Aerogel Rides Again

February 05, 1999

Everything in the universe, from planets to the particles of your skin, is composed of star dust. But even though star dust is the clay from which all things are formed, actually going into space to capture a pure sample is not an easy task. Yet NASA plans to do just that, using a lightweight insulating material called aerogel. Composed of over 95 percent air, aerogel is the lightest man-made material on earth. And although aerogel will be used in the depths of space to conduct research in astrophysics, it has hundreds of down-to-earth applications, as well.

To Catch a Falling Star
This Saturday, NASA will launch StarDust, a spacecraft designed to sample and return material from the comet Wild 2 (right). The comet travels a path from just outside Jupiter's orbit to just inside the orbit of Mars. StarDust will sweep through the comet's coma, the ball of gas surrounding the nucleus of the comet, at 136,000 miles per hour. NASA will use an aerogel "catcher's mitt" designed by Dr. Peter Tsou and his colleagues at the Jet Propulsion Laboratory in Pasadena, Calif. Rather than catch fly balls, this "mitt" will catch particles coming off the comet. After a 7-year journey, StarDust will return to Earth, dropping its newly obtained cargo onto the Utah desert. Because the comet originally came from the Oort cloud, which extends beyond the orbit of Pluto, the StarDust mission will bring back matter from the deepest recesses of our solar system.

Passing through cometary debris, however, can be more dangerous than standing in front of a machine-gun. The particles coming off the comet will probably be smaller than grains of sand, but they will be hitting the aerogel at an extremely high velocity.

This impact is so powerful that, with any substance other than aerogel, the particles would either vaporize upon impact or become so distorted that scientists couldn't study them.

"I like to call this NASA's version of collecting bugs on the windshield," says Dr. David Noever, an aerogel researcher at NASA's Marshall Space Flight Center. "In actuality, aerogel is low enough density to collect the "bugs" without destroying them, even though the satellite will be traveling through the comet debris at 20 times the speed of a bullet. That's a soft touch."

When the particles hit the aerogel, they will drill through the material, gradually slowing down, creating furrows that scientists will use to track the paths of the particles.

Orange Jello, Lemon Jello, and Aero Jello?
Aerogel is the result of a friendly wager between Dr. Steven Kistler and Dr. Charles Learned, two Stanford University scientists. They competed to see if one of them could replace the liquid inside a jelly jar with gas without causing any shrinkage. Kistler won the bet, and published his findings in a 1931 edition of the journal Nature.

But the potential of aerogel didn't come to light until the 1960s and 70s, when many in the aerospace industry were trying to develop an extremely lightweight, heat-resistant material to put on airplanes and spacecraft. To illustrate the impossibility of ever developing such a material, some in the aerospace industry dubbed it "unobtainium."

Modern silica aerogel starts as a liquid (mainly water, alcohol and silica), and then gels into something that looks like Jello. Just as Kistler replaced the liquid in jelly with gas without causing the jelly to shrink, the trick to making areogel is to dry the "aero-Jello" without collapsing it into a dense slab. This can be accomplished by exchanging the alcohol with liquid carbon dioxide, and then by removing the carbon dioxide at high pressure.

"The whole process is not that different from how coffee is decaffeinated," comments Noever. The end result is "frozen smoke," one of the lightest solid materials known. Researchers are currently developing techniques to dry aerogel under normal atmospheric pressure.

Although aerogel looks like it could float away, it has very high compression strength for its mass.

"But aerogel is not really so much of interest as a structural material," says Noever. "The real clincher is its incredible insulating effects on any kind of energy transfer: thermal, electrical or acoustic. Aerogel can damp out almost any kind of energy."

That's the primary reason aerogel was used as insulation on the Sojourner Mars rover in 1997. As night fell on Mars, the temperature dropped down to -67 C (-88 F). Although the temperature outside was colder than Antarctica in winter, it remained a balmy 21 C (70 F) inside the Rover, where sensitive electronics were protected from the hard freeze. The Rover performed spectacularly, gathering information on the surface of Mars for almost three months following the Pathfinder's landing on July 4.

A Toast to Jelly
Aerogel's superior insulation can be used down here on Earth, as well. Currently, a large portion of a home's heating bill literally goes out the window. A single one-inch thick windowpane of silica aerogel is equivalent to the insulation provided by 20 windowpanes of glass (R-20 insulation factor). Aerogel is such a good insulator that a house with aerogel windows could be kept warm and toasty by using only a fraction of the energy currently needed. Aerogel windows would help people save on their home heating bills,and subsequently would reduce the world-wide production of carbon dioxide and other greenhouse gases.

So why aren't we currently using aerogel windows? Because aeorgel is not perfectly transparent and is expensive to produce. Aerogel's slightly bluish cast presently makes it only practical for use in skylights or bathroom windows. According to Noever, "The holy grail of aerogel applications is transparent, superinsulating household windows."

The problem lies in the size of the material's pores. Most of the pores in aerogel are too small to scatter visible light, but once in a while a few of the pores are larger. The larger pores scatter light as it passes through aerogel, and this creates the hazy appearance. It is thought that the elimination of these larger pores would result in an aerogel of better optical quality.

NASA is developing techniques to produce a clearer aerogel. There is evidence that the pore irregularities are diminished when the substance is manufactured in the microgravity environment of space. Experiments on suborbital rockets have shown that producing aerogel in space can reduce the number of large pores that form.

"But the rocket only offered 7 minutes of reduced gravity and the results are not yet conclusive," says Dr. Laurent Sibille of the Universities Space Research Association, working at NASA/Marshall. "That is why we are repeating these experiments on the Space Shuttle."

Aerogel experiments were conducted on the recent STS-95 mission with Senator John Glenn, and are planned for the upcoming STS-93 mission, which will also launch the Chandra X-Ray Observatory.

"The goal of our research is to establish the role of gravity in the manufacturing of materials and suggest Earth-based techniques to modulate or counter these effects," states Sibille. By finding out how gravity affects the production of aerogel, researchers can develop better ways to produce it down here on Earth.

As progress continues, we can look forward to a future where the use of clear aerogel insulation will better our chances that neither hazy windows nor atmospheric pollution will cloud our view of the stars.

SuperHero meets SuperMaterial
One of the most amazing things about Wonder Woman was her invisible jet. The only indication that she wasn't flying seated through the air were the existence of faint lines tracing the outline of her plane. It was fantastic, bizarre and completely impossible.

With the invention of aerogel, however, Wonder Woman's plane looks less like pretend, and more like prophecy. Although current aerogel technology is not advanced enough to produce a product as amazing as Wonder Women's jet, there are some similarities between her aircraft and aerogel. Like aerogel, her aircraft was almost transparent. And aerogel would have allowed Wonder Woman to fly at the same altitude as a normal plane without feeling a chill. As research in aerogel technology develops, perhaps someday the "SuperMaterial" that comprised Wonder Women's jet will become an ordinary product.

Long before that day, however, aerogel will probably be a common household name. Although it is not yet ready for commercial use, Fortune magazine's "Technology to Watch" column mentioned hundreds of potential products that could be manufactured out of aerogel, citing everything from surfboards to satellites. With the use of aerogel in the Sojourner Mars rover, in the StarDust spacecraft, and in the hundreds of other possible products and applications, the "unobtainium" once only dreamed about may finally be just around the corner.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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