A new version of aerogel, the particle-collecting
substance on NASA’s Stardust spacecraft, has been recognized
by Guinness World Records as the solid with the lowest
density.

Dr. Steven Jones of NASA’s Jet Propulsion Laboratory,
Pasadena, Calif., a materials scientist who created the
aerogel used by Stardust, also created a lighter version that
weighs only 3 milligrams per cubic centimeter (.00011 pounds
per cubic inch.) The team received the official certificate
yesterday.

Guinness World Records approved the new aerogel’s
application for the least dense solid in March. Astronomer
David Hawksett, Guinness World Records’ science and technology
judge, decided that Jones’ aerogel beat out the previous
record holder, an aerogel that weighed 5 milligrams per cubic
centimeter (.00018 pounds per cubic inch.)

Aerogel is pure silicon dioxide and sand, just as is
glass, but aerogel is a thousand times less dense than glass
because it is 99.8 percent air. It is prepared like gelatin by
mixing a liquid silicon compound and a fast-evaporating liquid
solvent, forming a gel that is then dried in an instrument
similar to a pressure cooker. The mixture thickens, and then
careful heating and depressurizing produce a glassy sponge of
silicon.

What remains is sometimes called “solid smoke,” for its
cloudy translucent color and super-light weight. Surprisingly,
this seemingly brittle substance is durable and easily
survives launch and space environments.

“It’s probably not possible to make aerogel any lighter
than this because then it wouldn’t gel,” Jones said. “The
molecules of silicon wouldn’t connect. And it’s not possible
to make it lighter than the density of air, 1.2 milligrams per
cubic centimeter (.00004 pounds per cubic inch), because
aerogel is filled with air.” To change the density, Jones
simply changes the amount of silicon in the initial mixture.

Stardust will use aerogel to capture particles from comet
Wild 2 in 2004. NASA used aerogel for thermal insulation on
the Mars Pathfinder mission. It will also be used on the 2003
Mars Exploration Rover, and may aid a proposed fundamental-
physics testing mission and the Mars Scout Program.

More information is available at:
http://stardust.jpl.nasa.gov/tech/aerogel.html .

JPL is a division of the California Institute of
Technology, Pasadena, Calif.