would
also have a closer flyby of Ganymede, giving us a second chance to
examine its strange “snowmobile tracks.” The major loss, of course, was
Io, which would be seen from Voyager 2 only at distances of a million
kilometers or more.

    [Illustration: Io appeared in front of Jupiter as seen by Voyager on
    June 25, at a range of 12 million kilometers. At a resolution of
    about 200 kilometers, the bright and dark spots on the satellite are
    just beginning to be resolved, but it was not possible to determine
    if any eruptions were still in progress. [P-21719C]]

                             The Encounter

                           Wednesday, July 4.

(_Range to Jupiter, 5.3 million kilometers; range to Earth, 921 million
kilometers_). While most of the nation celebrated Independence Day with
picnics, sports events, and fireworks, the scientists and engineers at
JPL were working around the clock. Voyager 2 had already entered
Jupiter’s territory, crossing the bow shock for the first time on July 2
at a distance of 99 R_J from Jupiter, indicating that the magnetosphere
had expanded in the interval between the two encounters. At about noon
on July 3, the spacecraft encountered the magnetopause, but on July 4,
the data from the particles and fields instruments were ambiguous.
Apparently the magnetosphere was pulsating in response to changing
pressures, and the spacecraft was playing tag with the rapidly shifting
boundaries of the bow shock and the magnetopause.

As the low energy charged particle instrument began to measure particles
coming from inside the Jovian magnetosphere, it became apparent that
some important changes had taken place since Voyager 1’s encounter. From
the composition of the particles, it appeared that they were largely of
solar origin, unlike the heavy concentrations of ions of sulfur and
oxygen seen by Voyager 1. Scientists began to speculate that the Io
volcanoes, which presumably eject sulfur and oxygen into the
magnetosphere, might have declined in activity. In the evening, the
first images of Io at a resolution high enough to allow the volcanic
plumes to be seen would be beamed back to Earth.

     Object      Range to Center at Closest   Best Image Resolution
                    Approach (kilometers)       (km per line pair)

  Jupiter                            722 000                      15
  Amalthea                           558 000                      10
  Io                               1 130 000                      20
  Europa                             206 000                       4
  Ganymede                            62 000                       1
  Callisto                           215 000                       4

    [Illustration: Voyager scientists anxiously awaited the first views
    of Io that would show whether the volcanic eruptions seen in March
    were still active. This picture was taken on July 4, at a range of
    4.7 million kilometers, about the same as that of the volcano
    discovery picture on March 8. One large plume is clearly visible,
    rising nearly 200 kilometers above the surface. At the time of
    release of this picture on July 6, the scientists wrote, “The
    volcano apparently has been erupting since it was observed by
    Voyager 1 in March. This suggests that the volcanoes on Io probably
    are in continuous eruption.” [P-21738B/W]]

                           Thursday, July 5.

(_Range to Jupiter, 4.4 million kilometers_). The press room at Von
Karman Auditorium opened and the members of the press, most of them
veterans of the first encounter, arrived at JPL. Meanwhile, the
spacecraft continued to measure fluctuations in the magnetospheric
boundary. By noon, JPL had reported at least eleven crossings of the bow
shock as the solar wind flirted with Jupiter’s magnetosphere. Apparently
the solar wind was much more variable in July than it had been in March.
At times the bow shock seemed to be thicker than that experienced by
Voyager 1; one Voyager 2 crossing took ten minutes, whereas the longest
Voyager 1 crossing was only one minute long. Even though the processes
affecting the magnetosphere seemed more complex, the magnetosphere was
less compressed; when Voyager 2 actually entered the magnetosphere at a
distance of 62 R_J, it was much farther from Jupiter than Voyager 1 had
been at its final crossing (47 R_J).

Photos obtained the day before from over 4 million kilometers showed
that at least one of Io’s volcanoes was still active. A total of eight
ongoing eruptions had been seen by Voyager 1, and scientists were
anxious to see how many of these were still erupting four months later.

      Boundary            Day           Distance (R_J)

         S                7/02          99  (multiple)
         S                7/02          97
         S                7/03          87
         M                7/04          72  (multiple)
         M                7/05          71
         S                7/05          69
         S                7/05          67
         M                7/05          62


         M                7/23         169
         M                7/23         173
         M                7/24         174
         M                7/24         175
         M                7/24         176
         M                7/24         177
         M                7/25         184
         M                7/25         185
         M                7/27         213
         M                7/31         253
         M                8/01         258
         M                8/01         262  (multiple)
         M                8/03         279  (multiple)
         S                8/03         283  (multiple)

    [Illustration: Although Voyager 2 did not come as close to Io as had
    Voyager 1, some changes in the surface during the four months
    between encounters were so large that they could still be easily
    seen. These two pictures of the region of the volcano Pele were
    taken in early March and early July, respectively. The most dramatic
    change was the filling in of the indentation in the ejecta ring,
    turning the hoofprint into a symmetric oval. The oval is about 1000
    by 700 kilometers in outermost dimension, and the area that changed
    amounts to more than 10 000 square kilometers. [260-687AC]]

    [Illustration: Volcano Pele on Io.]

    [Illustration: Volcano Pele on Io.]

While attention at JPL focused on the unfolding drama of the Jupiter
encounter, many members of the world’s press seemed more interested in
the fate of Skylab, which was nearing its death plunge into the Earth’s
atmosphere. Launched in 1973, Skylab had been one of NASA’s more
successful projects. Three crews of astronauts had visited it, carrying
out intensive studies of the Sun and breaking one record after another
for the duration of manned space flight. Since the departure of the
final group of three astronauts in 1974, Skylab had been sinking
gradually lower as a result of friction with the extreme upper
atmosphere of Earth. During the past year, higher temperatures in the
atmosphere had increased this drag, and now the end was near. With a
strange fascination, the world watched the end of this old spacecraft,
almost seeming to forget the spectacular new results being transmitted
from Jupiter. To the frustration of the Voyager team and the press
“camped out” in Von Karman Auditorium for the second encounter, the
exaggerated stories of a possible Skylab disaster took precedence over
Voyager news. Ultimately, Skylab fell over the Indian Ocean and
Australia on Wednesday, July 11, just as the major findings of Voyager 2
were being released.

                            Friday, July 6.

(_Range to Jupiter, 3.5 million kilometers_). With the first satellite
encounter still two days away, Voyager 2 continued to make a variety of
measurements of Jupiter and all the Galilean satellites. As the distance
to Io decreased, it was possible to see detailed surface features as
well as to look for the volcanic plumes at the edge of the disk,
silhouetted against black space. By the end of the day, the Great Red
Spot loomed so large that six imaging frames (a 2 × 3 mosaic) were
required to encompass it and its immediate surroundings.

At the first formal press conference of the Voyager 2 encounter, Project
Scientist Ed Stone reviewed the progress of the mission. Because the
ailing spacecraft receiver was working so well, Ray Heacock, Voyager
Project Manager, announced that the major trajectory correction maneuver
at Jupiter had been rescheduled to take place only two hours after
closest approach. Since the geometry was especially favorable at this
time, the 76-minute rocket burn could put the spacecraft on its planned
route to Saturn with a minimum expenditure of fuel, thereby preserving
the option of sending the spacecraft on to Uranus.

The Io torus was under observation, both directly by the ultraviolet
spectrometer, and indirectly by the charged particle instruments. The
LECP instrument had begun to pick up sulfur ions, but at lower energies
and lower concentrations than those recorded during the first encounter.
In the ultraviolet, glows could be seen both from the torus and from
aurorae in the polar regions of Jupiter.

Photographs of Io showed that the heart-shaped feature surrounding Pele
(P₁), Io’s largest volcano, had changed shape. The indentation of the
heart had disappeared, making the heart into an oval. Apparently a new
deposit of volcanic ejecta had blanketed the surface, altering its
color. Perhaps an earlier obstruction in the volcanic vent, or the shape
of the vent itself, had caused the area surrounding Pele to look
heart-shaped. In any case, whatever had caused the indentation was now
gone. At the same time, new photos failed to show a plume above Pele,
and there was speculation that changes in this eruption might be related
to the altered population of charged particles in the magnetosphere.

    [Illustration: The Voyager 2 pictures of Callisto looked remarkably
    similar to those obtained of the other side of the satellite by
    Voyager 1. Seen from a distance of 2.3 million kilometers, the large
    craters (100 kilometers or more across) appear as light spots. No
    new major impact features such as Valhalla, discovered by Voyager 1,
    are visible on the hemisphere seen by Voyager 2. [P-21740C]]

                           Saturday, July 7.

(_Range to Jupiter, 2.6 million kilometers_). As the spacecraft rapidly
closed on Callisto, better and better photographs were taken of the
previously unseen hemisphere. As with the Voyager 1 observations,
however, the main impression was one of heavy cratering, unrelieved by
other geologic structures. Meanwhile, the coverage of Io had improved as
the satellite rotated to the point at which a census of the volcanic
eruptions seen in the first encounter began to emerge.

At the 11:00 a.m. press conference, Larry Soderblom announced that four
of the volcanoes discovered by Voyager 1 had been looked at again by
Voyager 2, and three of them—Prometheus (P₂), Loki (P₃), and Marduk
(P₇)—were still active. However, there was no trace of volcanic activity
coming from Pele, the source of the largest plume seen by Voyager 1. P₁
was either greatly subdued or had turned off completely.

Dr. Soderblom also announced that Voyager 2 images had detected another
giant ring structure on Callisto, bringing the total to three, and there
were probably more. This particular ring feature was estimated to be
about 1500 kilometers across. It was also noted that although Callisto
generally seemed to be saturated with shoulder-to-shoulder craters, the
crater density near the ring structures seemed to be lower.

Saturday was a fairly quiet time for the scientists but not for the
spacecraft or the spacecraft team. “We blocked out about 7½ hours,”
explained Michael Devirian, Ground Data Systems Development,
Integration, and Test Director, “in which we could send it a set of
commands and re-send it if necessary to make sure all close-encounter
commands were received by Voyager 2 until all the commands got through.
The whole thing went perfectly the first time.” So everything was “go”
for close encounter. The near encounter phase began at 6:36 p.m. PDT.

    [Illustration: A new face of Ganymede was revealed to Voyager 2.
    This image was taken July 7 from a distance of 1.2 million
    kilometers and clearly shows the large dark area Regio Galileo, as
    well as much of the lighter grooved terrain discovered by Voyager 1.
    The bright spots are impact craters. This image also shows what
    appear to be polar caps, extending down to about latitude 45° in
    both the northern and southern hemispheres. [260-670]]

                            Sunday, July 8.

(_Range to Jupiter, 1.5 million kilometers_). At 2:30 a.m. the first
long-exposure sequence of ring pictures was taken, and at 3:00 a.m. the
intensive period of the Callisto encounter began. Eighty high-resolution
images were obtained of the satellite, centered around closest approach
(215 000 kilometers) at 6:13 a.m. Incoming photos showed some features
that looked like double-walled craters, but no more giant ring
structures were seen. It appeared that there was an asymmetry in the
distribution of large impact features over Callisto’s surface. “Callisto
may turn out to be the most heavily cratered body in the solar system,”
Torrence Johnson remarked. Garry Hunt was to add later on, “There’s just
not room for another crater on that body—it’s totally full.”

At the press conference, Brad Smith confirmed the earlier finding that
Io’s volcano Pele was quite dead—at least for now. Although P₄ had not
yet been looked at, all other volcanoes discovered by Voyager 1 were
still active, but no new plumes had been found. However, new ultraviolet
images of P₂ (Loki) suggested that the eruption had increased in size.
(In a later report, the imaging team announced that P₂ had increased in
height to 175 kilometers and had changed to a two-column plume.) The new
photographs of Jupiter’s ring showed it to be quite narrow and
ribbonlike, Dr. Smith announced. The artist’s drawing (released during
the Voyager 1 encounter), intended to show the outer edge of the ring,
turned out to be a good representation of the actual ring, Dr. Smith

There seemed to be less high-speed sulfur and oxygen inside Jupiter’s
magnetosphere than there had been during the Voyager 1 encounter, George
Gloeckler announced. Voyager 2’s low energy charged particle instrument
was finding substantial amounts of carbon, silicon, magnesium, and other
elements of solar origin, but the Io-associated elements were almost
depleted. The ultraviolet instrument had found as much glowing sulfur in
Io’s torus as before, but less of it seemed to be raised to energies
high enough to leave the torus and be detected elsewhere in the

There were other indications of Jupiter’s changing weather. In a Voyager
report Sunday evening, Garry Hunt remarked, “One very exciting
observation came the other day which caused major excitement down in the
imaging area. We actually saw a white cloud starting to intrude across a
dark barge [large brownish oval-shaped feature in Jupiter’s northern
hemisphere]. Atmospheric scientists get very excited by that because
this is showing us how the colors layer themselves up—that white cloud
is clearly above the dark brown. We’re desperately trying to understand
the relationship of colors on Jupiter.”

    [Illustration: The first close-up views of Europa were both exciting
    and perplexing to Voyager scientists. The best Voyager 1 resolution
    had been only about 30 kilometers, but the Voyager 2 trajectory
    permitted a much closer flyby. These pictures, taken on July 9 at a
    range of 240 000 kilometers, have a resolution of about 5
    kilometers. The bright icy crust of Europa is covered with a
    spectacular series of dark streaks, giving the satellite a cracked
    appearance. In a few cases, narrower light lines run down the
    centers of the dark streaks, which are typically a few tens of
    kilometers in width. Very few, if any, impact craters are visible on
    Europa. [P-21760C and P-21764C]]

    [Illustration: Europa]

                            Monday, July 9.

(_Range to Jupiter at encounter, 722 000 kilometers_). Encounter day!
And not just one encounter, but a whole sequence: Ganymede, Europa,
Amalthea, Jupiter, and Io. By early Sunday evening, a wealth of new data
on Ganymede was pouring in. Not only was this a side of the satellite
not seen before, but Voyager 2 would pass closer to Ganymede than had
Voyager 1. Encounter took place at 1:06 a.m., at a range of 62 000
kilometers. Between 9:00 p.m. Sunday night and 1:30 a.m. Monday morning
a total of 217 photos, plus infrared and ultraviolet spectra, were
scheduled. Sixty-nine photos were sent back in real time; others were
recorded for playback later.

As the Ganymede pictures appeared on the TV screens, they revealed a
world of tremendous variety. Some regions were heavily cratered:
“Ganymede looks like Mercury or the highlands of the moon,” one Voyager
scientist remarked. Other parts of the surface, however, showed very
different features: long, parallel mountain ridges that looked like
grooves made with a giant’s rake; narrow, segmented lines; white ejecta
blankets from impacts that looked like a dazzling, snow-covered
landscape. Some of the pictures suggested cracking and slipping of
Ganymede’s crust, while others showed what appeared to be remnants of
ancient terrain unaffected by subsequent intense geologic activity. Many
of the highest resolution frames were not seen at this time; they were
recorded on the spacecraft for playback later.

Starting at about 8 a.m., Earth began receiving the first closeup views
of Europa. Europa “could be the most exciting satellite in the whole
Jovian system,” said Larry Soderblom, “because it’s sort of the
transition body between the solid silicate body, Io, and the ice balls,
Ganymede and Callisto.” The icy crust looked as though it “had been
ruptured all over—as though it was in pieces—just as though it had been
broken in place and left there.” At 11:43 a.m., closest approach took
place at a range of 206 000 kilometers. By this time the scientists were
dazzled by what they had seen; some were calling Europa the most bizarre
of all the Galilean satellites. In the Imaging Team viewing area, David
Morrison compared Europa’s surface to “a cracked egg,” and Gene
Shoemaker said, “It looks like sea ice to me.” When someone commented
that the canal-like streaks were reminiscent of Mars, Torrence Johnson
replied, “It looks like some pictures of Mars I’ve seen, but only on the
walls of Lowell Observatory.” Another quipped, “Where is Percival
Lowell, now that we need him!”

There were to be two press conferences: one to present spacecraft and
scientific results and one to celebrate the second successful flyby and
to talk of new goals—Saturn, and perhaps Uranus.

At the first conference, Ed Stone began by discussing the radiation
Voyager was experiencing. One of Jupiter’s surprises was that the
radiation environment was greater than had been anticipated, and this
caused problems with the radio receiver. The receiver frequency was
shifting “more rapidly than we had anticipated,” said Ray Heacock, “and
we have not been able to keep an uplink continuously with the
spacecraft.” The solution was to keep sending up commands at different
frequencies until a frequency the spacecraft would accept was found.
Just how bad were the radiation levels? Ed Stone commented, “The
penetrating radiation at a given distance is more intense now at this
distance than it was when Voyager 1 flew by.” From a preliminary
analysis it seemed that, overall, Voyager 2 would still be subjected to
lower radiation levels than Voyager 1 had been, but to higher levels
than had been expected. In addition, Voyager 2’s radio receiver was much
more sensitive. The higher than expected radiation intensity also led
Voyager scientists to have the ultraviolet spectrometer shut off, since
that instrument was also quite sensitive to the radiation.

The fourth member of the Galilean satellites had finally been seen, and
Larry Soderblom happily introduced Europa. “Well, some few months ago,
before the Voyager 1 encounter, we thought we had some idea of what
planets were like—at least the planets in the inner solar system: Mars,
Mercury, the Moon, the Earth. And we’ve discovered many times over in
the last couple of months how narrow our vision really was. Included in
the Jovian collection of satellites are the oldest (Callisto), the
youngest (Io), the darkest (Amalthea), the brightest (Europa), the
reddest (Amalthea and Io), the whitest (Europa), the most active (Io),
and the least active (Callisto). Today we found the flattest (Europa).”

In spite of the appearance of a cracked or broken surface, Europa showed
no topography at all. Toward the sunset line, where the low angle of
illumination should reveal even low relief, “the surface disappears—as
if it were the surface of a billiard ball.” It seemed clear that Europa
has much less relief than the other two icy satellites, Ganymede and
Callisto. But why can’t Europa’s surface support relief? Perhaps Europa
has a thick ice mantle—on the order of 100 kilometers. If Europa is
affected by tidal heating, then such an ice mantle might be “sort of
soft and slushy” rather than rigid as are the crusts of Ganymede and
Callisto. “The fact that the surface of Europa cannot support relief of
any substantial amount suggests that the surface must be soft.” But, Dr.
Soderblom added, there does not appear to be much lateral motion or
rotation causing the surface markings—they don’t seem to be
offset—rather, “it’s as if Europa had been cracked, broken, by some
process which crushed it like an eggshell and just left the pieces
sitting there. Expansion and contraction of ice and water are a good way
to crunch up the surface.”

    [Illustration: Regio Galileo is the largest remnant of the ancient,
    heavily cratered crust of Ganymede. This Voyager 2 color
    reconstruction was made from pictures taken at a range of 310 000
    kilometers; the scene is about 1300 kilometers across. Numerous
    craters, many with central peaks, are visible. The large bright
    circular features have little relief and are probably the remnants
    of old large craters that have been annealed by the flow of icy
    near-surface material. The closely spaced, arcuate linear features
    are analogous to features on Callisto, such as the “ripple” marks
    surrounding the ancient impact feature Valhalla. [P-21761C]]

    [Illustration: At high resolution, the grooved terrain on Ganymede
    shows a wonderful complexity. Surface features as small as 1
    kilometer across can be seen in this mosaic of Voyager 2 images
    taken July 9. The grooves are basically long, parallel mountain
    ridges, 10 to 15 kilometers from crest to crest—about the same scale
    as the Appalachian mountains in the Eastern United States. The
    numerous impact craters superposed on the mountain ridges indicate
    that they are old—probably formed several billion years ago.

