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AURORA WATCH: Earth is exiting a solar wind stream that sparked beautiful polar auroras on Oct. 25th. The chances for a follow-up display tonight are low: gallery.
EXPLODING COMET: Exploding Comet 17P/Holmes is now larger than Jupiter. Astronomer Eric Allen of Quebec's Observatoire du Cégep de Trois-Rivières combined images he captured on three consecutive nights (Oct. 25, 26 and 27) and placed them beside a picture of Jupiter scaled to the same distance as the comet:
The diaphanous and curiously spherical cloud surrounding the comet's core is now large enough to swallow the King of Planets! It's gotten so big, in fact, that many observers say they can see it without a telescope. "I estimate the comet's brightness at magnitude +2.2 and last night I could see the disk easily with the naked eye," reports Martin McKenna of Maghera, Northern Ireland. "I saw the disk, too," says Doug Zubenel of Lincoln County, Kansas. Bill Smith of puts the comet "at magnitude +2.1--nearly as bright as Polaris"--and notes that "to the naked eye, it is no longer a simple point of light." [sky map]
What would make a comet explode in this fashion? Actually, a better question is, what would make a comet explode twice in this fashion? This is the second time Comet 17P/Holmes has erupted. The first was in 1892, an outburst that led to the comet's discovery by British astronomer Edwin Holmes.
The answer may be sinkholes. There is growing evidence that some comets and asteroids may have a porous internal structure akin to, say, swiss cheese or a honycomb. Suppose one of the chambers of the honycomb suddenly collapsed, exposing many square kilometers of fresh cometary ice to sunlight for the first time. A flurry of sublimation would ensue with mega-jets of dusty gas emerging from the sinkhole to create a cloud around the comet much as we see now. Twice = two sinkholes, one in 1892 and one in 2007.
Because no one can see the comet's nucleus--it is small, far away and hidden inside a Jupiter-sized debris cloud--this "explanation" is no more than conjecture. The true events at the core of Comet Holmes remain unknown. Grab your telescope and savor the mystery!
Comet 17P/Holmes Photo Gallery
[sky map] [ephemeris] [3D orbit] [Night Sky Cameras]