Part 1

Voyager Encounters Jupiter

July 1979

NASA National Aeronautics and Space Administration

CONTENTS

Foreword 3 Introduction 4 Images of Jupiter and Its Satellites 6 The Voyager Mission 37 Scientific Highlights 39 Jupiter 39 Amalthea 39 Io 39 Europa 39 Ganymede 39 Callisto 39 The Magnetosphere 40 Scientific investigations of the Voyager mission 40

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

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.

Jupiter is the largest planet in our solar system, with a diameter 11 times that of Earth. Jupiter rotates very quickly, making one full rotation in just under ten hours. Composed primarily of hydrogen and helium, Jupiter’s colorfully banded atmosphere displays complex patterns highlighted by the Great Red Spot, a large, circulating atmospheric disturbance. Three of Jupiter’s 13 known satellites are also visible in this Voyager 1 photograph. The innermost large satellite, Io, can be seen in front of Jupiter and is distinguished by its bright, orange surface. To the right of Jupiter is Europa, also very bright but with fainter surface markings. Callisto is barely visible beneath Jupiter. These satellites orbit Jupiter in the equatorial plane and appear in their present position because Voyager is above the plane.]

The Great Red Spot on Jupiter is a tremendous atmospheric storm, twice the size of Earth, that has been observed for centuries. The Great Red Spot rotates counterclockwise with one revolution every six days. Wind currents on the top flow east to west, and currents on the bottom flow west to east. This Voyager 1 picture shows the complex flow and turbulent patterns that result from the Great Red Spot’s interactions with these flows. The large white oval is a similar, but smaller, storm center that has existed for about 40 years.]

A comparison of the Voyager 2 photograph above with the preceding Voyager 1 photograph shows several distinct changes in the Jovian atmosphere around the Great Red Spot. The white oval beneath the Great Red Spot in the first picture has moved farther around Jupiter, and a different white oval has appeared under the Great Red Spot in the Voyager 2 picture taken four months later. The disturbed cloud regions around the Great Red Spot have noticeably changed, and the white zone west of the Great Red Spot has narrowed.]

High-speed wind currents in the mid-latitudes of Jupiter are shown in this high-resolution Voyager 1 photograph. The pale orange line running diagonally to the upper right is the high-speed north temperate current with a wind speed of about 120 meters per second (260 miles per hour), over twice as fast as severe hurricane winds on Earth. Toward the top of the picture, a weaker jet of approximately 30 meters per second (65 miles per hour) is characterized by wave patterns and cloud features that rotate in a clockwise manner.]

The large brown-colored oval appearing in this Voyager 1 picture was selected as one of the targets to be photographed near closest approach to Jupiter because it is probably an opening in the upper cloud deck that exposes deeper, warmer cloud levels. Brown ovals (which can also be seen in the preceding and following photographs) are common features in Jupiter’s northern latitudes and have an average lifetime of one to two years.]

Jupiter’s Equatorial Zone is the broad, orange band that traverses the center of this Voyager 2 picture. This zone is characterized by the wispy clouds along its northern edge. The brown oval was observed by Voyager 1 four months earlier, illustrating the stability of this type of feature in the Jovian atmosphere. In contrast, the turbulent region in the lower right of the picture, which lies just to the left of the Great Red Spot, shows features that are relatively short lived. With the exception of the cooler Great Red Spot, as colors range from white to orange to brown, we are generally looking at deeper and warmer layers in the Jovian atmosphere.]

This Voyager 1 picture was also taken the same day, about one hour after the infrared image.]