A few dozen cubes are missing from the tray. Zolensky and his colleagues have cut out hundreds of small sections of those cubes. They remove an entire particle track by poking a pair of sharp glass fibers into the aerogel, a process that takes up to a day. The extracted piece looks like the clipped corner of a fingernail and has a particle at one end.
Seen through a microscope, the particle Zolensky shows me is jet black. Astronomers once pictured comets as gently disintegrating "fluffy ice balls." Then detailed photographs revealed that comets' outer rinds are blackened crusts, charred by the radiation of space. Ice and dust spew through fissures in the crust, eroding the comet's interior with each orbit. The Stardust samples—messengers from the inside of Wild 2—show that comets are dark through and through.
Under higher magnification, the dust particles look like exploded popcorn kernels. Stardust scientists were surprised to find that some of the comet's grains are made of minerals that form only at extremely high temperatures. It appears that these grains arose close to the Sun, inside the orbit of Mercury, in a blast furnace far removed from the calm, cold margins of the solar system where comets now drift.
No one had expected that the hot ingredients of the inner solar system mixed with the cold outer solar system billions of years ago. "It's a remarkable result," says planetary scientist Michael A'Hearn of the University of Maryland. "It's causing us to rethink how things got put together." It may mean that wind or radiation from the newborn Sun was stronger than expected, propelling the heat-forged grains deep into the solar system, where they merged with ices and frigid dust into comets.
The collection tray also captured evidence suggesting that comets may have helped seed life on Earth. NASA researchers found traces of glycine—one of the amino acids that make up the proteins in all living things—on the aluminum foil lining the sides of the aerogel cubes. The discovery, confirmed this past summer, suggests that comets contain some of life's basic molecules. Comets and meteorites colliding with the young Earth would have spread such compounds, possibly providing the ingredients for the Earth's first cells.
Stardust wasn't the only comet mission. NASA's Deep Impact spacecraft slammed a probe into Comet P9/Tempel 1 in 2005 and found that the crust was rigid but the layers underneath were weak and powdery. A European mission now en route, Rosetta, will try to land on a comet in 2014, scoop up some icy dirt and analyze it on the spot.
After we strip off our clean-room suits, Zolensky takes me downstairs to see the mission's return capsule. (It is now on display at the National Air and Space Museum.) The cone-shaped shield that protected Stardust's cargo during its fiery re-entry is about a yard wide. There's some dirt on it, Utah mud from the landing. The capsule's surface, a carbon composite mixed with cork, is burned and yields slightly to the touch. The spacecraft flew three billion miles—the most distant traveler ever to find its way home.
Robert Irion has written about black holes and new planets for Smithsonian.