Orbital motion - Higher
Gravity provides the force needed to maintain the stable orbit of planets around a star and also of moons and artificial satellites around a planet.
The moons around planets in the Solar System are natural satellites 鈥 they are made of the same material as other Solar System objects. Astronomers have sent artificial satellites (satellites that are manmade) to orbit several Solar System objects, including Jupiter, the asteroid Vesta and Mars.
Artificial satellites travel in one of two different orbits:
- polar orbits
- A satellite orbiting a planet at the same rate as the planet. A geostationary satellite orbiting Earth has a period of 24 hours. orbits
Polar orbits take the satellites over the Earth鈥檚 poles. The satellites travel very close to the Earth (as low as 200 km above sea level), so they must travel at very high speeds of nearly 8,000 m/s.
Satellites with polar orbits are used for monitoring the weather, military applications (spying) and taking images of Earth鈥檚 surface.
Geostationary satellites take 24 hours to orbit the Earth, so the satellite appears to remain in the same part of the sky when viewed from the ground. These orbits are much higher than polar orbits (typically 36,000 km) so the satellites travel more slowly, at speeds of around 3 km/s.
Satellites in geostationary orbits are used for communications and satellite television.
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 around or below 7,600 m/s.
- A stable orbit is one in which the satellite鈥檚 speed is just right 鈥 it will not move off into space or spiral into the Earth, but will travel around a fixed path.