Comet is 'hyperactive' say astronomers

Posted by Emma Woollacott

Comet Hartley 2 is an excitable little chap - in a class of its own, say astronomers.

NASA's Deep Impact spacecraft got a close-up look last fall during its EPOXI mission. And a new, in-depth analysis of the images and data taken during the flyby confirms that carbon dioxide is the fuel for Hartley 2's ice-spewing jets.

Unlike most comets, Hartley 2 blasts out powerful jets, and also spins and tumbles rapidly.

"Hartley 2 is a hyperactive little comet, spewing out more water than other comets its size," says University of Maryland astronomer Michael A'Hearn, who's principal investigator for the EPOXI and Deep Impact missions.

"When warmed by the sun, dry ice [frozen carbon dioxide] deep in the comet's body turns to gas, jetting off the comet and dragging water ice with it.

The strong activity in water release and carbon dioxide-powered jets aren't spread equally across the comet. During the flyby, carbon dioxide-driven jets were seen at the ends of the comet, with most occurring at the small end.

In the middle region, or waist, of the comet, water was released as vapor, with very little carbon dioxide or ice. This implies that material in the waist is probably a product of the activity at the ends of the comet, the researchers say.

"We think the waist is a deposit of material from other parts of the comet, our first evidence of redistribution on a comet," says professor Jessica Sunshine, deputy principal investigator for the EPOXI mission.

"The most likely mechanism is that some fraction of the dust, icy chunks, and other material coming off the ends of the comet are moving slowly enough to be captured by even the very weak gravity of this small comet. This material then falls back into the lowest point, the middle."