Comet Images

Many years ago people used to think that comets were huge balls of fire. In fact, they're solid, frozen, lifeless cosmic icebergs. Well, when they're not near the Sun anyway. Scientists believe that comets come from the Oort cloud - a ring of around 10 billion icy objects which resides around our Solar System, far beyond Pluto's orbit halfway between the Sun and another star (about 100,000 Au or 2 light years away). The comets all have a coma, inside which is a small bright nucleus usually about 10km in diameter. The whole thing (the coma and nucleus) make the head of the comet.

As I mentioned earlier, the comets come from the Oort cloud (which also links up with the Kuiper belt which, at one stage passes quite close to Neptune) and only come hurtling into the Solar System when they are dislodged (for example under the influence of a passing star). They accelerate all the time and so can be extremely dangerous when they reach the planets. Most of the time they just zip around the Sun and whizz back into outer space, but occasionally they get trapped by Jupiter's gravity and make repeated visits to the Solar System (eg Halley's comet - see below) until they finally evaporate.

We here on Earth can only see them when they come close to the Sun. This is because they themselves do not generate any light and it is only when they approach the Sun that they develop large tails of luminous material. Also, when they get hot enough and the nucleus unfreezes, a coma develops and the gas inside absorbs ultraviolet light, giving it a bright blue look.

Contrary to what many people seem to think, comets' tails don't point away from the direction they're travelling, they point away from the sun because of the solar winds.

The most famous comet is probably Halley's comet - named after Edmund Halley, the man who not only worked out that comets have orbits, but he also figured out that a comet he was observing (Halley's comet) would reappear every 76 years. He was right, and some evidence suggests that William the Conqueror saw it back in 1066. The last time it appeared was a little closer though, in 1986 and it is scheduled again in 2062.