Geminid meteor shower 2010: Why is the Phaethon asteroid going to pieces?

Jewitt and Dr. Li captured images of Phaethon with NASA's STEREO-A spacecraft – one of a pair of sun-watching probes. They observed the brightening and proposed that Phaethon's flash occurred as it shed rocky material fractured by the heat of its close approach to the sun.

In effect, the researchers say, the object is a "rock comet" rather than an icy "dirty snowball" or "snowy dirt ball" comet. The duo published its results in November in the Astronomical Journal.

As for Phaethon's origins, another team led by Julia Maria de León Cruz at the Institute of Astrophysics of Andalusia in Grenada, Spain, suggests that Phaethon may be a chip off Pallas, a 340-mile-wide asteroid in the main asteroid belt, which circles the sun between Mars and Jupiter.

Although Phaethon and Pallas don't share the same overall color, a first-cut clue as to their surface composition, Phaethon does share more-detailed spectral signatures of nine other, smaller asteroids near Pallas that are associated with it. They posit that Phaethon and its nine siblings constitute debris left over from a collision between Pallas and another object in an event that would have carved a sizable crater into Pallas.

The Grenada team published its study in April in the journal Astronomy and Astrophysics.

It's still unclear if Phaethon is shedding enough material to continually resupply the Geminid meteor stream, Jewitt acknowledges. The stream is about 1,000 years old. Phaethon would have to undergo at least 10 such shedding events each orbit over that time to provide enough material to sustain the Geminid shower that humans observe today. So far, astronomers have observed just one.

More broadly, Phaethon could be opening a window on a little-understood process that marks the end of the line for many asteroids and comets.

"We know small bodies can be destroyed in different ways," Jewitt says. Collisions can break them apart. Comets can run out of gas, their cores becoming dark hulks orbiting the sun. Asteroids and comets can end in fiery plunges into the sun. And comets can get disrupted by planets' gravity and break apart.

With Phaethon however, astronomers may be witnessing that Jewitt calls spontaneous disintegration. "Its a physical decay," he says. "They fall to bits, for reasons which are unclear."

Not a promising future from Phaethon's perspective. But for Earthlings, that slow-mo crumble can put on a good show.

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