Satellite

In the context of Space flight a satellite is an object which has been placed into Orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from Natural satellite such as the Moon.

History'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; also some satellites, notably Space station , 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 probe have been placed into orbit around other bodies and become artificial satellites to the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Sun.

Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites, Communications satellite , navigation satellites, weather satellites, and research satellites. Space station 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.

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.