Comets are made of ice, dust and gas. They orbit the Sun in very oval (elliptical) orbits. When they are a long way from the Sun they are very difficult to see because they have no tail. As they get closer to the Sun they are heated up, and all of the gases trapped inside begin to escape from the centre. These gases stream out into space to form a huge tail, millions of kilometres long. This makes the comet very easy to see. The main gases in a comet are called carbon dioxide and nitrogen.
The centre (called the nucleus) of the comet may only be a few tens of kilometres across. This is surrounded by a huge cloud of hydrogen gas called the coma which can be as big as 10 000 km across.
The tail always points away from the Sun, and has several different parts. The white coloured tail is made of dust. The ion tail is made of gas and is usually blue in colour. Finally there is sometimes a sulphur tail which is yellow and much smaller than the other two. Not all comets will have all of these tails - it depends what the comet is made of.
Most comets live in a large cloud of these objects called the Oort cloud. Scientists think that there are billions of these objects still sitting in the Oort cloud. Sometimes some of these comets hit each other, or are disturbed by a passing star and fall back towards the Sun. This is when they get their long tails which let us see them.
There are two types of comets – long period comets and short period comets. Short period comets take 200 years or less to go round the Sun, while long period comets take more than 200 years.
The most famous comet of all is Halley's comet, which goes round the Sun roughly every 76 years. Its last visit was in 1986 and we won’t see it again until the year 2061.
A comet is thought to have wiped out the dinosaurs when it struck the Earth 65 million years ago, but they do not always cause destruction. Comets may be the only reason that life exists on this planet at all. The early Earth had no water at all, and some scientists think that all of the water in the oceans and rivers and lakes today was brought to the Earth by comets.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001