PIONEER

any of the first series of unmanned U.S. deep-space probes designed chiefly for interplanetary study. With the exception of Pioneer 1, which was launched into lunar orbit on Oct. 11, 1958, all other probes in the series were sent to explore planetary bodies or to measure various interplanetary particle and magnetic field effects. Pioneer 6 (1965), for example, was injected into solar orbit to determine space conditions between the Earth and Venus. It transmitted much data on the solar wind and solar cosmic rays in addition to measuring the Sun's corona and the tail of Comet Kohoutek. Pioneer 10 (1972) flew by Jupiter in December 1973, the first space probe to do so, and discovered its huge magnetic tail, an extension of the planet's magnetosphere. Pioneer 11 (1973), also called Pioneer-Saturn, passed by Jupiter in December 1974 and flew within about 20,900 km (13,000 miles) of Saturn in September 1979. It transmitted data and photographs that enabled Earth-based scientists to identify two additional rings around the planet and the presence of radiation belts within its magnetosphere. Pioneer Venus 1 and Pioneer Venus 2 (1978) began orbiting Venus in December 1978. The first of these twin spacecraft sent back radio observations of the planet's lower atmosphere, and the second made radar photographs that revealed enormous mountains and deep basins on its surface.