Voyager Encounters Jupiter

Foreword

In late summer of 1977, the United States launched two unmanned Voyager spacecraft on an extensive reconnaissance of the outer planets, a decade-long odyssey that could take them to 3 planets and as many as 18 planetary satellites. The first encounter was with the giant Jovian planetary system, 645 million kilometers (400 million miles) away. Passing by Jupiter and its complex satellite system in 1979, the Voyager spacecraft have collected and returned to Earth an enormous amount of data and information that may prove to be a keystone in understanding our solar system.

This publication provides an early look at the Jovian planetary system and contains a selected sample from the more than 30,000 images collected during this phase of the Voyager mission. While Voyager achieved an impressive record of accomplishments, full realization of the scientific value of this program must await the remaining Voyager encounters with Saturn and perhaps Uranus, and a detailed analysis of the data from all the spacecraft investigations.

Robert A. Frosch, Administrator National Aeronautics and Space Administration

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Introduction

In March 1979 Voyager 1 swept past Jupiter, photographing both the giant planet and five of its moons. Four months later, a companion spacecraft, Voyager 2, made a similar encounter. Now, with Jupiter receding behind them, both spacecraft are headed toward the outer reaches of our solar system. In November 1980, Voyager 1 will fly past Saturn. Voyager 2, traveling at slower speeds, will reach the same way station in August 1981. Beyond there, the itinerary is less certain. In January 1986, eight years after its departure from Earth, Voyager 2 may sail within range of Uranus, taking closeup pictures of that distant planet for the first time. Long after they have exhausted their fuel supplies and their radios have fallen silent, both spacecraft will continue their traverse through space and beyond our solar system, on an endless journey.

Preliminary results of the Voyager encounters with Jupiter are presented in this booklet. As you examine the pictures, you will be participating in a revolutionary journey of exploration. Living in a society where many accomplishments and products are billed as “extraordinary,” “stupendous,” “once in a lifetime,” or “unique,” we sometimes lose our perspective. Conditioned to hyperbole, we fail to recognize those advances that are truly exceptional. We need a historian’s vantage point to identify the events that can literally change the course of civilization. So it is that every student of history recognizes the importance of the Renaissance, an extraordinary time when man looked outward, reaching beyond the traditions of the past to study his place in the natural world. The results were apparent in art, architecture, and literature, in new philosophic and governmental systems, and in the staggering scientific revolution exemplified by Galileo’s first examination of the heavens with a telescope, and in his stubborn support of the heretical assertion that the Earth was not the center of the solar system.

Historians writing a hundred or two hundred years from now may well look on the latter part of the twentieth century as another turning point in civilization. For the first time, we explored beyond Earth—first the Moon, then the neighboring planets, and finally the outermost planets, the very fringe of our solar system.

How will the historian evaluate this period of exploration? First, perhaps, he will describe the Apollo program as a visionary example of great cooperative ventures that can be accomplished by many individuals, private companies, and government institutions. He will describe the subsequent space ventures that weave a fabric of cooperation and goodwill between nations.

He will point out the technological advances incorporated in unmanned spacecraft, sophisticated robots able to control their own activities and solve their own problems. He will mention the revolution in microelectronics—the art of fabricating complex electrical control circuits so small the eye cannot perceive them, a revolution accelerated by the requirement to conserve weight and generate performance in interplanetary spacecraft. He will point to the introduction of new products, particularly in areas of communication, medical treatment, and energy conversion.

Turning his attention to the environment, the historian will almost surely suggest that the first widespread realization of the fragile natural balances on Earth came at a time when we were first able to see our Earth in its entirety. The impact of a picture of Earth from deep space, a luminously blue globe surrounded by darkness, has probably been more persuasive than lengthy treatises describing the complex ways in which our system of rocks, plants, animals, water, and air is interrelated. On a more practical level, the historian will point to the new understanding of our terrestrial environment. The composition and structure of other planetary atmospheres—on Venus, Mars, and Jupiter—provide important clues to what may happen in our own atmosphere, especially if we disrupt the chemical composition. Study of the primitive crusts of the Moon, Mars, and Mercury permits us to reconstruct the first billion years of Earth history, a time when chemical elements were being concentrated in activity ultimately leading to the formation of important ore deposits. Unmanned spacecraft missions to the Sun increase our understanding of that most fundamental of all energy sources, paving the way for the efficient conversion of solar energy into many practical applications, and releasing us from dependence on ever-decreasing reserves of fossil fuels. Spacecraft circling the Earth study the upper atmospheric processes that play major roles in controlling our weather. These same spacecraft look down on Earth, aiding us with increasingly accurate forecasts of weather and crop productivity.

