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Comets revolve around the Sun. Halley Comet completes one revolution around the Sun in an 5.3 billion km long orbit in 76 years. It consists of three parts-centre, coma and tail. The centre consists of dust particles whose sizes vary from sand particles to big houses. The coma is a small thing. Some times its size becomes as large as Jupiter. Its tail originates only when it gets closer to Sun. Sometimes it is without its trail and at times it develops two tails. It has not done any harm to the Earth. The Halley Comet was 32 million km away. When the comet is broken, it is converted into meteors. The meteors formed from the detritus of a comet begin burn as they enter the atmosphere of the Earth. Meteors begin to burn from a distance of 112 km from the surface of the Earth and are completely burnt by the time they are 6.5 km from the Earth. The burning meteors appear like fire works. Some meteors do not burn completely in atmosphere and reach the Earth but it is very rare. Recently meteors have fallen in Siberia in 1908 and 1947. Where meteors fell, all the vegetation was destroyed in an area having a radius of 20 km.

The meteors are mainly composed of iron. Traces of nickel, aluminum, sulphur, etc., have been found associated with the iron of the meteor.

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