Aerogel is a real solid material. It is rigid enough to hold its shape, transparent enough to see through, and 99.8 percent air by volume. A block large enough to span an adult's hand weighs about as much as a few feathers. It looks faintly blue and slightly translucent — visually closer to frozen smoke than to anything most people would recognize as a solid.

The first aerogel was reportedly produced in 1931 by an American chemist named Samuel Kistler at the College of the Pacific in California. According to the story he later told, the work was prompted by a bet at a dinner party with a colleague over whether the liquid inside a gel — a wobbly substance like jellied dessert — could be replaced with gas without collapsing the underlying solid network.

Kistler developed a technique called supercritical drying. The liquid in the gel is brought to a temperature and pressure where the distinction between liquid and gas disappears, then bled off. What remains is the original molecular framework of the gel, now filled with air.

For decades, silica aerogels were the lightest known solid material. Newer carbon and graphene aerogels developed in the 2010s are lighter still, but silica aerogels remain among the lightest solids ever made. They are also the best solid thermal insulators ever produced. A thin sheet of aerogel can stop a blowtorch on one side from warming a flower on the other side. They have been used as insulation on Mars rovers and in specialized cold-weather equipment.

NASA used a block of silica aerogel on the Stardust mission to capture interstellar and cometary dust traveling at over six kilometers per second. The aerogel slowed and trapped the particles intact, without vaporizing them.

A material that is almost not there, doing things no other solid can do.