You just know images of comets as bright spots with long tails? These images are normally taken from Earth and cover a huge area of the sky. You don't actually see the comet, just the tail which is caused by the effects of solar wind and solar radiation upon the icy nucleus. Here is a close-up view of comet Hartley 2, the object composed of ice, dust and rocky particles that causes the long tail. This is a medium sized comet with a length of about 2 kilometres. Comets have diameters of a hundred up to several tens of kilometres. A comet is suspected to be the originator of the famous Tunguska event in 1908.
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