Orbital motion
Gravity provides the force needed to maintain stable orbit of both planets around a star and also of moons and artificial satellites around a planet.
Explaining orbits
For an object to remain in a steady, circular orbit it must be travelling at the right speed. The diagram shows a satellite orbiting the Earth.
There are three possible outcomes:
- If the satellite is moving too quickly then the gravitational attraction between the Earth and the satellite is too weak to keep it in orbit. If this is the case, the satellite will move off into space. This occurs at speeds around or above 11,200 metres per second (m/s).
- If the satellite is moving too slowly then the gravitational attraction will be too strong, and the satellite will fall towards the Earth. This occurs at speeds below 7600 m/s.
- A stable orbit is one in which the satelliteâs speed is just right - it will not move off into space or spiral into the Earth, but will travel around a fixed path.