The other side of Ganymede presented quite a different face from the one
Voyager 1 had seen. Here were the dark ancient cratered terrains, the
shoulder-to-shoulder craters reminiscent of Callisto, and there was a
huge circular feature on Ganymede looking like the remnant of a
Callisto-style ringed basin, preserved in the ancient, dark terrain. The
very large dark feature revealed by Voyager in the northern hemisphere
which bears these impact scars was later named “Regio Galileo,” for the
discoverer of the Galilean satellites. It was seen in the low-resolution
Pioneer 10 picture of Ganymede taken in 1973, but its nature was not
understood. It is so large it has even been glimpsed on occasions of
exceptionally stable “seeing” with ground-based telescopes.

3:29 p.m. PDT—Jupiter Encounter! In the press room half a dozen cameras
clicked in unison as the universal clock declared the Voyager 2 had made
its closest approach to Jupiter—650 000 kilometers from the cloud tops,
zipping by at about 73 000 kilometers per hour—neither as close nor as
fast as Voyager 1. By the time of the special press conference at 4:30
p.m., everyone at JPL was in a party mood. Thomas A. Mutch, who had
replaced Noel Hinners as NASA Associate Administrator for Space Science,
Robert Parks, and Rodney Mills were the speakers.

The Jovian system is a place of “incredible beauty and mystery. Jupiter
has been a nice place to go by, but we wouldn’t want to stop there—we’re
going on to Saturn,” Rod Mills explained, and Bob Parks agreed.

Tim Mutch had a different perspective. “Although we have just heard
Jupiter somewhat downgraded in favor of Saturn, nonetheless what we have
been witnessing, first in March and now, in July, is a truly
revolutionary journey of exploration. We have gone beyond the familiar
part of the solar system to objects that are so exotic that their very
existence, at least as far as I’m concerned, was something I’d accepted
intellectually, but didn’t really accept in an immediate sense. We’re
starting out in our own space program on a new stage of space
exploration—on our own long journeys beyond the solar system to distant
lands. We never like to think, or rather, it’s statistically unlikely,
that we’re at a turning point in history. But if you look back at
history books, such events are clearly read into the record. And I
submit to you that when the history books are written a hundred years
from now, two hundred years from now, the historians are going to cite
this particular period of exploration as a turning point in our
cultural, our scientific, our intellectual development.”

Although everyone was already celebrating another successful mission,
the encounter was far from over. Data continued to come in; there was
still the ten-hour Io Volcano Watch, which had begun at 4:31 p.m.; there
were more observations of Jupiter, including scheduled ring observations
and dark side searches for aurorae and lightning bolts. There was a lot
of work and excitement yet to come. Jupiter had another surprise in
store for Voyager 2.

    [Illustration: During the 10-hour Io volcano watch on July 9, the
    spacecraft kept nearly the same face of Io in view. Most of the
    surface was turned away from the Sun, however, and only a thin
    crescent could be seen, shrinking as the observations continued.
    These four frames were all photographed with identical exposures
    from a range of about 1 million kilometers. These images show
    Amirani (P₅) and Maui (P₆) on the west edge, brightening as the Sun
    illuminates them more nearly from behind. [260-677]]

    [Illustration: Io volcanoes.]

    [Illustration: Masubi (P₈) is faintly visible in the crescent (above
    and below).]

    [Illustration: Io volcanoes.]

    [Illustration: Loki (P₂) rises 250 kilometers above the surface,
    catching the morning sunlight on the east edge of Io.]

                           Tuesday, July 10.

(_Range to Jupiter, 1.4 million kilometers_). The Io watch continued
through the night. As time passed, the satellite rotated in the same
direction as the motion of the spacecraft, keeping nearly the same side
in view. Because of this, a few volcanoes could be closely watched, but
most would be missed entirely. During the sequence, the illuminated
crescent steadily shrank, until at the end, volcanic plumes could be
seen on both edges, one illuminated by the setting Sun, the other
shining in the dawn light.

At the 11 a.m. press conference, Esker Davis announced that engineers
had lost contact with the spacecraft radio receiver Monday evening
(probably due to Jupiter’s radiation) and had to “chase it around most
of the night,” sending commands at various frequencies until they locked
on to the frequency the spacecraft would accept. The major trajectory
correction maneuver, begun at about the same time contact with the
receiver was lost, was successful. The 76-minute thruster firing, done
at periapsis instead of two weeks after encounter, enabled the
spacecraft to get a bigger “boost” from Jupiter than was originally
planned, amounting to a fuel saving of about 10 kilograms of hydrazine,
enough to preserve the option of going on from Saturn to Uranus.

Andrew Ingersoll discussed some results of the analysis of the Jovian
atmosphere. “At first, Voyager seemed to do nothing but emphasize the
chaos, not the order.” But, with the help of ground-based observations,
Reta Beebe found that there is a “regular alternation of eastward and
westward jets” underlying the seemingly chaotic visible features. “The
turbulence we see in the visible clouds seems to be a minor side show,
or a process without much energy or mass compared to the very great
energy and mass that might be moving around in the deep atmosphere.”

“We’re continuing to operate in our panic mode to try to get pictures to
the press,” Brad Smith said as he introduced new photographs of the
satellites. In earlier photos, Ganymede had seemed to have two different
kinds of terrain—an ancient, cratered, Callisto-like surface, and the
stranger, grooved terrain—terrains that might be representative of two
very different types of major episodes in Ganymede’s history. The most
recent images showed a much more confused picture, with several
additional types of surface geology.

At the daily project science briefing, another interpretation was being
discussed. Lyle Broadfoot reported that new measurements of the position
of the ultraviolet aurora demonstrated that it was caused by charged
particles from the Io torus, not from the outer parts of the
magnetosphere. Apparently these plasma particles arise in the volcanic
eruptions, are trapped for a time in the torus, and then fall into the
polar regions of Jupiter, where they excite auroral emissions. A
terrestrial aurora, in contrast, is caused by particles that originate
in the solar wind. Jim Sullivan of the plasma investigation estimated
that about two tons of material each second are fed from Io into the
plasma torus. This plasma, driven by the rotation of the Jovian magnetic
field, appears to be able to supply the million-million watts of power
radiated in the ultraviolet.

By 5:00 p.m. the excitement had died down; many of the scientists had
parties to attend that evening, and some members of the press were
planning parties of their own. The schedule of spacecraft activities
also seemed to have slowed. There were dark-side observations planned to
search for lightning and aurorae. There would be a few more ring
pictures—not too much to see on the monitors that night ... or so many
people thought. But a few people were waiting around, perhaps to catch a
glimpse of lightning or auroral activity, or to wait for another look at
Jupiter’s faint ring.

Between 5:52 and 6:16 p.m., six long-exposure, wide-angle photographs of
the dark side of Jupiter had been scheduled to search for aurorae and
lightning. The spacecraft was 1 450 000 kilometers from Jupiter and
about two degrees below the equatorial plane.

Shortly after 6 p.m., the first of these ring photographs appeared on
the TV monitors with unexpected brilliance. Taken in orange and violet
light, the images showed the outline of Jupiter and, protruding from it,
two narrow lines—one reaching all the way to Jupiter’s limb, the other
broken off, apparently hidden by the shadow of the giant planet. Seen
from the new perspective of the shadow of Jupiter, the tenuous rings
were remarkably clear. A sudden renewal of excitement surged through the
devotees remaining in the press room. About 6:15, Brad Smith came down
to join the press to watch the remainder of this series of pictures come
in. “Hey Brad, are you going to burn out the camera with the ring?”
someone joked. “Well, the rings do forward scatter nicely, don’t they?”
Dr. Smith replied. As the wide-angle pictures were followed by
narrow-angle views, more and more detail became apparent. For the first
time, a definite width for the ring could be seen, and there was even a
hint of additional material inside the main ring. All in all, Voyager
had provided one more splendid series of pictures before it took off for

    [Illustration: From a vantage point 2.5 degrees above the ring
    plane, Voyager 2 was able for the first time to determine the width
    of Jupiter’s ring. This picture shows that the ring is ribbon-like
    and only a few thousand kilometers wide, quite unlike the broad
    rings of Saturn. [P-21757B/W]]

                          Wednesday, July 11.

JPL Public Information Officer Frank Bristow opened the 10 a.m. press
conference with an announcement: “We’ll have the report from the Imaging
Team including the tremendous pictures that we received here last night
of the Jupiter ring that excited the entire team.”

Brad Smith showed the ring pictures. “As many of you who were here last
night know, we got some rather nice pictures of the ring of Jupiter.
It’s as though Voyager 2 was fearful that we might be becoming just a
little bit apathetic after this series of marvelous discoveries and felt
that it had to dazzle us one more time before it left for Saturn. The
rings appear very much brighter than we had expected them to be.” The
outer ring is about 6500 kilometers wide. There is material inside the
ring. There is a rather sharp outer boundary and a somewhat diffuse
inner region. “And it is now our belief that the material in the ring
goes all the way down to the surface of Jupiter.” There is a very narrow
relatively bright outer ring and an extremely faint inner ring that goes
all the way down to Jupiter’s cloud tops.

Larry Soderblom summarized the satellite data: With respect to the
Galilean satellites, “We’re in a relatively high state of ignorance.”

The Io Volcano Watch images seemed to indicate that plume P₂ was now the
highest volcano on Io, since P₁ seemed to have become quiet. Io may be
the easiest Galilean satellite to try to understand, because we can
actually see the geological processes that are shaping the planet. Io’s
“twin,” Europa, seems to be where “our highest state of ignorance” lies.
“The faint bright streaks which show some relief are evidently different
from the diffuse dark bands which don’t seem to show topography, but the
similarity of these forms [that both the light and dark markings are of
planetary scale] suggests that they must be related.”

Ed Stone speculated about the other two Galilean satellites. Ganymede
and Callisto are essentially identical in size, mass, and probably
composition. By examining them, we can perhaps learn what happens when
bodies with very similar chemistry have different “life histories” and
different surface properties (there are indications that Ganymede’s
crust may not have been as rigid as Callisto’s). Going further, he added
that Callisto and Mercury, the least dense and the most dense,
respectively, of the terrestrial-style planets, although totally
different in composition and density, seem to have similar surfaces and
similar histories. What would have happened to Mercury if it had been
made of ice, water, and rock as Callisto is? Would it have evolved as
Callisto did?

    [Illustration: One of the most spectacular of the Voyager 2 images
    was obtained from inside the shadow of Jupiter. Looking back toward
    the planet and the rings with its wide-angle camera, the spacecraft
    took these photos on July 10 from a distance of 1.5 million
    kilometers. The ribbon-like nature of the rings is clearly shown.
    The planet is outlined by sunlight scattered from a haze layer high
    in the atmosphere. On each side, the arms of the ring curving back
    toward the spacecraft are cut off by the planet’s shadow as they
    approach the brightly outlined disk. [P-21774B/W]]

    [Illustration: The rings of Jupiter proved to be unexpectedly bright
    when seen with the Sun nearly behind them. Strong forward scattering
    of sunlight is characteristic of small particles. These two views
    were obtained by Voyager 2 on July 10 from a perspective inside the
    shadow of Jupiter. The distance of the spacecraft from the rings was
    about 1.5 million kilometers. Although the resolution has been
    degraded by camera motion during the time exposures, these images
    reveal that the rings have some radial structure. [260-610B/W and

    [Illustration: Rings of Jupiter.]



The main atmospheric jet streams were present during both Voyager
encounters, with some changes in velocity.

The Great Red Spot, the white ovals, and the smaller white spots at
41°S, appear to be meteorologically similar.

The formation of a structure east of the Great Red Spot created a
barrier to the flow of small spots which earlier were circulating about
the Great Red Spot.

The ethane to acetylene abundance ratio in the upper atmosphere appears
to be larger in the polar regions than at lower latitudes and appears to
be 1.7 times higher on Voyager 2 than on Voyager 1.

An ultraviolet map of Jupiter shows the distribution of absorbing haze.
The polar regions are surprisingly dark, suggesting that the absorbing
material must be at high altitudes.

Equatorial ultraviolet emissions indicate planet-wide precipitation of
charged particles into the atmosphere from the magnetosphere.

The high-latitude ultraviolet auroral activity is due to charged
particles that originate in the Io torus.

                       Satellites and Ring System

The ring consists of a bright, narrow segment surrounded by a broader,
dimmer segment, with a total width of about 5800 kilometers.

The interior of the ring is filled with much fainter material that may
extend down to the top of the atmosphere.

Images of Amalthea in silhouette against Jupiter indicate that the
satellite may be faceted or diamond shaped.

Volcanic activity on Io changed somewhat, with six of the plumes
observed by Voyager 1 still erupting.

The largest Voyager 1 plume (Pele) had ceased, while the dimensions of
another plume (Loki) had increased by 50 percent.

Several large-scale changes in Io’s appearance had occurred, consistent
with surface deposition rates calculated for the large eruptions.

Europa is remarkably smooth with very few craters. The surface consists
primarily of uniformly bright terrain crossed by linear markings and
very low ridges.

There are four basic terrain types on Ganymede, including younger,
smooth terrain and a rugged impact basin first observed by Voyager 2.

Callisto’s entire surface is densely cratered and is likely to be
several billion years old.

Equatorial surface temperatures on the Galilean satellites range from 80
K (night) to 155 K (the subsolar point on Callisto).


The outer region of the magnetosphere contains a hot plasma consisting
primarily of hydrogen, oxygen, and sulfur ions.

The hot plasma generally flows in the corotation direction out to the
boundary of the magnetosphere.

Beyond about 160 R_J, the hot plasma streams nearly antisunward.

Outbound the spacecraft experienced multiple magnetopause crossings
between 204 R_J and 215 R_J.

The abundance of oxygen and sulfur relative to helium at high energy
increases with decreasing distance from Jupiter.

Measurements of high energy oxygen suggest that these nuclei are
diffusing inward toward Jupiter.

The ultraviolet emission from the Io plasma torus was twice as bright as
four months earlier and the temperature had decreased by 30 percent to
60 000 K.

The low-frequency (kilometric) radio emissions from Jupiter have a
strong latitude dependence and often contain narrowband emissions that
drift to lower or higher frequencies with time.

A complex magnetospheric interaction with Ganymede was observed in the
magnetic field, plasma, and energetic particles up to about 200 000
kilometers from the satellite.

[3]Adapted from a summary prepared by E. C. Stone and A. L. Lane for the
    Voyager 2 Thirty-Day Report.

    [Illustration: A new inner satellite of Jupiter, provisionally
    designated 1979J1, was discovered by David Jewitt and Ed Danielson
    of Caltech in these Voyager 2 ring photographs.]

    [Illustration: In a 15-second exposure with the wide-angle camera,
    the edge-on ring shows as a faint line, and the satellite is the dot
    indicated by the arrow. [260-807]]

    [Illustration: In a narrow angle 96-second exposure, the motion of
    the satellite can be seen. Again, the faint band is the ring,
    blurred by camera motion, and the arrow indicates the streak due to
    the satellite. A star streak is located above and to the left of the
    satellite; note that the length and angle of the two trails are
    different, owing to satellite motion. [P-22172]]

                            A New Satellite

One of the most fascinating discoveries of Voyager 2 was not recognized
at first. Graduate student David Jewitt of the California Institute of
Technology, working with Imaging Team member Ed Danielson, began a
detailed analysis of all the ring photos in late summer. In early
October he determined that a short streak on a photo taken July 8,
previously presumed to be an image of a star trailed by the time
exposure, did not correspond to any known star position. Perhaps this
was a new satellite! Additional sleuthing turned up a second image of
the same part of the ring that also showed the anomalous object,
together with trails due to known stars. The differing angles and
lengths of the trails of the object and the stars confirmed that this
was indeed a 14th satellite of Jupiter. Following the guidelines of the
International Astronomical Union, it was designated 1979J1, pending
later assignment of a mythological name. The proposed name is Adrastea,
a nymph who nursed the infant Zeus in Greek legend.

The newly discovered satellite orbits Jupiter at a distance of 58 000
kilometers above the equatorial cloud tops, placing it just at the outer
edge of the ring and much closer to the planet than is Amalthea,
previously thought to be the innermost satellite. It travels at 30
kilometers per second (nearly 70 000 miles per hour), circling Jupiter
in just seven hours and eight minutes. From its brightness, scientists
guessed that it might be 30-40 kilometers in diameter.

The proximity of Adrastea to the ring suggests a relationship between
the two. When the discovery was announced to the press in mid-October,
it was speculated that the ring material might originate on the
satellite, perhaps eroded away by the energetic charged particles in the
inner Jovian magnetosphere. Once again, Voyager had added to our
perspective on planetary processes, suggesting that undiscovered but
similar small satellites might also be associated with the rings of
Saturn and Uranus.

Voyager 2 had certainly added a few years’ of data of its own to Voyager
1’s “ten years’ worth of data.” It had given a different view of the
Jovian system, helping to solve some of the mystery surrounding Jupiter
and its satellites, and creating new mysteries. As Voyager 2 sped out
away from Jupiter, riding along the giant planet’s huge magnetotail,
attention turned to Saturn: What would Pioneer 11, the Pathfinder,
discover in September 1979? What would the Voyagers learn in November
1980 and August 1981? Would all go well? Would Voyager 2 fly on to

There was also a yearning to examine more closely, with the Galileo
Project, what had been unknown for so long, yet had become so familiar
in only a few months’ time—the little dark, red “potato” Amalthea, the
volcano-covered world Io, the mysterious “cracked billiard ball” Europa,
cratered and groovy Ganymede, ancient Callisto, and the king of the
planets itself, a colorful, banded world of stable climate and
ever-changing weather patterns.

    [Illustration: A fifteenth satellite of Jupiter was discovered in
    the spring of 1980 by Steven Synnott of JPL. It was first seen on
    this Voyager 1 image taken March 5, 1979, in which the
    75-kilometer-diameter satellite shows as a dark oval against the
    planet. Also visible is the shadow of the satellite, designated
    1979J2. This satellite orbits between Io and Amalthea with a period
    of 16 hours and 11 minutes. [P-22580B/W]]

    [Illustration: The Jupiter seen by the Voyager cameras is a
    cloud-belted world of rapid jet streams and complex cloud forms.
    Prominent in this Voyager 1 image, taken February 5 at a range of
    28.4 million kilometers, is the alternating structure of light zones
    and dark belts, and the Great Red Spot and numerous smaller spots.
    Also easily visible are the two inner Galilean satellites, Io and
    Europa. The resolution in this picture is 500 kilometers, about five
    times better than can be obtained from Earth-based telescopes.
    Callisto can be faintly seen at the lower left. [P-21083C]]

                               CHAPTER 8
                      JUPITER—KING OF THE PLANETS

                           A Star That Failed

More massive than all the other planets combined, Jupiter dominates the
planetary system. The giant revealed by Voyager is a gas planet of great
complexity; its atmosphere is in constant motion, driven by heat
escaping from a glowing interior as well as by sunlight absorbed from
above. Energetic atomic particles stream around it, caught in a magnetic
field that reaches out nearly 10 million kilometers into the surrounding
space, embracing the seven inner satellites. From its deep interior
through its seething clouds out to its pulsating magnetosphere, Jupiter
is a place where forces of incredible energy contend.

At its birth, Jupiter shone like a star. The energy released by
infalling material from the solar nebula heated its interior, and the
larger it grew the hotter it became. Theorists calculate that when the
nebular material was finally exhausted, Jupiter had a diameter more than
ten times its present one, a central temperature of about 50 000 K, and
a luminosity about one percent as great as that of the Sun today.

At this early stage, Jupiter rivaled the Sun. Had it been perhaps 70
times more massive than it was, it would have continued to contract and
increase in temperature, until self-sustaining nuclear reactions could
ignite in its interior. If this had happened, the Sun would have been a
double star, and the Earth and the other planets might not have formed.
However, Jupiter did not make it as a star; after a brief flash of
glory, it began to cool.

