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.