In less than 24 hours, a small, faint comet became 400,000 times brighter late last month, blossoming into a fuzzy, starlike apparition visible to the naked eye. Now, 3 weeks after its spectacular flare-up, Comet 17P/Holmes remains visible to the naked eye in the constellation Perseus, which stands nearly overhead from the United States soon after midnight.
Many comets brighten as they near the sun. Heat vaporizes volatile ices on a comet's surface, throwing out fine, highly reflective dust particles in the process. But Holmes, which has a 6.88-year orbit, never gets any closer to the sun than twice Earth's distance. Even more puzzling, the brightening took place about 5 months after the comet's closest approach.
The rapid brightening suggests that a layer of material lifted off the comet and disintegrated, says Zdenek Sekanina of NASA's Jet Propulsion Laboratory in Pasadena, Calif. The resulting dusty halo may be "microscopic dust grains originating from the cataclysmic breakup of the jettisoned layer," he notes in the Nov. 3 circular of the International Astronomical Union.
The entire nucleus of the comet may consist of many such fragile, stacked layers, cemented by ice, Sekanina speculates. In support of his model, he cites observations by ground-based telescopes of parallel streaks of material at some distance from the comet's nucleus. The streaks could be dust trails left behind by a disintegrating layer, he suggests.
The 5-month delay between the comet's closest approach and the outburst may represent the time required for the outer layer to soak up solar heat and transmit it to an underlying region of ice, Sekanina says. Only when the ice explosively vaporizes does the outer layer fly off.
An English astronomer discovered the comet in the fall of 1892, when it had undergone a similar sudden brightening about 5 months after its closest approach to the sun. About 2.5 months later, in January 1893, Holmes had an additional outburst. It then remained quiet until recently. For decades, the comet was so faint that astronomers lost track of it between 1906 and 1964.
Could the newly brightened Holmes, now fading, get a second wind this January, as it did 114 years ago?
Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., thinks the 1892–1893 events and the current outburst may be linked. One possibility is that some debris from the older set of outbursts fell back onto the comet, choking off activity for more than a century. Last month, internal pressure from vaporizing ice might have finally become strong enough to eject the fallen debris, says Hal Weaver of Johns Hopkins Applied Physics Laboratory in Laurel, Md.
Recent images of Holmes from the Hubble Space Telescope show three spurs of dust emanating from the nucleus on Oct. 29. An Oct. 31 observation indicates that the comet had another, much tinier, outburst of dust on Oct. 30, notes Weaver.
Brian G. Marsden
Harvard-Smithsonian Center for Astrophysics
60 Garden Street
Cambridge, MA 02138
NASA Jet Propulsion Laboratory
Mail Stop 183-501
4800 Oak Grove Drive
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Harold A. Weaver
Johns Hopkins University
JHU Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723-6099
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