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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 greater

amounts of gas and dust go into the coma. Even though the coma can get to be very

large, its size can actually decrease about the time it crosses the orbit of Mars. At this

distance the particles that drift out from the sun act as a powerful wind which blows the gas and

dust particles away from the nucleus and coma. This is the process which makes the comet's

tail. The hydrogen cloud is very large at millions of kilometers in diameter. But it

is only a very sparse body of neutral hydrogen. It was discovered from spectroscopy that

was carried out by satellites in 1970. Hydrogen was discovered in comets

Tago-Sato-Kosaka and Bennett. It is ionized hydrogen that forms the light that goes past the

coma. The reason why the hydrogen cloud was not discovered for a long time is because it is

not visible from Earth. Atomic hydrogen emits in the ultraviolet, but the ozone layer stops

the waves from entering. The hydrogen cloud can only be observed from space, with

satellites. The dust tail is usually up to 10 million kilometers long, and is composed of

smoke-sized dust particles that come off the nucleus by escaping gases. The dust tail is also

the most visible part of a comet to the naked eye. The tail has a potential to be long when

it enters the orbit of Earth. The record for the longest tail is the length of the Great

Comet of 1843; its tail extended more than 250 million kilometers. The ion tail, known as type

I or plasma, is made up of ions. It can be up to 100 million kilometers long and 100,000

kilometers wide. The tail is straight and always is opposed to the direction of the Sun. The

color of it, through a spectrum, is mostly blue. The reason why the tail is ionized is

because of solar wind. Solar wind, which flows at about 400 kilometers per second, is filled

with charged particles that are around the solar magnetic field. The gases in the tail

are ionized by the process of "photoionization...
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