At first Jupiter continued to collapse. Within the first ten million
years of its life, the planet was reduced to nearly its present size,
with only a few percent additional shrinkage during the past 4.5 billion
years. The luminosity also dropped as internal heat was carried to the
surface by convection and radiated away to space. After a million years
Jupiter emitted only one-hundred thousandth as much radiation as the
Sun, and today its luminosity is only one-ten billionth of the Sun’s.

Jupiter’s internal energy, although small by stellar standards, has
important effects on the planet. About 10¹⁷ watts of power, comparable
to that received by Jupiter from the Sun, reach the surface from the
still-luminous interior. The central temperature is still thought to be
about 30 000 K, sufficient to maintain the interior in a molten state.
Scientists generally agree that Jupiter is an entirely fluid planet,
with no solid core whatever.

                 Composition and Atmospheric Structure

Because of its great mass, Jupiter has been undiscriminating in its
composition. All gases and solids available in the early solar nebula
were attracted and held by its powerful gravity. Thus it is expected
that Jupiter has the same basic composition as the Sun, with both bodies
preserving a sample of the original cosmic material from which the solar
system formed.

    [Illustration: Jupiter is a gas giant, composed of the same elements
    as the Sun and stars—primarily hydrogen and helium. Its internal
    structure is dominated by the properties of hydrogen, its most
    abundant constituent and by the high temperatures in the deep
    interior that remain from its luminous youth. Most of the interior
    is liquid: metallic hydrogen at great depths and high pressures, and
    normal hydrogen nearer the surface. In the upper few thousand
    kilometers, the hydrogen is a gas. The primary known or suspected
    cloud layers are, from the top down, thin hydrocarbon “smog”;
    ammonia; ammonium hydrosulfide; water-ice, and liquid water.

  Cloud top-aerosols
  Ammonia crystals
  Ammonium hydrosulfide clouds
  Ice crystal clouds
  Water droplets
  Trace compounds
  Fluid molecular hydrogen
  Transition Zone
  Fluid metallic hydrogen
  Possible core

The primary constituents of Jupiter have long been suspected to be
hydrogen and helium, the two simplest and lightest atoms. However, it
has proved impossible to derive accurate measurements of the abundance
of these two elements from astronomical observations. On the basis of a
rather simple infrared measurement, Pioneer investigators found He/H₂ =
0.14 ± 0.08. On Voyager, IRIS was able to obtain much improved infrared
spectra, yielding an initial value of He/H₂ = 0.11 ± 0.3. Voyager
scientists expect that further analysis will reduce the uncertainty to
about ± 0.01. The ratio of 0.11 is in excellent agreement with the solar
value of about 0.12, supporting the idea that Jupiter and the Sun have
similar elemental compositions.

Astronomers have known for a long time that, in addition to hydrogen and
helium, the compounds methane (CH₄) and ammonia (NH₃) are present in the
visible atmosphere of Jupiter. In the 1970s, additional spectra in the
infrared resulted in the discovery of water (H₂O), ethane (C₂H₆),
germane (GeH₄), acetylene (C₂H₂), phosphine (PH₃), carbon monoxide (CO),
hydrogen cyanide (HCN), and carbon dioxide (CO₂). All these are trace
constituents, with two of them, ethane and acetylene, apparently formed
at high altitudes by the action of sunlight on methane.

A total of approximately 100 000 infrared spectra, many of small regions
on the disk, were obtained by IRIS. These spectra generally show
hydrogen, helium, methane, ammonia, phosphine, ethane, and acetylene. In
addition, excellent spectra were obtained in “hot spots,” regions in
which breaks in the upper clouds permit radiation from deeper layers to
escape. (The hot spots generally correspond to dark brown regions on
photographs of the planet.) IRIS measured temperatures in the hot spots
up to -13° C but no higher; apparently this temperature corresponds to
the top of a deeper cloud deck. Spectral features indicative of the
presence of water vapor and germane were clearly seen in the hot spots.

Further analysis of the IRIS spectra will be required to derive the
abundances of the gases detected. However, even the preliminary data
showed how variable Jupiter can be, especially in its upper atmosphere.
The two hydrocarbons, ethane and acetylene, vary in relative abundance
with latitude; there is less acetylene near the poles. In addition to
this planetwide trend, smaller variations were seen from place to place
and between the observations in March and July. All the variations will
eventually provide information on the processes of formation,
transportation, and destruction of hydrocarbons in the upper atmosphere.

      Element       Atomic          Instruments

  Hydrogen (H)            1  UVS, Plasma, LECP, CRS
  Helium (He)             2  Plasma, LECP
  Carbon (C)              6  LECP
  Nitrogen (N)            7  CRS
  Oxygen (O)              8  UVS, Plasma, LECP, CRS
  Neon (Ne)              10  CRS
  Sodium (Na)            11  LECP, CRS
  Magnesium (Mg)         12  CRS
  Silicon (Si)           14  CRS
  Sulfur (S)             16  UVS, Plasma, LECP, CRS
  Iron (Fe)              26  CRS

Voyager did not make any direct measurements of the chemical composition
of the clouds, but theorists generally agree that the uppermost clouds
are ammonia cirrus, and that layers of ammonium hydrosulfide (NH₄SH) and
water exist at deeper levels. All these clouds are formed in the
troposphere, the layer of the atmosphere in which convection takes
place. The top of the ammonia cloud deck is thought to have a pressure
of about 1 atmosphere and a temperature of about -113° C.

Ammonia cirrus is white, yet Jupiter’s clouds display a spectacular
range of colors. Voyager did not determine the nature of the coloring
agents; they may be minor constituents—trace impurities in a sea of
white clouds. Perhaps organic polymers, formed from atmospheric
chemicals such as methane and ammonia that have reacted with lightning,
are responsible for the oranges and yellows. The color of the Red Spot
could be caused by red phosphorus (P₄). According to this theory,
phosphine (PH₃) from deep in Jupiter’s atmosphere is brought to high
altitudes by the upwelling of the Great Red Spot. Ultraviolet light,
penetrating the upper reaches of the Red Spot, splits the phosphine
molecules, and, through a series of chemical reactions, converts the
phosphine into pure phosphorus. However, this theory fails to explain
the existence of the smaller red spots on Jupiter; these spots are not
at such high altitudes as the Great Red Spot (which is the highest and
coldest of Jupiter’s visible clouds), so it is unlikely that ultraviolet
light could react with any phosphine in these areas to produce red

    [Illustration: Although the Voyager spacecraft never flew over the
    poles of Jupiter, it is possible to reconstruct from several images
    the View that would be seen from directly above or below the planet.
    Note the absence of a strong banded structure near both poles. The
    regular spacing of cloud features is obvious. In the Southern
    hemisphere, the three white ovals are 90 degrees apart in longitude,
    but a fourth oval at the other quadrant is missing. The irregular
    black areas at each pole are places for which no Voyager data exist.
    The resolution of the original pictures from which these polar
    projections were made was about 600 kilometers.]

    [Illustration: North pole. [P-21638C]]

    [Illustration: South pole. [P-21639C]]

Various forms of elemental sulfur might be responsible for the riot of
color we see on Jupiter. Sulfur forms polymers (S₃, S₄, S₅, S₈,) that
are yellow, red, and brown, but no sulfur in any form has been detected
on Jupiter. “We never promised you we were going to identify the colors
on Jupiter with this mission,” one of the atmospheric scientists
remarked, “but we will have a probe that is going into the atmosphere in
the mid-1980s—Galileo.” Perhaps the mystery of the Jovian clouds will
have to wait till then.

Temperature maps of Jupiter were obtained by IRIS in radiation arising
at different levels above the clouds. Maps show temperatures at
pressures of 0.8 atmosphere near the clouds, and 0.2 atmosphere near the
top of the troposphere. In addition to the low temperatures over the
bright zones and the higher temperatures over dark belts, there is a
great deal of smaller scale structure. It is interesting that a cold
area corresponding to the Great Red Spot is clearly visible even near
the top of the troposphere, indicating that this feature disturbs the
atmosphere to very high altitudes.

The structure of the atmosphere of Jupiter above the troposphere was
investigated through the radio occultation experiment as well as by
IRIS. The level in which the minimum temperature of about -173° C occurs
has a pressure of 0.1 atmosphere. Above this point lies the
stratosphere, in which temperatures increase with altitude as a result
of sunlight absorbed by the gas or by aerosol particles resembling smog.
At 70 kilometers above the ammonia clouds, the temperature is about
-113° C. Above this level, the temperature stays approximately constant,
although at extreme altitudes the temperature again rises in the

    [Illustration: If one could “unwrap” Jupiter like a map, views such
    as these would be obtained. The comparison between the pictures
    shows the relative motions of features in Jupiter’s atmosphere. It
    can be seen, for example, that the Great Red Spot moved westward and
    the white ovals eastward during the time between the acquisition of
    these pictures. Regular plume patterns are equidistant around the
    northern edge of the equator, while a train of small spots moved
    eastward at approximately latitude 80° S. In addition to these
    relative motions, significant changes are evident in the
    recirculating flow east of the Great Red Spot, in the disturbed
    region west of the Great Red Spot, and as seen in the brightening of
    material spreading into the equatorial region from the more
    southerly latitudes. [P-21771C]]

    [Illustration: The planet as it appeared about March 1.]

    [Illustration: As it was in early July.]

                           Weather on Jupiter

The Voyager pictures reveal a planet of complex atmospheric motions.
Spots chase after each other, meet, whirl around, mingle, and then split
up again; filamentary structures curl into spirals that open outward;
feathery cloud systems reach out toward neighboring regions; cumulus
clouds that look like ostrich plumes may brighten suddenly as they float
toward the east; spots stream around the Red Spot or get caught up in
its vortical motion—all in an incredible interplay of color, texture,
and eastward and westward flows. Such changes can be noticed in the
space of only a few Jovian days.

    [Illustration: Differing characteristics of Jupiter’s meteorology
    are apparent in high-resolution images, such as this one taken by
    Voyager 1 on March 2 at a range of 4 million kilometers. The
    well-defined pale orange line running from southwest to northeast
    (north is at the top) marks the high-speed north temperate current
    with wind speeds of about 120 meters per second. Toward the top of
    the picture, a weaker jet of approximately 30 meters per second is
    characterized by wave patterns and cloud features which have been
    observed to rotate in a clockwise manner at these latitudes of about
    35°N. These clouds have been observed to have lifetimes of one to
    two years. [P-21193C]]

On a broader time scale, greater changes on the face of Jupiter can be
seen. Features drift around the planet; even the large white ovals and
the Great Red Spot slide along in their respective latitudes. Belts or
zones intrude upon each other, resulting in one of the banded structures
splitting up or seeming to squeeze together and eventually disappear.
Small structures form, then die. The largest spots may slowly shrink in
size, and the Red Spot itself changes its size and color.

The Jupiter of Pioneers 10 and 11 was quite unlike the planet seen by
Voyager 1. At the time of the Pioneer exploration, the Great Red Spot,
embedded in a huge white zone, was more uniformly colored, and pale
brown bands circled the northern hemisphere. In the intervening years,
the south temperate latitudes have changed completely, developing the
complex turbulent clouds seen around the Red Spot by Voyager 1. Yet,
even between the two Voyagers, Jupiter appeared to be undergoing a
dynamic “facelift.” At a quick glance, Voyager 2 photographs showed the
visage that had been familiar since early in 1979, but a closer look
showed that it is not quite the same. The white band below the Great Red
Spot, fairly broad during the first flyby, had become a thin white
ribbon where it rims the southern edge of the Spot. The turbulence to
the west of the Red Spot had stretched out and become “blander” than it
was before. Small rotating clouds seemed to be forming out of the waves
in this region. The cloud structure that had been east of the Red Spot
during the Voyager 1 flyby spread out, covering the northern boundary
and preventing small clouds from circling the huge red oval. The Red
Spot itself also changed. Its northern boundary seemed—at least
visually—to be more set off from the clouds that surround it, and the
feature appeared to be more uniform in color, perhaps reverting back to
the personality it had in Pioneer days.

    [Illustration: Jupiter’s cloud patterns changed significantly in the
    few months between the two Voyager flybys. Most of the changes are
    the result of differential rotation, in which the prevailing winds
    at different latitudes shift long-lived features with respect to
    those north or south. Thus, for example, the three large white ovals
    shifted nearly 90 degrees in longitude, relative to the Great Red
    Spot, between March and July. [P-21599]]

    [Illustration: Cloud patterns from Voyager 2.]

The most obvious features in the atmosphere of Jupiter, after the banded
belts and zones, are the Great Red Spot and the three white ovals. These
have often been described as “storms” in Jupiter’s atmosphere. The ovals
are about the size of the Moon, and the Red Spot is larger than the
Earth. Voyager has revealed that in many respects the white ovals, which
formed in 1939, resemble their ancient red relative. All four spots are
southern hemispheric anticyclonic features that exhibit counterclockwise
motion; hence they are meteorologically similar. Other smaller bright
elliptical and circular spots also exhibit anticyclonic motion, rotating
clockwise in the northern hemisphere and counterclockwise in the
southern hemisphere. In general, these features are circled by
filamentary rings that are darker than the spots they surround. Hints of
interior spiral structure can be seen in some of these spots. All the
elliptical features in the southern hemisphere lie to the south of the
strong westward-blowing jet streams. The spots tend to become rounder
the closer they are to the poles.

Along the northern edge of the equator are a number of cloud plumes,
which appear to be regularly spaced all around the planet. Some of the
plumes have been observed to brighten rapidly, which may be an
indication of convective activity; indeed, some of the plume structures
seem to resemble the convective storms that form in the Earth’s tropics.
The plumes travel eastward at speeds ranging from about 100 to 150
meters per second, but they do not move as a unit.

    [Illustration: The Great Red Spot of Jupiter is a magnificent sight,
    whether viewed in normal or exaggerated color. These pictures were
    taken by Voyager 1 at a range of about 1 million kilometers; the
    area shown is about 25 000 kilometers, with features visible on the
    originals that are as small as 30 kilometers across. The Red Spot is
    partly obscured on the north by a thin layer of overlying ammonia
    cirrus cloud. South of the Red Spot is one of the three white ovals,
    which are also anticyclonic vortices in the atmosphere.]

    [Illustration: This frame is in natural color. [P-21430C]]

    [Illustration: The red and blue have been greatly exaggerated in
    this frame to bring out fine detail in the cloud structure.

The most visible cloud interactions take place in the region of the
Great Red Spot. Material within the Red Spot rotates about once every
six days. Infrared measurements show that the Red Spot is a region of
atmospheric upwelling, which extends to very high altitudes; however,
the divergent flow suggested by this upwelling seems to be very
small—one bright feature was observed to circle the Red Spot for sixty
days without appreciably changing its distance from the spot’s center.
During the Voyager 1 flyby, spots were seen to move toward the Red Spot
from the east, flow along its northern border, then either flow on to
the west past the Red Spot or into the outer regions of its vortex. A
spot caught on the outer edge of the Red Spot flow might break in two as
it reached the eastern edge of the spot, with one piece remaining in the
vortex and the other moving off to the east. Alternatively, a spot
floating toward the Red Spot from the east might be pushed northward to
join the eastward current flowing north of the giant red oval.

By the time of the second Voyager encounter, a ribbon of white clouds
curled around the northern border of the Red Spot, blocking the motion
of small spots that might otherwise have been caught up in the vortex.
Spots approaching the Red Spot from the east just turned around and
headed back in the direction from which they had come.

    [Illustration: Voyager 2 captured the Red Spot region four months
    after Voyager 1, when some changes had taken place in the cloud
    circulation pattern around it. This is a mosaic of Voyager 2 frames,
    taken on July 6. The white oval to the south is not the same one
    that was present in a similar location during the Voyager 1 flyby,
    because of differential rotation at the two latitudes. [260-606]]

In the northern hemisphere, small brown anticyclonic features speed
around the planet, often colliding with one another. On collision, the
spots may combine and roll around together for a while. Ultimately, part
of the mass of the combined spots is ejected as a streamer, and the
remaining material continues on its eastward path.

                           Order out of Chaos

Despite all the turbulence in Jupiter’s atmosphere—this ever-changing
chaotic mixture of cyclonic and anticyclonic flows, of ovals and
filaments, of reds, browns, and whites—a pattern may be emerging: There
is an underlying order to the seemingly random mixing of patterns we see
in the Jovian atmosphere.

First, the changing weather patterns are in some sense cyclic. The fact
that Jupiter may be reverting to the appearance it presented at the time
of Pioneers 10 and 11 is not surprising. From Earth-based studies,
astronomers have found that the face of Jupiter often goes through a
major change every few years. The transition is very rapid, but the
planet maintains its new “look” for some time—until the next major
transition. At the time of the Pioneer flybys, “The Red Spot was
prominent, but it was surrounded by intense cloud. There was no visible
structure at all in the south tropical zone—it was totally bland. You
couldn’t see the turbulent area to the west,” explained Garry Hunt.
“And, I believe that the buildup of cloud that we’re seeing to the east
of the Red Spot is the beginning of the transition that will produce the
Pioneer look.”

There is even more order underlying Jupiter’s changing atmosphere. This
order is revealed in part in the alternating belts and zones. It is
believed that the cloud-covered zones are regions of rising air, and the
belts are regions of descending air. The internal energy of Jupiter
provides the power to maintain this pattern of slow vertical
circulation. In addition, there are horizontal or zonal flows that are
much more regular than the changing cloud patterns.

    [Illustration: The three large white ovals are the longest-lived
    features in Jupiter’s atmosphere, after the Great Red Spot. Like the
    Red Spot, they are anticyclonic, or high-pressure, regions. During
    Voyager 2 encounter, one of the ovals was just south of the Red
    Spot. This picture shows the other two ovals as they looked in early
    July. The clouds show very similar internal structures. To the east
    of each of them, recirculating currents are clearly seen.

    [Illustration: In this frame, a similar structure is seen to the
    west of the cloud.]

    [Illustration: A sequence of pictures of Jupiter, taken once per
    rotation (about ten hours), can be used to construct a time-lapse
    movie of the circulation of the Jovian atmosphere. These frames are
    from the Voyager 1 “Blue Movie” of the Great Red Spot region. Every
    odd Jovian rotation is shown, so the 24 frames correspond to 48
    Jupiter days, or about 20 Earth days. White spots can be seen
    entering the Red Spot from the upper right and being carried around
    by its six-day rotation until they are ejected toward the lower
    right. Above the Red Spot, the flow is toward the right; below it,
    toward the left. The rotation of the Spot is counterclockwise, or
    anticyclonic. [260-449]]

    [Illustration: At different latitudes, the strong prevailing zonal
    winds produce different apparent rotation rates. Plotted here are
    the horizontal velocities measured from a pair of Voyager images
    taken one rotation (about ten hours) apart. Also shown for
    comparison are older ground-based measurements obtained from careful
    timings of the apparent rotation rate at different latitudes. The
    excellent agreement of the two plots indicates the stability of the
    zonal winds. Also, the wind pattern shows much greater symmetry
    between northern and southern hemispheres than do the more
    superficial cloud patterns.]

“At first, Voyager seemed to do nothing but emphasize the chaos, not the
order in Jupiter’s atmosphere,” Andy Ingersoll stated. “There are
turbulent regions in which individual little spots seem to change every
Jovian rotation. And the whole texture in certain turbulent regions is
unrecognizable in one earth day.” With the Voyager spacecraft, more
detail could be seen than ever before; in addition, changes could be
observed on a small timescale, as they happened. “It became much more of
a mystery how large-scale order could exist in the face of all this
small-scale chaos. But I think we are beginning to see the order
underneath. What we are looking at when we observe Jupiter are minute
cloud particles representing only a small fraction of the mass of the
atmosphere.” The large-scale order the scientists had found was a
regular alternation of eastward and westward jets. “If we take all the
measurements from Earth-based observations over the last 75 years, we
find that every current that has ever been seen from the Earth over 75
years is visible in one ten-hour rotation. They’re all there—they were
just invisible.” The ever-changing appearance was dancing above a
regular, almost stationary pattern of alternating flows, which may come
from deep within Jupiter’s atmosphere. Why this alternating pattern
persists remains a mystery. Even if the underlying pattern can be
thought of as a sort of Jovian climate, it still does not explain the
mechanics of “the minor sideshow”—the changing weather patterns.
Analysis of the Voyager pictures is sure to keep planetary
meteorologists busy for many years to come.

