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.

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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.

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