“Oddball” asteroid – the third largest near-Earth rock -- is really a comet

Discovery could have implications for origin of water on Earth

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An international team of scientist s today said a the third largest near-Earth object-believed for 30 years to be an asteroid, is actually a comet.

Using the Spitzer Space Telescope operated by the NASA's Jet Propulsion Laboratory at the team -- led by Michael Mommert of Northern Arizona University and Joshua Emery, assistant professor of Earth and planetary sciences at the University of Tennessee - looked at images of the rocky object known as 3552 Don Quixote taken in 2009 when it was in orbit closest to the Sun and found it had a coma and a faint tail.

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About 5% of near-Earth objects are thought to be "dead" comets that have shed all the water and carbon dioxide in the form of ice that give them their coma -- a cloud surrounding the comet nucleus -- and tail.  The team found that Don Quixote is neither. It is, in fact, an active comet, thus likely containing water ice and not just rocks, the team concluded.

Emery said the researchers also reexamined images from 2004, when it was at its farthest distance from the Sun and found that the surface is composed of silicate dust, which is similar to comet dust.

He also determined that Don Quixote did not have a coma or tail at this distance, which is common for comets because they need the Sun's radiation to form the coma and the Sun's charged particles to form the tail. The researchers also confirmed Don Quixote's size and the low, comet-like reflectivity of its surface.

"Don Quixote has always been recognized as an oddball," said Emery in a statement. "Its orbit brings it close to Earth, but also takes it way out past Jupiter. Such a vast orbit is similar to a comet's, not an asteroid's, which tend to be more circular -- so people thought it was one that had shed all its ice deposits."

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What all of this means is that carbon dioxide and water ice might be present within other near-Earth asteroids, as well. It also may have implications for the origins of water on Earth as comets may be the source of at least some of it, and the amount on Don Quixote represents about 100 billion tons of water -- roughly the same amount that can be found in Lake Tahoe, according to Emery.

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