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Gravity is the force needed to maintain the orbit of planets, asteroids and comets around a star.

The attractive force of gravity also keeps moons and artificial satellites in orbit around a planet.

The Moon is a natural satellite.

An artificial satellite is a man-made body placed in orbit round the earth or another planet in order to collect information about it or for communication purposes.

The International Space Station is an example of an artificial satellite.

Key Point

Artificial satellites can be used for:

• communications – satellite television and phone calls;
• Earth observation - including weather forecasting, tracking storms and pollution, spying and satellite photography;
• navigation - including the Global Positioning System (GPS) on mobile phones and in cars;
• astronomy – looking into outer space from our solar system.

Examples of artificial satellites orbits include:

• polar orbits;
• geostationary orbits.

Polar orbits take the satellites over the Earth’s 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 - nearly 8000 m/s.

Polar orbits are often used for earth-mapping, as well as for some weather satellites.

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.

They orbit above the equator.

These orbits are much higher than polar orbits (typically 36,000 km) so the satellites travel more slowly (around 3 km/s).

Geostationary orbits are used for communication and broadcast satellites.