National Geographic
Naked Science
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                                   COMETS
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There are a reported 3 535 comets as of June 2008, of which several hundred are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion.

The number of naked-eye comets averages to roughly one per year, though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it is often considered a Great comet.


Scientists are only now starting to unravel the secrets of comets. Often referred to as dirty snowballs, they contain ice and elements from the very start of the universe. Some theorize that a comet, crashing into our planet, brought with it the organic material that started life on Earth. 

Spacecraft continues to offer new information on their makeup, from bringing back samples from a comet's tail to direct contact when a NASA-launched craft slammed into the comet Tempel-1.




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http://www.imdb.com/title/tt0989320/


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