Comet Outburst Spawns Mini-Comets

A comet recently spewed out a cluster of mini comets in a huge outburst that was the largest ever witnessed by astronomers.

A team of researchers began observing the comet 17P/Holmes in October 2007, after it was reported that the object, about 2.2 miles wide (3.6 km wide), had brightened by a million times in less than a day.

UCLA researcher Rachel Stevenson and colleagues noted multiple fragments flying rapidly away from the comet's nucleus. They continued observing for several weeks after the outburst using the Canada-France-Hawaii Telescope in Hawaii and watched as the dust cloud ejected by the comet grew to be larger than the sun.

The astronomers examined a sequence of images taken over nine nights using a digital filter that enhances small features. They found numerous tiny objects that moved away from the nucleus at speeds of up to 280 mph (125 meters per second). These objects were too bright to simply be bare rocks, but instead were more like mini comets, creating their own dust clouds as ice on their surfaces sublimated directly to vapor.

"Initially we thought this comet was unique simply because of the scale of the outburst," Stevenson said. "But we soon realized that the aftermath of the outburst showed unusual features, such as these fast-moving fragments, that have not been detected around other comets."

Although the outburst was impressive in the telescope images, it wasn't visible to the naked eye.

Scientists aren't sure of the exact cause of the outburst. Possibly, pressure inside the comet built up as it moved closer to the sun, until eventually part of the surface broke away, releasing a huge cloud of dust and gas, as well as larger fragments.

Even after ejecting mini comets, the solid nucleus of comet Holmes survived and continued on its orbit, seemingly unperturbed.

Holmes takes about 6 years to circle the sun, and travels between the inner edge of the asteroid belt to beyond Jupiter. The comet is now moving away from the sun but will return to its closest approach in 2014, when astronomers will examine it for signs of further outbursts.

Stevenson will present the findings at the European Planetary Science Congress in Potsdam, Germany on Wednesday.

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