ScienceShot: Comet Revealed as a Giant Dog Bone

At about 10 a.m. this morning, the EPOXI spacecraft screamed by the icy nucleus of comet Hartley 2 at 43,000 kilometers per hour. The fifth comet ever imaged so close up, Hartley 2 looks a bit like a dog bone, or a dumbbell, or, more technically, a highly elongated triaxial thingy. The shape of the comet, which is only about a kilometer in size, is reminiscent of Comet Borrelly, first sighted by the Deep Space 1 spacecraft in 2001, only more so. Presumably, Hartley 2 was once two separate bodies that came together. Or, it could have always been a solitary, barely compacted collection of rubble that came close to splitting in two when it spun too fast. Today, the sun's warmth is driving off water vapor, other gases, and dust—especially in bright jets seen streaming off the far end—that have been locked up in the comet since the solar system formed 4.6 billion years ago. This afternoon, team scientists will release processed images and perhaps make some sense of the rough ends and smooth neck between, something never seen before.

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