A Hybrid Asteroid-Comet

This weird space object observed repeatedly plunging close to the Sun has been puzzling astronomers: is it a comet or an asteroid? Now, it seems like a comet playing the asteroid. P/2007 R5 was first detected passing near the Sun in 1999 and spotted again in 2003 by the Solar and Heliospheric Observatory (SOHO) spacecraft.

Sebastian Hoenig of the Max Planck Institute for Radio Astronomy in Bonn, Germany, realized it could have been the same object and forecast with great precision its orbit and return in September 2007.

Officially, it was assigned as a short-period comet, orbiting near the Sun more than once and requiring less than 200 years to make one round. This is the first short-period comet found by SOHO.

When discovered, some astronomers saw P/2007 R5 as an asteroid rather than a comet, as it lacks the typical comet traits like a tail and a coma, the cloud of gas and dust wrapping a comet's body (nucleus).

Still, on its latest passage near the Sun, at only 15% of Mercury's distance from the star, it shone one million times stronger before fading again, a swing common for comets, not asteroids. This happens when the Sun's energy vaporises frozen water and carbon dioxide on the comet, blasting dust off its surface. The sunlight reflected by the dust produces the comet's brightness bursts.

"The object's orbit suggests it is a comet from the Kuiper belt, a reservoir of icy bodies out beyond the orbit of Neptune. Most of its surface ices were probably baked off during previous passes near the Sun, so that it now shows relatively little activity when it feels the Sun's heat," said Scott Sheppard of the Carnegie Institution of Washington in DC, US.

"It is quite possibly an extinct comet nucleus of some kind," said Karl Battams of the US Naval Research Laboratory in Washington, DC, who runs SOHO's comet discovery program.

The comet's nucleus is small, only 100 to 200 m (300 to 600 ft) across. The body behind the yearly Geminid meteor shower is believed to be another similar extinct comet, named 3200 Phaethon.

"Although P/2007 R5 has lost most of its comet-like flair, it should consider itself lucky. Many of the comets SOHO spots approaching the Sun get lost in its glare and never emerge again, presumably because the Sun's heat and gravity causes them to disintegrate," said Sheppard.

In the last years, astronomers detected other objects in the asteroid belt between Mars and Jupiter having comet-like tails.
"We're finding now that it's not so cut and dry - objects in the outer main belt of asteroids could easily have ices [in them]," Sheppard told New Scientist.

"Some people are calling them 'main belt' comets. There's probably not a sharp cutoff. Comets probably have a lot of rock in them and asteroids probably have a lot of ice in them as well."