Dates: Beginning on August 20, 1977
Definition: An uncrewed American exploratory mission to the outer solar system.
Significance: The Voyager Program probes executed the first “Grand Tour” in planetary exploration by successively encountering the outer planets: Jupiter, Saturn, Uranus, and Neptune. Such a tour, using the “planetary-gravity-assist” technique to travel from planet to planet, is possible only once every 175 years.
Introduction and Overview
The Voyager Program conducted the first planetary Grand Tour in history by sending two uncrewed spacecraft on a mission to explore the outer solar system. Preliminary de- sign began in 1969. After obtaining official approval in May, 1972, Voyager 1 was launched on September 5, 1977. Voyager 2 was actually launched a few weeks earlier on August 20, 1977. Both were launched from Cape Canaveral, Florida, by Titan III-E/Centaur rockets. Voyager 1 encountered Jupiter in 1979 and Saturn in 1980, before flying out of the plane of the solar system. Voyager 2 encountered Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. As of 2001, the two Voyager spacecraft are the most distant human-made objects from Earth and have provided humankind’s most detailed views of the outer solar system.
The Voyager Program was originally approved as a mission to Jupiter and Saturn only. However, the mission scientists and engineers knew that during the 1970′s, the configuration of the outer planets in the solar system provided a unique opportunity for a Grand Tour of the outer solar system. With this Grand Tour in mind, they designed the Voyager craft with the capability of extending the original mission should approval later be granted.
After Voyager’s early success, the National Aeronautics and Space Administration (NASA) officially approved the extension of the mission. Voyager 1 turned to fly out of the plane of the solar system after its encounter with Saturn. Voyager 2 continued on to explore Uranus and Neptune. The only possible trajectory that would have allowed Voyager 2 to continue its mission to Pluto went directly through the interior of the planet Neptune.
During each planetary flyby, the Voyager craft used the gravity-assist technique, a sort of gravitational slingshot effect from the giant planets that propelled the spacecraft on to the next planet. The Voyager spacecraft trajectories were very carefully selected so that as the Voyager fell toward a planet, the craft would pick up speed. The planet also deflected the trajectory so that the craft was aimed in the right direction for the next planetary encounter. These gravitational boosts shortened the time required for Voyager 2 to reach Neptune by nearly two decades and significantly reduced the amount of fuel necessary to propel the craft from planet to planet.
Envoys Beyond the Solar System
On February 17, 1998, at a distance of 6.5 billion miles, Voyager 1 exceeded the distance from the Sun of the slower Pioneer 10 spacecraft (launched in 1972) to become the most distant human-made craft from Earth. As of January, 2001, the Voyager 1 and 2 craft remained functional and were located more than 7.4 billion and 5.8 billion miles from Earth, respectively. They have enough power to last until about 2020, when they will both be more than 10 billion miles from Earth. During this time, they may cross the heliopause, the boundary between the solar system and the surrounding universe, to become the first human-made objects to leave the solar system. Voyager 1 will pass near a faint star in the constellation Camelopard is in about 40,000 years. In 296,000 years Voyager 2 will pass near the brightest star in the night sky, Sirius.
As humankind’s envoys beyond the solar system, both Voyager craft carry records containing pictures and sounds from Earth. The records are 12-inch copper disks plated with gold. The craft also contain needles for playing the records and illustrations of their use. More than one hundred pictures were included, twenty of which are in color. These pictures attempt to depict Earth and its rich variety of life, including human life and culture. The records contain spoken greetings in sixty different languages, natural and machine-made sounds of Earth, and a 90-minute selection of musical excerpts that represent a variety of cultures and forms.
Instrument Packages
Each Voyager craft, which is about the size and mass of a subcompact car, contains a suite of scientific instruments. The Voyager images were provided by two cameras on each craft. They have filter wheels to allow color images. The narrow-angle cameras, capable of resolving a newspaper headline from a distance of 1 kilometer, provided high-resolution images. The wide-angle cameras provided the global images at a lower resolution.
The Voyager crafts’ infrared and ultraviolet spectrometers and photopolari meters provided information about atmospheric and satellite compositions and structures. The planetary radio astronomy experiment measured the planetary radio emissions. The magnetometers measured and studied the planetary magnetic fields and their interactions with the solar magnetic field. Four experiments—the plasma-particles experiment, the plasma-waves experiment, the low-energy charged-particles experiment, and the cosmic-ray particles experiment—were designed to provide information about energetic charged particles at the planetary encounters and in interplanetary space.
Jupiter Encounter
Voyager 1 flew within 217,000 miles of Jupiter on March 5,1979. Voyager 2 followed on July 9,1979, flying within 449,000 miles. The detailed images and measurements taken during the Voyagers’ encounter with Jupiter vastly exceeded what had been possible to accomplish from Earth or on previous Pioneer missions and revealed much new information about Jupiter and its system of moons. Pictures of Jupiter revealed beautiful detail in the striped zone-and-belt structure of the planet’s cloud tops. The zones are the lighter-colored stripes, and the belts are the darker-colored stripes. The Voyagers showed that these zones and belts are manifestations of both east-west and up-down circulation patterns in the atmosphere. The high wind speeds are manifested by the obvious turbulence in the zone-belt interfaces. The previously known Great Red Spot is a centuries-old anticyclonic storm larger than Earth. It rotates fully counterclockwise within a period of four to six days. The Voyagers also measured a strong planetary magnetic field for Jupiter.
