On Sunday a capsule from the Stardust spacecraft became the fastest man-made object ever to return to Earth, clocking in at nearly 29,000 miles per hour before touching down in the Great Salt Lake Desert in Utah. On Tuesday NASA scientists became the first humans ever to behold comet dust when they opened the capsule and peered at its interstellar contents. The team reported on their initial observations yesterday, launching several months of analysis by research groups worldwide.
"Last Sunday, after seven years in space traveling nearly three billion miles, Stardust landed in the Great Salt Lake Desert with a treasure from when the solar system formed 4.6 billion years ago," says astronomer Donald Brownlee of the University of Washington, who led the Stardust team. "We should have more than one million particles larger than one micron in diameter."
The solar-powered Stardust collected interstellar particles on two trips around the sun while preparing for its rendezvous with Comet Wild 2 (pronounced "Vild") in 2004. The 600-pound spacecraft penetrated Wild 2's coma--the cloud of gas and debris surrounding a comet's nucleus--and snapped pictures of its inner terrain. It also harvested tiny bits of the comet itself in a tennis racket-shaped collector carrying an ice cube tray-like device filled with the lightest-weight, lowest-mass solid known: aerogel. This silicon-based solid composed of 99 percent empty space slowed and then cradled the particles for their long journey back to Earth. "We weren't sure whether we'd be able to see particle impacts [in the aerogel] with the naked eye," Brownlee recalls. "Just looking at it you can see hundreds of tracks."
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These carrot-shaped tracks are the marks left by the particles and, confounding expectations, the scientists even found some particles that were themselves visible. "It appears to be a transparent mineral grain," Brownlee explains, though the majority of particles look like tiny black rocks. "Locked within the cometary particles is unique chemical and physical information that could be the record of the formation of the planets and the materials from which they were made," he adds.
Comets contain abundant organics--carbon-based molecules that are not necessarily formed from life. Because Wild 2 has only circled the sun five times since settling into its new orbit in 1974 and therefore hasn't had much time to be transformed, the more than a million captured particles should contain clues to the origins of the solar system and Earth itself. "Much of the earth's water and organics perhaps came from comets," explains team member Michael Zolensky of the Johnson Space Center in Houston. "This can give us a history of organics in the solar system."
The aerogel collector also gathered microscopic interstellar dust that may have traveled from other galaxies, which could offer researchers a glimpse of the nuclear reactions in distant stars. These deep-space motes are so small, however, that even advanced microscopes have trouble finding them in the aerogel. The scientists have therefore enlisted the help of computer users worldwide via a project called Stardust@Home that employs their spare computing power to help pinpoint the particles. "We have more than 65,000 people signed up for this," Zolenski notes. "But we can always use more."