Fresh off a bonus flyby of comet Tempel 1 in February, NASA's Stardust spacecraft fired its four main engines for more than two minutes Thursday, draining its fuel tank as managers said goodbye to the well-traveled comet chaser after more than 12 years in space.

Artist's concept of the Stardust craft's main engines firing in space. Credit: NASA/JPL-Caltech
 
With Stardust's single hydrazine fuel tank emptied, the craft lost its ability to control its orientation and the probe's solar panels were expected to lose track of the sun, and officials anticipated the mission's battery charge would be exhausted within hours.

Stardust was also programmed to turn off its radio transmitters about 20 minutes after the burn, just in case it might interfere with some future mission using the same frequency.

NASA announced the last transmission from Stardust was received at 7:33 p.m. EDT (2333 GMT) Thursday. Officials monitored the burn from the Jet Propulsion Laboratory in California and issued commands from the Lockheed Martin Corp. mission support center in Denver.

Thursday's maneuver was the final exercise for Stardust, which has been running on low fuel as the craft was retasked for an extended mission after returning the first comet samples to Earth in 2006.

It turns out Stardust had just enough propellant to pull off the high-speed flyby of comet Tempel 1 Feb. 14, yielding a cache of images of the ice ball, which was the target of NASA's Deep Impact mission in 2005.

Deep Impact dropped a heavyweight mass that struck Tempel 1, and NASA dispatched Stardust to revisit the comet and see how the object changed in the ensuing five-and-a-half years.

Renamed Stardust-New Exploration of Tempel, or Stardust-NExT, the mission kicked off in 2007, one year after Stardust brought a hardened sample return capsule back to Earth with dust particles from comet Wild 2.

Stardust's engines fired for 146 seconds Thursday, according to Allan Cheuvront, the mission's program manager at Lockheed Martin. Officials expected the propulsion system would burn between a couple of minutes to more than 10 minutes.

The mission-ending burn was designed to empty Stardust's reservoir of propellant, giving engineers fresh data on how well their fuel projections matched up with reality.

"Stardust has been teaching us about our solar system since it was launched in 1999," said Tim Larson, Stardust-NExT's project manager from JPL in Pasadena, Calif. "It makes sense that its very last moments would be providing us with data we can use to plan deep space mission operations in the future."

Stardust didn't carry a fuel gauge, so ground controllers came up with three methods to estimate how much propellant was left after its 3.5 billion mile journey.

Stardust snapped this picture of comet Tempel 1 during its Feb. 14 flyby. Credit: NASA/JPL-Caltech/Cornell
 
Using a bookkeeping method, engineers simply add up the amount of fuel the spacecraft should have burned, then subtract that number from the level loaded before launch.

Controllers also monitor pressure gauges inside the fuel tank, which give markers indicating about how much propellant is inside.

A more exotic way to estimate fuel is by warming the tank to a predetermined temperature, then logging the vessel's thermal response as it cools, according to Cheuvront.

Stardust gave NASA and Lockheed Martin a rare opportunity to track an interplanetary probe as it purposely drained its fuel tank, letting engineers know just how much propellant was left inside to match that quantity with estimates.

"We'll crunch the numbers and see how close the reality matches up with our projections," Cheuvront said. "That will be a great data set to have in our back pocket when we plan for future missions."

Launched in February 1999, Stardust flew past asteroid Annefrank and then collected particle samples from comet Wild 2 in January 2004. Stardust jettisoned the sample container back into Earth's atmosphere in January 2006, but the mothership kept on flying through the solar system.

With Stardust still in good health and having just enough fuel to complete another comet rendezvous, NASA selected a proposal to send the spacecraft toward Tempel 1 to follow up on Deep Impact's collision.

The Stardust-NExT extension began in 2007 and used a series of orbit-adjusting engine burns to aim the probe for Tempel 1, which it successfully flew past Feb. 14.

With Stardust running on fumes, NASA and Lockheed Martin chose to make Stardust's final moments a valuable learning experience.

"This kind of feels like the end of one of those old Western movies where you watch the hero ride his horse towards the distant setting sun, and then the credits begin to roll," Larson said. "Only there's no setting sun in space."