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. [P-21334C]

NASA SP-439

Voyage To Jupiter

David Morrison
and Jane Samz

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

FOREWORD

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

INTRODUCTION

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 generations.

April 1980

Edward C. Stone

Voyager Project Scientist

ACKNOWLEDGEMENT

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.

CONTENTS

[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

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

Introduction

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 second).

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.

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?

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]

N 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 S

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.

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.

July 25, 1973

October 5, 1974

October 2, 1975

November 20, 1976.

January 28, 1978.

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 vapor.

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 ago.

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]

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 investigation.

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 effect.

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 each.

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 Jupiter.

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 present.

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.

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]

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.

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 needed.

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 Tour.

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.

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 matter.

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 planet.

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 taken.

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 system.

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 him.

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 Jupiter.

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.

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 counterclockwise.

Pioneer 11 Pioneer 10 Callisto Ganymede Europa Io Amalthea

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 beautifully.

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 thunderclouds.

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 experiment.

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.

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 0

PIONEER SCIENCE INVESTIGATIONS
Project Scientist: J. H. Wolfe, NASA Ames

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.

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.

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.

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.

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 program:

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.

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.

Project Scientist Edward C. Stone

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).

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.

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 Magnetometer 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 Photopolarimeter Infrared interferometer spectrometer and radiometer Low energy charged particles Thrusters 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.

Project Manager Ray Heacock

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.

VOYAGER’S GREETINGS TO THE UNIVERSE

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

Sumerian Akkadian Hittite Hebrew Aramaic English Portuguese Cantonese Russian Thai Arabic Roumanian French Burmese Spanish Indonesian Kechua Dutch German Bengali Urdu Hindi Vietnamese Sinhalese Greek Latin Japanese Punjabi Turkish Welsh Italian Nguni Sotho Wu Korean Armenian Polish Netali Mandarin Gujorati Ila (Zambia) Nyanja Swedish Ukrainian Persian Serbian Luganada Amoy (Min dialect) Marathi Kannada Telugu Oriya Hungarian Czech Rajasthani

SOUNDS OF EARTH ON VOYAGER RECORD

Whales Planets (music) Volcanoes Mud pots Rain Surf Crickets, frogs Birds Hyena Elephant Chimpanzee Wild dog Footsteps and heartbeats Laughter Fire Tools Dogs, domestic Herding sheep Blacksmith shop Sawing Riveter Morse code Ships Horse and cart Horse and carriage Train whistle Tractor Truck Auto gears Jet Lift-off Saturn 5 rocket Kiss Baby Life signs—EEG, EKG Pulsart

VOYAGER RECORD PHOTOGRAPH INDEX

Calibration circle Solar location map Mathematical definitions Physical unit definitions Solar system parameters The Sun Solar spectrum Mercury Mars Jupiter Earth 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 Conception Fertilized ovum Fetus diagram Fetus Diagram of male and female Birth 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) Seashore Snake River and Grand Tetons Sand dunes Monument Valley Forest scene with mushrooms Leaf Fallen leaves Sequoia Snowflake Tree with daffodils Flying insect with flowers Diagram of vertebrate evolution Seashell (Xancidae) Dolphins School of fish Tree toad Crocodile Eagle Waterhold Jane Goodall and chimps Sketch of Bushmen Bushmen hunters Man from Guatemala Dancer from Bali Andean girls Thailand craftsman Elephant Old man with beard and glasses (Turkey) Old man with dog and flowers Mountain climber Cathy Rigby Sprinters Schoolroom Children with globe Cotton harvest Grape picker Supermarket 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) Boston UN Building, day UN Building, night Sydney Opera House Artisan with drill Factory interior Museum X-ray of hand Woman with microscope Street scene, Asia (Pakistan) Rush hour traffic, India Modern highway (Ithaca) Golden Gate Bridge Train 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) Chakrulo 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”

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 BRAINS OF THE VOYAGER SPACECRAFT[1]

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 conditions.

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.

Project Manager Robert Parks

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

Introduction

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 mission.

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 objects.

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 wavelengths.

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 observations.

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.

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 Photopolarimeter Low energy charged particles detector Infrared interferometer spectrometer and radiometer Ultraviolet spectrometer Thrusters Star trackers Radioisotope thermoelectric generators Science instrument calibration panel and shunt radiator Thrusters Planetary radio astronomy and plasma wave antenna

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 medium.

The Voyager Imaging Science Team

Imaging

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.

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.

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.

Rudolph Hanel, infrared spectrometer Principal Investigator

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 lines.

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.

Lyle Broadfoot, ultraviolet spectrometer Principal Investigator

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.