artificial Satellite

Satellite, artificial, object placed in orbit as a satellite. First seriously proposed in the 1920s, they were impracticable until large enough rockets were developed. The first artificial satellite, Sputnik 1, was launched by the USSR on Oct. 8, 1957, and was soon followed by a host of others, mainly from the USSR and the United States, but also from the United Kingdom, France, Canada, West Germany, Italy, Japan, and China. These satellites have many scientific, technological, and military uses. Astronomical observations (notably X-ray astronomy) can be made unobscured by the atmosphere. Studies can be made of the radiation and electromagnetic and gravitational fields in which the earth is bathed and of the upper atmosphere. Experiments have been made on the functioning of animals and plants in space (with zero gravity and increased radiation). Artificial satellites are also used for reconnaissance, surveying, and meteorological observation, as navigational aids (position references and signal relays), and in communications for relaying television and radio signals. Manned satellites, especially the historic Soyuz and Mercury series, have paved the way for space stations, which have provided opportunities for diverse research and for developing docking techniques; the USSR Salyut and U.S. Sky lab projects are notable. The basic requirements for satellite launching are determined by celestial mechanics. Launching at various velocities between that required for zero altitude and the escape velocity produces an elliptical orbit lying on a conic surface determined by the latitude and time of launch. To reach any other orbit requires considerable extra energy expenditure. Artificial satellites require a power supply—solar cells, batteries, fuel cells, or nuclear devices; scientific instruments; a communications system to return encoded data to earth; and instruments and auxiliary rockets to monitor and correct the satellite's position. Most have computers for control and data processing, thus reducing remote control to the minimum.

See also: Satellite.