A large asteroid known for more than a century appears to actually be a comet in disguise, astronomers say. Most asteroids are chunks of metallic rock that have virtually no atmospheres. Tens of thousands of asteroids circle the sun inside what's known as the main asteroid belt, a doughnut-like ring that lies between the orbits of Mars and Jupiter. By contrast, most comets are loose clumps of dirt and ice thought to originate in the Kuiper belt, far beyond the orbit of Neptune. (source: news.nationalgeographic.com)
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