Aerogel was used on the Stardust spacecraft to capture comet particles from Comet Wild 2. This image shows Dr. Peter Tsou handling the so-called "solid blue smoke."
Aerogel is an incredibly light, extrtemely durable substance - .8 percent of the volume is empty space. By comparison, aerogel is 1,000 times less dense than glass, which is another silicon-based solid. When a particle hits the aerogel, it buries itself in the material, creating a carrot-shaped track up to 200 times its own length. This slows it down and brings the sample to a relatively gradual stop. Since aerogel is mostly transparent - with a distinctive smoky blue cast - scientists will use these tracks to find the tiny particles.
Though with a ghostly appearance like an hologram, aerogel is very solid. It feels like hard styrofoam to the touch.
Image Credit: NASA