What is a comet?

Astronomers have estimated that about 10 trillion comets make up the Oort Cloud, the enormous spherical cloud that surrounds our Solar System. The cloud is incredibly large, about 7.6 million million kilometers across. Each comet follows its own orbit around the Sun. They are all potato-shaped balls of snow and rocky dust, only kilometers across.

Comets are often described as 'dirty snowballs'. The solid center, or nucleus, of a comet consists mostly of ice mixed with sooty material. The nucleus is quite small, and is usually only a few kilometers across.

Comets travel around the Sun in an 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 System itself.

An example a a comet is the well-known Halley's Comet. Halley's Comet returns to Earth every 76 years after having travelled out close to the orbit of Pluto. Its last visit to earth was in 1986.