Looking beyond matters of technology and the environment, the historian may cite the latter part of the twentieth century as a time of explosive exploration, comparable to the 15th and 16th century exploration of the Earth’s oceans and the distant lands that bounded them. In a sense, exploration—whether it is physical or intellectual—provides its own rewards. The United States has always been a nation that moves forward, pushing back the frontiers of the West, pushing back the frontiers of social and economic development, and now pushing back the frontiers of space. It is arguable that this spirit of exploration is indispensable to a vigorous society, and that any society that ceases to explore, to inquire, and to strive is only a few years from decline.

And so the historian may recall the early days of lunar exploration, the Apollo project, the landing of unmanned Viking spacecraft on Mars, and the encounters of Voyager spacecraft with Jupiter and Saturn as the first steps in a sustained program of space exploration—a program that is profoundly changing man’s perspective of himself, of the Earth, and of the larger cosmos beyond.

Thomas A. Mutch, Associate Administrator for Space Science National Aeronautics and Space Administration

Images of Jupiter and Its Satellites

The date of each photograph and the distance of the spacecraft from the planet or satellite are included with each picture.

The Voyager Mission

The Voyager mission is focused on the exploration of the Jupiter and Saturn systems. The alignment of these large planets permits the use of a gravity-assist trajectory in which the gravity field of Jupiter and Jupiter’s motion through space may be used to hurl the spacecraft on to Saturn. In 1977, a rare alignment (once every 176 years) of our four outer planets—Jupiter, Saturn, Uranus, and Neptune—may permit a gravity-assist trajectory to Uranus and even to Neptune for Voyager 2.

Voyagers 1 and 2 began their journeys in the late summer of 1977, catapulted into space by a Titan/Centaur launch vehicle from Cape Canaveral, Florida. With them went the hopes and dreams of thousands of people who had worked to create them and their mission.

The Voyager spacecraft are unique in many respects. Since their journeys are taking them far from the Sun, the Voyagers are nuclear powered rather than solar powered. The Voyagers are the fastest man-made objects ever to have left Earth. In fewer than ten hours, they had crossed the Moon’s orbit. This compares to about three days for an Apollo flight and one day for the Mariner and Viking spacecraft. Their launches marked the end of an era in space travel—the end of the planned use of Titan/Centaur launch vehicles. With the advent of the Space Shuttle in the 1980s, future spacecraft will be launched from the Shuttle Orbiter.

Voyager 1 was launched 16 days after its sister ship, but because of a different trajectory, it arrived at Jupiter four months ahead of Voyager 2. Both spacecraft spent more than nine months crossing the asteroid belt, a vast ring of space debris circling the Sun between the orbits of Mars and Jupiter. During their 16- and 20-month journeys to Jupiter, the spacecraft tested and calibrated all of their instruments, exercised their scan platforms, and measured particles and fields in interplanetary space. As the spacecraft neared the planet, the cameras showed the dramatic visible changes that had taken place in the five years since Jupiter had been photographed by Pioneer 11. And for the first time, we got a close look at some of Jupiter’s moons: Amalthea, Io, Europa, Ganymede, and Callisto.

Targeted for the closest look at Io, Voyager 1 flew the more hazardous course, passing between Jupiter and Io, where the radiation environment is the most intense. Voyager 2’s flight path gave Jupiter and its intense radiation a much wider berth. Unlike Voyager 1, which encountered the five innermost satellites as it was leaving Jupiter, Voyager 2 encountered the satellites as it was approaching the planet, thus providing closeup photography of opposite sides of the satellites.

Arriving at Jupiter from slightly different angles, both spacecraft measured the large, doughnut-shaped ring of charged sulfur and oxygen ions, called a torus, encircling the planet at about the orbit of Io. Then, both spacecraft disappeared behind Jupiter, out of view of Earth and Sun, for about two hours. During this time, measurements were taken on the planet’s dark side. Each spacecraft took over 15,000 photographs of Jupiter and its satellites.