                        Lights in the Night Sky

Toward the end of the first encounter period, Voyager 1 flew behind
Jupiter, and the spacecraft’s wide-angle camera scanned the northern
hemisphere on the nightside of the planet, searching for aurorae and
lightning bolts. The most impressive darkside feature found was a
tremendous aurora in the north polar region. But this was not the first
time Jovian aurorae had been detected. Very-high-energy auroral
emissions resulting from ultraviolet glows of atomic and molecular
hydrogen had been detected prior to encounter on the bright side of
Jupiter by the ultraviolet spectrometer. The ultraviolet observations
indicate that atmospheric temperatures in the auroral regions are at
least 1000 K. In both the visible and the ultraviolet spectra, the
aurorae are confined to the polar regions and result from charged
magnetospheric particles striking the upper atmosphere. The ultraviolet
aurorae are created when high-energy particles from the Io plasma torus
spiral in toward Jupiter on magnetic field lines.

Several meteor trails were also evident in the darkside pictures of
Jupiter’s atmosphere. Traveling at roughly 60 kilometers per second as
they entered, these fireballs brightened quickly and seemed to survive
for about 1000 kilometers before they died.

Clusters of lightning bolts—indicative of electrical storms—were also
discovered on Jupiter’s nightside. This particular phenomenon does not
seem to depend on latitude. The Voyager 1 photograph that captured the
huge Jovian aurora also caught the electrical discharges of 19
superbolts of lightning, and Voyager 2 photographs located eight
additional flashes. Radio emission (whistlers) from lightning discharges
were also detected by the Voyager radio astronomy receivers and the
plasma wave instrument.

    [Illustration: The night side of Jupiter is not dull. A large aurora
    (northern light) arcs across the northern horizon, while farther
    south about twenty large bolts of lightning illuminate electrical
    storms in the clouds. Similar pictures also revealed fireballs, or
    large meteors, burning up in the atmosphere of Jupiter.

                             Magnetic Field

Deep in the interior of Jupiter, the pressures are so great that
hydrogen becomes an electrical conductor, like a metal. Currents driven
by the rapid rotation of the planet are thought to flow in this metallic
core. The result is a magnetic field that penetrates the space around

Direct measurements of the Jovian magnetic field were first made by the
Pioneers, and Voyager results generally confirm the initial findings.
The strength of the Jovian field is about 4000 times greater than that
of the Earth. The dipolar axis is not at the center of Jupiter, but
offset by about 10 000 kilometers and tipped by 11 degrees from the axis
of rotation. Each time the planet spins, the field wobbles up and down,
carrying with it the trapped plasma of the radiation belts. The Voyager
particles and fields instruments concentrated not on the planetary
magnetic field but on the processes taking place in the magnetosphere.

    [Illustration: A probe of the Jovian atmosphere is obtained each
    time a spacecraft passes behind the planet as seen from the Earth.
    Passage through the ionosphere and atmosphere alters the phase of
    the radio telemetry signal, and subsequent computer analysis allows
    members of the Radio Science Team to reconstruct the profile of the
    atmosphere. Shown here is the atmospheric temperature as a function
    of pressure as derived from the Voyager 1 X-band occultation data,
    corresponding to a point at latitude 12°S, longitude 63°. The two
    curves represent extreme interpretations of the same data; the best
    fit lies somewhere between. Accuracy is high at greater depths but
    poor at levels above a pressure of about 0.03 bar. Clearly shown is
    the temperature minimum near 0.1 bar and the steady increase of
    temperature with depth as the radio beam probed toward the cloud
    level near 1.0 bar.]

    [Illustration: At a wavelength near 5 micrometers, the primary gases
    in the atmosphere of Jupiter are particularly transparent, and the
    infrared radiation from the planet comes from relatively great
    depths. At these depths, it is possible to see evidence of gases
    such as water vapor that condense at higher altitudes where the
    temperatures are lower. This IRIS spectrum in the 5-micrometer
    spectral region shows features identified with water (H₂O), germane
    (GeH₄), and deuterated methane (CH₃D), as well as the more easily
    detected ammonia (NH₃).]

  Wavelength (µm)
    Voyager 1
    Voyager 2
  Wave number (cm⁻¹)
  Voyager 1 brightness temperature (K)
  Voyager 2 brightness temperature (K)

    [Illustration: The structure of the Jovian atmosphere can be derived
    from infrared spectra as well as from the radio occultation data.
    This profile of temperature as a function of pressure covers the
    same range of altitudes as does the preceding figure and is in good
    agreement. Both profiles locate a temperature minimum of about 110 K
    near a pressure of 0.1 bar (1000 mb = 1 bar = 1 atm). The IRIS data
    also show variation of structure with position, including a cooler
    minimum temperature (about 100 K) over the Great Red Spot.]

    [Illustration: The structure of the atmosphere can be inferred from
    IRIS spectra at many locations over the disk of Jupiter. Scientists
    are beginning to assemble this vast amount of information into maps
    that show the temperatures at a given pressure. The temperature
    contours are labeled in degrees Kelvin. The banded structure, with
    higher temperatures near the dark equatorial belt, is most clearly
    evident at the lower altitude. Surprisingly, the cool region
    associated with the Great Red Spot (latitude 23°S) is more apparent
    at high altitude.]

    [Illustration: The observed temperature at a depth near the cloud
    tops (0.8 bar).]

    [Illustration: The observed temperature at an altitude about 30
    kilometers higher (0.15 bar).]

                           The Magnetosphere

Giant Jupiter has an enormous realm—from the size of its satellite
system to its tremendous aurorae and superbolts of lightning, to the
huge planet-sized cloud features that surround its atmosphere. The most
gargantuan Jovian feature is its magnetosphere, which envelopes the
satellites and constantly changes in size, pumping in and out at the
whim of the solar wind. The Pioneer and Voyager spacecraft provided four
cuts through this dynamic region, showing that its borders in the upwind
solar direction lie between 50 R_J and 100 R_J from Jupiter. Downwind,
away from the Sun, the magnetosphere extends much farther; some
scientists postulate that a magnetotail may reach as far as the orbit of

Charged particles in the magnetosphere are subject to powerful forces.
Tightly embedded in Jupiter, the magnetic field spins with a ten-hour
period as the planet rotates. The particles are caught in the spinning
field and accelerated to high speeds. The result is a co-rotating plasma
in the magnetic equator of Jupiter, extending outward to at least 20
R_J. Beyond this distance, the flow breaks up and the magnetosphere is
more unstable. Within the co-rotation region, the spinning plasma sets
up a powerful electric current girdling the planet.

Charged particles can be accelerated in the magnetosphere to high
energies, corresponding to speeds tens of thousands of kilometers per
second. Some of these particle streams escape from the inner parts of
the magnetosphere and can penetrate the magnetopause and be ejected from
the Jovian system. On Voyager 1, the low energy charged particle
instrument began detecting these streams of “hot” plasma on 22 January,
when Voyager 1 was still 600 R_J (almost 50 million kilometers) from the
planet. Voyager 2 first detected Jovian particles at an even greater
distance, 800 R_J. Hydrogen and helium ions (protons and alpha
particles) dominate the magnetosphere at great distances from Jupiter,
but increasing amounts of sulfur and oxygen appeared as the spacecraft
crossed the magnetopause. The heavier ions presumably originate from Io.

In the inner magnetosphere, the Galilean satellites have a powerful
influence on the populations of fast-moving particles. During the
Voyager 1 encounter, the primary effect was observed at Io, where the
satellite apparently sweeps up energetic electrons. In the million-volt
energy range, these particles are depleted near Io, with peaks observed
both inside and outside the satellite’s orbit. Voyager 2 passed close to
Ganymede, and here also major effects were seen, with the satellite
apparently absorbing electrons. In the wake created by the motion of the
co-rotating plasma past Ganymede, the particle populations showed large
and complex variations.

    [Illustration: The magnetosphere of Jupiter can be “seen” from Earth
    by its emissions at radio wavelengths. The recent development of
    imaging radio telescopes in Great Britain, The Netherlands, and the
    United States allows frequent mapping of the large-scale features of
    the innermost magnetosphere, inside the orbit of Io. These six
    images showing one rotation of Jupiter were obtained near the time
    of the Voyager 1 flyby by Imke de Pater at the Westerbork Radio
    Observatory near Leiden. The brightest emission is shown by dark red
    or black. The size of the planet is indicated by a dotted white
    circle. The tilt of the magnetosphere relative to the rotation axis
    of the planet can be seen by the wobble of the magnetosphere as
    Jupiter rotates.]

Just inside the Jovian magnetosphere is the “hot spot” of the solar
system: a 300-400 million degree plasma detected by Voyager 1 while it
was still about 5 million kilometers from Jupiter. T. P. Armstrong
commented, “Even the interior of the Sun is estimated to be less than 20
million degrees.” S. M. Krimigis added that the temperature of this
plasma is “the highest yet measured anywhere in the solar system.”
Fortunately for Voyager, this region of incredibly hot plasma is also
one of the solar system’s best vacuums. The spacecraft was in little
danger because the bow shock protects this region from the solar wind,
and most of the particles in Jupiter’s magnetosphere are held in much
closer to the planet.

    [Illustration: Each Voyager passed through the boundaries of the
    magnetosphere—the bow shock (BS) and the magnetopause (MP)—on both
    the inbound and the outbound legs of its passage through the Jovian
    system. In this diagram, the heavy solid line represents the
    spacecraft trajectory, as seen looking down from the north. Also
    shown are the positions of the bow shock in March and of the
    magnetosphere in both March and July.]

    [Illustration: Several regions of plasma (charged particles) make up
    the Jovian magnetosphere. These sketches, based on Voyager data,
    show the magnetosphere as viewed from above (a) and as seen from the
    Jovian equatorial plane (b). Most of the plasma co-rotates with the
    planet and is confined near the magnetic equator, where it forms a
    broad plasma sheet about 100 R_J across.]

The very hot plasma in the outer magnetosphere discovered by Voyager is
thought to play an important role in establishing the size of the Jovian
magnetosphere. Although the density is low, only about one charged
particle per hundred cubic centimeters, this plasma actually carries a
great deal of energy because of the high speed of the particles. It is
this plasma pressure, rather than the magnetic field pressure, that
appears to hold off the pressure of the solar wind. However, the balance
between hot plasma inside the magnetopause and the solar wind outside is
not very stable. The Voyager experimenters suggest that a small change
in solar wind pressure can cause the boundary to become suddenly
unstable. A large quantity of the hot plasma can then be lost, producing
the bursts seen at large distances and permitting a sudden collapse of
the outer magnetosphere. Continued injection of hot plasma from within
would then reinflate the magnetosphere, which would expand like a
balloon until another instability developed. Processes of this sort may
be the cause of the rapidly varying magnetospheric boundaries observed
by both Voyager spacecraft.

    [Illustration: The rings of Jupiter are best seen when looking
    nearly in the direction of the Sun, since the small particles that
    comprise them are good forward scatterers of sunlight. This mosaic
    is of Voyager 2 images (two wide angle and four narrow angle)
    obtained from a perspective behind the planet and inside the shadow
    of Jupiter. The spacecraft was 2 degrees below the equator of
    Jupiter and 1.5 million kilometers from the rings. The shadow of the
    planet can be seen to obscure the near segment of the ring near the
    edge of the planet. The brightest region of the ring is about 1.8
    R_J from the center of Jupiter. [260-678B]]

                            Rings of Jupiter

One of the spectacular discoveries of Voyager was the existence of a
ring system of small particles circling Jupiter. Saturn and Uranus were
known to have rings, but none had been seen before at Jupiter.

As revealed by the Voyager cameras, the rings extend outward from the
upper atmosphere to a distance of 53 000 kilometers above the cloud
tops, 1.8 R_J from the center of the planet. The main rings, however,
are much narrower, spanning from 47 000 to 53 000 kilometers above
Jupiter. There are two main rings, a 5000-kilometer-wide segment, and a
brighter, outer 800-kilometer segment. The thickness of the rings is
unknown, except that it is certainly less than 30 kilometers, and
probably under 1 kilometer.

    [Illustration: Structure within the ring can be seen in the best
    Voyager 2 images, taken about 27 hours after closest approach to
    Jupiter. This enlarged portion of a wide-angle picture taken with a
    clear filter shows a bright core about 800 kilometers across with a
    dimmer region a few thousand kilometers across on the inside, and a
    narrow dim region on the outside. [260-674]]

The rings of Jupiter are quite tenuous, which explains why they are
invisible from Earth [although they have been detected from Earth since
Voyager]. Seen face on, the brightest part of the ring blocks less than
one part in ten thousand of the light passing through, making it
essentially transparent. In fact, the ring does not even offer much
resistance to a spacecraft; Pioneer 11 traversed the ring in 1974 with
no obvious ill consequences. Apparently the individual particles that
make up the ring are widely dispersed. They can be seen only when the
rings are viewed nearly edge-on, or toward the Sun, where they show up
well in forward scattered light. It is this extra brilliance when
backlit that created the excellent photos taken by Voyager 2 from inside
the shadow of Jupiter.

The individual ring particles are probably dark, rocky fragments that
are very small—essentially dust grains. They move around Jupiter in
individual orbits, circling the planet in 5 - 7 hours. Scientists
postulate that such orbits are not stable and that the particles fall
slowly in toward Jupiter. Apparently the rings are constantly renewed
from some source, which may be the satellite Adrastea (J14), discovered
by Voyager 2. There has also been speculation that Adrastea may
influence the ring structure by sweeping particles out of the ring. At
present the rings of Jupiter remain mysterious. They are clearly very
different from the rings of Saturn and Uranus, and reaching an
understanding of their origins and dynamics presents many challenges to
planetary scientists.

    [Illustration: The four Galilean satellites of Jupiter are
    planet-like worlds, revealed by Voyager to be as diverse and
    fascinating as the terrestrial planets Mercury, Venus, Earth, and
    Mars. In this Voyager 1 composite, all four are shown in their
    correct relative size, as they would appear from a distance of about
    1 million kilometers. Relative color and reflectivity are also
    approximately preserved, although it is not possible to show on a
    single print the full range of brightness from the dark rocky
    surface of Callisto to the brilliant white of Europa or orange of
    Io. [260-499C]]

    [Illustration: Io (longitude 140°).]

    [Illustration: Europa (longitude 300°).]

    [Illustration: Callisto (longitude 350°).]

    [Illustration: Ganymede (longitude 320°).]

                               CHAPTER 9
                            FOUR NEW WORLDS

                       Jupiter’s Satellite System

In a sense, the Voyager Mission revealed a new planetary system.
Astronomers had long been fascinated by the large Galilean satellites of
Jupiter, but they had only looked from afar, watching the dancing points
of light in their telescopes, and, occasionally, as the atmosphere
steadied, seeing these points resolve themselves into tiny disks before
dissolving again in the turbulence of the terrestrial atmosphere. Much
had been learned from telescopic studies, but not until the Voyager
flights had we truly seen the Galilean satellites. The historic hours as
Voyager 1 cruised past each satellite on March 5 and 6, 1979,
fundamentally altered our perspective. Four new worlds were revealed, as
diverse and fascinating as the more familiar terrestrial planets.
Although not yet household words, the names Io, Europa, Ganymede and
Callisto have now been added to Mercury, Venus, Moon, and Mars in the
lexicon of important “Earth-sized” bodies in the solar system.

Jupiter has fifteen known satellites, counting the two new satellites
discovered by Voyager. These moons vary greatly in size, composition,
and orbit. The four outermost satellites, Sinope, Pasiphae, Carme, and
Ananke, circle the planet in retrograde orbits of high inclination;
their distances from Jupiter vary between 20 and 24 million kilometers
(290 R_J to 333 R_J). These small bodies, none more than 50 kilometers
in diameter, require nearly two years for each orbit of Jupiter. It is
possible that they are captured asteroids, but so little is known about
them that astronomers cannot tell if their surface properties resemble
those of asteroids, or if these four satellites are even similar to each

The next group of Jovian satellites consists of four small
difficult-to-observe objects. These are Lysithea, Elara, Himalia, and
Leda, the latter discovered by Charles Kowal of Hale Observatories in
1974. They have similar orbits, varying in distance from Jupiter between
11 and 12 million kilometers (about 160 R_J). Like the outer group,
these satellites have orbits of high inclination; unlike the outer
group, they move in the proper, prograde direction around Jupiter. The
largest, Himalia (170 kilometers in diameter) and Elara (80 kilometers
diameter), are known to be very dark, rocky objects, and it seems
probable that the others are similar. It is unlikely that the census of
the outer groups of irregular satellites is complete, and new satellites
less than 10 kilometers in diameter will probably be discovered.

The Jovian system is dominated, of course, by the large Galilean
satellites, which vary in size from just smaller than the Moon (Europa)
to nearly as large as Mars (Ganymede). These satellites are in regular,
nearly circular orbits in the same plane as the equator of Jupiter, and
all four lie within the inner magnetosphere of Jupiter, where they
interact strongly with energetic charged particles and plasma. Most of
this chapter will be devoted to a discussion of these fascinating

We now know of three additional small satellites inside the orbit of Io,
orbiting close to Jupiter. The first, Amalthea, was discovered in 1892;
it orbits Jupiter in just twelve hours at a distance of 181 000
kilometers (2.55 R_J). A smaller object, Adrastea (officially 1979J1 for
the first new satellite of Jupiter discovered in 1979), is much closer,
at 134 000 kilometers (1.76 R_J). As described in Chapter 7, it skirts
the outer edge of the ring, circling Jupiter in just over seven hours.
The inner satellite moves faster than Jupiter’s rotation; seen from the
planet, it would rise in the west and set in the east. Both Amalthea and
Adrastea are buried deep within the inner magnetosphere where they are
continually bombarded by energetic electrons, protons, and ions.
Depletion of the Jovian radiation belt particles was observed at the
orbits of both satellites by Pioneer 11, which went much closer to
Jupiter than the Voyagers, testifying to the intensity of the
interaction between these objects and their surroundings.

    [Illustration: Callisto was revealed by the Voyager cameras to be a
    heavily cratered and hence geologically inactive world. This mosaic
    of Voyager 1 images, obtained on March 6 from a distance of about
    400 000 kilometers, shows surface detail as small as 10 kilometers
    across. The prominent old impact feature Valhalla has a central
    bright spot about 600 kilometers across, probably representing the
    original impact basin. The concentric bright rings extend outward
    about 1500 kilometers from the impact center. [260-450]]

                          SATELLITES OF JUPITER
                       Distance From Jupiter
      Name                10³       Jupiter      Period      Year of
                       kilometers    Radii       (days)     Discovery

  Adrastea      J14           134        1.76        0.30          1979
  Amalthea      J5            181        2.55        0.49          1892
  1979J2        J15           222        3.11        0.67          1980
  Io            J1            422        5.95        1.77          1610
  Europa        J2            671        9.47        3.55          1610
  Ganymede      J3           1070       15.10        7.15          1610
  Callisto      J4           1880       26.60       16.70          1610
  Leda          J13        11 110         156         240          1974
  Himalia       J6         11 470         161         251          1904
  Lysithea      J10        11 710         164         260          1938
  Elara         J7         11 740         165         260          1904
  Ananke        J12        20 700         291         617          1951
  Carme         J11        22 350         314         692          1938
  Pasiphae      J8         23 300         327         735          1908
  Sinope        J9         23 700         333         758          1914

    [Illustration: The state of the interiors of the Galilean satellites
    can be judged from their sizes and densities. These cross-sectional
    views represent the best guess following the Voyager flybys as to
    the composition and structure of the objects. Io, with a density
    equal to that of the Moon and a long history of volcanic activity,
    is a dry, rocky object. Europa is less dense, and it probably has a
    global ocean of ice as much as 100 kilometers thick over a rocky
    interior. Ganymede and Callisto both have densities near 2 grams per
    cubic centimeter, suggesting a composition about half water and half
    rock. There is probably a rocky core surrounded by an icy mantle.]

