Dust nabbed by spacecraft may be from outside the solar system

Microscopic grains of space dust captured by NASA’s Stardust spacecraft appear to have come from interstellar space. These fragile particles, perhaps the first directly captured from outside the solar system, could help researchers understand the building blocks of not only Earth and its siblings but also planets around other stars.

The sample — just seven particles — comes after years of collecting dust and more years of thousands of people analyzing the spacecraft’s take. “Any sane person asks: Why spend years doing this?” says Andrew Westphal, a planetary scientist at the University of California, Berkeley and a member of the Stardust team. “This is about our origin,” he says, “what materials formed the sun, planets and us.”

Launched in 1999, Stardust set out to collect interstellar dust, sootlike grains that fill the space between stars, as well as debris from a comet (SN: 1/10/04, p. 19). In 2006, the probe swung by Earth and tossed its cargo into a desert in Utah.

Eight years later, after sifting through the grains, planetary scientists think they’ve finally identified particles as messengers from outside the solar system. No one description captures all seven particles, Westphal’s team reports in the Aug. 15 Science. Some are blobs of silicate minerals; others contain crystals of olivine and spinel that appear to have been smoothed over by millions of years of space weathering. To collect samples, Stardust hung a net approximately the shape and size of a tennis racket out its door. To capture dust particles, the net was stuffed with 132 tiles of aerogel, a fluffy silicon-based foam designed to capture high-speed particles without damaging them.

The researchers took pictures of both the aerogel and the net’s aluminum frame with a powerful microscope. Then, more than 30,000 volunteers — nicknamed “dusters” — pored over the microscope images, looking for tracks and craters left behind by anything striking the net. Most of the scans contained nothing of interest. But with 96 particle candidates in hand, researchers measured the composition and structure of the materials embedded in the aerogel and splattered on the frame. Most particles were pieces of the spacecraft, whereas seven appeared to come from space.

Westphal and colleagues argue that the seven samples probably originated in interstellar space because of the orientation of the spacecraft. The aerogel tiles were pointed towards a known stream of dust that comes from the constellation Ophiuchus. The trajectories traced in the aerogel tiles line up with that dust stream.

The grains’ oxygen isotopes — forms of the element that vary in mass — look similar to those of material from within the solar system. But that doesn’t rule out an interstellar source of the dust, says Bruce Draine, an astrophysicist at Princeton University who was not involved with the study. The gas and dust that surrounds the solar system probably hasn’t changed too much in the 4.6 billion years since the sun and planets formed, he says.

The seven particles represent the first opportunity to directly examine debris that’s currently passing through the solar system, Draine says. Other researchers previously had found possible interstellar particles embedded in meteorites. But, he cautions, those grains have unusual chemical compositions and might not be representative of typical alien dust.

Some of the particles’ properties, such as the presence of silicates, match what astronomers deduce from observations of distant dust clouds. But the dust holds a couple of surprises: The speeds of the largest particles were lower than expected, for example, which Draine says suggests either that the particles had some sort of interaction with the solar wind or that their origin was actually within the solar system.

Westphal says they’ve produced microscope images of only about half the aerogel tiles. In addition to hunting for more dust grains, the team will also spend the next couple years figuring out how to extract more information out of the seven known particles to clarify their origin.

“We have to be super careful,” he says. With so few particles, the researchers don’t want to risk damaging a single one. NASA has no plans for a follow-up mission, so these seven may be the only samples from beyond the solar system for a long while.

Editor’s note: The second image originally published with this story was incorrect. The article was updated on August 15, 2014, to replace the image with the correct one.