What scientists hope to learn from a wisp of comet dust

Star dust from NASA's mission is expected to shed light on the genesis of the Earth's solar system.

For the first time since the end of the Apollo program more than 30 years ago, scientists have harvested bits of our cosmic neighborhood and brought them back to Earth.

Where Apollo retrieved the equivalent of salt and pepper in 842 pounds of moon rocks, NASA's Stardust mission brought back planetary saffron - dust from a comet and from distant stars, represented by a million tiny grains that would barely cover the bottom of a thimble.

Tuesday, scientists are set to get their first peek at collectors bearing the dust - the culmination of a seven-year, nearly 3 billion mile trip to explore Comet Wild 2. The mission came to a picture-perfect end in the predawn hours Sunday morning, when a capsule bearing the samples threaded its way between two major storm systems, parachuting onto the desert floor at the US Army's Dugway Proving Ground in Utah.

The samples Stardust has gathered are expected to shed light on the conditions in which the solar system formed some 4.5 billion years ago and on the basic building blocks available for its construction. Scientists have long studied the gases comets purge when their ices vaporize as they approach the sun. And other flyby missions have allowed scientists to draw some crude conclusions about cometary dust.

But this sample-return mission will allow for far more detailed studies using room-size microscopes and mile-long particle accelerators.

"Comets are like libraries, storing the record of our formation in tiny particles," says Donald Brownlee, astronomy professor at the University of Washington and the mission's lead investigator.

Far more difficult will be combing the collectors for stardust - material that forms after stars many times more massive than the sun explode. Chemical elements heavier than hydrogen and helium are forged in these explosions, then ejected into interstellar space. There, these elements can gather into dense clouds of dust and gas that give rise to new stars and planets.

The goal is to see how closely the composition of relatively recent stardust compares with that of Comet Wild 2 and samples of space dust that were collected over the years by high-flying jets. The results should yield information about the kind of processing solar-system material went through as the sun and planets evolved.

Compared with the comet dust, the sample of dust from interstellar space is expected to be minuscule, notes Andrew Westphal, associate director of the Space Sciences Laboratory at the University of California at Berkeley. The front of the 16-inch diameter collector is thought to have gathered a few thousand tiny clumps of comet dust and perhaps a million dust particles overall. Dr. Westphal says he expects only about 45 stellar dust grains to show up on the collector's back.

As a result, he and his team are enlisting volunteers to eye-ball roughly 1.5 million images of the aerogel collector - each roughly the size of a sand grain. Dubbed Stardust@home, the project is making a Web-based "virtual" microscope available to volunteers for the hunt. So far, Westphal says, some 20,000 people have "pre-registered" for the effort.

Meanwhile, Stardust's mother ship, which dropped its capsule off Sunday and headed toward its next orbit around the sun, remains available for additional comet flybys, NASA officials say. The craft and its instruments are in good condition, with enough fuel to tackle new targets.

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