Voyager probesTwo 825-kg US probes launched in 1977 toward the planets of the outer Solar System and making use of the rare planetary alignment that occurred at the beginning of the 1980s. Voyager 1, launched on Sept. 5 1977, reached Jupiter in Mar. 1979, moving in a path that took it close to the satellites Callisto and Ganymede, then quite near Europa, and close to Io, before it made its closest approach (280 000 km) to Jupiter on Mar. 5; it then passed Amalthea. Using the slingshot action of Jupiter, Voyager 1 was set on a course for Saturn, which it reached in Nov. 1980. It approached within 125 000 km of that planet on Nov. 12, dipping below Saturn's rings just after passing close to the satellite Titan. The spacecraft then headed out of the Solar System with no further planetary encounters.
Voyager 2, launched on Aug. 20, 1977, reached Jupiter in July 1979. It passed close to Callisto, Ganymede, and Europa, then passed Amalthea before its closest approach (714 000 km) to the planet on July 9. Since Voyager 1's encounter with Titan was successful, Voyager 2 was set on a path that took it past Saturn in Aug. 1981 and on toward Uranus and Neptune. It passed closer to several of Saturn's satellites (in particular Iapetus, Hyperion, Tethys, and Enceladus) than Voyager 1 had done, but farther from Titan. Moving above the plane of Saturn's rings it made its closest approach (101 300 km) to the planet on Aug. 25, 1981. Voyager 2 reached Uranus in Jan. 1986, making its closest approach (107 000 km) on Jan. 24 and discovering many hitherto unknown small satellites. It went on to Neptune, making its closest approach (5000 km) on Aug. 25, 1989. Voyager 2 then also made its way out of the Solar System.
The two spacecraft carried a variety of instruments for scientific investigations, and three engineering subsystems for on-board control of spacecraft functions. Trajectory correction maneuvers could be enabled only from Earth. The science instruments for viewing the planets and their satellites and ring systems were mounted on a platform capable of being pointed very precisely. The instruments included a narrow-angle and a wide-angle TV camera for high-resolution imaging of satellite surfaces, etc., an infrared spectrometer, interferometer, and radiometer, an ultraviolet spectrometer, and instruments for studying plasmas, low-energy charged particles, and cosmic rays. In addition there were magnetometers mounted on an extendable boom and two planetary radio astronomy and plasma wave antennas. A 3.7-meter aperture high-gain radio dish pointed continuously toward Earth, allowing two-way communications of commands and information. Power was provided by three radioisotope thermoelectric generators, and hot gas jets provided thrust for attitude stability and trajectory corrections. Not everything ran smoothly with the Voyager probes. During Voyager 2's Saturn encounter, its camera and science instruments platform became temporarily stuck. Another problem was that its radio antenna failed, and the team of Earth-based scientists were forced to use a less efficient backup system. Nevertheless, a wealth of information was returned to Earth from the two Voyager craft, revealing many unexpected aspects concerning the satellites (especially the volcanic activity on Io), the fine details of Saturn's rings, and the existence of new satellites and planetary ring systems. (Details are given at individual entries in the Dictionary.)
After Voyager 2's encounter with the Neptune system, NASA redesignated the continuing Voyager journeys as the Voyager Interstellar Mission (VIM). Both Voyager craft are still making and transmitting observations on the outer heliosphere, monitored by the Deep Space Network. They are heading out of the Solar System in opposite directions. Voyager 1 is now the most distant human-made object in the Universe. At the start of 2005 it was more than 14 billion kilometers (93 AU) from the Sun, traveling away from it at a velocity of about 17.184 km/s (3.5 AU per year) and heading north at an angle of 35.55° to the plane of the ecliptic. Voyager 2 is heading out in a southerly direction at a velocity of 15.64 km/s (about 3.13 AU per year) and at an angle to the ecliptic of 47.46°. On their way out of the Solar System, both spacecraft are likely to have to negotiate a passage through the Oort cloud, which could take as long as 20 000 years. In about 40 000 years time, Voyager 1 will, if it survives, pass within 0.49 parsec of the red dwarf star AC+79 3888 in the constellation of Camelopardalis. By about the same time, Voyager 2 will pass within 0.52 parsec of another red dwarf, Ross 248 in Andromeda, and in 296 000 years it will pass Sirius at a distance of 1.32 parsecs. Both craft carry disks containing information about the Earth that an alien civilization might someday access.