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December 19, 2003

Comet encounter is key moment in UW astronomer’s long scientific quest

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After a nearly five-year chase, the Stardust spacecraft will finally meet comet Wild 2 on the day after New Year’s. It’s a moment Donald Brownlee has anticipated for nearly 25 years.

“This could prove to be a pivotal time for science, a remarkable opportunity to gather evidence that might actually tell us how the planets formed and give us clues about how life on Earth began,” said Brownlee, a University of Washington astronomer and principal investigator for the Stardust mission.

On Jan. 2, Wild 2 (pronounced Vilt) will overtake Stardust about 242 million miles from Earth, on the other side of the sun. The spacecraft will capture tiny grains streaming through the coma, the gas-and-dust envelope around the comet’s icy nucleus. A tennis-racquet shaped collector, using a wispy material called aerogel, will harvest comet grains as they speed past. Meanwhile, a high-gain antenna will transmit close-up pictures and sensitive instruments will gather data about the comet. Though the encounter will last about 12 hours, the really intense activity will be over in a matter of minutes. The action will be monitored closely in Stardust’s nerve centers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and Lockheed Martin Space Systems in Denver.

The comet particles will be traveling five times faster than a bullet from a rifle, but the aerogel will stop them in a fraction of an inch. However, because aerogel is as much as 99.9 percent empty space, it will not damage the grains or appreciably alter their characteristics, Brownlee said.

The spacecraft is scheduled to return to Earth in January 2006 and a capsule containing its treasure – less than an ounce of comet dust – will parachute to the Utah desert. The contents will be sent to NASA’s Johnson Space Center in Houston, and then parceled out to scientists around the world, who will begin trying to unlock secrets of the early solar system.

Thousands of tons of microscopic comet particles blanket Earth each year.

“Unfortunately, they are difficult to find among the earthly materials,” Brownlee said. “And even when extraterrestrial particles can be found, they are cosmic orphans – there is no way to determine their origin.”

And such particles cannot give the same kind of information as those taken directly from a comet like Wild 2, he said. That’s because Wild 2 only recently started orbiting close to the sun and so there hasn’t been enough time for the sun’s heat to destroy characteristics that have been preserved in the cold of deep space for billions of years. Before 1974, the comet’s solar orbit extended from Jupiter to beyond Uranus. But Jupiter’s gravity altered Wild 2′s course, bringing it close enough – just beyond the orbit of Mars – to make the Stardust mission feasible.

“This gives us a real opportunity to find out if our long-held suspicions are right, that comets played a major role in the origin of life,” Brownlee said. “No one really knows how life began, but we’re certain that carbon was key to the process. Comets are the most carbon-rich materials in the solar system, and we know they are full of organic compounds that fall on the Earth all the time.”

In addition, comets delivered a significant share of the Earth’s water.

Brownlee, along with colleague Peter Tsou at the Jet Propulsion Laboratory, began considering the possibility of a comet mission in 1980. Five years later he and a team of NASA scientists tried to formulate a mission to Halley’s comet, but that proved to be unworkable. However, technological advances and the fortunate orbit change made a mission to Wild 2 possible.

Stardust, the fourth in the National Aeronautics and Space Administration’s series of Discovery missions and the first mission designed to return samples from beyond Mars, was launched from Cape Canaveral, Fla., on Feb. 7, 1999. It is currently on its third giant loop around the sun, and will have traveled some 3.1 billion miles by the end of its voyage.

In November 2002, the spacecraft successfully tested systems it will use in the Wild 2 encounter during a flyby of Asteroid 5535 Annefrank. During its nearly five years in space, it also has captured interstellar dust using the opposite side of the collector that will gather the grains from Wild 2.

Stardust is a collaboration of the UW, NASA and its Jet Propulsion Laboratory at the California Institute of Technology, and Lockheed Martin Space Systems. Other key members are The Boeing Co., the Max-Planck Institute for Extraterrestrial Physics, NASA Ames Research Center and the University of Chicago.
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For more information, contact Brownlee at brownlee@bluemoon.astro.washington.edu  or (206) 543-8575.