Silicon-based solid with a porous, sponge-like structure in which 99.8% of the volume is empty space. Aerogel is used to collect high velocity particles without damaging them and also is used to insulate the electronics of the Mars rovers. Aerogel is 1,000 times less dense than glass and has many unusual properties, such as extremely low thermal conductivity, refractive index, and sound speed. 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.