updated 11/8/2010 10:47:46 AM ET 2010-11-08T15:47:46

NASA's Deep Impact spacecraft is running on fumes after chasing down two comets in the last five years, but the probe's cosmic work may not be done yet.

Deep Impact cruised to within 435 miles (700 kilometers) of Comet Hartley 2 yesterday (Nov. 4) and beamed back thousands of photos of the icy wanderer. The pictures should help researchers gain a better understanding of comet structure and evolution. [ New close-up photos of Comet Hartley 2.]

Over the course of its 2.9 billion-mile (4.6 billion-km) journey to the comet, Deep Impact used up almost all of its remaining fuel, researchers said. The probe doesn't have enough juice left for another comet encounter, but it may not be heading for the space graveyard just yet.

Deep Impact could end up serving as a relatively stationary observing platform for new studies, NASA officials said.

"NASA's looking at future uses, but that won't be decided for a little while," said Tim Larson of NASA's Jet Propulsion Laboratory. Larson is project manager of Deep Impact's flyby mission to Hartley 2, which NASA called EPOXI.

No more comet encounters

While the probe's comet-hunting days are over now, researchers are thinking about ways they may be able to use the spacecraft's instruments.

"We're looking forward to hearing some good ideas," Larson said.

Whenever Deep Impact goes drifting off into the cold depths of space, it can do so with its head held high, NASA officials said. Only five comets have ever been imaged up-close by spacecraft, and Deep Impact photographed two of them the other being Comet Tempel 1, back in 2005.

The observations Deep Impact has made should help scientists gain a better understanding of the structure and behavior of comets, according to researchers. The Hartley 2 flyby, for example, should yield the best-ever extended look at a comet streaking through the inner solar system.

And Deep Impact has been a bargain. Its extended, post-Tempel 1 mission was relatively affordable compared to other missions, said Ed Weiler, associate administrator at NASA's Science Mission Directorate.

Its $45 million price tag is about 10 percent of what it would have cost to launch a whole new mission.

"In these hard economic times, that's a really good deal," Weiler said.

Deep Impact's double duty

Deep Impact originally launched in 2005. The $252 million spacecraft served as mothership and observer on a mission to crash an 820-pound (371-kilogram) probe into Comet Tempel 1.

The impact, which took place in July 2005, revealed a great deal of water inside and on the surface of Tempel 1, as well as many organic molecules the building blocks of life in its interior. Researchers also got glimpses of layered, primordial material within the comet, yielding clues about how it may have formed 4.5 billion years ago.

After that mission ended, NASA decided to squeeze some more life out of the Deep Impact spacecraft.

They planned to send it after a comet named Boethin, aiming for a close flyby in December 2008. But that didn't pan out because Boethin vanished, likely breaking up into many tiny pieces.

So researchers settled instead on Hartley 2, a small comet that makes a long, looping trip around the sun once every 6 1/2 years. The comet was discovered in 1986 by Australian astronomer Malcolm Hartley, who was in attendance at Deep Impact mission control at JPL yesterday during the close flyby.

Deep Impact has been photographing Hartley 2 since early September, and it'll keep its instruments trained on the fleeing comet through late November.

The spacecraft has put a lot of miles on its odometer during these two successful missions 268 million miles (431 million km) to get to Tempel 1, and another 2.9 billion miles (4.6 billion km) to intercept Hartley 2, NASA officials said.

So it shouldn't come as a huge surprise that the spacecraft has burned through almost all of its original 187 pounds (85 kg) of hydrazine fuel.

A week or so before its closest approach to Hartley 2, Deep Impact only had about 11 pounds (5 kg) of the propellant left, scientists said.

NASA's broad EPOXI mission has been using the Deep Impact spacecraft to track and study various celestial objects. The name "EPOXI" is derived from the mission's dual science investigations the Extrasolar Planet Observation and Characterization (EPOCh) and Deep Impact Extended Investigations (DIXI).

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