Satellite

An artificial satellite is an object that has been intentionally placed into orbit (as opposed to a natural satellite such as Earth's Moon).

In 1957, the Soviet Union launched the world's first artificial satellite, Sputnik 1. Since then, about 8,900 satellites from more than 40 countries have been launched. Currently, about 5,000 remain in orbit, with about 1,900 operational. The largest satellite is the International Space Station. Starlink hopes to eventually launch 42,000 satellites.

Satellites are used for many purposes. Attaching space telescopes, they can be used to make star maps and maps of planetary surfaces. Common types include observation, communications, navigation, and weather satellites. Space stations and spacecraft in orbit are also satellites.

Satellites are in geocentric orbit:


 * Low Earth orbit (LEO) - ranging in altitude from 160 km (circular orbital speed at 8,000 metres per second) to 2,000 km
 * Medium Earth orbit (MEO) - altitudes between 2,000 km and 35,786 km
 * Geosynchronous orbit (GEO) - altitude of 35,786 km, with orbital speed at 3,000 metres per second
 * High Earth orbit (HEO) - higher than that of the geosynchronous orbit