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Ashley Yeager
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Comet ISON was punier than previously thought

NASA’s Mars Reconnaissance Orbiter captured comet ISON from 8 million miles away on September 29. While less spectacular than other images, the photos helped scientists improve estimates of the comet’s size.

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Guest post by Gabriel Popkin

SAN FRANCISCO — ISON, the comet whose Thanksgiving encounter with the sun captivated astronomers and the public, was probably no wider than New York’s Central Park.

A previous estimate from Hubble Space Telescope images put the maximum diameter of the comet’s core at 4 kilometers. But new estimates from NASA’s Mars Reconnaissance Orbiter suggest that the comet was almost certainly between 100 meters and 1 kilometer, and probably between 300 and 600 meters.

University of Arizona planetary scientist Alfred McEwen reported the size downgrade December 10 at the American Geophysical Union meeting. Such small comets usually break up when they come as close to the sun as ISON did. Scientists at the meeting confirmed that ISON is indeed dead and is not coming back.

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