Sputnik, series of unmanned satellites launched by the Soviet Union. The first was launched on Oct. 4, 1957. It circled the earth once about every 95 minutes at a speed of 18,000 mph (29,000 kmph) until it fell to earth on Jan. 4, 1958. Nine more Sputnik missions were carried out from 1957 to 1961.
See also: Space exploration.