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_comets _
By: Brandy Finley
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 comets 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 comets 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 Halleys 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 Heavens 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. Works Cited
Franklin, William. Small Comets. Online. Internet. Available:
http://smallcomets.physics.uiowa.edu/ Hamilton, Calvin. "Asteroid
Introduction." Online. Internet. Available:
http://www.solarviews.com/eng/asteroid.htm Laborde, John. Comet West.
Online. Internet. Available: http://www.solarviews.com/cap/comet/west.htm
Rondlen, Michael. Comet. Online. Internet. Available:
http://windows.ivv.nasa.gov/ Sipes, Russell. Comet Hale-Bopp. Online.
Internet. Available: http://www.sipe.com/halebopp/"Comet Hale-Bopp"
Word Count: 2350
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