Aerogel
Aerogel is an ultra-light, porous glass foam used by the Stardust mission to catch fast-moving comet dust.
Aerogel is a solid made mostly of air. It is formed by drying a silica gel so that its liquid is replaced by gas, leaving a delicate network of glass. The silica aerogel used for Stardust was extremely low-density, and looks like frozen blue smoke.
Its value for space science is that it can stop fast-moving particles gently. A dust grain hitting a solid plate at 6 kilometres per second would vaporise, but in aerogel it slows over a long, carrot-shaped track, often surviving at the end. NASA's Stardust mission used aerogel tiles to collect grains from comet Wild 2 and from interstellar space.
After return, scientists found tracks by scanning the tiles with microscopes and then cut out each grain. Many volunteers helped search images for interstellar dust tracks through an online project. Aerogel is also used as insulation on Mars rovers, because it blocks heat so well.
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