    Active volcanoes
    Sulfur and frozen SO₂
    Molten silicate interior
    Global fracture patterns
    Ice crust
    Rocky interior
    Fresh craters expose ice
    Young grooved terrain with intricate fracture
    Old, dark cratered areas
    Ice crust
    Water or ice mantle
    Large basins reduced by ice flow
    Fresh craters expose ice
    Ice/rock crust
    Water or ice mantle

Long after the flybys of Jupiter, continued analysis of Voyager images
revealed another new satellite, Jupiter’s fifteenth. Initially
designated 1979J2, the unexpected new satellite orbits the planet at
3.17 R_J, between Io and Amalthea. Stephen Synnott of the JPL Optical
Navigation Team discovered the satellite on pictures taken during the
Voyager 1 events on March 5, 1979, while searching for additional images
of satellite 1979J1. It is about 75 kilometers in diameter, but nothing
else is known about its physical properties.

Together, the 15 satellites circling giant Jupiter form a mini-solar
system. Perhaps the outer, irregular satellites were captured or
resulted from the catastrophic collisions of one or more larger
satellites with passing asteroids. The inner seven satellites constitute
a coherent system, almost certainly formed together with Jupiter and
sharing a common 4.5-billion-year history. They are fascinating as
individual worlds, and also as brothers and sisters, and the study of
their interrelationships undoubtedly will provide insights into the
general problems of planetary formation and evolution.

       Name           Diameter     Density (grams per
                    (kilometers)    cubic centimeter)

  Io                         3640                  3.5
  Europa                     3130                  3.0
  Ganymede                   5270                  1.9
  Callisto                   4840                  1.8


Callisto is the least active geologically of the Galilean satellites.
Basically a dead world, it bears the scars of innumerable meteoric
impacts, with virtually no sign of major internal activity. Callisto is
a world of craters, and to understand it we must explore the role that
cratering plays in molding planetary surfaces.

The space between the planets is filled with debris, ranging from the
larger asteroids, hundreds of kilometers in size, down to microscopic
grains of dust. Inevitably, each planet collides with some of these
fragments. The smaller particles do little damage; in the case of a
planet with an atmosphere, like Earth, they burn up as meteors before
reaching the surface, whereas on an airless planet, they erode the
surface by sandblasting the exposed rock. The larger impacts are another
matter, and the craters they produce can be the dominant features on the
surface of a planet.

    [Illustration: Voyagers 1 and 2 photographed most of the surface of
    Callisto at resolutions of a few kilometers or better. Shown here is
    a preliminary shaded relief map. Additional measurements will
    improve the accuracy of the coordinate system. [260-672]]

    [Illustration: Callisto is a world of craters, as is well shown in
    this Voyager 2 photomosaic taken from a distance of 400 000
    kilometers. Craters about 100 kilometers in diameter cover the
    surface uniformly. Many have bright rims, perhaps composed of
    exposed water-ice. There are very few craters larger than 150
    kilometers in diameter, however, indicating that the scars of very
    large impacts do not survive on the surface of Callisto.

    [Illustration: The concentric rings surrounding Valhalla are perhaps
    the most distinctive geological feature on Callisto. This Voyager 1
    close-up shows a segment of the ridged terrain. The presence of
    superposed impact craters shows that the rings formed early in
    Callisto’s history; however, the density of craters is less here
    than on other parts of the satellite, where the surface is older.

We who live on Earth tend not to realize the importance of cratering,
for the simple reason that our planet has very few craters, and these
are frequently of volcanic rather than meteoric origin. Why are we so
favored? Is there an invisible shield to protect us from the cosmic
shooting gallery? Clearly not; the Earth has experienced just as many
cratering impacts as has the Moon or other planets. The difference is
not that craters are formed less often, but that the great geological
activity of Earth—erosion, volcanism, mountain building, continental
drift, etc.—erases craters as fast as they are formed. On the average, a
10-kilometer-wide crater is formed on Earth about once every million
years, but all those older than a few million years have been eroded
away, filled in, or crushed beyond recognition by crustal motion.

If a planet lacks great internal geologic forces, large craters can
survive almost indefinitely. Such is the case for the Moon. Most of the
volcanism and other activity on the Moon ceased 3½ billion years ago, as
the dating of lunar samples obtained by the Apollo astronauts showed.
Since that time, the lunar surface has been passively accumulating
impact scars. The longer any particular surface area has been exposed,
the more densely packed are the craters. Thus crater density is the
first thing a planetary geologist looks for in photos of a new world.
Craters are the touchstone of this field, revealing the degree of
internal activity and allowing the determination of the relative ages of
different surface units.

On Callisto the density of craters is very high. In some places they are
packed as closely as one can imagine, particularly for craters several
tens of kilometers in diameter. Although no one knows the exact rate of
formation of impact craters on the Jovian satellites, geologists on the
Voyager Imaging Team estimate that it would require several billion
years to accumulate the number of craters found on Callisto. They
therefore conclude that Callisto has been geologically inactive almost
since the time of its formation.

Although superficially similar to the heavily cratered surfaces of the
Moon and Mercury, Callisto is far from identical to these rocky worlds.
One of the most obvious differences is a lack of craters larger than
about 150 kilometers on Callisto, together with a tendency for large
craters to have much shallower depths. Apparently the ice-rock
composition of Callisto alters the ability of the crust to retain large
craters. Geologists speculate that the ice flows over many millions of
years, filling in crater floors and gradually obliterating the largest
craters. There is also a conspicuous absence of mountains on Callisto,
again suggestive of a weak, icy crust.

    [Illustration: The IRIS instrument measured the temperature of spots
    on the surface of Callisto as each Voyager sped past. The
    measurements shown here were all made at equatorial latitudes
    (between -10° and 25°). Shown are very low predawn temperatures
    (-190° C) followed by an increase to a noon-time maximum of about
    -120° C, and then a drop again as the Sun sets. [260-735]]

The most prominent features in the Voyager pictures are the ghost
remnants of what must have been immense impact basins. The largest of
these, the “bullseye” of the Voyager 1 images, has been named Valhalla,
for the home of the Norse gods. These ghost basins have lost nearly all
their vertical relief. What remains is a central, light-colored zone
(probably the location of the original crater), surrounded by numerous
concentric rings of subdued, bright ridges. Such features had never been
seen before on any planet, and they appear to be the characteristic
geologic feature of an ice-rock planet.

Little is known about the composition of Callisto’s surface, the
material from which sunlight is reflected. It appears to be primarily
dark rock or soil, but it lacks diagnostic spectral features, except for
one infrared band due to water molecules bound in the soil. The many
lighter spots and arcs that outline craters in the high-resolution
pictures may be regions in which the ice is showing through, but these
cover only a very small fraction of the exposed surface. (It should be
noted that, although Callisto is the darkest of the Galilean satellites,
the term “dark” is relative, for even Callisto is brighter than Earth’s

The daytime surface temperature of Callisto, observed both from the
ground and by Voyager, is about -118° C. The Voyager infrared
interferometer spectrometer also determined the minimum temperature,
reached just before dawn, of -193° C. No atmosphere is expected at these
cold temperatures, and none was seen.

Analysis of Voyager images provided an improved diameter for Callisto of
4840 kilometers, yielding an average density of 1.8 grams per cubic
centimeter. As noted previously, it is this low density that leads to
the conclusion that ice or water is an important component of the
interior of Callisto. The ice has never been detected directly, but the
peculiar nature of the craters seen by Voyager adds strong
circumstantial support to this conclusion.

Callisto, with its heavy cratering, is the most familiar-looking of the
Galilean satellites; if all of them had turned out to be as geologically
dead as Callisto, planetary geologists would certainly have been
disappointed. However, each satellite, progressing in toward Jupiter,
presents increasing evidence of internal activity.


The largest of the Galilean satellites (5270 kilometers in diameter),
Ganymede was expected to be similar to Callisto in many ways. Both have
low densities (for Ganymede, 1.9 grams per cubic centimeter), indicating
a bulk composition of about half rocky materials and half water. In
addition, their diameters differ by only eight percent, and both are far
enough from Jupiter to escape the severe pounding Io receives from
magnetospheric charged particles. Thus it was with great interest that
Voyager scientists looked at the differences that emerged between these
two satellites.

    [Illustration: Shaded relief map of Ganymede. [260-673]]

The surface of Ganymede as revealed by the Voyager cameras is one of
great diversity, indicating differing periods of geologic activity. At
one extreme there are numerous dark areas that resemble the surface of
Callisto in both albedo (reflectivity) and crater density. The largest
of these, Regio Galileo, stretches from the equator to latitude +45° and
is 4000 kilometers across, nearly as large as the continental United
States. This ancient terrain even preserves the remnants of a
Callisto-type impact basin in the form of a system of parallel, curving,
subdued ridges about 10 kilometers wide, 100 meters high, and spaced
about 50 kilometers apart. The central part of this ghost basin is
missing, however; it was presumably destroyed by subsequent geologic

Other regions of the surface of Ganymede are clearly the product of
intense internal geologic activity. Generally, these regions are of
higher albedo and consist of many straight parallel lines of mountains
and valleys. The Voyager geologists call these the grooved terrain
because of their appearance from a great distance. Typically, these
mountain ridges are 10-15 kilometers across and about 1000 meters high,
similar in scale to some sections of the Appalachian Mountains in the
Eastern United States. No higher relief exists, presumably for the same
reason it is absent on Callisto. In many places the grooved terrain
forms between areas of the older, darker surface, giving the appearance
of mountains extruded between separating plates of ancient crust. In
other areas the relationships are much more complex, with curved systems
of grooves and ridges overlying each other, displaying intricate
crosscutting relationships. Apparently Ganymede has experienced a series
of mountain-building events.

The grooved terrains show a substantial range in age, as indicated by
the crater densities. The oldest have nearly the same density as the
ancient, dark plains, suggesting that formation of the grooved terrain
began early, perhaps 4 billion years ago. The youngest grooved terrain
has only about one-tenth as many craters, but this is still as many as
are seen in the 3.5-billion-year-old lunar plains. The Voyager
geologists believe that even in these areas geologic activity ceased
billions of years ago.

    [Illustration: The hemisphere of Ganymede that faces away from the
    Sun displays a great variety of terrain. In this Voyager 2 mosaic,
    photographed at a range of 300 000 kilometers, the ancient dark area
    of Regio Galileo lies at the upper left. Below it, the lighter
    grooved terrain forms bands of varying width, separating older
    surface units. On the right edge, a prominent crater ray system is
    probably caused by water-ice splashed out in a relatively recent
    impact. [260-671]]

    [Illustration: The sinuous nature of some of the narrower Ganymede
    groove systems can be seen in this oblique view, obtained on March 5
    by Voyager 1. The area shown is about the size of California, with
    features visible as small as 5 kilometers across. The ridges appear
    to be the result of deformation of the crust of Ganymede. [P-21235]]

Other types of surfaces are seen on Ganymede. Some regions are lightly
cratered and smooth, with no indication of mountain building. In one
place, there is a rough mountainous area that looks more like the
jumbled lunar mountains than the long ridges and valleys of the rest of
Ganymede. Many of the larger craters are distinguished by brilliant
white halos and rays that suggest that impacts may have splashed large
quantities of water or ice over the surface.

Many of the geologic features seen on Ganymede appear to have been
caused by breaking, faulting, or spreading of the crust. In a few cases,
there even seem to be indications of transverse, or sideways, motion
along faults. This evidence is extremely exciting to geologists, since
similar crustal motion on Earth is associated with the drift of
continental plates, drawn by convection currents deep in the mantle.
Such activity has never been seen before on another planet.

Astronomers on Earth had known since 1971 that about half the surface of
Ganymede was covered with exposed water ice and about half with darker
rock. An examination of the albedo variations in the Voyager pictures
suggests that the ice is exposed near large craters and, to a lesser
extent, in the grooved terrain, but no direct measurements were made by
Voyager of the composition of different parts of the surface.

The presence of ice on the surface suggested to many astronomers that
Ganymede might have a very tenuous atmosphere of water vapor or oxygen,
which might be released by the breakdown of water vapor by sunlight.
During the Voyager 1 flyby, a sensitive test for an atmosphere was made
by the ultraviolet instrument from observations of the star Kappa
Centauri as it was occulted by Ganymede. No dimming of the starlight was
seen, yielding an upper limit for the surface pressure of the gases
oxygen, water vapor, or carbon dioxide of 10⁻¹¹ bar, or one
hundred-billionth the atmospheric pressure at Earth.

The differences between the geologic histories of Ganymede and Callisto
are surprisingly large. No one knows the reason. Perhaps only a small
increase in internal temperature is necessary to initiate geologic
activity in an icy planet, and for some reason Ganymede crossed this
threshold for a part of its history, whereas Callisto did not.

    [Illustration: The complex patterns of the grooved terrain on
    Ganymede are apparent in high-resolution images. This picture, taken
    by Voyager 1 on March 5, has a resolution of about 3 kilometers and
    shows a region about the size of the state of Pennsylvania. The
    mountain ridges are spaced about 10 to 15 kilometers apart and rise
    about 1000 meters, similar to many of the mountains of Pennsylvania.
    The transections of different mountain systems indicate that they
    formed at different times. A degraded crater near the left center of
    the picture is crossed by ridges, indicating that it predated the
    period of crustal deformation and mountain building. [P-21279]]

    [Illustration: Ray systems of exposed water-ice are visible in this
    high-resolution mosaic of Ganymede, obtained by Voyager 2 on July 9
    at a range of about 100 000 kilometers. The rough mountainous
    terrain at lower right is the outer portion of a large fresh impact
    basin that postdates most of the other terrain. At the bottom,
    portions of grooved terrain transect other portions, indicating an
    age sequence. The dark patches of heavily cratered terrain (right
    center) are probably ancient icy material formed prior to the
    grooved terrain. The large rayed crater at upper center is about 150
    kilometers in diameter. [P-21770B/W]]

    [Illustration: The many variants of smooth and grooved terrain on
    Ganymede suggest a complex geologic history for this satellite. Four
    high-resolution views by Voyager 2 are grouped together. [260-678A]]

    [Illustration: A band of low mountain ridges has apparently been cut
    and offset by a fault.]

    [Illustration: Multiple sets of mountain ridges transect at nearly
    right angles. Some impact craters were formed before, and some
    after, the grooved terrain.]

    [Illustration: Many short parallel ridges butt into each other,
    making a crazy-quilt pattern.]

    [Illustration: In the center of this frame is an unusual smooth
    area, perhaps the result of flooding of the surface by material that
    filled in the grooves.]


As one proceeds inward through the Galilean satellites, these worlds
become less and less familiar to the planetary geologist. This was an
unexpected effect. Callisto and Ganymede were expected to have unusual
properties as a result of their large percentage of ice. The densities
of Europa and Io are more normal for the smaller, terrestrial-type
planets, and before Voyager many scientists expected these two
satellites might look much like the Moon, which they resemble in size.

Europa, with a diameter of 3130 kilometers, is about 15 percent smaller
than the Moon. Its density is 3.0 grams per cubic centimeter, indicating
a basically rocky composition. However, cosmic mixtures of rocky and
metallic materials are often a bit denser than this, leaving room for a
substantial component of ice or water. Calculations indicate that if all
the ice were at the surface, it might form a crust up to 100 kilometers

    [Illustration: Shaded relief map of Europa. [260-659]]

Telescopic observations of Europa demonstrated long before Voyager that
this satellite is almost completely covered with ice. It is a white,
highly reflecting body, looking, from a great distance, like a giant
snowball. In the early Voyager pictures, Europa always showed a bland,
white disk, in striking contrast to the spottiness of Ganymede or the
brilliant colors of Io.

Voyager 1 never got closer to Europa than 734 000 kilometers, and at
that distance it remained a nearly featureless planet, with no obvious
impact craters or other familiar geologic structures. What did show in
the Voyager 1 pictures, however, were numerous thin, straight dark lines
crisscrossing the surface, some extending up to 3000 kilometers in
length. To the members of the Imaging Team, these features were
“strongly suggestive of global-scale tectonic processes, induced either
externally (as by tidal despinning) or internally (as by convection).”
It was with the greatest interest that the Voyager 2 images, taken from
about four times closer, were anticipated.

The spectacular pictures obtained of the satellite in July were perhaps
more confusing than clarifying. Europa is entirely covered with dark
streaks that vary in width from several kilometers to approximately 70
kilometers and in length from several hundred to several thousand
kilometers. Most streaks are straight, but others are curved or
irregular. The streaks lie on otherwise smooth, bright terrain,
featureless except for numerous random dark spots, most less than 10
kilometers in diameter.

Voyager 2 photos showed, in addition to the smooth terrain with its dark
streaks, regions of darker, mottled terrain. This mottled terrain
appears rough on a small scale, and it may contain small craters just on
the limit of resolution (about 4 kilometers). Only three definite impact
craters have been identified, each about 20 kilometers across. This
small number of craters suggests either that the surface is relatively
young or that craters are not preserved for long in the icy crust.

Although the dark streaks give Europa a cracked appearance, the streaks
themselves are not obviously cracks. They are not depressed below their
surroundings; in fact, they have no topographic structure whatever.
Europa is extraordinarily smooth, and the dark streaks look rather like
marks made with a felt-tipped pen on a white billiard ball. The streaks
are not even very dark; the contrast with adjacent smooth terrain is
only about 10 percent.

One of the most remarkable geologic phenomena discovered by Voyager is
the light streaks that appear on Europa. These are smaller than the dark
streaks, only about 10 kilometers in width, but much more uniform. Seen
at low Sun angle, they also show vertical relief of less than a few
hundred meters. These light ridges are seen best at low Sun and tend to
be invisible at higher illumination angles.

The most amazing thing about the light ridges is their form. Instead of
being straight, they form scallops or cusps, with smooth curves that
repeat regularly on a scale of 100 to a few hundred kilometers. In some
of the low-Sun-angle pictures, the surface of Europa seems to be covered
with a beautiful network of these regular curving lines. The impression
is so bizarre that one tends not to believe the reality of what is seen.
Nothing remotely like it has ever been seen on any other planet.

At present the geology of Europa remains beyond our understanding.
Presumably there is a thick ice crust, perhaps floating on a liquid
water ocean. Presumably there is sufficient heat coming from the
interior to have produced cracking or motion in the ice crust, and the
light and dark streaks preserve a pattern in some way related to this
internal activity. However, the actual mechanisms for producing the
observed features so lightly traced on this smooth white world remain
for scientists to decipher.

    [Illustration: Europa looks like a cracked egg in this computer
    mosaic of the best Voyager 2 images. In this presentation, the
    variation of surface brightness due to the angle of the Sun has been
    removed by computer processing, so that surface features can be seen
    equally well at all places. The many broad dark streaks show up
    well, but this presentation does not bring out the much fainter and
    more enigmatic light streaks. These pictures were taken from a
    distance of about 250 000 kilometers and show features as small as 5
    kilometers across. [260-686]]


The most spectacular of the Galilean satellites is Io. Even in
low-resolution images, its brilliant colors of red, orange, yellow, and
white set it apart from any other planet. The dramatic scale of its
volcanic activity confirms that Io is in a class by itself as the most
geologically active planetary body in the solar system.

