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Deep Impact sets path for asteroid encounter in 2020
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: December 17, 2011


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Flying on its last bit of fuel, NASA's Deep Impact probe is carefully reshaping its course toward a potentially hazardous near-Earth asteroid in hopes the spacecraft can survey the body in January 2020.


Artist's concept of the Deep Impact spacecraft. Credit: NASA/JPL-Caltech/UMD
 
Engineer's don't know if Deep Impact has enough fuel to reach the asteroid, and NASA officials in Washington have not committed to funding the extended mssion.

But that isn't stopping engineers from trying, according to Tim Larson, Deep Impact's project manager at the Jet Propulsion Laboratory in Pasadena, Calif.

"There is a lot of uncertainty whether we'll be able to pull this off," Larson said in an interview with Spaceflight Now.

Engineers estimate there are just 4.4 pounds, or 2 kilograms, of hydrazine fuel left inside Deep Impact's propellant tank. About 190 pounds of fuel were inside Deep Impact when it blasted off in January 2005.

NASA management in Washington gave the Deep Impact team authority to fire the spacecraft's thrusters for 140 seconds on Nov. 24, changing the probe's velocity 19.7 mph and changings its trajectory around the sun.

The burn started aiming Deep Impact toward asteroid 2002 GT, a mystical object that regularly crosses paths with Earth. It could be a target for future human expeditions and it has a risk of one day colliding with Earth.

Discovered in 2002, the asteroid is nearly one-half mile wide. But scientists do not know what it looks like or its composition.

A smaller burn by Deep Impact in October 2012 would finish targeting asteroid 2002 GT, setting up for a high-speed flyby in January 2020, Larson said.

"Unfortunately because of the small amount of fuel we have left, we're pretty limited on our choices of where we can go," Larson said.

Scientists at the Jet Propulsion Laboratory identified asteroid 2002 GT as the best opportunity to continue the Deep Impact mission, which completed its first extended phase early this year.

"We did a search of a catalog of available bodies that could be reachable with the fuel we have available," Larson said. "Out of that, about a half-dozen potential bodies came up that we may be able to get to. This is the one that seemed most feasible to get some decent observations and science."

Deep Impact is about the size of a sport utility vehicle. It fired a high-speed impactor into comet Tempel 1 in July 2005, blowing a hole in the nucleus and spraying icy debris into space. The Deep Impact mothership continued flying, and NASA offered the craft for scientists to propose new missions.


Photo snapped by Deep Impact of comet Hartley 2. Credit: NASA/JPL-Caltech/UMD
 
NASA approved a proposal named EPOXI to send Deep Impact by comet Hartley 2 and use the probe's high-resolution telescope for observations of planets circling other stars.

The space agency issued another announcement of a possible second extended mission last year, a month before it flew by comet Hartley 2 in November 2010.

NASA has not issued clearance for Deep Impact to continue on toward a third target, but senior officials authorized the Nov. 24 burn to keep the extended mission as an option.

"Whether not it happens in 2020 comes down to whether funding will be available for continued operations and whether the spacecraft will remain healthy," Larson said. "At that point, it would be about 15 years old. So it's hard to predict the condition of all the equipment on-board at that time. We also have to conserve the fuel on-board between now and then."

A senior review by independent scientists will recommend in early 2012 whether to proceed with Deep Impact's second extended mission, according to Michael A'Hearn, the principal investigator for Deep Impact at the University of Maryland.

The panel will weigh the scientific promise of the flyby, the cost of continued operations, and the odds of Deep Impact surviving until 2020.

Scientific observations of extrasolar planets and other comets and asteroids could be possible on Deep Impact's long-duration cruise to the flyby in 2020, scientists said.

"It is still unclear how NASA will proceed with selecting investigations and determining who should be in charge," A'Hearn said, adding he is willing to continue leading Deep Impact's research or step aside for someone else.

Larson said Deep Impact has most recently cost about $5 million to operate each year.

Almost all of Deep Impact's leftover fuel would be used on the flight to asteroid 2002 GT. Reaction wheels control the probe's orientation in space, but thrusters must occasionally fire to unload built-up momentum from the wheels, consuming some of the remaining propellant.

Larger burns next October and fine-tuning maneuvers closer to the flyby may also be required.

But because fuel gauges do not function in the microgravity environment of space, engineers cannot be certain how much hydrazine is left on-board Deep Impact. Controllers can subtract propellant from the tank as it is consumed, or monitor pressures and the thermal behavior of the fuel reservoir.

"There's quite a bit of uncertainty estimating remaining fuel on a spacecraft, particularly when you get this low," Larson said.

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