satellite

A satellite in polar orbit round the Earth. Satellites in polar orbit usually complete an orbit in two hours or less and as the Earth is rotating as they orbit, they cover a large part of the Earth's surface each day. Remote-sensing satellites, such as Landsat, are in polar orbit.

The British satellite Prospero continues to orbit the Earth every 100 minutes and will do so until 2040. With the correct equipment, the satellite's radio transmitter can still be heard broadcasting on 137.56 MHz.

Any small body that orbits a larger one. Natural satellites that orbit planets are called moons. The first artificial satellite, Sputnik 1, was launched into orbit around the Earth by the USSR in 1957. Artificial satellites can transmit data from one place on Earth to another, or from space to Earth. Satellite applications include science, communications, weather forecasting, and military use.

Space probes have been sent to natural satellites including the Earth's Moon, Mars's Deimos, and the moons of the giant planets Jupiter, Saturn, Uranus and Neptune.

At any time, there are several thousand artificial satellites orbiting the Earth, including active satellites, satellites that have ended their working lives, and discarded sections of rockets. The brightest artificial satellites can be seen by the naked eye. Artificial satellites eventually re-enter the Earth's atmosphere. Usually they burn up by friction, but sometimes debris falls to the Earth's surface, as with Skylab and Salyut 7.

Hundreds of millions of pieces of space junk, ranging from particles a millimetre across up to disabled satellites, are careering around the Earth. The US Space Command catalogues the larger items to make sure they are not mistaken for enemy missiles; currently about 10,000 items are listed.