A new version of aerogel, the substance that will be used to capture particles from comet Wild 2 in 2004 by the 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 developed the aerogel used by Stardust, created a version that weighs only 3 milligrams per cubic centimeter. This beat out the previous record holder, an aerogel that weighed 5 milligrams per cubic centimeter. The team received the official certificate yesterday.

Aerogel

Described as a solid smoke because it's 99.8 percent air, aerogel is actually a stiff foam made from silicon dioxide and sand - the same ingredients that make glass. However, aerogel is 1,000 times less dense than glass. Its amazing properties make it extremely lighteight but incredibly strong, able to withstand pressure thousands of times higher than its own mass. It melts only when temperatures hit 2200 degrees F (1200 C).

Aerogel 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.

"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, because aerogel is filled with air." To change the density, Jones simply changes the amount of silicon in the initial mixture.

Scientist Samual Kistler invented the original Aerogel, in 1932. Monsanto bought the rights to the material and underutilized it as an insulator in picnic coolers and as a thickening agent in napalm bombs. JPL realized the properties of aerogel made it ideal for space travel. 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.

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