The nucleus, or body, of a comet has been described as a dusty snowball consisting of tiny pieces of rocky material embedded in a mass of ice and frozen gases. The nuclei of most comets are believed to range from 1 - 10 kilometers (.6 - 6 miles) in diameter.
The Coma and Hydrogen Cloud
The coma is a cloud of gases that surrounds the nucleus of a comet. In some comets, a cloud of hydrogen gas surrounds the coma. This hydrogen cloud may be as big as 3 million kilometers (about 2 million miles) across.
When comets come near the Sun, their tails become visible because heat from the Sun vaporizes some of the icy nucleus or head and sunlight reflects from the vapor. Solar wind pushes the vapor in a direction away from the Sun to form the comet's tail. For this reason, comet tails generally point away from the Sun.
Comet tails are often very long. When the Great Comet of 1843 was nearest the Sun, its tail was perhaps as long as 475 million kilometers (300 million miles), or extending beyond the orbit of Mars.
Some comets also develop separate, curved tails or dust clouds. More than 1,000 tons of this dust escaped from the head of comet C/1973 El (Kohoutek) every second, and formed a "dust tail."
What are Comets Made Of?
From the study of spectrograms, it is known that comets are made largely of compounds of carbon, oxygen, nitrogen, and hydrogen. These compounds include water, ammonia, methane, carbon monoxide, and smaller amounts of other, more complex compounds. The dust in comets is composed of silicates and some metals. Recently, ethane gas was detected in comet C/1996 B2 (Hyakutake).