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Hartley 2, a tiny, hyperactive comet

Comparison of a small part of (Left) Tempel 1 with (Right) Hartley 2 at approximately the same image scale and with nearly identical instruments. (Left) Impactor Targeting Sensor (ITS) image iv9000675, 9.1 m pixel-1. (Right) MRI image mv5004032, 8.5 m pixel-1. Sun is to the right.
[Image courtesy of Science/AAAS]

A little comet called Hartley 2 has an unusually small, active center that is spewing out water vapor and ice chunks, researchers report.

These findings come from the EPOXI mission, which is an extension of an earlier mission called Deep Impact. Deep Impact slammed a probe into comet 9P/Tempel 1 to study the comet's interior. The spacecraft that flew by Tempel 1 to drop off the impactor was then renamed EPOXI and redirected to Hartley 2, which it flew past in November 2010.

The nucleus of Hartley 2 is the fifth to be observed up close by a spacecraft and is much smaller than the previous four.

In a study appearing in the 17 June 2011 issue of the journal Science, Michael A'Hearn of the University of Maryland, College Park and colleagues give an overview of the findings from this encounter.

Hartley 2 is a member of a class of hyperactive comets that release far more water from their surface than most other comets do. Typically, comets release water when ice on their surfaces turn to gas, but in the Hartley 2 nucleus, most of the outgassing seems to be occurring as the release of carbon dioxide and other volatile gases drags chunks of ice out of the nucleus.