Comet Hides in Plain Sight: Don Quixote Isn't an Asteroid

A large, near-Earth asteroid has flown through space for the past 30 years. Known as 3552 Don Quixote, this asteroid's journey stretches to the orbit of Jupiter. Now, scientists have made a surprising discovery. This asteroid isn't an asteroid. It is, in fact, a comet.

"Its orbit resembled that of a comet, so people assumed it was a comet that had gotten rid of all its ice deposits," said Michael Memmert, one of the researchers, in a news release.

So how did the researchers find out that this asteroid was actually a comet? During an observation of Don Quixote in 2009, the researchers noted that the object was far brighter than expected. Later, the scientists compared infrared images of the supposed asteroid. Surprisingly, they found that this "asteroid" had a coma, which is a comet's visible atmosphere, as well as a faint tail.

It turns out that this asteroid didn't have the dry surface that scientists first suspected. Instead, the third-biggest near-Earth asteroid, which skirts our planet with an erratic, extended orbit, is sopping wet. The low, comet-like reflectivity of its surface and the fact that this comet possesses water has major implications for the origins of water on Earth.

It's very possible in our planet's distant past, comets carrying water bombarded the surface of Earth. In fact, researchers believe the amount of water on Don Quixote represents about 100 billion tons of water--roughly the same amount found in Lake Tahoe. While not all of Earth's water came from comets, it's likely that at least some of it has celestial origins. In fact, water on Jupiter has its own origins from a comet that smashed into the planet about 20 years ago.

The findings don't just have implications for understanding how Earth received its water, either. The discovery implies that carbon dioxide and water ice also might be present on other near-Earth objects.

The findings were presented at the European Planetary Space Conference in London this week.