Comets are made of ice, dust and gas and orbit around the Sun in highly elliptical orbits. When they are a long way from the Sun they are dormant and very difficult to see. As they pass closer into the inner Solar System they are heated by the Sunís radiation, 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 emits light, making 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 this is surrounded by a cloud of hydrogen gas called the coma which can extend 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 consist 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 consists of ionised gas and is usually blue in colour. This will point directly away from the Sun. Finally there is sometimes a sulphur tail which is generally yellow and much smaller than the other two. This will also point directly away from the Sun. Not all comets will display all of the tails. This depends on the composition of each individual comet.
Comets are believed to be left over from the formation of the outer planets. The gravitational pull of Jupiter 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 believed to be billions of these objects still sitting in the Oort cloud. Disturbances in this cloud 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 have orbital periods of less than 200 years while long period comets have orbital periods of 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 have a reputation for causing destruction, for example a comet is thought to have wiped out the dinosaurs when it struck the Earth 65 million years ago, but they are probably the only reason life exists on this planet at all. Bombardment of the early Earth by comets 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