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comet are heavenly body and group of ice or snowballs which are left from planets with dust . if comets falls on earth it damaged it heavy but it has minimum chances  to not happen because comets are mostly revolve near the sun and if small comets enters the earth's atmosphere then it get incandescent and erased by the friction  . 
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what happens when big comet hits earth
when big comet hits earth then that comet is responsible for big damage and also comets work as eraser of life from earth .
how many area. tell me in
i don't know mch but when comet is of 4000km^2 with heavy weight and stikes the earth's surface then comet work as nuclear bomb .
It is a celestial object consisting of a nucleus of ice&dust and when near sun a tail of gas and dust particles pointing from the sun.  also it is an icy small solar system  body  that when passing close to sun heats up and begins to outgas. this phenomenon is due to the effects of solar radiation and  solar wind upon the nucleus of the comet
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