Introduction satellite, artificial,
object constructed by humans and placed in orbit around the earth or other celestial body (see also space probe
). The satellite is lifted from the earth's surface by a rocket
and, once placed in orbit, maintains its motion without further rocket propulsion. The first artificial satellite, Sputnik I,
was launched on Oct. 4, 1957, by the USSR; a test payload of a radio beacon and a thermometer demonstrated the feasibility of orbiting a satellite. The first U.S. satellite, Explorer I,
launched on Jan. 31, 1958, returned data that was instrumental in the discovery of the Van Allen radiation belts
. During the first decade of space exploration, all of the satellites were launched from either the United States or USSR. Today, there are more than three dozen launch sites in use or under construction in more than a dozen countries.
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