The twin Voyager spacecraft achieved their primary mission of exploration of the mammoth planets of Jupiter and Saturn with dramatic success, thereby changing and deepening scientists’ understanding of these planets and their satellites. When the Voyager mission was extended to include the distant planets Uranus and Neptune, new and surprising data were collected on these gas giants and their moons.
The United States made a preliminary decision to explore the outer planets of the solar system in 1969, when Americans first landed on the Moon, and subsequent events leading up to the Voyager missions were influenced by the “space race” between the United States and the Soviet Union. The United States had undertaken earlier planetary missions in response to Soviet missions; for example, both had competed in explorations of Venus and Mars. During the 1960’s, however, an emphasis on manned spaceflight led to the subordination of planetary programs in the United States. Many space scientists wanted to explore the outer planets, but projects with this aim languished until the late 1960’s, when the National Aeronautics and Space Administration (NASA) appointed a management team to study the details of carrying out such missions. Little was done for some time in the area of long-term planetary missions, however, because of budget cuts that were imposed on NASA after the shortened Apollo Moon program.
1977-Oct. 2, 1989: Voyagers 1 and 2 Explore the Outer Planets[02940]
When astronomers pointed out that the planets of Jupiter, Saturn, Uranus, Neptune, and Pluto would all be lined up in the late 1970’s in a unique arrangement that would not recur for another 179 years, enthusiasm for an “outer planets Grand Tour” escalated among scientists. An indication of the level of enthusiasm was the fact that more than five hundred scientists submitted proposals for projects after NASA officials began planning such a tour, in which spacecraft would use the assist provided by Jupiter’s gravity to travel on to Saturn, with similar assists for travel to Uranus, Neptune, and Pluto. In 1972, NASA appointed Edward C. Stone, a magnetospheric physicist at the Jet Propulsion Laboratory
Early plans specified twin launches to Jupiter, Saturn, and Pluto in 1976 and 1977 and twin launches of spacecraft to Jupiter, Uranus, and Neptune in 1979. This four-spacecraft, five-planet mission would cost $750 million. Reductions in NASA’s budget, however, necessitated a rethinking of the project with more modest goals. The new mission, initially named Mariner Jupiter-Saturn, was to cost $250 million. It retained the idea of a dual launch, but now the purpose was a flyby of Jupiter and then of Saturn. In 1977, the mission was renamed Voyager.
NASA’s space engineers designed the Voyager spacecraft to be larger, sturdier, and more sophisticated than the Mariner and Pioneer spacecraft of previous planetary explorations. Voyager 1 and Voyager 2 were twins, each weighing 1,808 pounds (815 kilograms) and each carrying an array of scientific instruments and cameras controlled by versatile computers that were sufficiently flexible to be reprogrammed as unexpected conditions dictated.
A Titan III-E Centaur rocket, NASA’s most powerful, propelled Voyager 2 into its long journey on August 20, 1977, followed sixteen days later by a similar launch of Voyager 1. Even though Voyager 1 left Earth later than Voyager 2, its trajectory, which was different from that of its twin, meant that it would arrive at Jupiter more than four months ahead of Voyager 2. The long trip was not without its problems. For example, Voyager 2’s primary radio receiver failed, and, because of a faulty capacitor, its backup could communicate with Earth only via a reduced bandwidth. Despite these and other problems, the Voyagers safely traversed the asteroid belt between Mars and Jupiter, and they serially arrived at Jupiter in good enough shape to send valuable information back to the Deep Space Network
So many color images were received on Earth that scientists were able to create a movie of Jupiter’s restless atmosphere. Cloud disturbances rotated clockwise in the planet’s northern hemisphere and counterclockwise in the southern hemisphere. The most dramatic disturbance was the southern hemisphere’s Great Red Spot, so large that it could comfortably encompass six Earths, which had an outer-edge rotation of four to six days, with smaller, random motions in its center. The Jovian atmosphere also exhibited lightning flashes and, near the poles, auroral displays in both ultraviolet and visible light.
Image of Saturn’s rings captured by Voyager 2 in 1981.
The biggest surprises from the Voyager visits occurred when the spacecraft’s instruments were trained on Jupiter’s moons. On Io, whose brightly colored surface of yellow, orange, red, brown, and white reminded observers of a pizza, several active volcanoes were discovered that were comparatively more numerous and energetic than those on Earth. Four months later, when Voyager 2 arrived at Jupiter and Io, six of these volcanoes were still erupting. Although not as spectacular, Jupiter’s other moons proved to be intriguingly different from one another. Ganymede, revealed by Voyager to be the solar system’s largest satellite, had a variegated surface of mountains, valleys, basins, and grooved terrain. Europa, most extensively photographed by Voyager 2, was the solar system’s smoothest object. Its whitish surface was crisscrossed with many lines, which scientists interpreted as depressions in a relatively thin crust of ice over a great ocean. Callisto, the fourth of the moons first seen by Galileo in the seventeenth century, became a candidate for the most cratered satellite in the solar system. To cap off these and other Jovian discoveries, the Voyagers found that Jupiter had a system of thin rings that had never been seen from Earth.
