Topics: Comet / Pages: 10 (2340 words) / Published: Oct 9th, 1999
The first written records of comets date back to nearly 3,000 years ago from

China and Europe. The accounts of these comets were believed to be the causes of

terrible events that occurred afterwards. In more recent times, however, astronomers have

found out what they really are. A comet is basically a mixture of ices, from both

water and frozen gases, and dust. They have also been given the names "dirty snowballs" or

"icy mudballs." The typical comet is less that 10 kilometers across. They spend most of

their time frozen solid in the outer parts of our solar system. Comets are composed of five

parts: the nucleus, coma, hydrogen cloud, dust tail, and ion tail. The nucleus is

pretty solid and stable, composed mostly of ice and gas with a small amount of dust and other

solids. The surface of the nucleus is best described as a black crust. Comet nuclei can

range from 1 kilometer to about 50 kilometers across. The black crust on the surface of

the nuclei helps the comet to absorb heat, which causes some of the ices under the crust to

turn to a gas. Pressure builds up underneath the crust and causes the surface to bubble up

in some places. Eventually, the weak spots of the crust break open from the pressure, and the gas

shoots outward; this is referred to by astronomers as a jet. Dust that had

been mixed in with the gas is also pushed out, and as more jets appear, a small gas and

dust shell forms around the nucleus, and this is called the coma. The coma, also called the

head, is a dense cloud of water, carbon dioxide and other gases and comes off of the nucleus.

They can be several thousand kilometers in diameter, depending on the comet's distance

from the sun and the size of the nucleus. The size of the nucleus is important because

since large nuclei have a greater surface area facing the sun, which is the side that is the

warmest, hence the side where most of the jets are coming from, it means more jets and

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