| The name "comet" comes from the Greek, kometes, which means "head of hair". Comets appear to be one of the most facinating objects in the sky. Despite its bright and impressive appearance when it gets near to the Sun, comets are actually small objects. They are small bodies made up of frozen gas and dust. They have been described as "... the nearest thing to nothing that anything can be and still be called something." Some of the comets have orbits that go beyond Pluto's orbit. When far from the Sun, the cometary body which are known as the nucleus, are about 10 kilometres on average. They are not massive, the average mass of a comet is approximately one ten-billionth that of Earth. It is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball". When a comet approaches the Sun, it appears to get larger and develop into a cloud of diffuse material called the coma. The fuzzy appearance of the coma is caused when solar energy heats volatile ices located within the nucleus. The ices change directly to a gas similar to the process when dry ice is changed from solid to gas. When a coma is near the Sun, it is typically 105 kilometres in diameter. When is approaches the Sun even further, a long tail pointing away from the Sun forms, extending over 100 million kilometres from the nucleus. The tail appears to be quite complex and is not formed by a single tail. It includes an ion tail and a dust tail. The tail aways points away from the sun due to radiation pressure and solar wind. The two forces that produce tails tend to act in different directions and to affect the gas and dust particles differently, that is why two tails are often observed. |
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