As planned, with a successful gravity assist from Jupiter, Voyagers 1 and 2 traveled on to fly by Saturn in 1980 and 1981, respectively. During the trip, NASA officials decided that Voyager 1 would not go on to explore Pluto but instead would concentrate on a detailed survey of Saturn and Titan, Saturn’s largest moon, and Voyager 2 would continue on to explore Uranus and Neptune. The instruments aboard Voyager 1 showed Saturn’s atmosphere to be very different from Jupiter’s; for example, Saturn’s zonal winds were four times faster than those on Jupiter. However, Voyager’s most spectacular discoveries concerned Saturn’s rings, which proved to be more extensive and diverse than previous observation through telescopes on Earth had revealed. The five rings visible from Earth turned, in Voyager’s images, into hundreds. Spokes spread radially outward across the B ring, and the F ring appeared to be braided. By the time Voyager 2 arrived, the F ring had five strands instead of the earlier-observed three.
Saturn’s moons, the number of which was expanded to eighteen by Voyager, contained fewer surprises than those of Jupiter, but they turned out to have their own fascination. For example, Titan, now the solar system’s second-largest satellite, whose nitrogen atmosphere is tinted orange by methane, was found to have a surface pressure one and one-half times that of Earth. One Saturnian satellite, Mimas, was found to have an enormous crater that is one-third the diameter of the moon itself (it reminded some observers of the Death Star in George Lucas’s film Star Wars). Following their encounter with Saturn, Voyager 1 left the plane of the ecliptic (the plane in which Earth orbits the Sun) while Voyager 2 remained within it to travel to Uranus and Neptune.
Voyager 2 encountered Uranus early in 1986, returning detailed images and other data on the planet, its moons, magnetic field, and dark rings. Uranus, which rotates nearly perpendicular to the Sun, was found to have a peculiar magnetic field that is tilted 59 degrees with respect to the planet’s axis of rotation. This makes it the most tilted magnetic field in the solar system. Voyager 2 also found new Uranian rings and ten new Uranian moons. It also revealed many details about the five large moons that had been visible from Earth. For example, Miranda,
When Voyager 2 arrived at Neptune in 1989, the planet was the solar system’s most distant member, because Pluto’s eccentric orbit had taken it closer to the Sun at that time. Voyager 2’s instruments showed that Neptune has an energetic weather system similar to Jupiter’s, including a Great Dark Spot, an anticyclonic system below the equator. Neptune’s magnetic field is also tilted from the planet’s spinning axis, although not as much as Uranus’s magnetic field. Voyager 2 discovered four new moons orbiting Neptune, but its most surprising discoveries concerned Triton, Neptune’s largest satellite. Triton
In addition, Voyager 2 was able to resolve a question some astronomers had about Neptune’s rings: Were they arcs or complete rings? Voyager’s instruments revealed them to be complete rings with intermittent beads of dark material. This finding naturally raised another question: Why had these rings not become uniformly smooth with time?
At its start, the Voyager mission was called the most ambitious project of its kind ever undertaken; by its end, it was recognized as one of the most successful projects in NASA’s space program. Indeed, with its extension to Uranus and Neptune, Voyager became, in the words of one scientist, “the most productive single planetary mission of all time.” It was the first to encounter, explore, and characterize many bodies in the solar system, and the massive amount of information both Voyagers beamed back to Earth has kept astronomers busy for decades.
By the time Voyager 2 had passed Neptune, the entire Voyager mission had cost $865 million. Most scientists felt that the new knowledge that had been gained more than justified the expense. Unfortunately, however, like the Apollo program before it, Voyager created a hiatus in planetary exploration. Because of decreased funding, Edward Stone stated, planetary exploration had reached a “critical point” below which scientists did not have viable missions. Nevertheless, the Voyager spacecraft have continued to travel beyond our planetary system at the rate of a million miles per day, beaming back data as they journey toward the heliopause, where interstellar space begins.
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Burgess, Eric. By Jupiter: Odysseys to a Giant. New York: Columbia University Press, 1982. After providing accounts of the Pioneer 10 and 11 missions, focuses on the explorations of Jupiter by Voyagers 1 and 2, with particular emphasis on the Voyager science experiments. Includes photographs and index. (The author continues the Voyager story in his Far Encounter: The Neptune System, 1991.) -
Burrows, William E. Exploring Space: Voyages in the Solar System and Beyond. New York: Random House, 1990. A science journalist examines the social and political context of space projects such as the Voyager mission. Includes extensive bibliography and index. -
Miner, Ellis D. Uranus: The Planet, Rings, and Satellites. 2d ed. New York: John Wiley & Sons, 1998. Presents a detailed description of what is known about Uranus, with a particular emphasis on the Voyager missions. Illustrated with many photographs and diagrams. Includes chapter bibliographies, suggestions for further reading, and index. -
Murray, Bruce. Journey into Space: The First Thirty Years of Space Exploration. New York: W. W. Norton, 1989. Provides a behind-the-scenes look at various space missions, including Voyager. The author was director of the Jet Propulsion Laboratory of the California Institute of Technology from 1976 to 1982. Includes bibliographic references and index. -
Washburn, Mark. Distant Encounters: The Exploration of Jupiter and Saturn. New York: Harcourt Brace Jovanovich, 1983. A science journalist tells the story of the Voyager explorations of the two largest planets and their satellites in terms accessible to general readers. Extensively illustrated with color and black-and-white photographs. Includes index.
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