From the moment of launch, the Voyager spacecraft have been monitored by a worldwide tracking system of nine giant antennas strategically located around the world in California, Spain, and Australia to ensure constant radio contact with the spacecraft as the Earth rotates. Radio contact with Voyagers 1 and 2 has not been instantaneous, however. When Voyager 1 flew past Jupiter, radio signals between Earth and the spacecraft took 37 minutes; when Voyager 2 arrived, the signals took 52 minutes because by then the planet was farther from Earth.

The pictures in this book were taken by a shuttered television-type camera. Each picture is composed of 640,000 dots, which were converted into binary numbers before being radioed to Earth. When the signals reached Earth, they were reconverted by computer into dots and reassembled into the original image. Most of the color pictures are composed of three images, each one taken through a different color filter: blue, orange, or green. The images were combined and the original color was reconstructed by computer. The computer eliminated many of the imperfections that crept into the images, and enhanced some of the images by emphasizing different colors.

Designed to provide a broad spectrum of scientific investigations at Jupiter, the science instruments investigated atmospheres, satellites, and magnetospheres. The scientific investigations for the Voyager mission and their Jovian encounter objectives are shown in the table on page 40.

After their closest approaches to Jupiter, both spacecraft fired their thrusters, retargeting for their next goal, the Saturn system. Scientists will still be studying the wealth of new information about Jupiter when Voyager 1 reaches Saturn in November 1980, and Voyager 2 follows in August 1981. After Voyager 1 encounters Saturn, Voyager 2 may be retargeted to fly past Uranus in 1986. Upon completion of their planetary missions, both spacecraft will search for the outer limit of the solar wind, that boundary somewhere in our part of the Milky Way where the influence of the Sun gives way to other stars of the galaxy. Voyagers 1 and 2 will continue to study interstellar space until the spacecraft signals can no longer be received.

Scientific Highlights

Some of the most important information gathered by Voyagers 1 and 2 on the Jovian system is presented pictorially in this book and is supplemented here with brief summaries of the major discoveries, observations, and theories.

Jupiter

The atmosphere of Jupiter is colorful, with cloud bands of alternating colors. A major characteristic of the atmosphere is the appearance of regularly spaced features. Around the northern edge of the equator, a train of plumes is observed, which has bright centers representative of cumulus convection similar to that seen on Earth. At both northern and southern latitudes, cloud spots are observed spaced almost all the way around the planet, suggestive of wave interactions. The cloud structures in the northern and southern hemispheres are distinctly different. However, the velocities between the bright zones and dark belts appear to be symmetric about the equator, and stable over many decades. This suggests that such long-lived and stable features may be controlled by the atmosphere far beneath the visible clouds. The Great Red Spot possesses the same meteorological properties of internal structure and counterclockwise rotation as the smaller white spots. The color of the Great Red Spot may indicate that it extends deep into the Jovian atmosphere. Cloud-top lightning bolts, similar to those on Earth, have also been found in the Jovian atmosphere. At the polar regions, auroras have been observed. A very thin ring of material less than one kilometer (0.6 mile) in thickness and about 6000 kilometers (4000 miles) in radial extent has been observed circling the planet about 55,000 kilometers (35,000 miles) above the cloud tops.

Amalthea

Amalthea is an elongated, irregularly shaped satellite of reddish color. It is 265 kilometers (165 miles) long and 150 kilometers (90 miles) wide. Just like the large Galilean satellites, Amalthea is in synchronous rotation, with its long axis always oriented toward Jupiter. At least one significant color variation has been detected on its surface.

Io

Eight active volcanoes have been detected on Io, with some plumes extending up to 320 kilometers (200 miles) above the surface. Over the four-month interval between the Voyager 1 and 2 encounters, the active volcanism appears to have continued. Seven of the volcanoes were photographed by Voyager 2, and six were still erupting.

