If you wanted to catch a few particles of comet dust speeding through the vacuum of space at 6 kilometers per second — without damaging or destroying those particles — how would you do it?
Faced with exactly this problem, scientists at the Jet Propulsion Laboratory focused on aerogel — an extremely lightweight, porous material that is chemically identical to glass, but weighs only a little more than air.
Aerogel is the lightest solid known to science. It’s also one of the most insulating materials on Earth, the most porous, and it’s nearly transparent. Those last two properties made it an ideal choice for catching flecks of comet and interstellar dust on the recently-returned Stardust mission launched by NASA and JPL.
Since the satellite returned to Earth on January 15, NASA scientists have been busy slicing open Stardust’s aerogel cells and carefully extracting the bits of dust it collected from Comet Wild 2.
“Aerogel is unique in having so many superlative properties, and a huge range of properties too,” said Donald Brownlee, a University of Washington astronomer and the principal investigator for Stardust.
Aerogel isn’t exactly space-age technology. It was invented in 1931 by Steven Kistler, in response to a bet made by a fellow scientist. Kistler found a way to remove the liquid from a silica gel without destroying the long silica molecule chains that gave the gel its structure.
Holding a piece of aerogel is an uncanny experience. It’s so light it feels nearly weightless, like a chunk of solidified fog or smoke. It feels a bit like Styrofoam, and it squeaks when you rub your finger on it. It’s strong enough to support many times its own weight if the load is distributed evenly. But bend it or squeeze it too hard, as one Wired News editor discovered, and a chunk of aerogel will shatter into tiny fragments.
Ordinary gels, like Jell-O, are comprised of tangled chains of molecules — polymers — surrounding empty pockets of a liquid, such as water. If you try to dry out a cube of Jell-O at room temperature, the surface tension of the liquid will cause the polymer structures to collapse as the liquid evaporates. The result is that the gel cracks, shrinks and eventually crumbles to dust.
Modern scientists make aerogel by pressurizing and heating an ordinary gel to its “supercritical” point, where the liquid’s fluid and gaseous phases are indistinguishable, and then draining off the supercritical liquid. Because there’s no gas-liquid interface, there is no surface tension and so the liquid can be removed without destroying the gel’s polymer structure. With the liquid gone, air fills up the spaces between the polymers, and the result is a meringue-like aerogel.
Scientists aren’t sure why aerogel works so well as a cosmic duster. One theory, says Brownlee, is that the porousness of the material gives particles a chance to slow down as they smash through the nanometer-scale silica structures. As they go, the particles pick up a “paint” of melted glass on their front edge, which protects them from further collisions with the structure until they come to rest.
The transparency of aerogel was also critical to the Stardust mission because it allowed scientists to find the particles by following their tracks through the material.
Back on Earth, that porousness (aerogel can be up to 99-percent air) makes aerogel an ideal thermal insulator. So it’s no surprise that companies are investigating commercial uses for this material, ranging from windows to home insulation to clothing.
Such a wealth of useful properties makes aerogel interesting not only to rocket scientists, but to entrepreneurs and venture capitalists, who sunk $50 million last year into Aspen Aerogels, a company devoted to commercializing aerogel.
“As an insulator, aerogel is two to four times more efficient than anything else out there,” said George Gould, the director of research for Aspen Aerogels.
Aspen Aerogels makes economical aerogel textiles by impregnating “blankets” of fabric with silica gel, then pressurizing the impregnated fabric and extracting the now-supercritical liquid. The result is a flexible fabric with aerogel integrated into its matrix.
Prices for the material vary, but a typical price is a few dollars per square foot for quarter-inch thick material. When Aspen Aerogel’s second factory is completed later this year, Gould said, the company will be able to produce 100 million square feet per year of its aerogel textiles, bringing costs even lower.
The problem these companies face is that, while aerogel is a vastly superior insulator, the alternatives (like fiberglass or plain glass windows) are dirt-cheap.
The high pressure needed to create aerogel (around 800 pounds per square inch) means that producing even a tiny amount requires costly lab equipment. You can buy aerogel samples on eBay but they cost around $30 to $50 for small, nickel-sized chunks.
That means aerogel is unlikely to play a major role in construction or clothing unless its makers can bring the price down much further — or capitalize on its space-age reputation enough to make customers willing to pay extra for cachet.
“The costs are not necessarily prohibitive,” said Gould. “Relative to something like fiberglass, the costs are certainly greater but a lot of it has to do with capacity.”
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