<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> In the context of spaceflight, a satellite is an artificial object which has been intentionally placed into orbit . Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as Earth's Moon . </P> <P> In 1957 the Soviet Union launched the world's first artificial satellite, Sputnik 1 . Since then, about 6,600 satellites from more than 40 countries have been launched . According to a 2013 estimate, 3,600 remained in orbit . Of those, about 1,000 were operational; while the rest have lived out their useful lives and become space debris . Approximately 500 operational satellites are in low - Earth orbit, 50 are in medium - Earth orbit (at 20,000 km), and the rest are in geostationary orbit (at 36,000 km). A few large satellites have been launched in parts and assembled in orbit . Over a dozen space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Mercury, Venus, Mars, Jupiter, Saturn, a few asteroids, a comet and the Sun . </P> <P> Satellites are used for many purposes . Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and space telescopes . Space stations and human spacecraft in orbit are also satellites . Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways . Well - known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit . </P>

How many satellites are in the earth's atmosphere
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