Astronomers peer into the past. They observe galaxies forming millions of years after the fact. They hunt for traces of water that flowed eons ago on moons and other planets.
Even those who study asteroids — rocky objects that whiz through Earth's neighborhood on a regular basis — ponder ancient history as they examine scattered debris to reconstruct events from millions, or even billions, of years ago. So it has been a rare treat for them to track the aftermath of a recent asteroid collision — one that happened about Feb. 10, 2009, to be precise.
"It's the first time we've ever seen an asteroid collision happening," said Colin Snodgrass, a research fellow at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, who led one of two teams that described the collision Thursday in the journal Nature.
It started out as a case of mistaken identity. At first P/2010 A2, as the object is called, didn't appear to be the remnants of an asteroid at all.
First seen by a robotic telescope on Jan. 6, 2010, it had a glowing tail, which led observers to think it might be an unusual type of comet. But UCLA astronomer David Jewitt had his doubts.
"It didn't look like any comet I'd seen," he said. "The designation and the orbit didn't jibe." This object's location in the inner asteroid belt — between the orbits of Mars and Jupiter — was much closer to the sun than was typical for a comet, and its orbit was more circular than that of a comet.
On Jan. 9, Jewitt asked a colleague to take pictures of the alleged comet with a ground-based telescope. P/2010 A2's "nucleus" appeared to be detached from its tail.
This too signaled that it was not a comet.
Jewitt thought it might be a cloud formed by an asteroid collision just days old — and likely to dissipate rapidly. He put together an urgent request for some of Hubble's precious observing time and began taking detailed pictures of P/2010 A2 with the Hubble Space Telescope on Jan. 25.
His photos, shot over a four-month period, showed that a collision had been likely, and that it probably occurred in February or March of 2009, his team reported in Nature.
The Hubble pictures also showed a weird X-shaped formation where the nucleus of the "comet" had been thought to be. Asked what he thought the "X" might be, Jewitt let out a long sigh. "I wish I knew," he said.
Some Star Trek fans have joked that it has something to do with Klingon spaceships, Jewitt said. But the most likely answer is that "the X is basically a splash pattern from the impact" of two wayward asteroids, he said.
Meanwhile, in Germany, Snodgrass had initiated his own investigation. He commandeered a tiny camera on the European Space Agency's Rosetta spacecraft, which was approaching the asteroid belt for a flyby past the asteroid Lutetia.
P/2010 A2 is orbiting the sun at an angle very similar to that of the Earth's orbit, Snodgrass said. As a result, any images of the asteroid taken from Earth can only provide a two-dimensional view that leaves out crucial information about its trajectory.
It's like trying to decipher a drawing on a piece of paper lying flat on a table at eye level, said astronomer David Nesvorny of the Southwest Research Institute in Boulder, Colo., who was not involved in the research. From that vantage point, only the edge of the paper is visible. To see what's on the paper, one needs to stand above it.
Rosetta's camera, which was designed to take pictures of asteroids at close range, could offer the viewing angle Snodgrass wanted. Its images captured in March weren't as sharp as Jewitt's Hubble photos, but they provided enough information for a computer to figure out that only one scenario could have created a trail like the one observed from both Earth and from Rosetta — an abrupt event that produced a bunch of dust "in one go," like a collision or explosion, Snodgrass said.
He said he was "reassured" to have reached the same result as Jewitt, and that he found the precision of their calculations quite satisfying.
"To be able to say this happened on Feb. 10 — that's big," he said.
Nesvorny, who wrote an editorial in Nature accompanying the papers, said the discovery had already added to scientists' knowledge about asteroid composition and the origins of the "zodiacal cloud" of dust in the solar system.
Astronomers had thought the dust might have been created by millions of asteroid collisions. Now, judging by the tiny amount of material produced by the collision that led to P/2010 A2, Jewitt and Snodgrass think that is unlikely.