It is believed that a comet is made up of snow, ice, methane, ammonia, carbon dioxide, and other organic chemicals. It contains three parts: the nucleus, the coma, and the tail.
The nucleus usually has a diameter of one or two miles and is irregularly shaped. At the nucleus goes near to the Sun, the outer surface is melted, forming a gas. Dust is then freed. A dense cloud similar to fog is formed around the nucleus.
The coma – a larger, but less dense cloud – lies beyond this. It may be 50,000 to 60,000 miles in diameter, reflecting sunlight so that the nucleus is obscured.
The tail is formed when ultraviolet light breaks down the gas to form charged particles. These are carried by solar winds straight back as much as 10×106 or more miles. The tail points away from the sun.
Comets move in elliptical orbits. Because they are not massive, they can be overcome by the gravity of such large bodies as Jupiter and Saturn. Most scientists believe in the existence of the Oort Cloud that contains more than a trillion comets.
This is a list of some known comets. Distance is measured in Astronomical Units.