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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.

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