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Exploding comet in process of dying, astronomers say

Comet Holmes was once among the infinitesimal number of relatively small, nondescript celestial bodies that loop endlessly around the sun.
As with all comets, its nucleus formed from a mixture of primitive rock and ice some 4.5 billion years ago in a place where the planets of our solar system were born — in the flattened disc of gas and dust that once swirled around the sun. The comet spent most of its long life in the deep-freeze zone beyond Neptune until something deflected it closer to the sun. Then, like all comets, it began to disintegrate in the heat, slowly vaporizing into gas and ejecting dust, producing its characteristic comet tail.
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Comet Holmes (left), as observed from the Canada-France-Hawaii telescope, is seen next to an image of its inner structure after a digital filter was applied.
 But on Oct. 24, 2007, for reasons yet unknown, Comet Holmes blew up with such intensity that it became a million times brighter in less than a day, a phenomenon that had never been witnessed before. Once only visible through a telescope, it could now be seen with the naked eye.
"It was the most dramatic explosion ever seen in a comet, a very violent eruption," said David Jewitt, an astronomer who studies comets. He recently came to UCLA under a joint appointment with the Department of Earth and Space Sciences and the Institute of Geophysics and Planetary Physics.
The outburst, initially observed by chance by an amateur astronomer in Europe, immediately caught the attention of Jewitt, graduate student Rachel Stevenson and Jan Kleyna of the University of Hawaii. Two days later, they had Comet Holmes in the sights of some of the biggest and best telescopes in the world, including the Canada-France-Hawaii Telescope atop the summit of Mauna Kea. Over the next several months, they watched a tremendous sphere of gas and dust surrounding the comet's nucleus expand outward until it surpassed the size of the sun before fading away.
Astronomer David Jewitt.
"For a while, it was the biggest thing in the solar system," Jewitt said.
Much of the mystery surrounding the comet's explosion remained unsolved until two years later when Stevenson, who is now a UCLA doctoral candidate in planetary science, Jewitt and Kleyna reported on their study of images taken by the Mauna Kea telescopes' wide-field cameras, measurements of the nucleus and fragments, and the use of filters that strip away the gases and dust to reveal structures hidden inside.
Their findings were widely reported by National Geographic, Popular Science, Asian News International and, among other publications, after Stevenson presented a paper last month at the European Planetary Science Congress 2009 in Potsdam, Germany.
What they witnessed, they believe, is a comet in the process of dying as it ejects fragments of itself into space.
"This is probably one important way in which comets fall apart," Jewitt said. "The old idea was that they die by vaporizing material away at a slow, steady rate. It may be, in fact, that comets more usually have hiccups and burps before they 'vomit' all this stuff impulsively into space. Eventually, they run out of material to eject."
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The explosion caused the coma (left), the cloud of dust surrounding the comet, to grow to approximately 870,000 miles in diameter, making it larger than the sun (right) and Saturn, shown in the corner.
Stevenson said that Comet Holmes lost about 1 percent of its mass when it exploded — a tiny fraction of itself.  She was able to measure 16 distinct fragments that were expelled from the comet at about 200 miles an hour.
"These fragments were, in essence, mini-comets — some were the size of a bus. Each fragment was also releasing dust and gas, creating its own tail in the dusty atmosphere around it," Stevenson said. 
The explosion, which took place five months after the comet passed its closest point to the sun, was most likely triggered by sunlight. "Somehow it's able to store that energy and use it to make this dramatic event," the astronomer said.
"But we really don't know why it exploded," Jewitt said. It's even more puzzling because prevailing opinion is that the comet's nucleus is porous and not very strong, with the consistency of talcum power and the density of balsa wood. "So how can that sustain such a huge explosion? We have guesses, but we don't yet understand what happened."
Jewitt and Stevenson will now have to wait until 2014 when the comet once again comes closest to the sun.
"We're going to see if it will do it again," Stevenson said.
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