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

By: Katherine Butler on Jan. 11, 2012, 10:35 a.m.
comet C/2001 Q4

Photo: NASA/Loke Kun Tan

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Cool cousins

Comets are the rogue cousins of our solar system family, flashing about space with brilliant tails streaming behind them. Made up of rock, dust and ice, comets were formed with the rest of the solar system some 4.5 billion years ago. Unlike asteroids, which formed closer to the sun, comets came together in space so cold that their icy particles would not melt. They circle the sun just like planets but in an elongated, elliptical orbit. Pictured here is the comet C/2001 Q4, whose last close encounter with Earth was almost four years ago. It was visible on Earth by the naked eye in the Southern Hemisphere in May 2008. (Text: Katherine Butler)