Thanks to a massive effort by 30,716 volunteers, scientists have pinpointed what appear to be seven precious specks of dust from outside the solar system, each bearing unique stories of exploded stars, cold interstellar clouds and other past cosmic lives.
The Herculean effort began eight years ago after NASA's Stardust robotic probe flew by Earth to deposit a capsule containing samples from a comet and dust grains from what scientists hoped would be interstellar space. The spacecraft was outfitted with panels containing a smoke-like substance called aerogel that could trap and preserve fast-moving particles.
Stardust twice put itself into position to fish for interstellar grains, which are so small that a trillion of them would fit in a teaspoon. The only way scientists back on Earth would be able to find them was by the microscopic trails the grains made as they plowed into the aerogel.
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"When we did the math we realized it would take us decades to do the search ourselves," physicist Andrew Westphal, with the University of California, Berkeley, told Discovery News.
The team used an automated microscope to scan the collector and put out a call for volunteers.
"This whole approach was treated with pretty justifiable criticism by people in my community. They said, ‘How can you trust total strangers to take on this project?'" Westphal said.
"We really didn't know how else to do it. We still don't," he added.
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Recruits were trained and had to pass a test before they were given digital scans to peruse. Scientists sometimes inserted images with known trails just to see if the volunteers, known as "dusters," would spot them.
"We were very pleased to see that people are really good at finding these tracks, even really, really difficult things to find," Westphal said.
More than 50 candidate dust motes turned out to be bits of the spacecraft itself, but scientists found seven specks that bear chemical signs of interstellar origin and travel.
The grains are surprisingly diverse in shape, size and chemical composition. The larger ones, for example, have a fluffy, snowflake-like structure.
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Additional tests are needed to verify the grains' interstellar origins and ferret out their histories. But the grains are so tiny that with currently available technology, additional analysis would mean their demise.
"It'll probably be years before we can do a lot more with these samples," said space scientist Mike Zolensky, who oversees NASA's collection of cosmic dust, moon rocks and other extraterrestrial samples at the Johnson Space Center in Houston.
"But we've got them safely tucked away and we can hang on to them until those techniques come along," Zolensky said.
The research appears in this week's Science.