At first, observers thought the object designated P/2013 P5 was a comet lurking among the asteroids. But two sets of Hubble images show instead that's a bizarre asteroid that's shedding tails of dust into space.
David Jewitt is a veteran solar-system observer with a special interest in "main-belt comets," coma-shrouded objects that orbit among the asteroids between Mars and Jupiter. In fact, the UCLA astronomer has tracked down all 10 of the known ones.
So, in mid-August, when a team of observers led by Marco Micheli (University of Hawaii) discovered an 11th candidate, which was given the periodic-comet designation P/2013 P5 (PanSTARRS), Jewitt jumped into action. He quickly got approval to image the new object with the Hubble Space Telescope on September 10th and 23rd — and the resulting images were incredible.
Instead of a simple object with a single tail streaming behind it, P/2013 P5 turned out to have six tails radiating away from the central body. Moreover, the tails' appearance changed dramatically in the two weeks between Hubble's photo sessions. "We were literally dumbfounded when we saw it," Jewitt notes in a press release.
The tails weren't created all at once. Instead, based on modeling by team member Jessica Agarwal (Max Planck Institute for Solar
System Research), they appear to have been shed by the central body on six specific dates: April 15th, July 18th, July 24th, August 8th, August 26th, and September 4th. The tails then stretched out in a way that suggests their tiny particles range from 10 to 100 microns in size.
But P/2013 P5 probably isn't a comet. For one thing, it's in an orbit shared by the Flora family of asteroids, which formed roughly 200 million years ago, so it's likely just a chip off this grouping's parent body. Nor is it plausible that a rogue comet somehow became captured into asteroid-belt residency. A collision could certainly raise a cloud of dust — but not six times over the past half year.
Instead, Jewitt and his collaborators conjecture (in November 20th's Astrophysical Journal Letters) that the central mass is simply shedding dust every now and them. Using the Hubble images and knowing that Flora-family asteroids have fairly reflective surfaces, they estimate that the "nucleus" is no bigger than 1,600 feet (0.5 km) across — and probably somewhat smaller. So there's barely enough gravity to hold itself together.
If P/2013 P5 spins rapidly, which is likely a common trait within the Flora family, then dust on its surface must be sliding toward its equator, where it piles up and sloughs off into space. Sounds a little far-fetched, but the team finds this the most plausible explanation. And since the tails together represent no more than about 0.1% of the body's mass, they'll likely keep appearing until the near-surface dust runs out.