Team extends Stardust's fuel mileage for comet mission
BY STEPHEN CLARK
Posted: December 23, 2010
DENVER -- A lean team of Lockheed Martin engineers is taking fuel efficiency to new heights.
Flight controllers here are meticulously managing what little fuel is left inside the Stardust spacecraft speeding toward a Valentine's Day rendezvous with Tempel 1, a ball of ice and rock bombarded by a manmade impactor five years ago.
Stardust still has "positive margin" in its single hydrazine fuel tank. That's space terminology for having plenty of gas, even after journeying nearly 4 billion miles since launch.
"We're riding above empty, but we're managing that," said Allan Cheuvront, Stardust's program manager and senior engineer at Lockheed Martin Corp. "Right now, we have unallocated fuel. As long as we have unallocated fuel, we're happy."
DC Agle, a spokesperson at NASA's Jet Propulsion Laboratory, pegged the remaining fuel aboard Stardust at about 3.5 kilograms, or 7.7 pounds. Stardust was loaded with about 187 pounds of hydrazine fuel before launch in early 1999.
Even planning for unlikely trajectory burns and other contingencies, Stardust still has enough of a fuel budget to accomplish the precision flyby in a tight corridor just 120 miles from Tempel 1.
"We're going to finish up the mission a little on the plus side," Cheuvront said in an interview.
Engineers have turned to three different fuel accounting methods to make triple-sure the probe has enough gas to guide itself on a razor-thin trajectory toward the comet.
Like many interplanetary spacecraft, there is no fuel gauge aboard Stardust.
"It would be really nice to have a gauge," Cheuvront said. "There are a couple of ways to do it. We do a bookkeeping method, where we know every thruster pulse, we know we started with, and we know the 'on' time of the thrusters. Each pulse is about 15 milliseconds of firing."
Using the 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.
The methods result in slightly different fuel levels, but Cheuvront said all three estimates indicate Stardust can make it to Tempel 1 and beyond.
Managers have penciled in three trajectory correction maneuvers between now and the Feb. 14 flyby. The thruster firings will fine-tune Stardust's approach to Tempel 1, ensuring it hits an aimpoint 120 miles from the comet's nucleus at a relative speed of 6.77 miles per second, or more than 24,300 mph.
Stardust, which is about the size of an office desk, has a modest propulsion system. Its largest thrusters produce just 1 pound of thrust.
Engineers instituted new measures to more carefully limit propellant usage. According to Cheuvront, Stardust now burns up to 40 percent less fuel than it did early in its prime mission.
The spacecraft is already snapping pictures of Tempel 1 to help navigators on Earth develop accurate guidance solutions. The first optical navigation imagery was downlinked to the ground last week, and Stardust captured another set of pictures Tuesday, according to Joseph Veverka, principal investigator for the Stardust-NExT mission.
Stardust will continue taking optical navigation images about twice a week through Jan. 4, when the craft will step up its picture-taking to a rate of once every two hours for scientific purposes. The camera will snap images at a rapid minute-by-minute pace as the probe makes its closest approach to Tempel 1.
The mission's primary camera is a wide-angle unit left over from the development of NASA's Voyager program in the 1970s, but with updated component designs from the more recent Galileo, Cassini and Deep Space 1 missions.
Scientists hope Stardust will yield a crucial second data point on Tempel 1 after NASA dispatched the first-of-a-kind Deep Impact mission to the comet in 2005.
Tempel 1 was intentionally struck by a 820-pound copper-plated spacecraft in July 2005, blasting a hole in the comet to give scientists an unprecedented peek inside. A mothership behind the impactor captured riveting imagery of the high-energy collision as it flew about 300 miles away from the comet.
Deep Impact's best views of the comet lasted only a matter of minutes, giving researchers just a single snapshot into the comet's history stretching billions of years back to the dawn of the solar system.
Now scientists tracing the arc of solar system history are on the cusp of a second chance to study Tempel 1, an icy chunk of rock nearly 5 miles in diameter believed to be a relic left over from the formation of the planets.
NASA assigned Stardust with the job of chasing down Tempel 1.
Cheuvront leads the team overseeing Stardust's journey through the inner solar system. He has been with the mission since 1996 and oversees a handful of control personnel. The flight operations team will grow to about 11 members by the time of flyby early next year.
Officials track the spacecraft's status and issue commands from a Lockheed Martin control center here.
"I have an amazing team," Cheuvront said. "Stardust would not be success it is without this team."
Launched in February 1999, Stardust set off toward comet Wild 2, where it soared through a cloud of dust and vapor in 2004 to scoop up microscopic samples. In 2006, Stardust released a small capsule containing dust particles from Wild 2 for a blazing reentry and landing in Utah.
Researchers found compelling organic molecules, the building blocks of life, amid the samples Stardust ferried back to Earth.
But Stardust wasn't finished.
NASA issued a request for proposals to the science community for concepts to reuse Stardust's main spacecraft, which continued circling the sun after dropping its sample canister at Earth.
Veverka, a researcher at Cornell University, responded with the Stardust-NExT mission. NExT stands for New Exploration of Tempel 1.
NASA selected the NExT mission in July 2007 and Cheuvront's team took Stardust out of hibernation a month later.
Deep Impact's mothership also got orders for an extended mission, which is now concluding after a successful flyby of comet Hartley 2 in November.
Stardust-NExT's primary goal is to revisit Tempel 1 and chronicle changes to the comet over the last five years.
Tempel 1 is a Jupiter-family comet, meaning its path around the sun is susceptible to perturbations induced by the immense gravity of the solar system's largest planet. The comet currently completes a circuit around the sun every five-and-a-half years.
Scientists want Stardust to observe much of the same surface area studied by Deep Impact. The objective is to see at least 25 percent of the same territory.
But predicting Tempel 1's uncertain rotation is an inexact science. Based on fleeting observations by Deep Impact, researchers believe the comet spins once every 40 hours, but that could have changed since 2005.
Forecasting whether Stardust will see the crater formed by Deep Impact is a challenge. Minor errors in rotation estimates or navigation could place the crater on the wrong side of the comet as Stardust nears.
"The bottom line is we have to get really lucky because we just flat out don't know," Cheuvront said.
Stardust will also measure the density, composition and size of dust from Tempel 1's coma.
The probe will continue comet observations for two months after the February encounter. NASA and Lockheed Martin, which built the spacecraft in the 1990s, are targeting April 28 for the end of the mission.
Because the craft will be low on fuel, officials do not foresee an opportunity for another extended mission. Instead, engineers could command Stardust's thrusters to fire until its tank is empty.
The depletion burn could help verify propellant estimation models for future missions by giving teams data on exactly how much fuel was left inside Stardust.
"It will be an extremely sad day," Cheuvront said.