Twin robotic space probes, Voyager 1 and Voyager 2, were launched by the United States in 1977. Their original mission was to fly by Jupiter and Saturn, but the journey of Voyager 2 was successfully extended to Uranus and Neptune. The Voyagers were the most scientifically fruitful space mission ever launched, collecting, among other data, high-quality photographs of four planets and dozens of moons, most of which were previously known only as specks of light in telescopes. The Voyagers are still functioning today, returning data to Earth as they coast toward interstellar space.
The structural foundation of each Voyager is a puck-shaped frame or "bus" about 6 ft (1.8 m) diameter. To this bus are attached 11 science instruments, communications antennae, computers and a power-generation unit. A high-gain dish antenna 12 ft (3.7 m) in diameter is mounted directly onto the bus and devoted to receiving command signals from Earth. A radioiosotope thermoelectric generator provides each Voyager with electric power derived from the heat given off by several kilograms of plutonium-238. Voyager's cameras and spectrometers are mounted on a scan platform attached to a gearbox at the end of a 10-ft (3-m) boom. The gearbox makes it possible to select
observational targets, within limits, without having to rotate the entire spacecraft. Each Voyager also possesses several computers to handle scientific data, coordinate its own subsystems, and control its position.
Since it was known that the Voyagers would eventually leave the solar system, they are also equipped with messages for any extraterrestrial beings that might encounter them, perhaps millions of years hence. These messages are preserved on gold-plated audio discs of the now-obsolete type that encodes sound as sinuous grooves on a surface. The Voyager records include a wide variety of Earthly sounds (e.g., rain, a kiss, the rock-and-roll classic "Johnny B. Goode" by Chuck Berry) and images (e.g., a snowflake, Australian hunters, rush-hour traffic), and are supplied with needles, cartridges, and playback instructions in pictorial form.
Voyager 1 made its closest approach to Jupiter in March, 1979, taking detailed pictures of the planet and several of its moons; Voyager 2 encountered Jupiter in July of the same year. Voyager photographs revealed that the moon Io is the most volcanically active body in the solar system, with its interior kneaded to hot liquid by periodic gravitational tugs from the moon Europa and its surface pockmarked by hundreds of volcanoes. Some of these volcanoes squirt liquid sulfur compounds at 0.6 miles per second (1 km/sec) up to 190 mi (300 km) above the surface, forming umbrella-shaped plumes that can be easily seen from space. Europa was found to be among the smoothest bodies in the solar system, covered with a network of cracks suggesting a relatively thin layer of ice over a watery world-ocean. Ganymede and Callisto, consisting mostly of ice, were found to contain fascinating geology of their own. Callisto, thanks to data obtained in 1998 by the space probe Galileo, is now thought to also possess a world-ocean of saltwater, albeit under a thicker crust than Europa's.
Both spacecraft received a gravitational assist from Jupiter that increased their speed and redirected them toward Saturn, which Voyager 1 reached in November, 1980, and Voyager 2, 10 months later. Their observations showed that Saturn's ring structure was more complex and finely divided than was suspected. Close observation of Saturn's moon Titan, the largest satellite in the solar system, revealed little because of its dense, hazy atmosphere.
After Saturn, Voyager 1 proceeded to head out of the solar system, but Voyager 2 continued on what was termed the Grand Tour—a course that would take it to Neptune and then, with yet another gravitational assist from that planet, to Uranus. A Grand Tour is only available to spacecraft when the outer planets are in a certain alignment; this alignment was present when the Voyagers were launched in the 1970s, but will not recur for another 150 years. In January, 1986, Voyager 2 swept through the Uranian system of moons, which is oriented at right angles to the plane of the ecliptic so that Uranus, with its system of moons, moves as if rolling along its orbit. Voyager 2, moving along the ecliptic, shot through the Uranian system like a dart through a bull's eye, gathering detailed images of its five previously known large moons—all of which revealed unique geology—and discovering ten new, lesser satellites. In August, 1989, Voyager 2 encountered Neptune, outermost of the major planets, passing within a mere 3,000 mi (4,800 km) of its north pole. It made thorough observations of Neptune's ring system (similar to Saturn's, but less spectacular) and observed bizarre nitrogen geysers on its largest moon, Triton.
For its encounters with Uranus and Neptune, Voyager 2 was ingeniously reprogrammed to cope with conditions it had not been designed to face. The power yielded by its thermoelectric generators had declined, forcing controllers to dole it out by switching essential systems on and off in an intricate sequence. Additionally, the Sun's light is much dimmer at Uranus and Neptune—the former being four times and the latter six times as far from the Sun as is Jupiter—necessitating lengthy camera exposures (over a minute in some cases). In order to keep its instruments steady for such long periods, the slight jerk caused by the onboard tape recorder starting up was compensated for by milliseconds-long steering-rocket blasts.
Voyager 1 and Voyager 2 continue to relay data on charged particles, plasma, and magnetic fields from approximately 70 and 80 AU (astronomical units) away, respectively, where 1 AU equals the average distance of the Earth from the Sun. They will have enough electrical power and hydrazine steering-rocket propellant to remain functional until about 2020, at which time, after over 40 years of productive science, their mission will cease. Voyager 1 will be the first human-made object to pass into true interstellar space when it crosses the heliopause (the limit of the solar wind's influence) some time before the year 2013.
Swift, David W. Voyager Tales: Personal Views of the Grand Tour. Reston VA: American Institute of Aeronautics and Astronautics, 1997.
Harwood, William, and Stephen P. Maran. Space Odyssey: Voyaging Through the Cosmos. Washington, DC: National Geographic, 2001.
National Aeronautics and Space Administration. "Voyager: Celebrating 25 Years of Discovery." Jet Propulsion Laboratory, California Institute of Technology. November 15, 2002 [cited December 30, 2002]. <http://voyager.jpl.nasa.gov/>.