Comets are made of ice, dust and gas and orbit around the Sun in very elliptical orbits. When they are a long way from the Sun they are inactive and very difficult to see. As they pass closer into the inner Solar System they are heated by the Sun, and the trapped gases begin to escape from the centre, or nucleus, of the comet and stream out into space to form a huge tail, millions of km long. This gas becomes ionised and gives out light, this makes the comet very easy to see. The main gases in a comet are carbon dioxide and nitrogen.
The nucleus of the comet may only be a few tens of km across, but it is surrounded by a cloud of hydrogen gas called the coma which can go on for 10 000 km. The picture on the right is of the nucleus of Halley's comet, which was intercepted by Giotto when it last passed through the Solar system in 1986. The white areas are where the Sun has heated the nucleus and the gas is escaping to create the tail. The tail always points away from the Sun, and can be made up of several parts. The white coloured tail is the dust tail that curves slightly back along the path that the comet has travelled. The ion tail is ionised gas and is usually blue in colour. This points directly away from the Sun. There is also sometimes a sulphur tail which is usually yellow and much smaller than the other two. This will also point directly away from the Sun. Not all comets will have all of the tails. This depends on what each individual comet is made of.
Comets are thought to be left over from the creation of the outer planets. The pull of Jupiter's gravity threw many of them out of the inner Solar System, and the ones without enough speed to completely escape the Sunís gravity formed a large cloud called the Oort cloud. There are thought to be billions of these comets still sitting in the Oort cloud. Disturbing this cloud can cause some of the objects to fall back towards the Sun. These are seen in our sky as comets.
There are two types of comets Ė long and short period. Short period comets orbit in less than 200 years while long period comets take more than 200 years.
Perhaps the most famous comet of all is Halley's comet, which visits our region of space approximately every 76 years. Its last visit was in 1986 and we wonít see it again until the year 2061.
Comets are known for causing destruction, for example a comet is thought to have killed the dinosaurs when it hit the Earth 65 million years ago, but they are probably the only reason life exists on this planet at all. Comets hitting the early Earth is the most likely way that water was brought to our planet, and is the only reason that life was able to evolve.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001