Section 1 of 4What Orbits What
Planets
- Orbit the Sun, held in place by the Sun's gravitational attraction.
Moons
- Orbit planets as natural satellites, held in orbit by the planet's gravity.
Comets
- Orbit the Sun on very elongated paths, which take them far away from it at times.
Asteroids
- Mostly orbit the Sun in the asteroid belt, which lies between Mars and Jupiter.
Artificial satellites
- Man-made objects that orbit the Earth or other bodies, held in orbit by gravity.
| Body or object | What it orbits |
|---|---|
| Planet | Sun |
| Moon | Planet |
| Comet | Sun |
| Asteroid | Sun |
| Artificial satellite | Earth |
Section 2 of 4Geostationary and Polar Orbits
Geostationary and Polar Orbits
Geostationary
- Orbits above the equator at about 36 000 km with a period of 24 hours.
Geostationary use
- Used for radio and telecommunication broadcasting, since it stays above one spot on Earth.
Polar
- Orbits round the north and south poles at a much lower height of a few hundred kilometres.
Polar use
- Used for weather monitoring and imaging the Earth's surface, with clearer images and shorter signal delay.
Section 3 of 4Choosing an Orbit
1
Choose a geostationary orbit when a satellite must stay over one point for continuous communication.
2
Choose a low polar orbit when detailed images are needed and the signal delay must be short.
3
A polar system usually needs several satellites to give continuous coverage of any one area.
4
The height of each orbit is set by the balance between gravity and the speed needed for a circular path.
Section 4 of 4Exam Checklist & Tips
8.2.3 – Key Points for the Exam
- Geostationary orbit A high orbit above the equator with a period of 24 hours.
- Polar orbit A low orbit that passes over the north and south poles.
- Natural satellite A moon that orbits a planet.
- Artificial satellite A man-made object that orbits a larger body.
- Orbital period The time taken for an orbiting body to complete one full orbit.
Common Mistakes
- Saying that a geostationary satellite is always above the poles; it is above the equator.
- Stating that moons orbit the Sun directly; moons orbit planets.
- Assuming a polar satellite stays above one point on the Earth;
- it moves over the surface as the Earth turns beneath it.