Space Fills Void Aerogel Created

Able to stop a speeding bullet! More insulating than ordinary fiberglass! So lightweight it’s almost as light as air!

For some space applications, scientists can’t rely on man or steel. Instead, it’s aerogel to the rescue.

Sometimes called “solid smoke” because of its extraordinarily low density and the bluish cast it takes when light shines on it, aerogels once were little more than a scientific curiosity.

The material, created in the 1930s, is made of silicon dioxide, the basic material in glass. It is extremely light and strong--able to support more than 4,000 times its own weight. One variety of aerogel developed at the Jet Propulsion Laboratory holds the Guinness record for world’s least dense solid, weighing in at 3 milligrams per cubic centimeter.

In addition, aerogels are excellent thermal insulators, can withstand wild temperature fluctuations and resist the effects of ultraviolet light.

“It has all these unique properties all wrapped up into one material all at once,” said Steven Jones, the scientist at the Jet Propulsion Laboratory who developed the Guinness record version. “It’s a solution waiting for a problem.”

Space exploration has supplied the problems. The characteristics of aerogel, to take one example, make it the perfect candidate to go where no space mission has gone before: to the tail of a comet.

Stardust, an unmanned spacecraft launched in 1999, will use aerogel to capture fragile dust from the tail of Comet Wild-2 in 2004. Scientists predict the aerogel will trap particles as small as 1/50 the width of a human hair moving six times faster than a rifle bullet.

Aerogels have other uses in space exploration. Consider insulation. When engineers set out to design a rover to survey the surface of Mars, they found the machine was several kilograms too heavy. Using lightweight aerogels for the insulation solved the problem. The materials also will be used on two larger Mars vehicles that are planned.

What is it about aerogel that gives it those amazing properties?

It’s a solid foam, said Sidney Perkowitz, professor at Emory University in Atlanta and author of “Universal Foam: From Cappuccino to the Cosmos” (Walker & Co., 2000).

A foam, says Perkowitz, is simply open spaces of one kind of material embedded in another material. Swiss cheese, for example, is a solid foam. Meringues and souffles also owe their light and airy textures to a foamy structure.

Foams have unique properties that are different from their individual parts, Perkowitz said. Soap suds offer an example. “Air is transparent, water is transparent,” he said. “You mix both together, and light behaves differently.”

Aerogels have a microscopic “open-pore” design, reminiscent of a sponge or a net, which makes the materials only slightly denser than air.

To make aerogels, scientists mix liquid silica with a solvent to form a Jell-O-like solid. Scientists then remove the solvent using extreme heat and pressure to preserve the gel network.

The structure of aerogel interferes with the way the substance transfers light, sound and thermal energy. Although 99% of the mass of an aerogel is made of air filling its empty pores, each individual pore is so small it prevents the air molecules within it from interacting with other molecules.

On Stardust’s mission to Wild-2 (pronounced “vilt,” after its Swedish discoverer), blocks of aerogel arrayed in a honeycomb-like tray on a collector shaped like a tennis racket will trap dust particles from the comet’s tail. After making a couple of swings around Earth, Stardust will fly within 93 miles of Comet Wild-2 in 2004.

The aerogel will act like shrink-wrap to preserve the particles’ original “fluffy” shape and chemical composition, said Peter Tsou, a deputy investigator for the project.

Each bit of dust will carve a track shaped something like a carrot into the block of aerogel, melting the material and encasing itself in a little glass shell. Other cushiony materials such as polystyrene would disintegrate into a powder when exposed to ultraviolet light in space, Tsou said.

When Stardust returns home in 2006, researchers will extract the particles for study.

Maintaining the original structure and composition is important, Tsou said, just as they are to an archeologist excavating a site. If you were a researcher, “would you prefer a chip of a vase or a bottle of atoms of a vase?” he asked.

Comets essentially are hunks of ice and rock that are left over from the solar system’s formation, so analyzing the comet dust “will tell us about the start of the solar system,” said Donald Brownlee, a University of Washington professor and the primary investigator of the mission.

But not just any comet dust will do.

“More importantly, we want fresh comet,” Tsou said.

Kept in the deep freeze far away from the sun, comets retain their original primordial compositions. But when some gravitational disturbance knocks them out of their distant orbits and sends them plunging toward the sun, each pass they make through the inner solar system melts some of the solids and ice. Wild-2 is ideal for the mission, as it has made only a few orbits.

The aerogel collector also will bring back clues about stars outside of our solar system by collecting interstellar dust. Scientists believe tons of the stuff rains down on the planet every year, but its composition changes as it burns through the atmosphere.

While aerogel is invaluable for such scientific missions, the materials, as yet, have few commercial applications.

Researchers have studied aerogels’ potential use in windows. The foam diffracts light, however, bending rays rather than allowing them to pass straight through as glass does.

Also, aerogels need to be handled with care. Although able to support great weights, the material is fragile enough to shatter in the grip of a human hand unless pressure is applied evenly.

It also is difficult to mass produce, Jones said. “You can’t make it on a conveyor belt.”

One company, Boston-based Cabot Corp., plans to launch custom insulation products this fall using a particle version of aerogel. The material could be used to fill the space between double-pane glass, for example, providing diffuse light with as much insulation as an opaque wall.

“There’s a lot of interest in energy-efficient ways to bring daylight into places,” said David Kaul, manager of Cabot’s aerogel unit.

Aspen Aerogels in western Massachusetts developed aerogel blankets with a number of uses, including insulation for wrapping pipes or even to reduce the infrared emissions of Army helicopters, said Chris Blair, director of sales and marketing.

It also is used for extreme-weather clothing. Corpo Nove, an Italian clothing design company, began using Aspen blankets to line jackets in 1999, said Susan Clowe, spokeswoman for the company. The lining provides three times the warmth of Thinsulate, but the jackets cost more than $2,000.

And don’t think you’ll want to snuggle directly under an aerogel blanket any time soon. The material has many amazing properties, but comfort is not one of them. A blanket of aerogel “leaves a horrible feeling on your skin,” Clowe said. “It has to be encased between two other layers.”