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Captured cosmic dust may have interstellar origins

The multiyear effort that identified the candidate interstellar grains was, in part, a triumph of citizen science.

En route to a 2004 rendezvous with the comet Wild 2, NASA’s Stardust probe spent nearly 200 days with its cosmic dust collector—a large paddle tiled with silica aerogel—facing the interstellar wind. That collector was eventually retrieved, and now researchers led by Andrew Westphal (University of California, Berkeley) and Rhonda Stroud (Naval Research Laboratory) have discovered among its contents seven motes that likely originated from outside our solar system. Three of the particles were identified by the micron-sized tracks they left as they plowed to a stop in the aerogel. (One such track is shown in the figure and its inset.) To find the tracks, the researchers enlisted the help of more than 30 000 citizen scientists, who collectively pored over millions of microscope images online. The other four grains were identified in scanning electron microscope images by the tiny craters they made as they collided with aluminum foil on the collector paddle. Although the particles’ provenance remains to be confirmed via isotope analysis, their estimated impact speeds (around 10 km/s), diameters (0.2–2 µm), trajectories, and compositions are consistent with an interstellar origin. Curiously, the total harvest was an order of magnitude smaller than expected, given previous estimates of the flux of interstellar dust through the solar system. The paucity may be a consequence of the particles’ unexpectedly high porosity: Lower-density particles are more susceptible to the Sun’s radiation pressure and therefore less likely to penetrate the solar system’s inner reaches, where Stardust collected its samples. (A. J. Westphal et al., Science 345, 786, 2014.)

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