Surrounding Jupiter, the Voyager mission discovered a ring system, albeit much less extensive than that of Saturn, as well as three small, previously undiscovered moons. Both craft observed Jupiter’s previously known moons in unprecedented detail, and Voyager 1 made the unexpected observation that Jupiter’s closest major moon, Io, had nine active volcanoes at the time of the Voyager 1 encounter. The next major moon, Europa, has an icy crust covered with a large number of intersecting cracks, beneath which there may be a liquid ocean. Voyager found two distinct types of terrain, grooved and cratered, on Jupiter’s next major moon, Ganymede. The final major moon, Callis to, has an ancient surface saturated with craters.
Saturn Encounter
On November 12, 1980, Voyager 1 flew to within 77,000 miles of Saturn, followed by the Voyager 2 on August 25, 1981, which flew to within 63,000 miles of Saturn. Both craft returned unprecedented data. Pictures of Saturn revealed a zone-and-belt structure to the cloud tops similar to that of Jupiter. However, Saturn’s zones and belts did not have the richly detailed structure found on Jupiter, despite Saturn’s higher measured wind speeds, which can exceed 1,000 miles per hour. Apparently, this zone-and-belt structure is slightly deeper on Saturn than on Jupiter and covered by a hazy layer that masks its detail. The Voyagers also measured Saturn’s magnetic field and confirmed that it is the only planetary magnetic field almost perfectly aligned with the rotation axis.
Saturn’s most beautiful feature is its extensive ring system, the only one that is directly visible from Earth. Voyager photographs revealed surprisingly detailed and completely unexpected structures. The major rings, called the A-, B-, and C-rings, consist of hundreds of smaller individual rings. They are apparently caused by the combined gravitational forces of Saturn’s many moons on the individual ring particles orbiting Saturn. The faint outer F-ring was revealed to consist of apparently twisted strands. Dark spokes were found in the B-ring. Some of these spokes rotate with Saturn’s magnetic field rather than at the orbital speed of ring particles.
The Voyager mission discovered six new moons of Saturn and studied the planet’s previously known moons. Saturn’s largest moon, Titan, was found to have a significant atmosphere, consisting primarily of nitrogen, like Earth’s atmosphere, but much colder. Further studies may help scientists understand the chemistry of Earth’s primitive atmosphere. With a diameter of only about 300 miles, Enceladus should be too small to be geologically active, yet it surprisingly proved to be the most geologically active of all Saturn’s moons, except Titan. The source of this activity is poorly understood. The moon Mimas is slightly less than 250 miles in diameter, but it has a 6-mile deep impact crater that is 80 miles in diameter. This crater also has a central mountain comparable in size to Mount Everest. With this crater, Mimas resembles the Death Star of the motion picture Star Wars.
Uranus and Neptune Encounters
After the Saturn encounter, Voyager 1 exited the plane of the solar system. Voyager 2 continued on its Grand Tour to explore Uranus and Neptune. The challenges of the outer solar system necessitated extensive reprogramming during the four-year voyage from Saturn to Uranus. At a distance of nearly 2 billion miles from the Sun, it is much darker at Uranus. Hence, long time exposures are needed for the images. Because Voyager 2 was speeding by on its outward journey, it was not possible to mount the cameras on a steady tripod, as is usually done for timed exposures. In a difficult maneuver, called image-motion compensation, the craft rotated just the right amount to compensate for its motion. This technique worked to produce clear sharp images of Uranus and Neptune. In addition, with only a 23-watt radio transmitter the method used to transmit data back to Earth had to be revised, and the receiving antennaes on Earth had to be linked to pick up the distant signal.
On January 24, 1986, Voyager 2 flew to within 67,000 miles of Uranus, taking images that revealed a nearly featureless pale blue planet. With extensive computer processing, the images show a barely visible cloud structure on Uranus. Three years later, on August 24-25,1989, Voyager 2 flew to within only 3,000 miles of Neptune’s north pole. In contrast to Uranus, Neptune showed considerable atmospheric activity. There was a dark blue spot called the Great Dark Spot, similar to the Great Red Spot of Jupiter, and a few smaller dark spots. There were also white lenticular-shaped clouds above the Great Dark Spot.
Both Uranus and Neptune have fairly strong magnetic fields, with large tilts relative to their planetary spin axes. The source of these magnetic fields is poorly understood. The Voyager cameras also studied the thin ring systems around both planets. Voyager 2 studied the previously known moons around both planets and discovered ten new moons around Uranus and six around Neptune.
Context
On February 14, 1990, at a distance of 3.7 billion miles from Earth, Voyager 1 took a unique family portrait of the solar system. A series of39 wide-angle images shows the Sun and all but the three smallest planets, Mercury, Mars, and Pluto. The narrow-angle camera also took images of the six visible planets. From this perspective, Earth is a faint dot.
The success of the Voyager mission far exceeded expectations. On their twelve-year mission to the outer solar system the Voyager craft sought out and explored many strange new worlds. Following the Pioneer 10 and 11 missions, the Voyagers were not the first missions to Jupiter and Saturn. However, the Voyager mission was the first to distant Uranus and Neptune. The Voyager mission provided the first detailed look at all these planets. These hardy craft will also be the first human-made objects to leave the solar system during the first two decades of the twenty-first century.
