What is a Satellite?
If you want to be really technical then a satellite is just an object that orbits another object, held in place by the force of gravity between the two. That means that the Earth is a satellite of the Sun and that the Sun is in turn a satellite of the Milky Way Galaxy. More usually a satellite is defined as an object that orbits a planet. The object can be either natural (such as our Moon) or artificial (such as the International Space Station). The planet doesn't have to be the Earth either and so the Cassini mission to Saturn is also a satellite as it is in orbit around the ringed planet. The Voyager missions of the 1970s aren't satellites as they flew past the planets and did not enter orbit.
The First Artificial Satellites
Image: Courtesy of NSSDC
The first artificial satellite was launched by the Soviet Union (a former union of countries led by Russia) on the 4th of October 1957. Sputnik 1 was an 83 kg, 58 cm diameter sphere, which transmit a regular bleep from space as it orbited the Earth. As this was during the height of the Cold War, Americans were horrified and the Space Race was under way. Because of the mutual distrust between America and the Soviet Union, most of the early efforts in space were guided by the military organisations of both countries. These included spy satellites to take photographs of enemy troop movements and special detectors used to check if the other side were breaking the nuclear test ban treaties by exploding their bombs on the far side of the Moon (governments really were this paranoid but no nuclear explosions were detected).