The relative smoothness of Io’s surface and its volcanic activity suggest that it has the youngest surface of Jupiter’s moons. Its surface is composed of large amounts of sulfur and sulfur dioxide frost, which account for the primarily yellow-orange surface color. The volcanoes seem to eject a sufficient amount of sulfur dioxide to form a doughnut-shaped ring (torus) of ionized sulfur and oxygen atoms around Jupiter near Io’s orbit. The Jovian magnetic field lines that go through the torus allow particles to precipitate into the polar regions of Jupiter, resulting in intense ultraviolet and visible auroras.

Europa

Europa, the brightest of Jupiter’s Galilean satellites, may have a surface of thin ice crust overlying water or softer ice, with large-scale fracture and ridge systems appearing in the crust. Europa has a density about three times that of water, suggesting it is a mixture of silicate rock and some water. Very few impact craters are visible on the surface, implying a continual resurfacing process, perhaps by the production of fresh ice or snow along cracks and cold glacier-like flows.

Ganymede

Ganymede, largest of Jupiter’s 13 satellites, has bright “young” ray craters; light, linear stripes resembling the outer rings of a very large, ancient impact basin; grooved terrain with many faults; and regions of dark, heavily cratered terrain. Among the Galilean satellites, Ganymede probably has the greatest variety of geologic processes recorded on its surface and may be the best example for studying the evolution of Jupiter’s inner satellites. Imbedded within Jupiter’s magnetosphere, Ganymede is subjected to the influences of the corotating charged-particle plasma and an interaction may exist with this plasma. No atmosphere has been detected.

Callisto

The icy, dirt-laden surface of Callisto appears to be very ancient and heavily cratered. The large concentric rings indicate the remains of several enormous impact basins, created by huge meteors crashing into the surface, and since erased by the flow of the crust. Callisto’s density (less than twice that of water) is very close to that of Ganymede, yet there is little or no evidence of the crustal motion and internal activity that is visible on Ganymede.

The Magnetosphere

Perhaps the largest structure in the solar system is the magnetosphere of Jupiter. This is the region of space which is filled with Jupiter’s magnetic field and is bounded by the interaction of that magnetic field with the solar wind, which is the Sun’s outward flow of charged particles. The plasma of electrically charged particles that exists in the magnetosphere is flattened into a large disk more than 4.8 million kilometers (3 million miles) in diameter, is coupled to the magnetic field, and rotates around Jupiter. The Galilean satellites are located in the inner regions of the magnetosphere where they are subjected to intense radiation bombardment. It appears that Io is a source of the sulfur and oxygen ions which fill the magnetosphere. Another magnetospheric interaction is the electrical connection between Io and Jupiter along the magnetic field lines that leave Jupiter and intersect Io. This magnetic flux tube was examined by Voyager 1 and a flow of about five million amperes of current was measured, which was considerably more than anticipated. Voyager also discovered a new low-frequency radio emission coming from Jupiter, which is possibly associated with the Io torus.

Scientific investigations of the Voyager mission

Investigation Typical Jovian encounter objectives

Imaging science High-resolution reconnaissance over large phase angles; atmospheric dynamics; geologic structure of satellites Infrared radiation Atmospheric composition, thermal structure and dynamics; satellite surface composition and thermal properties Photopolarimetry Atmospheric aerosols; satellite surface texture and sodium cloud Radio science Atmospheric and ionospheric structure, constituents, and dynamics Ultraviolet spectroscopy Upper atmospheric composition and structure; auroral processes; distribution of ions and neutral atoms in the Jovian system Magnetic fields Planetary magnetic field; magnetospheric structure; Io flux tube currents Plasma particles Magnetospheric ion and electron distribution; solar wind interaction with Jupiter; ions from satellites Plasma waves Plasma electron densities; wave-particle interactions; low-frequency wave emissions Planetary radio astronomy Polarization and spectra of radio frequency emissions; Io radio modulation process; plasma densities Low-energy charged particles Distribution, composition, and flow of energetic ions and electrons; satellite-energetic particle interactions Cosmic ray particles Distribution, composition, and flow of high-energy trapped nuclei; energetic electron spectra

National Aeronautics and Space Administration

Jet Propulsion Laboratory California Institute of Technology Pasadena, California

JPL 400-24 7/79

Transcriber’s Notes

—Silently corrected a few palpable typos.

—Moved captions nearer the relevant images; tweaked image references within captions accordingly.

—Added a Table of Contents.

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