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 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
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"