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Comet flyby demonstrates science on shoestring budget
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: November 5, 2010


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Thursday's daring flyby of comet Hartley 2 not only collected a treasure trove of spectacular imagery, it also served as a model for future opportunities to reuse old NASA spacecraft for new missions.


Comet Hartley 2. Credit: NASA/JPL-Caltech/UMD
 
The Deep Impact spacecraft's encounter with Hartley 2 at about 1400 GMT (10 a.m. EDT) went off without a hitch. The car-sized probe hit its marks, missing its aimpoint by less than one-half of a percent as it soared by at a distance of approximately 700 kilometers, or 435 miles.

NASA immediately released five up-close shots of the never-before-seen comet, but the best imagery is still being processed, scientists said in a press conference Thursday afternoon.

After tense moments during the flyby, the images triggered applause in the EPOXI control room at the Jet Propulsion Laboratory in Pasadena, Calif.

"I remember in the seconds before the images came down, I thought to myself, 'This is an exploration moment,'" said Ed Weiler, associate administrator of NASA's science directorate. "Here we are sitting in this little building at JPL, and we're going to see something that no other humans in the history of humankind have seen, the nucleus of Hartley 2."

The probe's medium-resolution camera snapped the pictures unveiled Thursday. Researchers must carefully process high-resolution images to overcome a defect in the on-board instrument.

The imagery revealed a peanut-shaped rocky nucleus spewing water and other gases into deep space, creating a cloudy coma, or tail, around Hartley 2's core.

Only the fifth comet ever imaged by a nearby spacecraft, Hartley 2 doesn't fit the mold of other icy bodies closely studied on previous missions. Its nucleus is barely one mile across -- much smaller than other surveyed comets -- and it is much more active.

"While it's the smallest (comet), no doubt about it, I think it's undoubtedly the most interesting," said Jessica Sunshine, deputy investigator for the EPOXI mission. "And for its size, it is the most active."

Scientists hope to determine which features of Hartley 2 are left over from its ancient genesis, separating those characteristics from changes during the comet's evolution.

According to NASA, the EPOXI mission costs about $42 million from its inception to the end of operations. That is about one-tenth the cost of a new Discovery-class science mission, Weiler said.

The low cost makes EPOXI a "darn good bargain," Weiler told reporters Thursday afternoon.

"I would like to do that again," Weiler said. "We're going to be looking at other satellites. In these days of hard economic times, this is a really good deal."

Deep Impact was originally selected as a Discovery mission in 1999.

Discovery-class missions are relatively low-cost science-based projects geared for solar system exploration.


Artist's concept of the Deep Impact spacecraft approaching Hartley 2. Credit: NASA/JPL-Caltech/UMD
 
Deep Impact dropped a high-speed projectile on the surface of comet Tempel 1 in July 2005, blasting a hole in its nucleus and sending shards of icy debris into space. After it completed that mission, the spacecraft was ready for a new assignment.

The agency selected the EPOXI mission to use Deep Impact's twin telescopes to help search for planets around other stars. At the same time, navigators charted a course through the inner solar system to reach Hartley 2, using a series of small engine burns and three gravity assist flybys of Earth.

Anticipating another possible extension, NASA issued a request for information in early October, soliciting ideas from scientists for another reuse of Deep Impact.

According to Weiler, the probe no longer has enough fuel to change its trajectory and reach other comet. But it does have enough propellant for other missions.

Weiler identified hunting for asteroids or astronomy research as potential applications for Deep Impact, which carries two of the most sensitive telescopes currently in deep space.

The spacecraft still has about 10 pounds of maneuvering fuel, enough to maintain its orientation in space for roughly 11 years on a science mission, according to the request for information document released Oct. 5.

"We'll be entertaining those ideas in the next few months," Weiler said. "What I do know is it doesn't cost a lot to use a satellite a second, third, or even fourth time."

Tim Larson, the EPOXI project manager at JPL, said the spacecraft will continue observing Hartley 2 until the end of November, then perform an instrument calibration. Funding for the EPOXI mission is expected to run out around the end of 2010, he said.

Stardust, another retired Discovery mission, is also operating in an extended phase.

The Stardust spacecraft is heading for a Feb. 14 encounter with comet Tempel 1. The flyby will be the second look at the comet since Deep Impact blasted a hole in its nucleus in 2005.

During its original mission, Stardust collected dust from comet Wild 2 and dropped a return capsule to Earth with the samples. The Stardust mothership continued on in space, presenting an opportunity for a new objective.

NASA also recycled two small probes built to study Earth's magnetic field and sent the craft to the moon, where they arrived in a preliminary orbit in August and October. The satellites are in a bonus phase to observe the solar wind's interaction with the moon.