A puzzling object that seemed to be a comet flying inside the solar system’s asteroid belt is no comet at all, but the remains of a violent collision between two fossil rocks that populate the belt, astronomers say.
The object was first sighted in early January by astronomers at the Air Force LINEAR project telescope in New Mexico, who reported it as a comet that must have flown into the asteroid belt from the solar system’s outer reaches, as all comets do. It was the fifth presumed comet to be reported in the unlikely region, they said, and they gave it the code name P/2010 A2.
The main asteroid belt lies between the orbits of Mars and Jupiter, and the strange object was spotted there again later in the month by astronomers at the Kitt Peak National Observatory in Tucson.
David Jewitt, a leading expert on comets and asteroids at UCLA, was intrigued by the reports of a possible comet inside the asteroid belt where millions of rocky objects fly in orbit around the sun, constantly colliding and grinding each other down.
He quickly obtained time for the Hubble Space Telescope’s wide field camera to home in on the object, and the images showed a stony nucleus just beyond an X-shaped halo of rocky filaments and a long, streaming tail of dust and gravel.
“Its appearance is very, very strange,” Jewitt said in an interview. “Strictly speaking it’s a hybrid – a comet because of its tail, but with no gas in the tail, it has to be a crash between two small asteroids, and we’re seeing it as it happened.”
The collision – which probably happened only a few weeks ago – reduced both bodies to a cluster of smashed-up rocks and gravel that under the radiation pressure of sunlight streamed out into a long, long tail looking just like a comet’s, Jewitt said.