Silica aerogel is 99.8% air — but not lighter than air

Silica aerogel is a solid that is up to 99.8% air, about a thousand times less dense than glass, and in 2002 Guinness World Records named NASA JPL’s version, at 3 milligrams per cubic centimetre, the world’s least dense solid. It is made from the same silicon dioxide as glass: a wet gel is dried by careful heating and depressurising, leaving a ghostly glassy sponge.

Is aerogel lighter than air?

Not the silica kind. Air weighs about 1.2 mg/cm³, so even JPL’s record aerogel was more than twice as dense, and NASA describes typical aerogel as about three times the density of air. JPL’s Steven Jones said at the time that aerogel could never be made lighter than air, “because aerogel is filled with air.”

The current Guinness record holder complicates the picture. A graphene aerogel made at Zhejiang University and announced in February 2013 is listed at 0.16 mg/cm³ — roughly a seventh of air’s density — and Guinness calls it “lighter than air itself.” Guinness does not reconcile that with Jones’s point: weighed with the air that fills its pores, no aerogel can come out lighter than air itself.

What is aerogel used for?

NASA has used it as an insulator and as a gentle trap. That emptiness makes silica aerogel exceptional at holding in heat: NASA says a one-inch aerogel window insulates as well as fifteen panes of glass and trapped air, which is why aerogel kept the Sojourner rover from freezing through Martian nights in 1997.

NASA’s Stardust spacecraft flew through the dusty coma of comet Wild 2 in January 2004, its aerogel panels “soft-catching” grains of dust without destroying them. The return capsule landed in Utah on 15 January 2006 carrying more than 10,000 particles larger than a micrometre — the first comet samples ever brought to Earth.