What is a comet?

Comets are often described as 'dirty snowballs'. The solid centre, or nucleus, of a comet consists mostly of ice mixed with sooty material. The nucleus is quite small and is usually only a few kilometres across. Comets travel around the sun in and elongated orbit. They plunge out into deep space beyond the farthest planet before diving back into the Solar System and passing close to the sun. As the comet's nucleus comes closer to the sun it becomes smaller and may eventually break up into small fragments. Comets are thought to be as old as the Solar Sytem itself.