Book Report on Comets

This essay has a total of 2390 words and 10 pages.

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
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