How Dust Is Captured

Catching comet dust is no easy feat. When the comet zooms past the Stardust spacecraft, the particles will be moving at up to nine times the speed of a bullet fired from a rifle. Although the captured particles will each be smaller than a grain of sand, high-speed capture could alter their shape and chemical composition -- or vaporize them completely.
To collect the particles without damaging them, Stardust will use a foamed glass substance called Aerogel.

Aerogel Factoids
Light and airy: Composed of 99.8 percent air, Aerogel is the lightest-weight, lowest-mass solid known to science, with the feel of a soft foam rubber. Mostly transparent, it is sometimes called "blue smoke."
Making tracks: When particles hit Aerogel, they become embedded, leaving tiny, carrot-shaped tracks easily spotted under a microscope. Using any other known material would make finding the minuscule dust as unlikely as finding a needle in a haystack.
Excellent insulation: It's 39 times more effective than the best fiberglass.