Aerogel is almost entirely air, yet it can stop a comet particle

Aerogel is made from the same stuff as glass and sand — silicon dioxide — but with the liquid drained from a wet gel and replaced by gas, leaving a ghostly skeleton that is up to 99.8% air. The result is a solid roughly a thousand times less dense than glass, light enough to have been crowned the world’s lightest solid at just 3 milligrams per cubic centimeter.

The magic is in the drying. Let the liquid simply evaporate and surface tension drags the delicate pores inward, collapsing the structure into ordinary powder. The chemist Samuel Kistler solved this in 1931 with supercritical drying: raise the temperature and pressure until the liquid and gas phases become indistinguishable, so there is no surface tension to pull anything down. The nanostructure survives intact — “frozen smoke.”

A material more cloud than stone.

That emptiness makes it a phenomenal insulator. NASA notes a one-inch aerogel window insulates as well as fifteen panes of glass, which is why aerogel kept the Sojourner, Spirit, Opportunity and Perseverance rovers warm through frigid Martian nights.

It is also tough where it counts. On the Stardust mission, blocks of aerogel caught dust from comet Wild 2 slamming in at about 6 kilometers per second. Each grain plowed a carrot-shaped track roughly 200 times its own length, decelerating gently instead of vaporizing, and the samples came home to Earth in 2006.

The brittleness that long limited aerogels is fading too. Newer flexible polymer and graphene aerogels bend rather than crumble, opening uses far beyond spacecraft — building insulation, thickening agents, even soaking up oil spills.