After successfully sweeping up the first particles ever captured from a stream of interstellar dust, the Stardust spacecraft today safely stored its microscopic treasures. It's now zooming toward a January 2004 encounter with Comet Wild-2. When Stardust returns to Earth in 2006, a comparison of particles from the two locations will reveal whether comets coalesce out of interstellar dust.
NASA launched Stardust in February 1999, sending it toward one of the many swirls of dust that thread the Milky Way. Dust like this may have glommed together over millions of years to form comets. Mission planners knew the dust would smash into the collector at about 26 kilometers per second, so they coated a tennis-racket-shaped arm with aerogel, a porous solid that resembles frozen smoke in an ice-cube tray. The dust particles crashing into the collector would embed themselves in the aerogel and remain there until Stardust returns home.
But that's not Stardust's only trick. Scientists want to keep the samples from the dust stream and the comet separate, so the racket has two collecting surfaces. On 22 February, Stardust extended just the backside of the racket into the dust stream. Early today the craft retracted the arm. When Stardust arrives at Comet Wild-2, the arm will extend again with the front side of the collector turned toward the comet by flipping the racket.
For the moment, however, the Stardust team is happy to have negotiated the first in-flight mechanical test of the collector arm. "This was a major milestone," says principal investigator Don Brownlee of the University of Washington, Seattle. The sense of relief was shared by David Kring, a planetary scientist at the University of Arizona, Tucson. "Stardust is a critically important mission," says Kring, who studies comet chemistry. If the collector hadn't opened and retracted correctly, he says, "the mission would have been over."