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 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 “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 is a particle of solar wind
that excites an electron of the atom or molecule concerned. This electron
reaches a level at which it is stable, goes down again and releases its energy
in the form of a photon, a particle of light, of a well determined energy and
thus, of a specific color. When our solar system began, it was just a vast cloud
of gas and dust. Several billion years ago, the cloud slowly rotated around the
sun, which was very young, and particles within the cloud collided with one each
other. During this time some objects were shattered by these collisions, while
others grew in size and were to later become the planets. Throughout this early
period, comets probably filled the solar system. Their collisions with the early
planets played a major part in the growth and evolution of each of the planets.
The ice that makes up comets seems to have been what formed the first
atmospheres of the planets, and scientists now very strongly believe that it was
the collisions of comets that brought water to our world, and made life able to
begin. Over the years, comets actually became more rare within our solar system.
They do not fill our skies as they did about 4 billion years ago. Also today, a
comet that can be seen with the naked eye can be expected only about once in a
whole decade. Astronomers with powerful telescopes can see many more comets, but
even in this case it is still not common for as many as 15 or 20 comets to be
able to be seen in the sky at one time. Today, most comets are located outside
of our solar system in part of the original cloud of dust and gas that has
stayed pretty much untouched for billions of years. These regions are called the
Oort Cloud and the Kuiper Belt. The Dutch astronomer Jan Oort first proposed the
theory of the Oort Cloud in 1950. His study of the orbits of comets with very
long orbital periods made him believe that a large cloud of comets existed far
outside the solar system, possibly within the range of 5-8 trillion kilometers
(or more) from the sun. The total number of comets within this belt was
estimated as a trillion. It is thought that objects within this cloud are
occasionally ejected either by collision with one another, or by the
gravitational forces of stars. Many of the ejected objects probably never cross
the paths of the planets, and still more do not come close enough to be seen
with even the largest telescopes. However, a few do manage to travel into the
inner solar system and are subsequently seen from Earth. This cloud remains a
theory only, as it has never been directly detected. The Kuiper Belt is a region
that was first proposed by the Dutch-American astronomer Gerard Kuiper in 1951.
Seeing that Oort's cloud of comets did not really explain the reason for the
population of comets with short orbital periods (making complete orbits around
the sun in less than 200 years), Kuiper thought that a belt of comets probably
existed outside the orbit of Neptune within the range of 30 to 50 astronomical
units (2.8 to 4.6 billion miles) from the sun. Collisions and perturbations by
the planets of our solar system are believed to be the reasons for the ejection
of bodies from this belt. Around 1988, astronomers David Jewitt (University of
Hawaii) and Jane Luu (University of California at Berkeley) began searching for
members of the Kuiper belt using modern electronic cameras attached to a large
telescope on Mauna Kea, Hawaii. The equipment was capable of detecting extremely
faint objects. After nearly 5 years of systematic searching they found a
distinct image on 1992 August 30, which was subsequently designated 1992 QB1.
The object was moving very slowly, and calculations eventually revealed the
object took 291 years to orbit the sun at an average distance of 43 AU. Since,
the discoveries of that object over three dozen additional objects had been
found as of the end of 1996. Some astronomers estimate that there are over
30,000 icy objects bigger than 100 kilometers in diameter in the Kuiper belt.
The total mass of the belt is believed to be hundreds of times bigger than the
asteroid belt between Mars and Jupiter. Comets are one of the more exciting
things to study in astronomy because of a number of reasons. One of these
reasons is because they are unpredictable. Comets can suddenly brighten or fade
away in just a few hours. They can also lose their tail, or even develop more
tails. Another thing is they can split into pieces, so multiple comets can be
observed traveling together. Another reason why comets are interesting to study
is because they have some of the oldest and untouched objects in the solar
system. The comets’ composition represent how things were originally, and also
what made the sun and the planets how they are today. Studies have recently
shown that comets are what formed life on Earth. Collisions between Earth and
comets in earlier times brought water to the Earth, which resulted in oceans
forming. After the oceans were formed, it enabled life to begin. However, as
easily as comets can begin life, they just as easily can end it. The way that
the dinosaurs became extinct was from collisions from comets. Most of the comets
that are seen from Earth only come by once every few millions of years. Some do,
however, come back within around 200 years. These types of comets are called
short-period comets. These comets, which have shorter orbits, are believed by
scientists to come from the Kuiper belt. The reason why this is believed is
because there are some small, icy objects that orbit near and beyond Pluto that
have been detected. Since the orbits of short-period comets are shorter, they
pass the Sun more often, which makes it start to disappear. The ice and gasses
start evaporating, which leaves the dust and other solids left. When this
happens, meteors are formed. The orbit of planets and comets are alike because
they travel in an ellipse with the sun as the center point. However, for the
planets the orbit is more in a circular shape. Also, the planets orbit the sun
on the same plane. However, most comets, including comet Hyakutake, which was
visible in 1996, and Hale-Bopp that was in 1997, have elliptical orbits of that
are very large in size, and are shaped more like an oval than a circle shape.
The foci of these comets are very far apart from each other. The plane of comet
Hyakutake's orbit intersected the planetary orbit plane at a steep angle. The
plane of Hale-Bopp is nearly perpendicular to the plane of earth's orbit. One of
the most well known comets is Halley’s comet. This comet has been known since
around 240 BC, and maybe even as early as 1059 BC. Its most famous appearance
was in the year 1066 AD, when it was seen right before the Battle of Hastings.
The comet was named after Edmund Halley, who is the one who calculated its
orbit. He figured that the comets that were seen in 1531 and 1607 were the same
thing, which meant it had an orbit of 76 years. Halley died, however in 1742, so
he never lived to see his if he was right. His prediction did come true though
when the comet came back on Christmas Eve in 1758. Halley's Comet came in the
years 1835 and 1910. Then in 1984 to 1985, five spacecraft from the USSR, Japan
and Europe were sent to make observations and study Halley's Comet in 1986. One
of the deep space satellites from NASA was changed so it could observe the solar
wind upstream from the comet Halley. Only three comets have ever been studied
from a spacecraft. Comet Giacobini-Zinner was one of the comets studied from
space; it was in 1985. Comet Halley was studied in 1986. Comet Grigg-Skjellerup
was studied on July 10th, 1992. The study of comets is very important because it
is important to know what they are, and also it can explain a lot of unanswered
questions about the origins of life. Some people who do not fully understand
what comets are and what they do can be led to believe in many bad things, which
can result in a tragedy. An example of such a tragedy would be the “Heaven’s
Gate” incident. There were many people who believed that the comet Hale-Bopp
was going to cause something very bad to happen, so they all committed suicide.
If people can have a better understanding of what comets are, things like this
could possibly be prevented. The roles that comets have played in the earth and
in life as it is today are really big and important. If it had not been for
comets, then life could possibly not have even existed now. Also, many
creatures, for example the dinosaurs, have become extinct because of the comets
colliding with the earth. If the comets had never made these creatures go
extinct, then life would without a doubt be different. 
Bibliography
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