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File:NASA Earth-observing Fleet June 2012.ogv 
NASA's Earth-observing fleet as of June 2012.
An animation depicting the orbits of GPS satellites in medium Earth orbit.
A full-size model of the Earth observation satellite ERS 2
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 the Moon.
The world's first artificial satellite, the Sputnik 1, was launched by the Soviet Union in 1957. Since then, thousands of satellites have been launched into orbit around the Earth. Some satellites, notably space stations, have been launched in parts and assembled in orbit. Artificial satellites originate from more than 50 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently operational, whereas thousands of unused satellites and satellite fragments orbit the Earth as space debris. A few space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, MercuryVenusMarsJupiterSaturnVestaEros, and the Sun.
Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites,communications satellitesnavigation satellites, weather satellites, and research satellites. Space stations and human spacecraftin 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 orbitpolar orbit, and geostationary orbit.
About 6,600 satellites have been launched. The latest estimates are that 3,600 remain in orbit.[1] Of those, about 1000 are operational;[2][3]the rest have lived out their useful lives and are part of the space debris. Approximately 500 operational satellites are in low-Earth orbit, 50 are in medium-Earth orbit (at 20,000 km), the rest are in geostationary orbit (at 36,000 km).[4]
Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control and orbit control.


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