The Stardust mission has, with the help of thousands of citizen scientists, found probable evidence of its own namesake — interstellar dust.
Launched in February 1999 on a 2.88-billion-mile journey that sent it in three giant loops around the sun, the Stardust spacecraft encountered comet Wild 2 in 2004. It returned to earth in January of 2006 bearing tiny samples from that comet, which researchers then began to analyze. University of Washington astronomer Don Brownlee was the mission’s principal investigator.
The spacecraft caught the small samples in aerogel, a material that reduced the stress of impact. The carrot-shaped tracks of the larger specks were easily visible to the eye.
But comet particles were not all that Stardust sought. At times during the spacecraft’s trips toward the sun, mission scientists used the back side of its aerogel collector to try and catch smaller and rarer motes of interstellar dust.
Now, astronomers at the University of California, Berkeley and other institutions, including the UW — together with thousands of “Stardusters” or at-home volunteers — report they have found seven dust motes among the samples that probably came from outside the solar system, and leaving much smaller tracks. They would be the first confirmed samples of contemporary interstellar dust.
The comet and interstellar dust collectors, tennis racket-sized mosaics of 132 aerogel tiles, were on opposite sides of the spacecraft’s collector arm. They were inside a return capsule that dropped by parachute as the Stardust spacecraft flew by Earth in 2006.
“They are very precious particles,” said Andrew Westphal, University of California, Berkeley, physicist and lead author (with 61 others, including Brownlee) of a report on the particles appearing in the Aug. 15 issue of the journal Science. Brownlee is also a coauthor of 12 other papers about the particles to be published soon in the journal Meteoritics & Planetary Science.
Westphal said additional tests are needed to confirm that the motes indeed are pieces of debris from interstellar space — a bit like finding needles in a massive haystack. If they are, the particles could help explain the origin and evolution of interstellar dust that until now could only be guessed from astronomical observations.
Two of the motes were found by the 30,000-some volunteer “Dusters” who scanned more than a million images through Stardust@home, a UW Berkeley citizen-science project. Once several Dusters tagged a likely track, Westphal’s team vetted it. About 30 other tracks proved to have come from the spacecraft itself.
Only 77 of the 132 aerogel panels have so far been scanned. Westphal expects to find no more than a dozen particles of interstellar dust in all, only about one-millionth of the comet material that Stardust collected.
The interstellar dust particles are “a factor of a hundred times” smaller than the comet motes, Brownlee said. He credits Westphal for his innovative use of the volunteer Dusters to help identify the interstellar motes. “If you look at the number of pictures that they looked at, it’s impressive.”
Astronomers have long been interested in interstellar dust, Brownlee said, and it can be seen by the naked eye if you look up on a clear night.
“Go outside the city, get away from the lights, and you’ll see the Milky Way, which is our galaxy — edge-on — we’re in the middle of it. And you’ll notice along the middle there’s a dark band — that’s interstellar dust.”
It has been modeled and viewed from a distance, he said. “But nonetheless, looking at something and imagining what it is, is very different from having it in hand.”
“We are very interested in interstellar dust because we are made out of interstellar dust,” said Brownlee. “All the atoms in our body were originally dust.”
The Stardust analysis was funded primarily by NASA, with additional resources from the Department of Energy.