Planetary scientists have known for decades that there's no neat division between comets and asteroids. Some objects look rocky and inert when discovered, only to develop big, obvious gas-and-dust emissions later on (2060 Chiron comes to mind). Then again, occasionally a fuzzy, cometlike object ends up being a lifeless dud (such as "Comet" Wilson-Harrington).
Still, planetary astronomers acknowledge the existence of a handful of legitimate comets in the asteroid belt, and initially it appeared that the count had risen to five when the LINEAR survey spotted something with a tail last January 6th. Within hours it earned the designation P/2010 A2, indicating its stature as a periodic comet.
But this one seemed odd from the get-go, particularly due to its lack of a coma (cloud) surrounding its presumed nucleus and due to an orbit wholly within the inner asteroid belt. Suspicions grew deeper when David Jewitt (UCLA) and others wrangled some time on the Hubble Space Telescope to get a better look. The Hubble image revealed a bizarre X-shaped feature, strange striations in the dust tail, and (again) no hint of a coma.
The nucleus, only 24th magnitude in the Hubble view, corresponds to a modestly reflective object about 400 feet (120 m) across. "It's completely asteroidal," Jewitt told a gathering of planetary scientists last week. Seeing Hubble's view, he realized it was "some kind of rock that looks like a comet."
But how — when — and why?
With the release today of a new series of Hubble images (choreographed by Jewitt's team) and an additional view from the European Space Agency's Rosetta spacecraft, it's now clear that P/2010 A2 resulted from an asteroid's abrupt breakup in early 2009 — a full year before astronomers picked up the pieces.
The eight Hubble images, taken this year between January 25th and May 9th, reveal a straight dust tail that gradually dimmed as the object drifted farther from Earth. It's crisscrossed with streaks that appeared to have be arcs of dust shed by the mysterious X and pushed outward by solar radiation pressure. According to team member Jessica Agarwal (European Space Agency), the 100,000 tons of ejected dust is mostly millimeters to centimeters in size — far coarser than the usual cometary fluff. No one can yet explain how the X-shaped feature came about.
Working backward to get this Humpty Dumpty back together, Jewitt and his team calculate that this could have been the result of that 400-foot asteroid with a much smaller renegade rock about 15 feet (5 m) across. Jewitt also cautioned his audience, "There is no proof that this was caused by an impact." However, his Plan B would be equally bizarre: the asteroid could have spun up so quickly (due to its interactions with sunlight) that it literally tore itself apart.
But the smart money is backing an outright collision. At the same meeting, Colin Snodgrass (Max-Planck-Institute for Solar System Research) and others deconstructed images of P/2010 A2 taken by the ESA's Rosetta spacecraft on March 16th. En route to its flyby of the large asteroid 21 Lutetia, Rosetta was positioned 10° above the object's orbit plane.
Thanks to Rosetta's fortunate positioning, the European researchers concludes that the shape of the dust trail couldn't have come from typical comet, which ejects dust and gas over an extended period and creates distinctive fan-shaped dust tails. Instead, P/2010 A2's tail was straight with parallel edges. The implication is that all the dust came out at once, and over time the force of sunlight pushed the tiniest flecks farthest downstream but left the bigger chunks near the scene of the crime. All these grains, the team concludes, must have been released together within a week of February 10, 2009.
If an impact really was the cause, and because the debris remained visible a year after the fact, Jewitt notes that a similar small-scale smashups should occur annually. "One of these should be happening and observable at all times," he concludes. "The door is open to real-time disruption studies."