The diameter of Io is 3640 kilometers, and its density is 3.5 grams per
cubic centimeter. Both values are nearly identical to those of our Moon.
Were it not for Io’s proximity to Jupiter, it would probably be a dead,
rocky world much like Earth’s satellite.

Careful examination of all the Voyager images of Io, some of which have
resolutions as good as 1 kilometer, has failed to reveal a single impact
crater. Yet the flux of crater-producing impacts at Io must be even
greater than for the other Galilean satellites, because of the focusing
effect of Jupiter’s gravity. The absence of craters alone would indicate
that Io has an extremely young and dynamic planetary surface, even
without the observation of active volcanoes. Calculations indicate that
craters on Io must be filled in or otherwise obliterated at a rate
corresponding to the deposition of at least 100 meters per million
years, and quite probably a factor of ten greater, or 1 meter every
thousand years.

    [Illustration: One of the most remarkable of all the Voyager
    discoveries was the arcuate white ridges on Europa. Visible only at
    very low Sun angle, these curved bright streaks are 5 to 10
    kilometers wide and rise at most a few hundred meters above the
    surface. Their graceful scalloped pattern is unique to this planet
    and has defied explanation. Also visible in this view, taken by
    Voyager 2 on July 9 at a range of 225 000 kilometers, are dark
    bands, more diffuse than the light ridges, typically 20 to 40
    kilometers wide and hundreds to thousands of kilometers long.

    [Illustration: Shaded relief map of Io as it appeared in early March
    1979. [260-634BC]]

    [Illustration: The great erupting volcanoes on Io produce
    distinctive surface markings. These are three views of Prometheus
    (P₃). The bright ring on the surface rims the areas of fallout from
    the plume, and it is probably an area in which sulfur dioxide frost
    is being deposited on the surface. [260-451]]

    [Illustration: Prometheus.]

    [Illustration: Prometheus.]

    [Illustration: Prometheus.]

    [Illustration: These views are of Loki (P₂) as seen by Voyager 1.

    [Illustration: The asymmetric structure of the plume can be seen.]

    [Illustration: Loki.]

    [Illustration: An ultraviolet image has been used to produce a
    false-color composite; the large ultraviolet halo above the
    visible-light plume may be due to scattering from sulfur dioxide gas
    rather than solid particles.]

    [Illustration: Perhaps the most spectacular of all the Voyager
    photos of Io is this mosaic obtained by Voyager 1 on March 5 at a
    range of 400 000 kilometers. A great variety of color and albedo is
    seen on the surface, now thought to be the result of surface
    deposits of various forms of sulfur and sulfur dioxide. The two
    great volcanoes Pele and Loki (upper left) are prominent. [260-464]]

In place of impact craters, the surface of Io has a great many volcanic
centers, which generally take the form of black spots a few tens of
kilometers across. In a few cases, high-resolution pictures show the
characteristic shapes associated with volcanic calderas on Earth and
Mars, and, if the other volcanic centers are similar, about 5 percent of
the entire surface of Io is occupied by calderas. These are extremely
black, reflecting less than 5 percent of the sunlight; often they are
surrounded by irregular, diffuse halos nearly as black as the central
spot. The calderas seem more like the Valles caldera in New Mexico,
which is associated with vents that produced large quantities of ash,
than with those of Hawaiian-type shield volcanic mountains.

There is evidence in many of the Voyager photos of extensive surface
flows on Io. These originate in dark volcanic centers and either spread
to fan shapes, typically 100 kilometers across, or else snake out in
long, twisting tentacles. Some of the flows are lighter than the
background and some are darker. Most are red or orange in color, often
outlined by fringes of contrasting albedo.

The equatorial regions of Io are quite flat, with no vertical relief
greater than about 1 kilometer high; indeed, many of the volcanic
centers do not appear to correspond to mountains or domes at all. There
are, however, a number of long, curvilinear cliffs or scarps and narrow,
straight-walled valleys a few hundred meters deep. These appear to be
places in which the crust has broken under tension, somewhat similar to
terrestrial faults and the valleys called graben. A few rugged mountains
of uncertain origin are visible in low Sun elevation pictures.

Near the poles of Io the terrain is more irregular. There are few
volcanic centers, but more mountains, some with heights of several
kilometers. In addition, there are regions that appear to be made of
stacked layers of material. These so-called layered terrains are
revealed when erosion cuts into them, exposing the layers along the
cliff or scarp. The largest such plateau or mesa has an area of about
100 000 square kilometers. The scarps sometimes intersect each other,
suggesting a complex history of deposition, faulting, and erosion.
Voyager geologists believe that these scarps may be areas in which the
release of liquid sulfur or sulfur dioxide has undercut cliffs,
analogous to internal sapping by groundwater at similar scarps on Earth.

Perhaps the most distinctive surface features on Io are the circular or
oval albedo markings that surround the great volcanoes. The first of
these to be seen was the 300-kilometer-wide white donut of Prometheus,
on the equator at longitude 150°. Much more spectacular is the hoofprint
of Pele, about 700 by 1000 kilometers. These symmetric rings mark the
locations of the kinds of eruptions that generate large fountains or
plumes, and may be produced by condensible sulfur or sulfur dioxide
raining down from the volcanic fountain. At least one new ring appeared
during the four months between the Voyager encounters, centered at
longitude 330°, latitude +20°, but by the time Voyager 2 photographed
this area, no plume remained active.

During the Voyager 1 flyby, temperature scans of the surface of Io were
made with the infrared interferometer spectrometer (IRIS). A number of
localized warm regions were found, the most dramatic being just south of
the volcano Loki. Here the images showed a strange, U-shaped black
feature about 200 kilometers across. The IRIS team interpreted its data
to indicate a temperature for the black feature of 17° C (or room
temperature), in contrast to the surrounding surface at -146° C. Perhaps
the dark feature was some sort of lava lake, either of molten rock or
molten sulfur. The melting point of sulfur is 112° C. If there were a
scum of solidifying sulfur on top of the “lake,” this interpretation
might well be the correct one.

The brilliant reds and yellows of the surface of Io immediately suggest
the presence of sulfur. When heated to different temperatures and
suddenly cooled, sulfur can assume many colors, ranging from black
through various shades of red to its normal light-yellow appearance.

Even before Voyager, laboratory studies had shown that sulfur matches
the overall properties of the spectrum of Io, including the low albedo
in the ultraviolet and the high reflectivity throughout the infrared.
Contemporary with the Voyager flybys, additional telescopic observations
and laboratory studies by Fraser Fanale at JPL and Dale Cruikshank at
the University of Hawaii identified another component on Io, sulfur
dioxide. Sulfur dioxide is an acrid gas released from terrestrial
volcanoes, where it combines with water in the Earth’s atmosphere to
produce sulfuric acid. At the temperature of the surface of Io, sulfur
dioxide is a white solid. Researchers guessed that the extensive bright
white areas in the Voyager pictures of Io might be covered with sulfur
dioxide frost or snow. The presence of this material on Io was confirmed
when the infrared IRIS instrument obtained a spectrum of sulfur dioxide
gas over the erupting volcano Loki during the Voyager 1 encounter.

    [Illustration: Close-ups of Io reveal a wide variety of volcanic
    phenomena. This Voyager 1 view of an equatorial region near
    longitude 300° shows several large surface flows that originate in
    volcanic craters or calderas. At the right edge is a light flow
    about 250 kilometers long. Another dark, lobate flow with bright
    edges is just left of center, with an exceedingly dark caldera to
    its left. [260-468A]]

The discovery of the ongoing eruptions on Io, made shortly after the
Voyager 1 flyby, did much to clarify the confused evidence pouring in
concerning the apparent youth of Io’s surface. Here, under the very eyes
of Voyager, eruptions were taking place on a scale that dwarfed anything
ever seen before. The discovery picture alone, taken from a distance of
4 million kilometers, showed two eruptions (Pele and Loki), each of
which was much larger than the most violent volcanic eruption ever
recorded on Earth.

Voyager 1 found eight giant eruptions, with fountains or plumes rising
to heights of between 70 and 280 kilometers. To reach these altitudes,
the material must have been ejected from the vents at speeds of between
300 and 1000 meters per second, several times greater than the highest
ejection velocities from terrestrial volcanoes. Although widely spaced
in longitude, these volcanoes were concentrated toward the equator;
seven of the eight were at latitudes between +30° and -30°, and the
eighth at -44°.

When Voyager 2 arrived four months later, it was able to reobserve seven
of the eight volcanoes. (To be identified reliably, the volcanic plumes
must be silhouetted against dark space at the edge of the disk.) Six of
these were still erupting; one, Pele, the largest plume seen by Voyager
1, had ceased activity. The plume associated with Loki had also changed
markedly, increasing in height from 100 to 210 kilometers in visible
light. (All the plumes appear larger when viewed in the ultraviolet.)
Loki had developed a more complex structure; in March it appeared to
originate near the south end of a 250-kilometer-long dark feature, but
in July there was a double plume, with activity at both ends of the dark

    [Illustration: Differences in surface elevation can clearly be seen
    in a few of the Io close-ups from Voyager 1. This remarkable picture
    is of the center of the great volcano Pele, at latitude 15°S and
    longitude 224°. A low mountain with flow features can be seen. In
    the background, there are several large irregular depressions with
    flat floors that appear to be the result of collapse. The diffuse
    dark features in the center are probably the ejecta plumes being
    erupted from the Pele vent. [P-21220B/W]]

    [Illustration: At the highest resolution obtained by the Voyager
    cameras, Io revealed some landscapes that looked familiar to
    terrestrial geologists. This picture, taken by Voyager 1 at a range
    of only 31 000 kilometers, shows a region about the size of the
    state of Maryland at a resolution of 300 meters. Clearly seen is a
    volcano not too different from some of those on the Earth or Mars.
    At the center is an irregular composite crater or caldera about 50
    kilometers in diameter with dark flows radiating from its rim. The
    style of volcanism illustrated here is quite different from the
    explosive plumes or fountains with their associated rings of bright
    material deposited on the surface. This volcano is located at about
    longitude 330°, latitude 70°S. [260-502]]

    [Illustration: In addition to its giant volcanic plumes or
    fountains, Io possesses other indications of current volcanic
    activity. One of these takes the form of intermittent blue-white
    patches that may be caused by gas venting from the interior. In this
    pair of photographs, the same region of the surface is shown about
    six hours apart. On the right, there is an arcuate bright gas cloud;
    on the left the same region is black. It is believed that the
    venting gas is sulfur dioxide, and that the condensation of this gas
    produces fine particles of “snow” that look blue. [260-508]]

    [Illustration: Some of the most dramatic changes in the surface of
    Io between March and July took place in the vicinity of the volcano
    Loki, at longitude 310° and latitude 15°N. On the left is a Voyager
    1 view; on the right, one from Voyager 2. The “lava lake” associated
    with Loki has become less distinct, apparently as a result of
    deposits that fell on the northern part of the dark U-shaped
    feature. Perhaps the surface had also cooled between these photos.
    In the upper left center, a new dark volcanic caldera with bright
    spots near it and a large, faint bright ring had appeared by July,
    although it was not active at the time Voyager 2 flew by.

No new eruptions were seen by Voyager 2; between them, the two
spacecraft effectively surveyed the whole surface of Io for plumes down
to 40 kilometers height. Interestingly, the smallest plume seen was 70
kilometers high; there appeared to be a real absence of smaller

From the number and size of the observed eruptions, it is possible to
calculate the resurfacing rate for Io due to these plumes. The result is
that each plume is erupting about 10 000 tons of material per second, or
more than 100 billion tons per year. This quantity corresponds to about
10 meters of deposition over the whole surface in a million years. When
additional note is taken of surface flows, the deposition rate could
easily be ten times higher, or 100 meters per million years, in
agreement with the rate estimated from the absence of impact craters.

                      Energy for the Io Volcanoes

Clearly, something extraordinary is happening to Io to generate the
observed level of volcanism. The primary heat source for the interiors
of the terrestrial planets is the decay of the long-lived radioactive
elements thorium and uranium. But Io would have to be supplied with a
hundred times its quota of these elements to explain the observed

A way out of this difficulty was provided by a theoretical investigation
carried out by Stanton Peale of the University of California at Santa
Barbara and Pat Cassen and Ray Reynolds at the NASA Ames Research
Laboratory. Working in the months before the first Voyager flyby, they
calculated that the tidal effects of Jupiter on Io could generate
large-scale heating of the satellite. Io is about the same distance from
Jupiter as the Moon from Earth, but the much greater mass of Jupiter
raises enormous tides in its satellite. These tides distort its shape,
but no other effect would be present if Io remained at a constant
distance from Jupiter. What Peale, Cassen, and Reynolds realized was
that the distance of Io from Jupiter varies as the result of small
gravitational perturbations from the other Galilean satellites.
Therefore the tidal distortions also vary, in effect squeezing and
unsqueezing Io each orbit. Such flexing pumps energy into the interior
of Io in the form of heat; theorists calculated that the heat supplied
could be as high as 10¹³ watts. They predicted, in a paper published
just three days before the Voyager flyby of Io, that “widespread and
recurrent surface volcanism might occur,” and that “consequences of a
largely molten interior may be evident in pictures of Io’s surface
returned by Voyager.”

    [Illustration: Voyager 2 obtained beautiful views of the volcanic
    eruptions during its ten-hour Io volcano watch on July 9. On the
    edge of the crescent image are the volcanoes Amirani (P₅) and below
    it Maui (P₆), each sending up fountains about 100 kilometers above
    the surface. The blue color is probably the result of sunlight
    scattered by tiny particles of sulfur dioxide snow condensing in the
    erupting plume. [P-21780]]

                           VOLCANIC ERUPTIONS ON IO
  Plume     Name           Location        Height During   Activity During
  Number             (latitude/longitude)    Voyager 1     Voyager 2 Flyby

       1 Pele             -20°/255°                   280 ceased
       2 Loki             +20°/300°                   100 increased
       3 Prometheus        -5°/155°                    70 increased
       4 Volund           +20°/175°                    95 no data
       5 Amirani          +25°/120°                    80 similar
       6 Maui             +20°/120°                    80 similar
       7 Marduk           -25°/210°                   120 similar
       8 Masubi           -40°/ 50°                    70 similar

    [Illustration: Between the two encounters, the volcanic eruption at
    Loki (P₂) changed character. The single plume emanating from the
    western end of an apparent dark fissure seen in March was joined by
    a second fountain of similar size about 100 kilometers to the east.
    The plume also increased in height, from about 120 kilometers in the
    Voyager 1 image (left) to 175 kilometers in the Voyager 2 image
    (right). [260-662A and B]]

    [Illustration: The detailed structure near the volcano Loki is like
    nothing seen elsewhere on Io.  [260-642B]]

    [Illustration: When this Voyager 1 picture was taken, the main
    eruptive activity (P₂) came from the lower left of the dark linear
    feature (perhaps a rift) in the center. Below it is the “lava lake,”
    a U-shaped dark area about 200 kilometers across. In this specially
    processed image, detail can be seen in the dark surface of this
    feature, possibly due to “icebergs” of solid sulfur in a liquid
    sulfur lake.]

    [Illustration: (Bottom) The IRIS on Voyager 1 found this “lava lake”
    to be the hottest region on Io, with a temperature about 150° C
    higher than that of the surrounding area.]

    [Illustration: One model for the structure of Io indicates that an
    ocean of liquid sulfur with a solid sulfur crust covers most of the
    satellite. Heat escapes from the interior in the form of lava, which
    erupts beneath the sulfur ocean. Secondary eruptions in the sulfur
    ocean heat liquid sulfur dioxide, which is mixed with solid sulfur
    in the crust. The rapid expansion of sulfur dioxide gas then
    produces the great eruptive plumes, which consist of a mixture of
    solid sulfur, sulfur dioxide gas, and sulfur dioxide snow.]

  Solid sulfur + solid SO₂
  Solid sulfur + liquid SO₂
  Molten sulfur
  Solid silicate

The Voyager observations appear to confirm the theoretical calculations.
The tidal heat source has presumably been acting since Io was formed
more than 4 billion years ago. With a totally molten interior and
continuing large-scale volcanism, Io has had an opportunity to
thoroughly sort out its composition. In the process it would have lost
all the volatile gases such as water and carbon dioxide, explaining why
Io now has no appreciable atmosphere in spite of the outpouring of
material from the interior. In addition, most of the sulfur from the
interior could have risen to the surface, where it would be constantly
recycled through volcanic activity.

The presence of large amounts of sulfur on the surface may help explain
the extraordinary nature of the Io volcanoes. One model considered for
the satellite postulates that it is covered by a sea of liquid sulfur
several kilometers deep, with a crust of solid sulfur and, below the
surface, liquid sulfur dioxide. Calculations by Sue Kieffer of the U.S.
Geological Survey and others indicate that the expansion of the sulfur
dioxide in such a model can explain the observed eruption velocities of
up to a kilometer per second.

The volcanic plumes on Io appear to be made primarily of sulfur and
sulfur dioxide. Both are molten as they emerge from the vent, but they
quickly cool as the plume rises 100 or more kilometers into the near
vacuum of space. Unlike terrestrial volcanoes, there is almost no gas in
the plumes. It requires about half an hour for the fine particles of
solidified sulfur and sulfur dioxide snow to fall back to the surface,
where they form the colorful rings that mark the major eruptive sites.

Almost all the roughly 100 000 tons of material erupted each second by
the Io volcanoes snows back to the surface. But apparently a
part—perhaps 10 tons per second—escapes from Io and is captured by the
Jovian magnetosphere. Another part contributes to an ionosphere—a
tenuous atmosphere of electrons and ions—that surrounds Io. The
injection of several tons of particles each second into the
magnetosphere has dramatic consequences that can be seen even from

    [Illustration: Direct evidence of an atmosphere on Io was obtained
    during the Voyager 1 flyby by the IRIS. In the region near the
    volcano Loki and its associated “lava lake,” infrared spectra
    clearly showed the signature of sulfur dioxide gas. It is not known
    whether this gas was a temporary feature associated with the
    eruption of Loki or if it might be present on Io more generally.
    Other evidence, however, points to the sulfur dioxide atmosphere as
    a transient feature. A small amount of the sulfur dioxide escapes
    and is broken apart by sunlight to provide the oxygen and sulfur
    ions observed in the Io torus.]

  Sulfur dioxide gas cloud
  -235° F
  Hot surface areas (45° F)

                              The Io Torus

Surrounding Jupiter at the distance of Io is a donut-shaped volume, or
torus, of plasma that originates at the satellite. At first, the atoms
escaping from Io expand outward as a gas, but soon they are stripped of
electrons and become electrically charged. Some of these gases, such as
sulfur dioxide, apparently originate in the large volcanic eruptions;
other, such as the sodium cloud being studied with Earth-based
telescopes, result from sputtering of surface materials by energetic
particles in the magnetosphere. After they are ionized by the loss of
one or more electrons, the atoms are caught by the spinning magnetic
field of Jupiter and become a part of what is called a co-rotating
plasma, spinning at 74 kilometers per second with the same ten-hour
period as Jupiter itself.

The Io torus was easily detected on Voyager by the ultraviolet
spectrometer, even from a distance of 150 million kilometers. The
strongest ultraviolet radiation comes from twice-ionized sulfur (atoms
that have lost two electrons) (S III), emitting a wavelength of 69
nanometers or about one-eighth the wavelength of visible light. The
spectrometer also detected glows from atoms of triply ionized sulfur (S
IV) and twice-ionized oxygen (O III).

