In the wee hours of December 11th, University of Arizona astronomer Steve Larson was on cosmic patrol, taking images in northern Leo with the Catalina Sky Survey's 26-inch (0.7-m) Schmidt telescope.

Outburst from asteroid Sheila

Asteroid 596 Scheila, as recorded on December 15th by Alex Gibbs with the 1.5-m reflector atop Mount Lemmon, Arizona. This is a composite of thirty 20-second exposures. North is up, and the field is about 6 arcminutes wide.

Catalina Sky Survey

That's when he noticed something odd about the appearance of the main-belt asteroid 596 Scheila. It didn't look like its expected starlike pinpoint. Instead, the asteroid was clearly fuzzy, with a soft glow extending a few arcminutes to the west and north. Other astronomers quickly confirmed the cometary appearance, spotting a distinct arc of matter to the north and a smaller one to the south.

In the week since Larson's discovery, the mystery concerning Scheila has only deepened. Past sightings of this main-belt body, which averages 2.9 astronomical units (about 270 million miles) from the Sun in an out-of-round, 5-year-long orbit, showed nothing special. A series of carefully taken images in 2005-06 by Brian Warner reveal a well-behaved object that spins every 15.9 hours.

But something has definitely happened to Scheila in the past month. Alex Gibbs, a member of the Catalina team, rechecked images taken in November and found that the asteroid had already brightened by a few tenths of a magnitude. By December 3rd, it was showing signs of diffuseness and shining a whole magnitude brighter than normal.

Over the last week the mysterious cloud has dissipated considerably. But even though Scheila is currently 225 million miles (365 million km) away, a number of amateurs have acquired praiseworthy images of the strange development. A few examples are here, here, and here (all taken December 12th).

"We are studying the evolution of the dust cloud to determine if it is the result of a single impulse, or a more sustained process," Larson says. In principal, spectra of the outburst could distinguish between those two possibilities.

So Maryland amateur John Menke and his 18-inch Newtonian gave it their best shot on the 16th, recording a spectrum that was unexpectedly dimmed at the red end and displayed a few provocative emissions near the blue end. "Sure looks like more than dust to me!" he comments.

An earlier spectrum of Scheila suggests that it's a rare T type asteroid, which have dark surfaces that are a close spectral match to that of a bare cometary nucleus. If Scheila is truly a long-dormant comet, then it's a big one: current estimates put its diameter at 70 miles (113 km).

Cometary specialists David Jewitt (University of California, Los Angeles) and Hal Weaver (Applied Physics Laboratory) have snagged some discretionary time on the Hubble Space Telescope later this month to try to determine what's going on. Meanwhile, large ground-based telescopes are attempting to detect emission from CN, a molecule that would unambiguously point to outgassing from the object's interior.

"It's a main-belt comet, although I don't know what type yet," Jewitt explains. He says it could have resulted from an impact (as occurred earlier this year with P/2010 A2) or outgassing (as occurs on 133P/Elst-Pizarro).

One other kind of observation might distinguish a cometlike gush from an accidental whack. If it's the latter, notes dynamicist David Nesvorny (Southwest Research Institute), Schiela's orbit and/or its spin rate might undergo "a measurable change if the impactor was massive enough." For example, he calculates that a 1-km object with one-millionth of Scheila's mass, striking at 5 km per second, would tweak the orbital semimajor axis by about 50 miles (75 km) — 50 times larger than the orbit's current uncertainty.

Learning the source of Scheila's shroud now falls to professional observers and their big guns, and I expect they'll have news for us in the next week or two. So stay tuned!


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