Close examination of wide-field, high resolution images of outburstingcomet 17P/Holmes show a set of fragments which rapidly recede fromthe nucleus and fade.
Astronomers from the University of California, Los Angeles and theUniversity of Hawaii have discovered multiple fragments ejected duringthe largest cometary outburst ever witnessed. Rachel Stevenson, JanKleyna and David Jewitt began observing comet Holmes in October 2007soon after it was reported that the small (3.6 km wide) body hadbrightened by a million times in less than a day. They continuedobserving for several weeks after the outburst using the Canada-France-Hawaii Telescope in Hawaii and watched as the dust cloud ejectedby the comet grew to belarger than the Sun.
The astronomers examined a sequence of images taken over nine nights inNovember 2007 using a digital filter that enhances sharpdiscontinuities within images. The filter, called a Laplacian filter,is particularly good at picking out faint small-scale features thatwould otherwise remain undetected against the bright background of theexpanding comet. They found numerous small objects that moved radiallyaway from the nucleus at speeds up to 125 m/s (280 mph). These objectswere too bright to simply be bare rocks, but instead were more likemini-comets creating their own dust clouds as the ice sublimated fromtheir surfaces.
Caption: Animated movie showing the expansion of the comaof comet Holmes over 9 nights in 2007 November. The images havebeen spatially filtered to reveal fine structure. Inside the expanding envelope of the dust coma, a set of faintobjects and their associated dust trails can be seen receding from the nucleus. Black/white circles that jump from image toimage are background stars.
While cometary outbursts are common, their causes are unknown. Onepossibility is that internal pressure built up as the comet movedcloser to the Sun and sub-surface ices evaporated. The pressureeventually became too great and part of the surface broke away,releasing a huge cloud of dust and gas, as well as larger fragments.Surprisingly, the solid nucleus of comet Holmes survived theoutburst and continued on its orbit, seemingly unperturbed.
The orbit period of comet Holmes is about 6 years, putting it in theclass of Jupiter Family Comets whose orbits are strongly influenced byJupiter. These objects are thought to have spent most of the last 4.5billion years orbiting the Sun beyond Neptune, in a region known as theKuiper Belt. Holmes probably was deflected into its present orbitwithin the last few thousand years and is losing mass as it evaporatesin the heat of the Sun. In another few thousand years it is likelyeither to hit the Sun or a planet, be ejected from the Solar system, orsimply die by running out of gas.
The comet is now moving away but will return to itsclosest approach to the Sun in 2014, when astronomers will examine itfor signs of further outbursts.
Caption: (Left) Image of comet Holmes from the3.6-meter Canada-France-Hawaii telescope on Mauna Kea showing the large expanding dust coma. On the left, a 'raw' image is shown, in whichthe brightness reflects the distribution of dust in the coma of the comet (the nucleus is in the bright, point-like region to the upper left of center). On theright is shown the same image after application of the Laplacian spatialfilter, to emphasize fine structures. The white/black circular objects arebackground stars enhanced by the Laplacian filter.
[The same image isavailable here as a300dpi tif file.]
David Jewitt
