An asteroid cruising through the solar system six years ago seemed just another silent ship sailing in the eternal darkness, until it flared up with the startling brightness of a comet's halo.
Just like that, the space rock known as NEO 2001 OG108 was re-classified as C/2001 OG108 in 2002, from asteroid to comet. Scientists now suspect that 5 to 10 percent of other Near Earth Objects (NEOs) may also be comets lurking in disguise as asteroids.
"That was the first real evidence we have of objects that look like asteroids but are comets in the NEO population," said Paul Abell, a planetary scientist with the Planetary Science Institute who is located at NASA's Johnson Space Center in Houston. Now he's heading a NASA-funded study to sort out which are which.
Telling apart comets and asteroids matters more than just to sticklers. Knowing the composition of NEOs is crucial to preventing possible collisions with Earth, especially when a collection of comet pieces bursting in the atmosphere can have far deadlier consequences than an asteroid. Finding out about the materials in comets and asteroids also provides hints about the early evolution of the solar system.
Abell's research may even uncover future targets for spacecraft to investigate, similar to the Deep Impact and Stardust missions. He is working with Faith Valis, director of the Multiple Mirror Telescope Observatory at Mount Hopkins, Ariz., to try and identify suspect objects for sure as comets or asteroids.
The most mysterious objects don't give up their secrets easily. Far-off comet bodies resemble dirty snowballs that lack the halo or "coma" they get once they approach the warmth of the sun. As a result, such objects can appear "blacker than coal" to telescopes because they reflect just 3 percent of light that hits them, Abell told SPACE.com.
The former NEO 2001 OG108 got as close to us as the orbit of Mars before acquiring its coma, which kept astronomers guessing up until then. Another object that continues to arouse controversy is 3200 Phaethon, a suspected asteroid that some observers believe to be the husk of a burned-out comet.
The uncertainty goes to the heart of comet evolution. Scientists argue about whether some comets simply lose their surrounding cloud of dust and gas, or form hardened shells to contain the loose icy material that makes up comets.
So far, Abell has set his sights on three comet culprits in the crowd of space rocks. NASA's Infrared Telescope Facility at Mauna Kea in Hawaii allows his group to see the chemical composition of different objects, and even find the unique fingerprints of comets depending on their origins.
For instance, some comets come from the Kuiper Belt, a disk-shaped icy cloud past the orbit of Neptune. Other passing comets such as Halley's Comet start from much further away in the Oort Cloud, which lies far beyond Pluto's orbit at 1,000 times the distance from the sun to the Kuiper Belt.
"There are some indications that there may be spectral differences between things that come from the Oort Cloud and things that come from the Kuiper belt," Abell noted.
The survey has a long way to go after analyzing just three objects, Abell said, but that's how science works. You get a result, and then you can start asking better questions.