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# Write a note on Comets.

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* In our solar system, there is a group of about a million celestial objects beyond Pluto which is called 'Cloud of Urt' [Wikipedia mentions it as : Cloud of Oort].
* These objects revolve around sun in a highly elliptical orbit and are called Comets.
* Comets are made up of dust and icy rocks.
* When a comet comes closer to sun, the ice gets vapourised forming a luminous tail.
* When the comet is nearest to sun, the tail is the longest and away from the sun.
* As the comet moves away from sun, the tail starts becoming smaller and ultimately disappears.
* Due to its tail a comet is called 'Tailed Star' despite the fact that it is neither a star nor is its tail permanent.
* The orbits of more than 750 comets are known.
* The most famous Halley's comet appears after a period of 76 years. It was last seen in 1986 AD and will be seen next in 2062 AD. It was named after Edmond Halley who had studied it in detail.
* In 1997 AD, a brilliant comet 'Helibob'[Wikipedia mentions it as : Hale Bopp] was seen for about 19 months even with naked eyes.
* A comet is basically made of water, carbon dioxide, ammonia, other frozen gases and dust particles.
* A detailed study has shown that the tail of a comet contains molecules of CO and HCN. These molecules are essential for the formation of certain complex compounds required for the origin of life.
* Some scientists believe that these molecules might have come to the earth from the tails of comets and resulted in the origin of life.
* Ancient people believed that the arrival of a comet was responsible for war, epidemic and natural calamities like flood. Modern science has, however proved that the arrival of a comet is a normal event and there is nothing to panic about.