Hartley 2's hyperactive（极度活跃的） state, as studied by NASA's EPOXI mission, is detailed¹ in a new paper published in this week's issue of the journal Science by an international team of scientists that includes Lucy McFadden of NASA's Goddard Space Flight Center in Greenbelt, Md. After visiting a comet and imaging distant stars for hints of extrasolar（太阳系以外的） planets, you could say the spacecraft used for EPOXI had seen its fair share of celestial² wonders. But after about 3.2 billion miles (5.1 billion kilometers) of deep space travel, one final wonder awaited the mission's project and science teams. On Nov. 4, 2010, the EPOXI mission spacecraft flew past a weird³ little comet called Hartley 2.

"From all the imaging we took during approach, we knew the comet was a little skittish⁴ even before flyby," said EPOXI Project Manager Tim Larson of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It was moving around the sky like a knuckleball（不旋转球） and gave my navigators fits, and these new results show this little comet is downright（明白的，直率的） hyperactive."

The EPOXI mission found that the strong activity in water release and carbon dioxide-powered jets did not occur equally in the different regions of the comet. During the spacecraft's flyby of the comet -- with closest approach of 431 miles (694 kilometers) -- 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 vapor⁵（蒸汽，烟雾） with very little carbon dioxide or ice. The latter findings indicate that material in the waist is likely material that came off the ends of the comet and was redeposited.

"Hartley 2 is a hyperactive little comet, spewing out more water than most other comets its size," said Mike A'Hearn, principal investigator⁶ of EPOXI from the University of Maryland, College Park. "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."

Although Hartley 2 is the only such hyperactive comet visited by a spacecraft, scientists know of at least a dozen other comets that also are relatively⁷ high in activity for their size and which are probably driven by carbon dioxide or carbon monoxide（一氧化物） .

"These could represent a separate class of hyperactive comets," said A'Hearn. "Or they could be a continuum in comet activity extending from Hartley 2-like comets all the way to the much less active, "normal" comets that we are more used to seeing."