The Jet Propulsion Laboratory has beaten itself into the Guinness World Records, again for creating the world's lightest solid.

Described as a solid smoke because it's 99.8 percent air, the material called Aerogel is actually a stiff foam made from silicon dioxide and sand - the same ingredients that make glass, but Aerogel is one thousand times lighter.

Its density is just 3 milligrams per cubic centimeter, 2 milligrams lighter than the previous world record holder, an older formulation of Aerogel.

This Aerogel is more than just lightweight. It withstands pressure thousands of times higher than its own mass, and melts only when temperatures hit 2200 F (1200 C).

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 and it has been used on the Mir Space Station as well as on the Mars Pathfinder missions. It is now being used to collect tiny cosmic particles on the Stardust spacecraft.

Aerogel is acting as a cometary dust-bin for the craft, which was launched in Feb 1999. So far, Stardust has collected several samples of stellar dust, and will encounter Comet Wild 2 in 2004.

Stardust's Aerogel collection unit will return to Earth in 2006 with the world's first samples of cometary dust - and the only space matter brought to Earth other than from the Moon.

Scientists are interested in the dust because comets are the oldest, most virgin materials in the solar system. Learning more about them will make for a more complete picture of Sun and planetary evolution.

It takes an out of this world material like Aerogel to capture a comet.

The dust of Comet Wild 2 will fly off of it six times faster than a rifle bullet. Materials that get in the way of such particles are usually destroyed, or the impact obliterates the dust itself. One comet-spying craft, CONTOUR, is covered in the bulletproof material Kevlar, to shield against such impacts.

But Stardust's Aerogel unit will act as a gentle catcher's mitt. Because Aerogel is only 0.02 percent matter, it will gently put the brakes on the dust of Comet Wild 2. Finding the cometary bits in Aerogel will be as easy as following the carrot-shaped trails that the grains will leave in the material.

The newest Aerogel, although not used in any spacecraft yet, is likely to hold the title of least dense material for a good while. The creator of the record-breaking Aerogel, materials scientist Steven Jones, believes that a limit has been reached in eliminating silica from the formula.

"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."

More JPL accomplishments are likely to appear in Guinness World Records. JPL software engineer Ron Baalke said that after he saw the Aerogel in an older record book, he made a quick call to Jones to see if the material had been made any lighter. Baalke then added the new Aerogel density to a submission list of 59 other JPL achievements he'd been compiling for Guinness.

"I was always interested in space history," Baalke said. Now, he's making some of his own.