Comet Hale Bopp
Comet Hale Bopp
Credit: © 1997 Loke Kun Tan
Comets are large balls of ice, rock and dust. You can think of them as large dirty snowballs that hurtle around the Solar System. Comets are much smaller than planets or moons. Most are less than a few kilometres across. 
Astronomers have discovered about 4,000 comets in our Solar System so far. However, most comets come from beyond Pluto, in the Kuiper Belt and Oort Cloud. These areas of space probably contain billions of comets we have not discovered yet!

Comets have very elliptical orbits. This means that they go around the Sun in a large, stretched out oval, rather than a circle. The furthest point from the Sun in an orbit is called aphelion. The aphelion of most comets is way beyond the planet Neptune. The closest point from the Sun in an orbit is called perihelion. The perihelion of some comets can be closer to the Sun than the planet Mercury!

Comets' orbits take them far away from the Sun and then back towards it. Every time a comet gets close to the Sun it gets smaller as some ice "boils" off. This means if a comet passes close to the Sun too many times it will eventually break up into pieces!

Comet Path
The path of a comet around the Sun (not to scale)
Credit: NASA

The main, central part of a comet is called its nucleus. During most of their orbit, comets are hard to see. This is because they are small and made of dusty ice so do not reflect much light from the Sun. As a comet get closer to the Sun, the ice in the comet heats up. Some of this ice turns into a gas. The gas (plus dust and some water) forms a cloud around the nucleus, called a coma. The coma gets lit up by the Sun's light making it easier to spot with a telescope.

The Sun's solar wind also pushes the coma into a large tail which stretches out from the comet. This is why this tail always points away from the Sun. The gas in the tail contains changed particles called ions. This is why this tail is sometimes called the 'ion tail'. Some comet's ion tails are over 1 million km long!

Comets also have another tail. They leave a trail of small rocks and dust in their path. This is sometimes known as the 'dust tail'. When the Earth passes through a comet's dust tail, we get meteor showers.
Scientists think that comets may have hit the Earth when the Solar System was very young. Comets may have brought water and organic compounds with them. These are the ingredients needed for life. The ESA ROSETTA mission sent a spacecraft to a comet. The mission orbited the comet and was the first mission to land a space probe on a comet.