If successful, NASA's Stardust probe will be the first ever to carry matter from a comet back to Earth for examination by scientists. It would also be the first time that any material has been deliberately returned to Earth from deep space.

And one wouldn't merely wax poetic to say that in those tiny grains of comet dust, one could find clues to the origin of our world and perhaps to the beginning of life itself.

Comets are like frozen time capsules from the time when our solar system formed. Drifting in the cold outer solar system for billions of years, these asteroid-sized "dirty snowballs" have undergone little change relative to the more dynamic planets. Looking at comets is a bit like studying the bowl of leftover batter to understand how a wedding cake came to be.

Indeed, evidence suggests that comets may have played a role in the emergence of life on our planet. The steady bombardment of the young Earth by icy comets over millions of years brought some of the water that makes our brown planet blue. And comets contain complex carbon compounds that might be the building blocks for life.

Launched in 1999, Stardust will rendezvous with comet Wild 2 (pronounced "Vilt" after its Swiss discoverer) on January 2, 2004. A rendezvous with a comet is a little like a rendezvous with a Gatling gun on a foggy night. As Stardust plunges through the hazy clouds of gas surrounding Wild 2's core, dust grains will fly by the spacecraft at about 13,000 mph, or six times faster than a speeding bullet. The "eyes" of Stardust, an onboard camera, will peek out from the body of the craft through a periscope to avoid damage. A Whipple Shield--a stack of five sheets of carbon filament and ceramic cloths each spaced 2 inches apart-- protects the rest of the spacecraft.

Stardust will use a material called aerogel to capture some of the fast-moving grains. Aerogel is a foam-like solid so tenuous that it's hardly even there: 99 percent of its volume is just air. The ethereal lightness of aerogel minimizes damage to the grains as they're caught. Mission planners hope to catch more than one thousand grains larger than 15 microns in the aerogel.

Wild 2 orbited the sun beyond Jupiter until 1974, when it was nudged by Jupiter's gravity into a Sun-approaching orbit--within reach of probes from Earth. Since then the comet has passed by the Sun only five times, so its ice and dust ought to be little altered by solar heating. Pristine dust from Wild 2 can tell us what the solar system was like before it was baked by 4.5 billion years of sunshine and radiation.

After the encounter, Stardust will loop around the Sun on a two-year journey back to Earth. In January 2006, home again, the spacecraft will eject the Sample Return Capsule (SRC), which looks like a miniature Apollo capsule. The SRC will parachute to Earth and, if all goes as planned, land in Utah where scientists will be waiting...

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