Objects that travel around a planet are called satellites. Each complete journey a satellite makes around the planet is called an orbit.
Our Earth has one natural satellite – the Moon. However, since October 1957 many thousands of artificial (human-made) satellites have been placed in orbits around the Earth.
To stay in orbit, a satellite has to travel at a very high velocity, which depends on the height. So, typically, for a circular orbit at a height of 300 km above the Earth's surface, a speed of 7.8 km/s (28,000 km/h) is needed. At this speed, the satellite will complete one orbit around the Earth in 90 minutes.
Satellites have to move so quickly in order to compensate for the pull of Earth’s gravity. This is similar to someone throwing a cricket ball or baseball. The harder the ball is thrown, the further it will travel before it reaches the ground.
If a player could throw the ball hard enough so that it reaches the necessary velocity, the ball would go into orbit. It would never fall back to Earth. Throw it harder still, so that it reaches a speed of 11.2 km/s (40,300 km/h) – known as 'escape velocity' - and the ball will leave the Earth altogether. It will then become a 'satellite' of the Sun.
Last modified 12 October 2011