NASA kills off comet hunter

22:46 25 March 2011 by David Shiga

NASA deliberately killed off its Stardust spacecraft on Thursday, burning up its last fuel reserves in an experiment designed to help engineers plan future missions.

Stardust launched in 1999 and in 2004 became the first and so far only mission to collect samples of comet dust when it flew by comet Wild 2. The samples parachuted to Earth in a capsule in 2006.

But the Stardust spacecraft itself kept flying. Because it was still healthy, NASA decided to repurpose it, tweaking its orbit to meet comet Tempel 1 on 14 February of this year.

NASA's Deep Impact mission had punched a crater in Tempel 1 in 2005, but had failed to obtain any images of it. So scientists were delighted when Stardust imaged the crater during its recent flyby.

But on Thursday, NASA commanded the spacecraft to burn up its remaining fuel in a suicidal blaze of glory. Without fuel, it will no longer be able to control its orientation in space to keep its solar panels pointed at the sun. It is expected to run out of power as a result.

'Quite melancholy'

"It was quite a melancholy kind of thing," says Donald Brownlee of the University of Washington in Seattle, who was the chief scientist for Stardust's mission to Wild 2 and a science team member for the Tempel 1 visit. "But on the other hand, it's far outlived its design – it flew past an asteroid and two comets and lasted 12 years in space." (Stardust made a side trip to asteroid Annefrank in 2002 on its way to comet Wild 2.)

Though using up Stardust's fuel means it is now a useless piece of space junk, the exercise will provide valuable information for future missions.

It is difficult to gauge how much fuel a spacecraft has left in the microgravity of space. Mission planners estimate how much is left based on how much a spacecraft started with and how long its engines have burned.

Prior to the burn, mission planners knew it had already used up nearly all of its hydrazine fuel. They estimated it had enough to fire the engines for somewhere between 2 and 10 minutes.

Betting pool

On Thursday at the control centre in Denver, Colorado, where the spacecraft's builder Lockheed Martin managed Stardust's operations, "they had a chalkboard and a pool of people who had put down guesses about how much was left", Brownlee says.

The lower estimates turned out to be right. Stardust's engines burned for only 2 minutes and 26 seconds before running out of fuel. Knowing this will help mission planners refine fuel calculations for future missions.

NASA commanded Stardust to turn off its radio transmitter on Thursday to eliminate the remote possibility that it could transmit signals in future and interfere with communications between Earth and other spacecraft. The last transmission from the spacecraft was received at 1633 Pacific Daylight Time (2333 GMT).

The final burn has barely changed Stardust's orbit around the sun. Its elliptical path brings it close to the orbits of both Earth and Mars, but there is no chance of a collision with either planet in the next 100 years. Stardust will get no closer than 2.7 million kilometres from Earth's orbit and 20 million kilometres from Mars's orbit during that time, the mission team says.

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