Not long ago, many people thought that comets were a sign that something bad was about to happen to them. Long ago people didn't understand about how objects in the sky moves, so the sight of a comet must have been very disturbing. There are many historical records and works of art which record the appearance of comets and link them with terrible events such as wars or plagues.
Now we know that comets are lumps of ice and rock that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips. When comets get close enough to the sun, heat makes them start to evaporate. Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long.
In 1985-1986, a spacecraft called Giotto visited the most famous comet Halley, on Halley's most recent visit to the inner solar system. In 1993, comet Shoemaker-Levy became trapped by the gravity of Jupiter and plunged into Jupiter's atmosphere!
In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp, Hale-Bopp was one of the brightest comets ever seen on Earth.
The solid part of a comet (the nucleus) is surrounded by a cloud of glowing gases, called the Coma.
The coma stretches out into a tail that can become broken up into streamers, with delicate twists and swirls.
Seeing a comet
Comets can be seen from Earth only when they are fairly near to the Sun. Most look like smudges of light, even through telescope.
Some comets have very long orbits which take them far out into the rest of the Solar System. They may only be seen from Earth once every few thousand years. Comets with shorter orbit can be seen more often and it is easier to predict their return.
Between 1995 and 1997, comet Hale-Bopp came into view. It was the clearest comet for nearly a hundred years. Its nucleus may be as wide as 40 km (25 miles).
Bright comets usually have two main tails. First there is a straight, bluish tail which stretches behind the comet, pointing away from the Sun. This tail is gas, blown off the comet by the solar wind.
There may also be a yellow-white dust tail which arches out in another direction. This traces out the path of the cometís orbit.
Some comets have more than two tails. A comet known De Cheseaux comet had seven, fanning out like a peacockís tail.