Scientists say samples returned to Earth by a NASA space mission may contain the oldest known dust particles from outside our own solar system.
Researchers analyzing microscopic particles gathered by the space agency's Stardust comet-chasing mission say just seven of the particles may have originated from beyond the solar system.
"They are very precious particles," says research leader Andrew Westphal, a physicist at the University of California, Berkeley.
The analysis didn't happen overnight; it involved thousands of volunteers looking at particles returned to the Earth in 2006 in a capsule detached from the robotic Stardust probe containing comet and dust samples scientists thought might display interstellar origins.
The particles were so tiny the single way to identify them was by their microscopic trails left in a special substance known as aerogel on the surface of the probe, Westphal said.
"When we did the math we realized it would take us decades to do the search ourselves," he said.
So the scientists put out a call for volunteers to sift though the millions of images an automated microscope made of the aerogel in the search for just a few dozen of the tiny interstellar grains.
"This whole approach was treated with pretty justifiable criticism by people in my community," Westphal said. "They said, 'How can you trust total strangers to take on this project?'"
However, more than 3,000 citizen-scientist volunteers, whom the scientists dubbed "dusters," came though in identifying particle trails in the gel, the researchers reported in the journal Science.
While more than 50 candidate dust particles were determined to be tiny specks of the spacecraft itself, the effort yielded seven dust motes that bore chemical signs of interstellar origin and travel, NASA researchers said.
Additional testing would be needed to confirm an interstellar origin for the particles; a final proof would be in the amounts of different isotopes of oxygen within them. Different concentrations than those found in the Solar System would be strong evidence of interstellar origin.
However, the seven particles are all so tiny any current testing technology would destroy them, the researchers said.
"It'll probably be years before we can do a lot more with these samples," says Mike Zolensky, who keeps an eye on the space agency's collection of cosmic dust, moon rocks and other extraterrestrial samples, all housed at Houston's Johnson Space Center.
"But we've got them safely tucked away and we can hang on to them until those techniques come along," he says.
