Comets
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 mud balls." The typical comet is
less than 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; astronomers refer this to 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 cometís 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 cometís 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 "photo ionization of the neutral molecules under the action of the
solar ultraviolet radiation", or "under the action of the solar wind by a
phenomenon where a proton removes an electron from an atom." The speed at
which the ions are moving is what causes the tail to be straight. The light from
the tail is emitted by "fluorescence," which