Pioneer

First series of unmanned U.S. deep-space probes. Pioneers 0–4 were all intended to be lunar missions, but only Pioneer 4 succeeded in escaping Earth's gravity (1964). Pioneer 1 was the first spacecraft launched by NASA (1958). Pioneers 6–9 were successfully launched into solar orbits (1965–68) to study the solar wind, solar magnetic field, and cosmic rays. Pioneer 10, launched in 1972, was the first space probe to traverse the asteroid belt and the first to fly by Jupiter (1973), where it discovered Jupiter's huge magnetic tail. It then became the first man-made object to exit the solar system. Pioneer 11's data and photographs (1979) enabled scientists to identify two previously undiscovered moons and an additional ring around Saturn and radiation belts in its magnetosphere. Two Pioneer Venus spacecraft, the Orbiter and the Multiprobe (Pioneer 12 and 13, respectively), reached Venus in 1978. The Orbiter observed the planet's clouds and atmosphere and mapped its surface by radar; the Multiprobe released four instrument packages into the atmosphere at different locations.

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