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Comets 101: A primer on the 'dirty snowballs' of space

Comet Lulin, C/2007 N3

Photo: NASA, Swift, Univ. Leicester, DSS (STScI/AURUA), Dennis Bodewits, et al.

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The comet Lulin

The Comet Lulin, otherwise known as C/2007 N3, is seen here as it approached Earth in February 2009. It’s named for the Lulin Observatory in Taiwan, where astronomers first spotted the comet in 2008. Here it is observed by NASA's Swift satellite. Astronomers believe that many comets come from the Oort Cloud, “a vast, extremely distant, spherical shell of icy bodies surrounding the solar system.” The Oort Cloud is a sort of borderlands in space, where our sun is too distant to invoke a strong gravitational pull on the objects. The Oort Cloud is believed to contain 0.1 to 2 trillion icy bodies in solar orbit, according to NASA. It is believed that tidal interactions from a passing star or the Milky Way itself can shift the contents of the Oort Cloud and in turn throw comets off their usual paths.