    [Illustration: At the time of the Voyager 1 encounter, the most
    abundant heavy ions in the Jovian magnetosphere were sulfur and
    oxygen. Multiply ionized sulfur and oxygen both emit strongly in the
    ultraviolet, where they could be observed by the ultraviolet
    spectrometer. This spectrum of the Io torus registers the tremendous
    amount of ultraviolet energy (about a million million watts) being
    radiated. To emit so strongly, the temperatures in the torus must be
    near 100 000 K.]

    [Illustration: Direct measurement of the heavy ions associated with
    the Io torus were made by the Voyager 1 LECP instrument. Here the
    amounts of various elements are shown for two cases: the Jovian
    inner magnetosphere (solid line) and a typical solar event. Both the
    Jovian and the solar particles have been scaled to show similar
    amounts of oxygen, but the solar particles are also rich in carbon
    and iron, whereas Jupiter has a great deal of sulfur. Evidence such
    as this demonstrates that the sulfur does not come from the Sun;
    rather, the sulfur and most of the oxygen appear to be the product
    of the Io sulfur dioxide volcanic eruptions.]

Scans across the torus showed that it had a thickness of 1.0 R_J and was
centered at a distance of 5.9 R_J from Jupiter. The torus is centered on
the magnetic, rather than the rotational, equator of the planet. To
produce the intense glow observed, the electron temperatures in the
torus must be 100 000 K, with an electron density of about 1000 per
cubic centimeter. The brightness in the ultraviolet corresponds to a
radiated power of more than a million million (10¹²) watts. This
enormous amount of energy must be continuously supplied by the

The ultraviolet emissions from the Io torus seen by Voyager were
dramatically different from those seen in 1973 and 1974 with the simpler
ultraviolet instrument on board Pioneers 10 and 11. These changes
correspond to more than a factor of 10 in brightness. As noted by the
Voyager Team, “Because of the remarkable differences we conclude that
the Jupiter-Io environment has changed significantly since December
1973. The observed differences are so spectacularly large that this
conclusion does not depend on a detailed comparison of the two
instruments, their calibrations, or the observing geometry.” The reason
for this change, or the degree to which it reflects a large-scale
variation in the volcanic activity of Io, is one of the major questions
arising from the Voyager mission.

The Io torus can also be observed from the ground. In 1976, spectra
showed the glow of singly ionized sulfur (S II), and in 1979 singly
ionized oxygen (O II) was detected. The observations of S II and O II
are particularly interesting because they provide a measure of the
density and temperature of the plasma. In April 1979, between the two
Voyager flybys, Carl Pilcher of the University of Hawaii succeeded in
obtaining a telescopic image of the torus in the light of S II. He
measured nearly the same ring diameter (5.3-5.7 R_J) as had Voyager;
interestingly, both agree that the sulfur torus is centered slightly
inside the orbit of Io (6.0 R_J).

    [Illustration: The emission of light from sulfur ions in the Io
    torus is so strong it can be measured from the Earth. In these
    pictures, University of Hawaii astronomer Carl Pilcher photographed
    the torus in the light of ionized sulfur on the night of April 9,
    1979. As the planet rotates, the torus is seen first partly opened,
    then edge-on, and again opened in the opposite direction. The dark
    band on the right of each image is due to light from Jupiter
    scattered in the telescope, as shown in the bottom picture, which
    contains the scattered light only.]

Direct measurements of the torus were made from Voyager 1 as the
spacecraft passed twice through this region, once inbound and once
outbound. The low energy charged particle instrument and the cosmic ray
instrument both determined that the composition of the ions in the Io
torus was primarily sulfur and oxygen. Ionized sodium was also observed.
For several years, ground-based telescope observations had revealed a
cloud of neutral sodium around Io; the Voyager instruments picked up
these atoms after they had each lost an electron and become trapped in
the magnetosphere. These instruments also derived the electron and ion
density (about 1000 per cubic centimeter) and confirmed that the ions
were co-rotating with the inner magnetosphere.

Another Io-associated phenomenon searched for by Voyager was the Io flux
tube. As a conductor moving through the Jovian magnetic field, Io
generates an electric current, estimated to have a strength of about 10
million (10⁷) amperes and a power of the order of a million million
(10¹²) watts. The region of space through which this current flows from
the satellite to Jupiter is called the flux tube.

Voyager 1 was targeted to fly through the Io flux tube. This was an
important decision, since this option precluded the possibility of
obtaining occultations by either Io or Ganymede. The event was to take
place on March 5, just after closest approach to Io. The effects of the
flux tube were clearly observed by the magnetometer, the LECP
instrument, and other particle and field instruments; however,
subsequent analysis indicated that the spacecraft had not penetrated the
region of maximum current flow; it probably missed the center of the
flux tube between 5000 and 10 000 kilometers.

The flux tube is not the only connection between Io and Jupiter. Radio
emissions from the atmosphere are triggered by Io’s orbital position,
and the aurorae that illuminate Jupiter’s polar regions are the result
of charged particles falling into the planet from the Io torus. Other
charged particles can occasionally escape outward and be detected as far
away as Earth.

Io is unquestionably a remarkable world. The only planetary body known
to be geologically more active than the Earth, it provides many extreme
examples to test the theories of geoscientists. Its intimate
interconnections with the Jovian magnetosphere and the planet itself
provide a unifying theme to the complex processes taking place in the
inner parts of the Jovian system.

    [Illustration: The Galileo Probe will make a fiery entry into the
    Jovian atmosphere, carrying a payload of scientific instruments for
    the first direct sampling of the atmosphere of a giant planet. Shown
    here is the moment, at a pressure level of about 0.1 bar, when the
    parachute is deployed and the still-glowing heatshield drops free
    from the Probe.]

                               CHAPTER 10
                           RETURN TO JUPITER

                         A Successor to Voyager

The spectacular discoveries of the Voyagers did not exhaust our interest
in the Jovian system. Both the giant planet and its system of satellites
will almost certainly play a central role in any future program of solar
system exploration and research. Thus, even as the two Voyager
spacecraft directed their attention further outward toward Saturn, NASA
had begun development of the next Jupiter mission, named Galileo.

Galileo is an ambitious, multiple-vehicle planetary mission. It has two
major interlocking elements: a probe to be placed in the atmosphere of
Jupiter and an orbiter to explore Jupiter, its satellites, and its
magnetosphere. By using individual satellite flybys to alter its orbit,
the Galileo spacecraft can carry out a satellite “tour” consisting of
flybys of the Galilean satellites at different geometries and a deep
penetration into the magnetosphere in the unexplored region of space
behind Jupiter.

Both an atmospheric probe for the planet and a long-lived orbiter to
study the satellites and magnetosphere are logical successors to
Voyager. In 1974, three years before Voyager launch, the Space Science
Board of the National Academy of Sciences was already emphasizing the
scientific advantages of both of these approaches. In suggesting goals
for 1975-1985, the Board wrote, “We recommend that a significant effort
in the NASA planetary program over the next decade be devoted toward the
outer solar system. Jupiter is the primary object of outer solar system
exploration.” Looking at specific mission goals, the Board recommended
that “the primary objectives in the exploration of Jupiter and its
satellites for the period 1975-1985 in order of importance are (1)
determination of the chemical composition and physical state of its
atmosphere, (2) the chemical composition and physical state of the
satellites, and (3) the topology and behavior of the magnetic field and
the energetic particle fluxes. In order to carry out this program, it
will be necessary to utilize orbiting spacecraft and probe-delivering

In the same period NASA carried out studies of two possible orbiter and
probe missions. Working through the Ames Research Center, a scientific
panel chaired by James Van Allen explored the adaptation of the Pioneer
10 and 11 spinning spacecraft to carry a probe to Jupiter and to carry
out an orbiter mission emphasizing magnetospheric studies. William B.
Hubbard of the University of Arizona chaired a JPL-based panel
investigating the use of a Mariner-class fully stabilized spacecraft
similar to Voyager to carry out a satellite-oriented orbiter mission. In
1976 these concepts were combined in a study, again chaired by Dr. Van
Allen, of a Voyager-type orbiter with probe-carrying capability. This
mission concept was given the name JOP, for Jupiter Orbiter Probe, and
lead responsibility was assigned by NASA to JPL, with Ames carrying out
the design of the probe.

In 1977, as Voyager activity was building toward autumn launch, a
struggle was underway in Washington to obtain approval for the new
Jupiter orbiter and probe mission. Budgeting authority was requested in
the President’s Fiscal Year 1978 budget, but only after extensive
testimony and several Congressional votes was the mission approved. The
official new start for JOP, soon to be renamed Galileo, was set for July
1, 1977, and the scientific investigators and their instruments were
selected in August.

At JPL, many members of the Voyager Team made a smooth transition to the
Galileo Project. Much of the knowledge that had gone into the design of
the Voyager spacecraft and its subsystems was now incorporated into
Galileo. Similarly, at Ames the knowledge gained from the design of the
Pioneer Venus probes, which were launched to Venus in 1978, a year after
Voyager launch, was applied to design of a Jupiter probe. Among the
individuals who brought their Voyager experience to Galileo were John
Casani, who left the position of Voyager Project Manager to become
Galileo Project Manager, and Torrence Johnson of the Voyager Imaging
Team, who became Galileo Project Scientist.

                  The Scientific Capability of Galileo

The investigations of Jupiter and its system planned for the Galileo
Project represented substantial advances over those carried out by
Voyager. In part, this was the result of new spacecraft capabilities,
particularly the atmospheric entry probe. It also represented increasing
sophistication in scientific instrumentation over the seven-year
interval between the selection of the payloads for the two missions.

The main emphasis in the study of Jupiter itself is on direct
measurements with the Probe. For the first time it will be possible to
examine directly the atmosphere of a giant planet. By measuring the
temperature and pressure as it descends through the clouds, the Probe
can determine the structure of the atmosphere with much higher precision
than could ever be obtained from remote observations. The structure, in
turn, provides information on dynamics—the circulation and heat balance
of the Jovian atmosphere. In addition, the Probe can make direct
measurements of the composition of the gases, with sensitivity in some
cases to quantities as low as a few parts per billion. In addition to
the elemental abundance, the amount of different isotopes can also be

Direct studies of the clouds of Jupiter can be made from the Galileo
Probe. With a device called a nephelometer (literally, cloud-meter), the
sizes and compositions of individual aerosol particles will be
determined. An infrared instrument will determine the temperatures of
the cloud layers and measure the amounts of sunlight deposited in
different regions of the atmosphere. Another instrument will search for
lightning; it has the ability to detect both the flash of light and the
radio static generated by each bolt.

Additional studies of the atmosphere, similar to those of Voyager, can
be carried out from the Galileo Orbiter. Television pictures,
ultraviolet and infrared spectra, and measurements of the polarization
of reflected light will all be obtained with the same scan platform
instruments that are used to study the surfaces of the satellites.

A full battery of fields and particles instruments is planned for the
Galileo Orbiter. Many of these are direct descendants of Voyager
instruments. In general, their capabilities have been improved,
particularly their ability to determine the composition of charged
particles. There is a steady progression from Pioneer to Voyager to
Galileo: The early measurements were concentrated on particle energies,
but more sophisticated instruments yield the composition of the ions and
the details of their motion.

Many of the advances expected from Galileo in magnetospheric studies
result from the Orbiter’s ability to explore many parts of the
environment of Jupiter. The Pioneer and Voyager spacecraft made single
cuts through the magnetosphere, and often it was difficult to
distinguish temporal from spatial effects. Galileo will repeatedly swing
around Jupiter, sampling conditions at many distances from the planet
over a time span of two years or more. In addition, it is planned to
adjust the orbit of Galileo to swing out into the magnetotail, the
turbulent region of the magnetosphere that stretches “downwind” from
Jupiter for hundreds of Jupiter radii. No flybys can reach the
magnetotail; an orbiting spacecraft is required.

The Galilean satellites naturally will be a primary focus of Galileo
science, particularly after Voyager. It is planned to have as many as a
dozen individual encounters, most of them at much closer range than the
Voyager flybys. To take advantage of these opportunities, the Galileo
scan platform will carry two new remote sensing systems.

    [Illustration: The Orbiter section of the Galileo spacecraft will
    carry both remote sensing and direct measuring instruments for the
    study of Jupiter, its satellites, and its magnetosphere. Several
    remote sensing instruments—an imagery system, a near infrared
    mapping spectrometer, an ultraviolet spectrometer, and a
    photopolarimeter/radiometer—will be mounted on a scan platform. The
    particles and fields instruments will be on a spinning section of
    the spacecraft. The Orbiter is expected to operate for at least two
    years around Jupiter, providing one close flyby of Io and several
    each of Europa, Ganymede, and Callisto. [P-20772]]

Instead of the vidicon television camera on Voyager, Galileo imaging
will be done with a new solid-state detector called a charged coupled
device (CCD). The CCD has a wider spectral response and greater
photometric accuracy. In addition, its increased sensitivity permits
shorter exposures, so that even on very close flybys the pictures will
not be blurred by spacecraft motion. Substantial coverage at a
resolution of 100 meters should be possible, compared to Voyager’s best
resolution of 1 kilometer for Io and 4 kilometers for Europa.

    [Illustration: Galileo will be launched by the Space Shuttle, the
    central element of the new NASA Space Transportation System.
    Together with its upper stage launch rocket, Galileo will be placed
    into Earth’s orbit in the large Shuttle bay, about 20 meters long
    and 5 meters in diameter. After releasing Galileo, the Shuttle will
    be piloted back to a landing at Cape Canaveral, to be used again for
    many future flights. [5-78-23599]]

    [Illustration: The Space Shuttle and the Inertial Upper Stage of
    Galileo as they will appear in Earth orbit. The upper stage has been
    released from the Shuttle bay and is being prepared for launch to

To determine the composition of satellite surface materials, Galileo
will also carry a near-infrared mapping spectrometer (NIMS). This
instrument will obtain measurements over the visible and infrared
spectra of areas as small as 10 kilometers across. With NIMS, it should
be possible to investigate the composition of individual features as
small as the volcanic calderas on Io or the ejecta blankets of
Ganymede’s craters.

                         Galileo Mission Design

The Galileo Orbiter and Probe are to be launched with NASA’s new Space
Shuttle and Inertial Upper Stage. To carry the maximum possible payload
to Jupiter, a close flyby of Mars is planned en route. The gravitational
field of Mars will give a boost to Galileo, just as that of Jupiter was
used by Voyager to swing on to Saturn.

The exact launch date and trajectory for Galileo have not yet been
specified, but if all goes well, the Orbiter spacecraft will approach
Jupiter from the dawn side of the planet sometime in the mid-1980s. It
will not be moving as fast as Voyager, since it must be placed into
orbit around Jupiter rather than flashing past on its way to the outer
solar system. On its initial trajectory, Galileo will probably come
within 5 R_J of Jupiter, slightly closer than Voyager 1. At this time it
will fire its rocket engines (supplied by the Federal Republic of
Germany in a cooperative program with NASA) to shed excess speed and let
itself be captured by Jupiter’s gravity. The first pass will also be the
time for a close flyby of Io.

    [Illustration: The most critical period of the Galileo flight will
    be the Probe entry at Jupiter. The Probe must strike the atmosphere
    at precisely the correct angle and speed to be slowed down without
    being destroyed. At a pressure level of about 0.1 bar the rapid
    deceleration period ends and the heat shield is released. A
    parachute is deployed to slow the descent further, and the Probe
    then has a period of nearly an hour to study the atmosphere and
    clouds of Jupiter. The Probe mission ends when its batteries run
    down or when it is crushed by the pressure of the Jovian atmosphere
    near the 20-bar level, whichever comes first. [SL78-545(3)]]

                      Probe Scientist: L. Collin, NASA Ames
        Investigation         Principal Investigator      Primary Objectives

    Atmospheric structure     A. Seiff, NASA Ames       Measure temperature,
                                                        density, pressure,
                                                        and molecular weight
                                                        to determine the
                                                        structure of
                                                        Jupiter’s atmosphere.
    Neutral mass              H. B. Neimann, NASA       Measure the
    spectrometer              Goddard                   composition of the
                                                        gases in Jupiter’s
                                                        atmosphere and the
                                                        variations at
                                                        different levels in
                                                        the atmosphere.
    Helium abundance          U. von Zahn, Bonn U.      Measure with high
    interferometer            (Germany)                 accuracy the ratio of
                                                        hydrogen to helium in
                                                        Jupiter’s atmosphere.
    Nephelometer              B. Ragent, NASA Ames      Determine the sizes
                                                        of cloud particles
                                                        and the location of
                                                        cloud layers in
                                                        Jupiter’s atmosphere.
    Net flux radiometer       R. W. Boese, NASA Ames    Measure energy being
                                                        radiated from Jupiter
                                                        and the Sun, at
                                                        different levels in
                                                        Jupiter’s atmosphere.
    Lightning and radio       L. J. Lanzerotti,         Measure lightning
    emission                  Bell Labs                 flashes in Jupiter’s
                                                        atmosphere, from the
                                                        light and radio
                                                        transmissions from
                                                        those flashes.
    Energetic particles       H. M. Fischer, U.         Measure energetic
                              Kiel (Germany)            electrons and protons
                                                        in the inner regions
                                                        of the Jovian
                                                        radiation belts and
                                                        determine their
                                                        spatial distributions.

Because of the intense radiation environment, the Galileo Orbiter will
not be able to spend much time in the inner magnetosphere, near the
orbit of Io. To do so would risk damage to the spacecraft electronics
and a premature end to the mission. Additional thruster firing during
the first orbit can be used to raise the periapse to 10 R_J or greater.
No more close passes by Io will be possible, but studies of this
satellite can be made on each subsequent orbit with imaging resolutions
of about 10 kilometers, sufficient to see details of the volcanic
eruptions and monitor volcano-associated changes in the surface.

At each subsequent orbit, Galileo will be programmed for a close flyby
of one of the other satellites. Several passes each of Callisto,
Ganymede, and Europa should be possible. The satellite tour does not
need to be fully planned in advance; by adjusting the spacecraft
trajectory with small bursts of the thruster motors, navigation
engineers can modify the orbit to permit adaptation to scientific needs.
As the Orbiter mission progresses, the spacecraft will also sample many
parts of the magnetosphere, including one long excursion, at least 150
R_J, into the magnetotail.

The total duration of the Orbiter mission is planned to be at least 20
months. Additions to the basic mission are possible if the spacecraft
remains healthy and fuel reserves are adequate. In contrast, the Galileo
Probe mission lasts only a few hours.

As the Probe approaches the atmosphere of Jupiter at the awesome speed
of 26 kilometers per second, it will be traversing a region of space
never before explored. An energetic particle detector will investigate
the innermost magnetosphere before the entry begins. Then, within a
period of just a few minutes, friction with the upper atmosphere must
dissipate the Probe energy until it is falling gently in the Jovian air.

Jupiter, being the largest planet, presents the most challenging
atmospheric entry mission ever undertaken by NASA. The design of the
Galileo Probe calls for a massive heat shield to protect the instruments
during the high-speed entry phase. After the Probe has slowed to
subsonic velocities, a parachute will be deployed, and the heat shield,
having done its job, will be dropped free.

