Magazine article Science News

Flare-Up: Comet Holmes' Surprise Bloom

Magazine article Science News

Flare-Up: Comet Holmes' Surprise Bloom

Article excerpt

In less than 24 hours, a small, faint comet became 400,000 times brighter late last month, blossoming into a fuzzy, star-like 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.

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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. …

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