There are hundreds of comets in our solar system, but many people have never seen one. Most comets do not come close enough to the Earth to be seen without a telescope.
People of long ago were very much frightened when there was a comet bright enough to be seen. They thought it was a sign that something terrible was about to happen. Today people have given up that old idea.
Comets have paths around the Sun just as the Earth has. But their paths differ in shape. They are like a long oval. The paths of many comets cross the Earth's path. Some day the Earth may have a head-on collision with a comet. It has already gone through the tail of one. This happened on May 18, 1910. Newspapers carried the news in advance. Many people feared the world would end. But nothing happened at all. If the news had not been in the papers, few people would have known that the Earth had had a strange adventure.
No one knows what would happen if the Earth ran into the head of a comet. The head of a comet is very big, but it is not a solid ball. Scientists believe it is made up of millions of chunks of rock and iron surrounded by gas. Possibly the Earth would be badly damaged if it did hit the head of a comet.
When a comet is far away from the Sun, it does not have a tail. But when a comet comes close to the Sun, the light of the Sun drives out some of the gas in the head. This forms a tail which shines in the sunlight. A comet's tail always streams out away from the Sun.
Halley's comet is the most famous comet. It was named for Edmund Halley, a famous English astronomer. Halley's comet comes close to the Earth every 76 years. It was through the tail of this comet that the Earth passed in 1910. This same comet was seen during the Crusades when the Christians were fighting the Turks. One of the Christians' prayers was "Lord, save us from the Devil, the Turk, and the Comet."
Sometimes a comet goes too close to the Sun or a large planet and is pulled to pieces. Nothing is left of it but bits of rock and iron. All the comets may finally be destroyed.