News in Science

Comet outburst sends fragments flying

Astronomers measuring the speed of fragments surrounding a comet that produced the largest outburst ever recorded, believe they have identified the source.

Comet 17P/Holmes presented spectacular viewing in October 2007, when the 3.6 kilometre-wide comet split open, creating a sphere of debris the size of the Sun. The comet brightened by a factor of a million, from magnitude 17 to magnitude 3, in less than a day.

Dr Rachel Stevenson of the University of California, Los Angeles and colleagues observed the comet over nine nights in November 2007 using the Canada-France-Hawaii Telescope in Hawaii.

Their research was presented today at the European Planetary Science Congress in Potsdam, Germany.

The researchers used a digital filter, known as a Laplacian spatial filter, which enhances discontinuities within the image, enabling them to pick out faint, small-scale structures.

They discovered a number of small fragments spiralling from the comet's central nucleus at speeds of up to 450 kilometres per hour.

The brightness of the fragments revealed they were not rocks but rather 'mini comets' creating their own coma, a halo of dust and gas lit by sunlight, the researchers say.

Blast from within

Stevenson says this suggests the fragments were not blasted from the comet by a collision, but from its own internal pressure.

"There are not many ways to eject fragments to such high speeds, so the discovery of these fragments allows us to rule out some possible causes of the outburst, such as a collision with another object in space, or material being flung off as the comet spun rapidly," she says.

"We suggest that pressure from gas built up inside the comet and eventually was strong enough to blast away some of the comet's surface."

Typically a comet's surface is dark. But gas released from the comet's core as it is heated by the Sun produces a glowing coma. Dust and the effects of solar radiation produce the characteristic tail.

"Initially we thought this comet was unique simply because of the scale of the outburst," says Stevenson.

"But we soon realised that the aftermath of the outburst showed unusual features, such as these fast-moving fragments, that have not been detected around other comets."

Surprisingly, the comet nucleus survived the outburst, as it did during a similar event in November 1892. The short-period comet orbits the Sun every six years and is due to return in 2014.

Not unique

Comet hunter Gordon Garradd of the Australian National University's Siding Spring Observatory says the cause of comet outbursts is unknown, but could be a combination of solar heating and gravitational effects.

"Comets are thought to be very friable, fragile objects that are not held together very tightly," he says. "Solar heating disrupts the surface allowing fissures to develop with further outgassing."

Garradd believes Comet Holmes' behaviour isn't unique and expects most comets to exhibit similar behaviour.

"It can happen to any comet, there's nothing specific about Holmes."