EPOXI spacecraft encounters comet

Flyby snaps pix of peanut-shaped Hartley 2

Small but spunky, hyperactive Comet Hartley 2 is shaped like a peanut and spews several jets of gas and dust on both its day and night sides, NASA’s EPOXI spacecraft has revealed. Intriguingly, the activity of the comet appears to be driven by the sudden venting into space of frozen carbon dioxide — dry ice — rather than frozen water, as has been found to be the case with many other comets, said principal mission scientist Mike A’Hearn  of the University of Maryland in College Park.

COMETARY CLOSE-UP This close-up image of Comet Hartley 2 was captured by the EPOXI spacecraft’s medium-resolution instrument during the craft’s closest approach on November 4. JPL/NASA, UMD

FLYING BY This series of images of Comet Hartley 2 was recorded by NASA’s EPOXI mission on November 4 as the craft flew under the icy body. The images start at top left and progress clockwise. NASA/JPL-Caltech,UMD

It’s the emission of carbon dioxide, rather than frozen water, that waxes and wanes most dramatically as the active portions of the comet rotate in and out of sunlight, he noted during a November 4 press briefing at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., just a few hours after the craft passed within 700 kilometers of the elongated object.

“This is the type of moment that scientists live for,” said planetary scientist Don Yeomans of JPL.

The new images, taken just after the closest approach of Hartley 2 to the sun, are allowing scientists for the first time to connect jet activity on a comet with specific topographic features on its surface, said EPOXI scientist Jessica Sunshine of the University of Maryland. In particular, the ends of the comets where jets are hurled into space are rough while the middle of the comet appears to be a valley where fine-grained material kicked up by the jets collects, creating a smoother surface. The brightness of the jets illuminates the nucleus and highlights the surface features, she said.

Four other comets have been visited by spacecraft, and although Hartley 2 is the smallest of these “it’s the most interesting and for its size, the most active,” Sunshine noted.

The released images all come from by EPOXI’s medium resolution camera. Pictures from the craft’s high resolution camera have already been beamed to Earth, but because that camera was out of focus, researchers are still using software to sharpen those images.

EPOXI is the new name for the Deep Impact mission, which in 2005 visited and blasted a projectile into Comet Tempel 1, a much larger and less active body than Hartley 2. The craft, which has also been used for extrasolar planet studies and has observed Earth, has too little fuel left to visit another solar system body.

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