Largest Comet Outburst Sent "Mini Comets" Flying

comet holmes outburst picture
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September 16, 2009—Like a mushroom shooting out spores, a well-known comet was seen firing multiple "mini comets" that went sailing away at up to 280 miles (451 kilometers) an hour, astronomers have announced.

The fragments were recently revealed in high-resolution images of comet Holmes, a relatively small body discovered in 1892 that mysteriously erupted in 2007, when the above images were taken. (Black-ringed dots moving in the images are background stars.)

Over several days astronomers using the Canada-France-Hawaii telescope on Mauna Kea watched the 2.2-mile-wide (3.5-kilometer-wide) cloud of dust surrounding the comet swell to become larger than the sun.

Later, closer looks at the high-resolution images revealed that the comet also sent fragments, each with their own dust clouds and icy tails, shooting away from the main body.

Altogether the extra material caused the comet to brighten by a million times in less than 24 hours—producing the largest known cometary outburst in history.

"Normally you need a large telescope to see comet Holmes, but during the outburst it was visible with the naked eye," said team member Rachel Stevenson of the University of California, Los Angeles.

Comet's Internal Pressure

Comet Holmes is a so-called Jupiter Family comet, one with an orbit that crosses so near Jupiter's that the comet is affected by the massive planet's gravity. Most such comets are doomed to break apart or crash into a planet or the sun.

For instance, comet Shoemaker-Levy 9 broke up in 1994 and its pieces crashed into Jupiter. At least one other comet, Kushida-Muramatsu, was briefly captured by the gas giant in 1949 and became a Jovian moon for 12 years.

In October 2007 an amateur astronomer first reported that comet Holmes had unexpectedly brightened. Upon hearing the news, Stevenson and her colleagues quickly trained the Canada-France-Hawaii telescope on the erupting object.

Based on what they saw, the team thinks the outburst was probably triggered five months before it was spotted, when the comet's elliptical orbit brought it closest to the sun, or about 186,000,000 miles (300,000,000 kilometers) away.

"We think that the interior of the comet was heated, vaporizing ice and creating a build up of pressurized gas inside," Stevenson said. By October the pressure had become too great and the comet erupted, releasing its gas in one go.

Stevenson presented the team's findings this week at the European Planetary Science Congress 2009 in Potsdam, Germany.

Despite the drama, the comet survived the explosion, Stevenson added, and it's now making its way back toward Jupiter. Its next close approach to the sun will be in 2014.

—Kate Ravilious

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