Twin robotic, unmanned interplanetary space probes, Voyager 1 and Voyager 2, were launched by the United States in 1977; on September 5, 1977 and August 20, 1977, respectively. Their original mission was to fly by the planets Jupiter and Saturn, but the journey of Voyager 2 was successfully extended to Uranus and Neptune. The Voyagers are 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. They are the longest-lasting missions ever launched into space. They are controlled by the Jet Propulsion Laboratory, a project of the National Aeronautics and Space Administration (NASA) and the California Institute of Technology (Caltech), near Pasadena, California.
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 eleven 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, ten 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 2 discovered ten moons of Uranus, along with studying the planet’s atmosphere and ring system. Because of Uranus’ axial tilt of 97.77°, which is unique in the solar system, the probe found that the tail of its magnetic field is also tilted by 60° from the planet’s rotational axis. The probe also discovered the magnetic field of Uransus. On its trip past Neptune, the probe discovered the Great Dark Spot, which has seen been studied by astronomers with the Hubble Space Telescope. Voyager 2 found out that Neptune has some of the strongest winds of all the gas planets in the solar system; sometimes over 994 mi (1,600 km) per hour.
As of September 2013, Voyager 1 was located over 125 astronomical units (AU) from the sun. An AU is a measure of length equal to approximately 93,000,000 mi (149,600,000 km), or the approximate average of the distance between the sun and Earth.Voyager 1 is in the sun’s heliosheath, which is the termination shock region between the solar system and interstellar space. According to NASA, the probe entered the termination shock sometime around May 2005. The heliopause is considered the official end of the solar system and the beginning of interstellar space. When Voyager 1 enters this space, it will be the first human-made object to pass into interstellar space when it crosses the heliopause (the limit of the solar wind’s influence) sometime in 2013 to 2015. Voyager 1 is expected to generate power, so be able to communicate with Earth, to the year 2020.
As of September 2013, Voyager 2 was a distance of over 102 AU from the sun. It travels about 3.3 AUs each year; being about twice the distance from the orbit of the dwarf planet Pluto but not yet having reached the orbit of dwarf planet Eris. The probe is expected to continue transmitting to Earth into the mid–2020s.