INTERPLANETARY EXPLORATION

use of unmanned space probes to investigate the other planets and other bodies in the solar system. (See also space exploration.) As in lunar exploration, the Soviet Union was the first nation to attempt interplanetary studies. In October 1960 the Soviets fired two probes to Mars, but these failed. Early in 1961 the Soviet Union scored a qualified success with its first Venus probe. The spacecraft called Venera 1 passed within 99,000 km (62,000 miles) of Venus, but no data were transmitted owing to a telemetry failure. The Soviets continued to launch planetary probes in 1962. Only one, Mars 1, flew toward its designated target, but it too was plagued by radio failure and sent back no data. The United States made its first attempt with Pioneer 5, launched on March 11, 1960. Operating flawlessly, the 43-kilogram (94.8-pound) vehicle passed near Venus and sent back the first deep-space measurements (e.g., cosmic-ray and magnetic-field intensities). The first probe to return data about another planet was the U.S. Mariner 2. Launched Aug. 26, 1962, Mariner passed within 34,600 km of Venus. The U.S. space probe Mariner 4, launched Nov. 28, 1964, marked the first successful mission to Mars: the probe sent back the first photographs of the planet, from within 9,900 km of its surface. The Soviets made much headway in their efforts to explore Venus during the last half of the 1960s. In 1966 they launched Venera 3, which became the first spacecraft to crash-land on another planet. Venera 4, which parachuted a capsule of instruments to the surface of Venus in 1967, was the first probe to transmit information on the planet's atmosphere. The United States continued with its Mariner and Pioneer programs. Mariner 5 (1967) measured the magnetic field and atmosphere of Venus, and Mariner 6 and 7 (1969) analyzed and photographed the surface of Mars. Pioneers 6, 7, and 9 (196568) collected information about deep-space conditions. The 1970s saw impressive gains in interplanetary exploration, particularly by the United States. The U.S. Mariner 10 became the first probe to reach Mercury. On March 29, 1974, it flew to within 740 km of the planet and transmitted high-quality photographs of the surface. More spectacular were those missions involving the U.S. Viking 1 and 2 probes (1975). Both of these twin spacecraft consisted of an orbiter and lander, the latter equipped with various devices specially designed to search for traces of bacterial life on Mars. For months the probes relayed detailed, close-up pictures of the Martian terrain and precise measurements of its meteorological conditions. Results of the biological tests, however, proved inconclusive. Other important planetary flights were those of U.S. Pioneer Venus 1 and 2, which took radar photographs of Venus' surface and analyzed the chemical composition of its atmosphere in December 1978. That same month, the Soviet probes Venera 11 and 12 transmitted data on the planet's lower atmosphere before landing. Exploration of the larger, more distant planets was also undertaken. The United States launched a total of four planetary probes toward Jupiter between 1971 and 1977, three of which flew on to Saturn after reaching their initial destination. In December 1973, Pioneer 10 became the first spacecraft to pass by Jupiter. Pioneer-Saturn flew to within 42,000 km of Jupiter in December 1974 and photographed its polar regions. In September 1979 the probe reached the vicinity of Saturn and discovered two additional rings around the planet. Voyager 1 and Voyager 2 flew past Jupiter in March and July 1979, respectively; the first probe detected a thin ring around the planet. Voyager 1 sped by Saturn in November 1980, taking photographs of the planet from a distance of 124,000 km. These pictures revealed that the structure of Saturn's rings was far more complex than the best telescopes on Earth could discriminate. Voyager 2 passed Saturn in August 1981 and recorded information about the atmospheric gases of the planet and of its largest satellite, Titan. The probe passed within 81,000 km of Uranus in January 1986. It made many significant discoveries, including the presence of a substantial magnetic field around the planet and 10 additional satellites. Voyager 2 passed within 4,800 km of Neptune in August 1989. Among its major discoveries were the presence of a magnetic field and three rings around the planet, intense atmospheric storms, and at least six additional satellites. The probe Galileo, launched in 1989, explored Jupiter and its moons in July 1995 and became the first artificial satellite to orbit that planet. Much information about comets was obtained from spacecraft exploration of Halley's Comet in 1986 during its closest approach to Earth. The Giotto probe of the European Space Agency (ESA) returned close-up photographs of the comet's nucleus and coma, while the Soviet Vega spacecraft analyzed the composition of its dust tail.