Orbits

There are eight planets in our solar system, which orbit our nearest star, the Sun.

The planets further from the Sun take longer to complete one orbit.

The time for a planet to complete one orbit of the Sun is called its year.

We can see planets because they reflect light from the Sun.

The orbits of the planets are slightly squashed circles called ellipses.

The Sun is quite close to the centre of the ellipse.

A comet is a lump of frozen dust and gases a few kilometres in diameter.

Comets travel around the Sun in very elliptical orbits.

As they approach the Sun, they warm up and some of the frozen gases evaporate forming a tail of dust and vapour, which can be thousands of kilometres long.

Light from the Sun reflects off the comet and tail.

Any object that orbits another is called a satellite.

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Gravity

The Earth, Sun, Moon and all other bodies attract each other with a force called gravity.

The force of gravity between two bodies decreases as the distance between them increases, so the closer together two objects are, the bigger the force of attraction between them.

Comets orbit the Sun due to gravity. As they approach the Sun, they accelerate and as they travel away from the Sun, they slow down.

A smaller body stays in orbit around a bigger one due to its high speed and the force of gravity between the bodies.

In order to stay in orbit at a particular distance, smaller bodies like the planets and satellites must travel at a particular speed around the larger body.

For a satellite to stay in orbit just above the Earth’s atmosphere, it must travel at about 8 000 m/s.

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Communications Satellites

Communications satellites are used for television, radio and phone systems.

We need to know where they are all the time so we can send signals to them and receive signals from them at any time.

They are usually put into a high orbit above the equator so that they move around the Earth in exactly the same time as the Earth spins, 24 hours.

This means that they are always in the same position when viewed from Earth.

These are called geostationary orbits.

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Monitoring Satellites

Monitoring satellites, like weather and spy satellites, are usually put into a low polar orbit. This means they pass over the North and South poles.

They are equipped with cameras or other detectors for scanning and surveying the Earth.

As the Earth spins beneath them, they can scan the whole Earth each day, from a much closer range than a geostationary satellite.

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