Comet Hartley 2 is an odd, dumbbell-shaped object that rotates as it tumbles along its orbit. One end spews carbon dioxide gas so violently that it regularly throws off chunks of ice as it travels around the sun every six and a half years or so. Astronomers are scratching their heads over how such a small object could still have so much energy and material after billions of years of existence.

NASA’s EPOXI mission flew near enough in November 2010 to capture impressive images of Comet Hartley 2 ejecting its gases. Much of the data it collected has since been analyzed. The University of Maryland’s Michael Ahearn told recently, “Among the comets visited by spacecraft, Hartley 2 is in a class by itself.”1

First, the comet looks young. reported:

For starters, its nucleus contains an abundance of carbon dioxide (CO2—or, in solid form, dry ice). This is a volatile material—it burns easily—and so scientists would expect much more of it to have burned away in the 4.5 billion years since the comet formed along with the rest of the solar system.1….

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