Using the Right Bait to Catch a Comet

By CHRIS DIXON

Published: January 20, 2004

PASADENA, Calif. &emdash; On Jan. 2, NASA's Stardust spacecraftflew through a 14,000-mile-an-hour hail of debris from the Wild 2comet. Under this withering barrage, the ship's objective was notonly to survive, but to use a collector made of a bizarre substanceto gather an unaltered sample of five-billion-year-old dust and gasejected by the comet.

This seemingly impossible task has been the 20-year quest of Dr.Peter Tsou, the deputy principal investigator for the Stardust. On aquiet afternoon at the Jet Propulsion Laboratory, Dr. Tsou showed offa three-centimeter cube of silica aerogel, the substance that he usedto catch the comet's tail.

At around 0.003 grams per cubic centimeter of material &emdash;only about three times as dense as air &emdash; aerogel is puresilicon dioxide. Not only is it the least dense solid in existence,it is also such a remarkable insulator that a layer of it surroundsthe most vital electronics on the Mars rover Spirit. But the moststriking feature, at least to the naked eye, is that up close, thecube looks like a blurry hologram.

"When you look at this," says Dr. Tsou, holding the aerogel up,"you don't know where to focus. That's why some people call it solidsmoke."

Made of 99.6 percent empty space, the little cube is indeedbarely there, with a density one-hundredth that of the hand thatholds it.

To make this strange material, scientists start with a liquidalcohol like ethanol and mix it with silicon dioxide to form a gel.Then, through a process called supercritical drying, the alcohol isforced out of the gel, typically with high-pressure carbon dioxide.With this drying process, the gel does not collapse or lose itsvolume. It appears holographic because the silicon dioxide scattersshorter wavelengths of light much like air in the daytime sky.

In the mid-1980's, Dr. Tsou ventured to the Los Alamos NationalLaboratory in search of a material that would allow him to collectparticles moving at three to six miles per second without destroyingeither the particles or their collector. "I approached manyscientists," he said. "They said, `Come on, that's ridiculous.' AtJ.P.L., we have a tech guru. I paid him to do a report, and he said,`It's not possible.' "

Dr. Tsou had considered using many thin foil layers or a polymerfoam to catch the particles, but in space, radiation and temperatureextremes quickly degrade foams. Because foams and foils are opaque,finding the captured interstellar particles would have been aproblem.

While at Los Alamos, he said, he noticed a cube of an oddmaterial on a laboratory windowsill. It was a form of aerogel thatLos Alamos had tested and rejected for its nuclear fusionexperiments.

The material was not new. In 1931, Steven S. Kistler was apioneer in making the substance at the College of the Pacific inStockton, Calif., now the University of the Pacific. But, Dr. Tsousaid, the material was not used much, except in powdered form as anontoxic anti-caking agent for food.

In the 1980's, Dr. Tsou and others began to work with thematerial. "It has 14 Guinness Book of World Records-type properties,"Dr. Tsou said. "It's the lowest density of any solid, and it has thehighest thermoinsulation properties. Though it would be veryexpensive, you could take a two- or three-bedroom house, insulate itwith aerogel, and you could heat the house with a candle. Buteventually the house would become too hot."

Additionally, aerogel slows soundwaves to about 10 percent oftheir speed in air, and because it has such a vast surface area forits volume, its use as a filtration agent could increase the capacityof desalination plants a thousandfold.

Because aerogel is transparent and releases light when struck bycertain high-energy radiation, it provides an excellent means ofcounting atomic particles. It also has incredible compressivestrength. "It can take 2,000 times its body weight without damage,"Dr. Tsou said. NASA's Web site shows a 2-gram cube of aerogel (lessthan 0.1 ounce) supporting a 2.5-kilogram brick (about 5.5 pounds).

"It's better than anything you can think of," said Glenn Tsuyuki,a manager with the Mars rover project.

Because of its ability to keep electronics on both the 1997 MarsPathfinder and the current rover, Spirit, at room temperature in theface of minus-100-degree cold, aerogel will probably remain theinsulator of choice on Mars missions for some time, Dr. Tsuyukisaid.

He added that he had proposed aerogel as the collection mediumfor a future flight studying dust in the upper Martian atmosphere.

"I would imagine that there are ideas out there that we haven'teven thought of," he said.