What is a Comet?

At one time, the appearences of a comet caused people to tremble with fear. They believed that comet s were evil omens fortelling plagues, wars, and death.Today, we have a pretty good idea of what comets are, though we still don't have all the answers about them. When a comet first appears, it is seen as a tiny point of light, though it may be thousands of miles in diameter. This point of light is "the head", or nucleus, of the comet. Scientists thick it is probably made of a great swarm of bits of solid matter, combined with gases. Where this matter originally came from is what is still a mystery.

As the comet approaches the sun, a tail usually appears behind it. The tail consists of very thin gases and fine particles of matter that are shot off from the comet's nucleus when it comes under the influence of the sun. Surrounding the nuleus of the comet is a third portion, known as "the coma". It is glowing cloud of matter that sometimes reaches a diameter of 150,000 miles, or even more.

Comet tails are very different in shape and size. Some are short and stubby. Others are long and slender. They are usually at least 5,000,000 miles in length. Sometimes they are almost 100,000,000 miles long! Some comets have no tail at all.

As the tail grows, the comet gains in speed beacuse it is nearing the sun, moving towards it head first. Then a curious thing happens. When the comet goes away from the sun, it goes tail first with head following. This is because the pressure of light from the sun drives off the very small particles from the comet's head to form its tail, always in a direction away from the sun.

As a result, when the comet goes away from the sun, its tail must go first. During its journey away from the sun, the comet gradually slows down and then disappears from sight. Comets may remain out of sight for many years, but most of them reappear eventually. Comets make trip after trip around the sun, but they may require a long time to make a single revolution. Halley's Comet, for example, takes about 75 years to make iits trip around the sun.

At present, astronomers have listed almost 1,000 comets, but there must be several hundered thousand comets in our solar system which remain unseen!