The image displays Don Quixote's orbit. Credit: Josh Emery

Some things are not always what they seem—even in space. For 30 years, scientists believed a large near-Earth object was an asteroid. Now, an international team including Joshua Emery, assistant professor of earth and planetary sciences at the University of Tennessee, Knoxville, has discovered it is actually a comet.

Called 3552 Don Quixote, the body is the third largest near-Earth object—mostly rocky bodies, or asteroids, that orbit the Sun in the vicinity of Earth. About 5 percent 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 finding will be presented at the European Planetary Science Congress 2013 in London today, Sept. 10. The discovery could hold implications for the origin of water on Earth.

"Don Quixote has always been recognized as an oddball," said Emery. "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."

Using the Spitzer Space Telescope operated by the Jet Propulsion Laboratory at the California Institute of Technology under contract with NASA, the team—led by Michael Mommert of Northern Arizona University—reexamined images of Don Quixote from 2009 when it was in the part of its orbit closest to the Sun, and found it had a coma and a faint tail.

Emery also reexamined images from 2004, when it was at its farthest distance from the sun, and determined 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.

This is Don Quixote's coma and tail (left) as seen in infrared light by NASA's Spitzer Space Telescope. In after-image processing (right), the tail is more apparent. Credit: NASA/JPL-Caltech/DLR/NAU

"The power of the Spitzer telescope allowed us to spot the coma and tail, which was not possible using optical telescopes on the ground," said Emery. "We now think this body contains a lot of ice, including carbon dioxide and/or carbon monoxide ice, rather than just being rocky."

This discovery implies 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, California's.

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