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.
UC Berkeley’s Take:
Since 2006, when NASA’s Stardust spacecraft delivered its aerogel and aluminum foil dust collectors back to Earth, a team of scientists has combed through the collectors in search of rare, microscopic particles of interstellar dust.
The team now reports that they have found seven dust motes that probably came from outside our solar system, perhaps created in a supernova explosion millions of years ago and altered by eons of exposure to the extremes of space. They would be the first confirmed samples of contemporary interstellar dust.
“They are very precious particles,” said Andrew Westphal, a physicist at UC Berkeley’s Space Sciences Laboratory and the lead author – with 65 coauthors – of a report on the particles appearing in the Aug. 15 issue of the journal Science. Twelve other papers about the particles are now available online and will appear next week in the journal Meteoritics & Planetary Science.
Westphal cautioned that additional tests must still be done before the team can say definitively that these are pieces of debris from interstellar space. But 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.
Specifically, these particles are much more diverse in terms of chemical composition and structure than previously thought; the small ones are much different from the big ones, and may have had different histories; and many of the big ones have a fluffy structure, like a snowflake, he said.
“The fact that the two largest fluffy particles have crystalline material – a magnesium-iron-silicate mineral called olivine – may imply that these are particles that came from the disks around other stars and were modified in the interstellar medium,” he added. “We seem to be getting our first glimpse of the surprising diversity of interstellar dust particles, which is impossible to explore through astronomical observations alone.”
Needles in a haystack
Two particles, each only about two microns (thousandths of a millimeter) in diameter, were isolated from the light, fluffy aerogel detectors after their impact tracks were discovered by volunteers calling themselves “Dusters” who scanned more than a million images through Stardust@home, a UC Berkeley citizen-science project that proved critical to finding these needles in a haystack. A third track was made by a particle coming from the right direction – the flow of the interstellar wind – but apparently was going so fast, more than 15 kilometers per second (10 miles per second), that it vaporized. Another 29 tracks discovered by volunteers were determined to have been kicked out of the spacecraft into the collectors.
An additional 100 tracks found by Dusters have yet to be analyzed, and only 77 of the 132 aerogel panels have been scanned to date. Westphal expects to find no more than a dozen particles of interstellar dust in all – a millionth the amount of cometary material picked up by other collectors that were on board Stardust.
Four of the particles reported in Science were found in aluminum foils located between aerogel tiles on the collector tray. Although the foils were not originally planned as collection surfaces, an international team led by physicist and nanoastronomer Rhonda Stroud of the Naval Research Laboratory searched the foils for the smallest grains that might be captured, too tiny to image in the aerogel. The team identified four pits lined with partially melted material composed of elements that fit the profile of interstellar dust particles.
“They were splatted a bit, but the majority of the particles were still there at the bottom of the crater,” said Stroud. “Their diversity was a surprise, but also these fluffy particles, sort of like a tossed salad, were complex, an agglomeration of other particles, rather than one dense particle suggested by the simplest models of interstellar particles.”
Three of these particles, just a few tenths of a micron across, also contained sulfur compounds which some astronomers have argued do not occur in interstellar dust particles. Stroud and other members of the preliminary examination team plan to continue analysis of the remaining 95 percent of the foils, in hopes of finding enough particles to understand the variety and origins of interstellar dust.
The two aerogel-embedded particles – dubbed Orion and Hylabrook by their Duster discoverers and paper coauthors – are destined for further tests to determine their oxygen isotope abundances, which could provide even stronger evidence for an extrasolar origin. Supernovas, red giants, and other evolved stars produce interstellar dust and generate heavy elements like carbon, nitrogen and oxygen that are necessary for life.
Interstellar snowstorm
Stardust was launched in 1999 to fly through the debris sloughed off by comet Wild-2 and capture cometary dust with aerogel tiles and aluminum foils mounted on the front of a two-sided collector. Collectors mounted on the rear were designed to catch particles from the “snowstorm of interstellar dust streaming through the galaxy,” said UC Berkeley research physicist Anna Butterworth.
“This dust is relatively new, since the lifetime of interstellar dust is only 50 to 100 million years, so we are sampling our contemporary galaxy,” said Butterworth.
The separate comet and interstellar dust collectors, each a tennis-racket sized mosaic of 132 aerogel tiles, were dropped by parachute as Stardust flew by Earth in 2006, and a consortium of scientists led by Westphal proceeded to analyze the interstellar collectors. Scientists at the Johnson Space Center in Houston have scanned half the panels at various depths through the transparent aerogel and turned these scans into movies. Westphal and his team tailored the movies for their virtual microscope, allowing Dusters – some 30,000 in all – to go online and search them as if focusing a microscope at different depths.
“We expected to find grains less than a micron across that would leave a track a couple of microns wide. That is about one-fiftieth the width of a human hair. We might not see the particles in an optical microscope, so the Dusters are looking for the impact tracks they made,” Butterworth said.
Once several Dusters tagged a likely track, Westphal’s team vetted them. In the million frames scanned, each a half-millimeter square, Dusters found 69 tracks, while Westphal found two.
Thirty-one of these were extracted along with surrounding aerogel by scientists at Johnson Space Center and shipped to UC Berkeley to be analyzed by a scanning transmission x-ray microscope, or STXM, attached to a synchrotron beam line at the Advanced Light Source at Lawrence Berkeley National Laboratory. STXM used soft x-rays to probe the chemical composition, and ruled out 29 grains because they contained aluminum metal, which does not occur in space, or other substances probably knocked out of the spacecraft and embedded in the aerogel.
Stardust@home will continue to analyze the remaining detector aerogel tiles once Phase 7 starts on Aug. 15. Foil analysis will also be incorporated into the project soon.
“As one of the first citizen-science projects, Stardust@home has been an amazing success,” said Butterworth, who is first author of one of the 12 MAPS papers about the project. “If we had had one person searching the aerogel 40 hours per week, they would have taken three years to cover once the same area searched multiple times by the Dusters.”
The Stardust analysis was funded primarily by NASA, with additional resources from the Department of Energy.