An Aerogel Lighter Than Air Caught Comet Dust at 6 Kilometers a Second

In 1931, a chemist named Samuel Kistler made a bet with a colleague, Charles Learned, that he could replace the liquid inside a jelly with gas without collapsing the structure. He won by carefully bringing the liquid past its critical point so it left as vapor rather than evaporating, leaving behind a fragile lattice of silica with 97 percent of its volume occupied by air. He called the result aerogel, and most chemists ignored it for the next sixty years.

What Kistler made is bizarre material. The lightest silica aerogels weigh about 1 mg per cubic centimeter — less than the surrounding atmosphere — so a slab of it actually has neutral buoyancy in air. It conducts heat about 0.003 watts per meter-kelvin, an order of magnitude better than fiberglass batting. And it has the highest known impact tolerance per unit mass for catching tiny fast-moving objects, because particles plunge into it and decelerate gently across millimeters of foam instead of stopping abruptly on a surface.

That last property is why NASA designed an entire mission around it. The Stardust spacecraft, launched in 1999, flew through the dust cloud of comet Wild 2 in January 2004 with a tennis-racket-shaped collector packed with blocks of aerogel. The dust grains, hitting at about 6 kilometers per second, carved tapered tunnels into the gel and stopped intact. Stardust's capsule landed in Utah in January 2006 carrying the first cometary material ever returned to Earth.