Comet chaser sets sights on asteroid

PASADENA, Calif., Oct. 28 (UPI) -- A NASA probe launched in 1999 to fetch samples from a comet will be put through a full dress rehearsal this week in preparation for the one-shot flyby of Comet Wild-2 in January 2004, mission officials told United Press International.

The spacecraft, called Stardust, is being prepared for a flyby of asteroid Annefrank, a 2.5-mile wide rock named for the famed Holocaust victim. No science is expected from the pass -- at best, the probe might be able to capture low-resolution, black-and-white pictures of the asteroid as it whips by it at 4 miles per second. Stardust will be nearly 2,000 miles away from the asteroid at its closest approach.


Stardust will keep its distance to assure it is not damaged by an undiscovered Annefrank companion asteroid or any nearby dust or debris. The point of the exercise is to uncover any problems with Stardust's target acquisition, autonomous navigation and science instrument operations before the critical comet flyby.


"This is a great opportunity for us," Stardust principal investigator Donald Brownlee said in an interview. "It's like dress rehearsal for a wedding. You expect everything to go as planned, but just in case you'd like to know ahead of time," said Brownlee, an astronomy professor at the University of Washington in Seattle.

Stardust is an ambitious but low-cost mission to capture the first samples from a comet, as well as grains of interstellar dust, and return them to Earth. If successful, the probe will pass by the planet and parachute its collection back to the surface in January 2006.

The Stardust team had to scrimp and save to come up with the hundreds of thousands of dollars needed to plan, test and support the asteroid Annefrank flyby. With mission costs capped at $200 million, project managers limited communication sessions to just a few hours per week to cut the number of people and costs needed to support the mission during its cruise phases.

"We probably track our spacecraft less than anyone," project manager Thomas Duxbury told UPI. "In the last year, we've talked to it maybe two-to-four hours per week. If you do that you don't need a lot of flight team on duty. We worked very hard to juggle our resources to fit the test into our budget. We then had to convince our management that even though we are doing this on a shoestring budget, we weren't putting the spacecraft at risk."


The scrutiny intensified after the loss earlier this year of Stardust's sister comet probe, Contour, now thought to have been destroyed after a failed engine burn.

Still, Duxbury, who is with NASA's Jet Propulsion Laboratory in Pasadena, said he would have considered it a personal failing of his management if Stardust had missed the opportunity to test its systems on a celestial practice target. The probe's launch was timed so that if money were available, the asteroid encounter could serve as an inflight engineering test.

"It was my absolute goal to not let this opportunity go by without as thorough a testing as possible," said Duxbury. "What we've found on just about every spacecraft we've ever flown is that the first time you try anything major, you run into a problem. We don't want to try its exotic imaging system and letting the spacecraft control itself when we're inside the coma of a comet because if it does even the slightest thing wrong, the spacecraft could be destroyed."

During the test, Stardust will run through the exact sequence planned for the comet encounter, with science instruments all running and relaying data at high speeds for the first time since before the satellite's launch.


"Ideally, everything will work, but my guess is we'll have some lessons learned and then we'll have over a year to fix it before we get to Comet Wild-2," said Duxbury.

Stardust will begin receiving its operating instructions Monday. The probe will be instructed to take pictures of the asteroid as it approaches on Wednesday, Thursday and Friday. The images will not make front page news, however, Duxbury warned. At best, they will be only a pixel, or single digital image unit, in size. Even at its closest approach, the asteroid images will be just 10 to 20 pixels, he said.

"This is where I think we're going to disappoint a lot of people," said Duxbury. "We plan to take 60 or more images during the flyby, but we'll be very far out. It will look like nothing more than a sliver of new moon. But we'll be jumping up and down and patting each other on the back and doing high-fives if we get those images because it would mean that all of our hard work and testing were successful. Even a crummy image, and we'll be thrilled."


(Reported by Irene Brown, UPI Science News, at Cape Canaveral, Fla.)


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