                   Project Scientist: T. V. Johnson, JPL
        Investigation        Principal Investigator   Primary Objectives

 Solid state imaging         M. J. S. Belton, Kitt  Provide images of
                               Peak Observatory       Jupiter’s
                               (Team Leader)          atmosphere and its
                                                      satellites; study
                                                      structure and
                                                      dynamics on
                                                      investigate the
                                                      composition and
                                                      geology of the
                                                      satellite surfaces;
                                                      study the active
                                                      volcanic processes
                                                      on Io.
 Ultraviolet spectrometer    C. W. Hord, U.         Study composition and
                               Colorado               structure of the
                                                      upper atmospheres
                                                      of Jupiter and its
 Near-infrared mapping       R. W. Carlson, JPL     Provide spectral
   spectrometer (NIMS)                                images and
                                                      reflected sunlight
                                                      spectra of
                                                      indicating the
                                                      composition of
                                                      their surfaces;
                                                      measure reflected
                                                      sunlight and
                                                      thermal emission
                                                      from Jupiter’s
                                                      atmosphere to study
                                                      composition, cloud
                                                      structure, and
                                                      profiles; monitor
                                                      hot spots on Io.
 Photopolarimeter/radiometer J. E. Hansen, NASA     Measure temperature
                               Goddard                profiles and energy
                                                      balance of
                                                      atmosphere; measure
                                                      Jupiter’s cloud
                                                      characteristics and
 Magnetometer                M. G. Kivelson, UC     Measure magnetic
                               Los Angeles            fields and the ways
                                                      they change near
                                                      Jupiter and its
                                                      satellites; measure
                                                      variations caused
                                                      by the satellites
                                                      interacting with
                                                      Jupiter’s field.
 Plasma particles            L. A. Frank, U. Iowa   Provide information
                                                      on low-energy
                                                      particles and
                                                      clouds of ionized
                                                      gas in the
 Energetic particles         D. J. Williams, NOAA   Measure composition,
                               Space Environment      distribution, and
                               Lab                    energy spectra of
                                                      particles trapped
                                                      in Jupiter’s
 Plasma waves                D. A. Gurnett, U. Iowa Investigate waves
                                                      generated inside
                                                      magnetosphere and
                                                      waves radiated by
                                                      possible lightning
                                                      discharges in the
 Dust detection              E. Grün,               Determine size,
                               Max-Planck-Institut    speed, and charge
                               (Germany)              of small particles
                                                      such as
                                                      near Jupiter and
                                                      its satellites.
 Celestial mechanics         J. D. Anderson, JPL    Use the tracking data
                               (Team Leader)          to measure the
                                                      gravity fields of
                                                      Jupiter and its
                                                      satellites; search
                                                      for gravity waves
                                                      propagating through
                                                      interstellar space.
 Radio propagation           H. T. Howard,          Use radio signals
                               Stanford U. (Team      from the Orbiter
                               Leader)                and Probe to study
                                                      the structure of
                                                      the atmospheres and
                                                      ionospheres of
                                                      Jupiter and its
 Interdisciplinary           F. P. Fanale (JPL), P. J. Gierasch (Cornell
   Scientists:                 U.), D. M. Hunten (U. Arizona), H.
                               Masursky (U.S. Geological Survey), M. B.
                               McElroy (Harvard U.), D. Morrison (U.
                               Hawaii), G. S. Orton (JPL), T. Owen (SU
                               New York), J. B. Pollack (NASA Ames), C.
                               T. Russell (UC Los Angeles), C. Sagan
                               (Cornell U.), F. L. Scarf (TRW), G.
                               Schubert (UC Los Angeles), C. P. Sonett
                               (U. Arizona), J. A. Van Allen (U. Iowa).

The Probe will spend nearly an hour descending from a pressure level of
about 0.1 bar, where the heat shield is jettisoned, to a depth of 10-20
bars. During this time it will make most of its scientific measurements,
relaying them back to Earth via the Probe carrier. Designers expect the
Probe to sink through regions of ammonia clouds, ammonium hydrosulfide
clouds, and ice and water clouds during this hour.

By the time it has descended below the water clouds, the increasing
pressure will exceed the strength of some Probe components. Engineers
expect the Probe to have completed its mission, exhausted its battery
power, and been crushed by the atmospheric pressure before the 20-bar
level is reached. Lifeless, it will then sink on into the thick, hot
lower atmosphere of Jupiter.

    [Illustration: During its two years in orbit, the Galileo Orbiter
    will carry out many investigations of the planet, the Galilean
    satellites, and the Jovian magnetosphere. Repeated close flybys of
    the satellites are used to modify and shape the orbit to provide
    additional flybys at an optimum viewing geometry. Initially, the
    orbit is a long loop that extends in the general direction of the
    sunset side of Jupiter. The orbit is then contracted, and the
    encounters with the satellites rotate it behind the planet for a
    long excursion into the magnetotail late in the tour.]

                             Beyond Galileo

After Galileo, the future cannot be predicted. Perhaps there will no
longer be a program of planetary exploration. But if humanity still has
the vision to seek a future in the stars, there will surely be other
Jupiter missions.

Perhaps the next mission will concentrate on Jupiter itself. Probes
could be built to withstand pressures as high as several hundred bars,
feeling their way deep into the murky depths of the planet. Or a hot-air
balloon could be deployed from a probe to carry instruments for
long-term studies of the atmosphere. A number of proposals have also
been made for additional satellite missions, including orbiters or
landers for Ganymede and Callisto. Or perhaps it will be desirable to
land a vehicle on one of the satellites and collect a sample and return
it to Earth for laboratory analysis.

Whatever the future holds, it is clear that the Pioneer and Voyager
missions blazed the path to Jupiter and beyond. The little Pioneers
proved that it could be done, and the Voyagers expanded their vision,
exploring and discovering new worlds more remarkable and exciting than
anyone could have imagined.

                               APPENDIX A

These maps were prepared for the Voyager Imaging Team by the U.S.
Geological Survey in cooperation with the Jet Propulsion Laboratory,
California Institute of Technology and the National Aeronautics and
Space Administration. Copies are available from Branch of Distribution,
U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202, and
Branch of Distribution, U.S. Geological Survey, Box 25286, Federal
Center, Denver, CO 80225.

                 Preliminary Pictorial Map of Callisto

  Atlas of Callisto
  1:25,000,000 Topographic Series
  Jc 25M 2RMN, 1979

This map was compiled from Voyager 1 and 2 pictures of Callisto.
Placement of features is based on predicted spacecraft trajectory data
and is highly approximate. The linkage between Voyager 1 and Voyager 2
pictures is particularly tenuous. Placement errors as large as 10° are
probably common throughout the map, and a few may be even larger.
Feature names were approved by the International Astronomical Union in
1979. Airbrush representation is by P. M. Bridges.

Jc 25M 2RMN: Abbreviation for (Jupiter) Callisto, 1:25,000,000 series,
      second edition, shaded relief map, R, with surface markings, M,
      and feature names, N.

    [Illustration: North polar region; South polar region]

                   Scale 1:13 980 000 at 56° latitude
                     Polar stereographic projection

    [Illustration: Callisto]

                   Scale 1:25 000 000 at 0° latitude
                          Mercator projection

                 Preliminary Pictorial Map of Ganymede

  Atlas of Ganymede
  1:25,000,000 Topographic Series
  Jg 25M 2RMN, 1979

This map was compiled from Voyager 1 and 2 pictures of Ganymede.
Placement of features is based on predicted spacecraft trajectory data
and is highly approximate. The linkage between Voyager 1 and Voyager 2
pictures is particularly tenuous. Placement errors as large as 10° are
probably common throughout the map, and a few may be even larger. A
large unresolved discrepancy exists in the area bounded by the -45° and
-55° parallels between 120° and 180° longitude. Relative placement of
features is distorted in that area. Feature names were approved by the
International Astronomical Union in 1979. Airbrush representation is by
J. L. Inge.

Jg 25M 2RMN: Abbreviation for (Jupiter) Ganymede, 1:25,000,000 series,
      second edition, shaded relief map, R, with surface markings, M,
      and feature names, N.

    [Illustration: North polar region; South polar region]

                   Scale 1:13 980 000 at 56° latitude
                     Polar stereographic projection

    [Illustration: Ganymede]

                   Scale 1:25 000 000 at 0° latitude
                          Mercator projection

                  Preliminary Pictorial Map of Europa

  Atlas of Europa
  1:25,000,000 Topographic Series
  Je 25M 2RMN, 1979

This map was compiled from Voyager 1 and 2 pictures of Europa. Placement
of features is based on predicted spacecraft trajectory data and is
highly approximate. Feature names were approved by the International
Astronomical Union in 1979. Airbrush representation is by J. L. Inge.

Je 25M 2RMN: Abbreviation for (Jupiter) Europa, 1:25,000,000 series,
      second edition, shaded relief map, R, with surface markings, M,
      and feature names, N.

    [Illustration: North polar region; South polar region]

                   Scale 1:13 980 000 at 56° latitude
                     Polar stereographic projection

    [Illustration: Europa]

                   Scale 1:25 000 000 at 0° latitude
                          Mercator projection

                    Preliminary Pictorial Map of Io

  Atlas of Io
  1:25,000,000 Topographic Series
  Ji 25M 2RMN, 1979

This map was compiled from Voyager 1 and 2 pictures of Io. Placement of
features is based on preliminary control information provided by M. E.
Davies of the Rand Corporation, Santa Monica, California, and is
probably accurate within 50 to 100 km. Feature names were approved by
the International Astronomical Union in 1979. Airbrush representation is
by P. M. Bridges.

Ji 25M 2RMN: Abbreviation for (Jupiter) Io, 1:25,000,000 series, second
      edition, shaded relief map, R, with surface markings, M, and
      feature names, N.

    [Illustration: North polar region; South polar region]

                   Scale 1:13 980 000 at 56° latitude
                     Polar stereographic projection

    [Illustration: Io]

                   Scale 1:25 000 000 at 0° latitude
                          Mercator projection

                               APPENDIX B
                         VOYAGER SCIENCE TEAMS

                            Imaging Science

  Bradford A. Smith, University of Arizona, Team Leader
  Geoffrey A. Briggs, NASA Headquarters
  A. F. Cook, Smithsonian Institution
  G. E. Danielson, Jr., California Institute of Technology
  Merton Davies, Rand Corp.
  G. E. Hunt, University College, London
  Tobias Owen, State University of New York
  Carl Sagan, Cornell University
  Lawrence Soderblom, U.S. Geological Survey
  V. E. Suomi, University of Wisconsin
  Harold Masursky, U.S. Geological Survey

                             Radio Science

  Von R. Eshleman, Stanford University, Team Leader
  J. D. Anderson, Jet Propulsion Laboratory
  T. A. Croft, Stanford Research Institute
  Gunnar Lindal, Jet Propulsion Laboratory
  G. S. Levy, Jet Propulsion Laboratory
  G. L. Tyler, Stanford University
  G. E. Wood, Jet Propulsion Laboratory

                              Plasma Wave

  Frederick L. Scarf, TRW Systems, Principal Investigator
  D. A. Gurnett, University of Iowa

                  Infrared Spectroscopy and Radiometry

  Rudolph A. Hanel, Goddard Space Flight Center, Principal Investigator
  B. J. Conrath, Goddard Space Flight Center
  P. Gierasch, Cornell University
  V. Kunde, Goddard Space Flight Center
  P. D. Lowman, Goddard Space Flight Center
  W. Maguire, Goddard Space Flight Center
  J. Pearl, Goddard Space Flight Center
  J. Pirraglia, Goddard Space Flight Center
  R. Samuelson, Goddard Space Flight Center
  Cyril Ponnamperuma, University of Maryland
  D. Gautier, Meudon, France
  S. Kuman, University of Southern California

                        Ultraviolet Spectroscopy

  A. Lyle Broadfoot, Kitt Peak National Observatory, Principal
  J. L. Bertaux, Service d’Aeronomie du CNRS, France
  J. Blamont, Service d’Aeronomie du CNRS, France
  T. M. Donahue, University of Michigan
  R. M. Goody, Harvard University
  A. Dalgarno, Harvard College Observatory
  Michael B. McElroy, Harvard University
  J. C. McConnell, York University, Canada
  H. W. Moos, Johns Hopkins University
  M. J. S. Belton, Kitt Peak National Observatory
  D. F. Strobel, Naval Research Laboratory
  Sushil Atreya, University of Michigan
  William R. Sandel, University of Southern California
  Donald Shemanski, University of Southern California


  Charles W. Hord, University of Colorado, Acting Principal Investigator
  D. L. Coffeen, Goddard Institute for Space Studies
  J. E. Hansen, Goddard Institute for Space Studies
  K. Pang, Science Applications Inc.

                       Planetary Radio Astronomy

  James W. Warwick, University of Colorado, Principal Investigator
  Anthony Riddle, Radiophysics, Inc.
  Jeffrey Pearce, Radiophysics, Inc.
  J. K. Alexander, Goddard Space Flight Center
  A. Boischot, Observatoire de Paris, France
  W. E. Brown, Jet Propulsion Laboratory
  T. D. Carr, University of Florida
  Samuel Gulkis, Jet Propulsion Laboratory
  F. T. Haddock, University of Michigan
  C. C. Harvey, Observatoire de Paris, France
  Y. LeBlanc, Observatoire de Paris, France
  R. G. Peltzer, University of Colorado
  R. J. Phillips, Jet Propulsion Laboratory
  D. H. Staelin, Massachusetts Institute of Technology

                            Magnetic Fields

  Norman F. Ness, Goddard Space Flight Center, Principal Investigator
  Mario H. Acuna, Goddard Space Flight Center
  K. W. Behannon, Goddard Space Flight Center
  L. F. Burlaga, Goddard Space Flight Center
  R. P. Lepping, Goddard Space Flight Center
  F. M. Neubauer, Technische Universitat, F.R.G.

                             Plasma Science

  Herbert S. Bridge, Massachusetts Institute of Technology, Principal
  J. W. Belcher, Massachusetts Institute of Technology
  J. H. Binsack, Massachusetts Institute of Technology
  A. J. Lazarus, Massachusetts Institute of Technology
  S. Olbert, Massachusetts Institute of Technology
  V. M. Vasyliunas, Max Planck Institute, F.R.G.
  L. F. Burlaga, Goddard Space Flight Center
  R. E. Hartle, Goddard Space Flight Center
  K. W. Ogilvie, Goddard Space Flight Center
  G. L. Siscoe, University of California, Los Angeles
  A. J. Hundhausen, High Altitude Observatory

                      Low-Energy Charged Particles

  S. M. Krimigis, Johns Hopkins University, Principal Investigator
  T. P. Armstrong, University of Kansas
  W. I. Axford, Max Planck Institute, F.R.G.
  C. O. Bostrom, Johns Hopkins University
  C. Y. Fan, University of Arizona
  G. Gloeckler, University of Maryland
  L. J. Lanzerotti, Bell Telephone Laboratories

                               Cosmic Ray

  R. E. Vogt, California Institute of Technology, Principal Investigator
  J. R. Jokipii, University of Arizona
  E. C. Stone, California Institute of Technology
  F. B. McDonald, Goddard Space Flight Center
  B. J. Teegarden, Goddard Space Flight Center
  James H. Trainor, Goddard Space Flight Center
  W. R. Webber, University of New Hampshire

                               APPENDIX C
                        VOYAGER MANAGEMENT TEAM

                      NASA Office of Space Science

  Thomas A. Mutch, Associate Administrator for Space Science
  Andrew J. Stofan, Deputy Associate Administrator
  Adrienne F. Timothy, Assistant Associate Administrator
  Angelo Guastaferro, Director, Planetary Division
  Rodney A. Mills, Program Manager
  Milton A. Mitz, Program Scientist
  Walter Jakobowski, Viking and Flight Support Manager

             NASA Office of Space Tracking and Data Systems

  William Schneider, Associate Administrator of Space Tracking and Data
          Systems Acquisition
  Charles A. Taylor, Director, Network Operations and Communication
  Frederick B. Bryant, Director, Network System Development Programs
  Maurice E. Binkley, Director, DSN Systems

              NASA Office of Space Transportation Systems

  John F. Yardley, Associate Administrator for Space Transportation
  Joseph B. Mahon, Director, Expendable Launch Vehicles
  Joseph E. McGolrick, Chief, Small and Medium Launch Vehicles
  B. C. Lam, Titan III Manager

            Jet Propulsion Laboratory, Pasadena, California

  Bruce C. Murray, Laboratory Director
  Gen. Charles H. Terhune, Jr., Deputy Laboratory Director
  Robert J. Parks, Assistant Laboratory Director for Flight Projects
  Raymond L. Heacock, Project Manager
  Esker K. Davis, Deputy Project Manager
  Peter T. Lyman, Deputy Project Manager
  Richard P. Laeser, Mission Director
  George P. Textor, Deputy Mission Director
  Charles E. Kohlhase, Mission Planning Office Manager
  James E. Long, Science Directorate Manager
  Charles H. Stembridge, Deputy
  Arthur L. Lane, Assistant Project Scientist for Jupiter
  Francis M. Sturms, Sequence Design and Integration Directorate Manager
  Robert K. Wilson, Deputy
  Michael J. Sander, Development, Integration and Test Directorate
  Robert G. Polansky, Deputy
  Michael W. Devirian, Space Flight Operations Directorate Manager
  Raymond J. Amorose, Deputy
  Marvin R. Traxler, Tracking and Data System Manager
  Kurt Heftman, Mission Control and Computing Center Manager

        California Institute of Technology, Pasadena, California

  Edward C. Stone, Project Scientist

                           ADDITIONAL READING


_Jupiter_, T. Gehrels, Ed., U. of Arizona Press, Tucson, 1254 pages

_Space Science Reviews_, special Voyager instrumentation issues, _Vol.
21_, No. 2, pgs. 75-232 (November 1977); _Vol. 21_, No. 3, pgs. 234-376
(December 1977).

“Melting of Io by Tidal Dissipation,” by S. J. Peale, P. Cassen, and R.
T. Reynolds, _Science, Vol. 203_, pgs. 892-894 (2 March 1979).

_Science_, special Voyager 1 issue, _Vol. 204_, pgs. 945-1008 (1 June

_Nature_, special Voyager 1 issue, _Vol. 280_, pgs. 725-806 (30 August

“Jupiter’s Ring,” by T. Owen et al., _Nature, Vol. 781_, pgs. 442-446
(11 October 1979).

_Science_, special Voyager 2 issue, _Vol. 206_, pgs. 925-996 (23
November 1979).

_Geophysical Research Letters_, special Voyager issue, _Vol. 7_, pgs.
1-68 (January 1980).


“The Solar System,” special issue of _Scientific American, Vol. 223_,
No. 3 (September 1975).

“The Galilean Satellites of Jupiter,” by D. P. Cruikshank and D.
Morrison, _Scientific American, Vol. 234_, No. 5, pgs. 108-116 (May

_Pioneer Odyssey_, by R. O. Fimmel, W. Swindell, and E. Burgess, NASA
SP-396, 217 pages (1977).

_Murmurs of the Earth: The Voyager Interstellar Record_, by Carl Sagan
et al., Random House, New York, 1978.

“Jupiter and Family,” by J. Eberhart, _Science News, Vol. 115_, pgs.
164-173 (17 March 1979).

“The Far-Out Worlds of Voyager,” by J. K. Beatty, _Sky and Telescope,
Vol. 57_, pgs. 423-427 (May 1979) and pgs. 516-520 (June 1979).

“Return to Jupiter and Co.,” by J. Eberhart, _Science News, Vol. 116_,
pgs. 19-21 (14 July 1979).

“Voyage to the Giant Planet,” by C. Sutton, _New Scientist, Vol. 83_,
pgs. 217-220 (19 July 1979).

“Voyager Views Jupiter’s Dazzling Realm,” by R. Gore, _National
Geographic, Vol. 157_, No. 1, pgs. 2-29 (January 1980).

“The Galilean Moons of Jupiter,” by L. A. Soderblom, _Scientific
American, Vol. 242_, No. 1, pgs. 88-100 (January 1980).

“The Great Red Spot,” by D. Schwartzenburg, _Astronomy, Vol. 8_, pgs.
6-13 (July 1980).

“Four New Worlds,” by D. Morrison, _Astronomy, Vol. 8_ (September 1980).

    [Illustration: National Aeronautics and Space Administration]

                          Transcriber’s Notes

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—Silently corrected a few palpable typos.

—Moved captions nearer the relevant images; tweaked image references
  within captions accordingly.

—In the text versions only, text in italics is delimited by

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