Solid Smoke

Aerogel was created in 1931 as the result of a bet. Steven Kistler wagered a colleague that he could replace the liquid inside a jam jar without producing any shrinkage. In other words, he thought he could fill the volume with something that had the same struc­tural properties but was completely dry. The material that won the bet was 99.8 percent air and has the lowest density of any known solid. Aerogel is made by extracting the liquid from a gel by super­critical drying. This allows the liquid to be slowly drawn off with­out the solid matrix of the gel collapsing under its capillary action, as would occur during evaporation. The first aerogels were made of silica; more recent ingredients include alumina and carbon.11

If you touch an aerogel, it feels like Styrofoam or the green foam that flowers are often pressed into. Pressing on it softly doesn’t leave a mark and pressing on it firmly leaves a slight depression. But pressing down sharply enough will cause a catastrophic break­down of the dendritic structure, making it shatter like glass. It’s light and strong, supporting four thousand times its own weight. Aerogel is a thousand times less dense than glass, and the myriad tiny cells of air trapped inside the material make it one of the best insulators known. Engineers at NASA’s Jet Propulsion Lab learned how to make extremely pure aerogel and it was used as an insula­tor on the Mars Pathfinder mission. Peter Tsou is the wizard at JPL who fabricates aerogels; because of the importance of his skills, he was named deputy principal investigator on Stardust.

With Stardust, the challenge was to capture small particles mov­ing at six times the speed of a rifle bullet without vaporizing them or altering them chemically. Aerogel is perfect for this job; the rigid foam that’s not much denser than air slows the particles down and brings them to a relatively gentle halt, each one leaving a carrot­shaped wake two hundred times its size. Imagine firing bullets into a swimming pool filled with Jello. Stardust’s aerogel was fitted into a module the size and shape of a tennis racket that swung out when the spacecraft approached the comet. One side was turned to face Wild 2, and the other side was turned to face interstellar dust encountered on the journey. Before and after use, the module was stored in its protective Sample Return Capsule (plate 9).12

Stardust flew within 150 miles of the comet on January 2, 2004 and headed back to Earth with its precious cargo trapped like tiny flies in a silica spider web. On January 15, 2006, Stardust returned home after seven years and nearly 3 billion miles of traveling. First, the mission controllers did a short rocket burn to divert the space­craft from hitting the Earth, leaving it with just 20 kg of fuel. Then they fired two cable-cutters and three retention bolts to release the 46-kg return capsule and watched as springs on the spacecraft pushed the capsule away. The capsule streaked into the pre-dawn California sky at 29,000 mph, faster than any man-made object had ever been returned to Earth. The heat shield and parachutes worked flawlessly and the capsule landed in the Utah desert at 5:10 a. m. The few people up and outside that morning saw a fire­ball and heard a sonic boom.

Within two days, the package containing the aerogel was opened in a clean room at the Johnson Space Center in Houston. Stardust was subject to the maximum contamination restrictions, since it returned material from an extraterrestrial object with the potential to host life. In practice, the risk of “infecting” the Earth with alien life was low, since any known organism would almost certainly be destroyed by the high impact speeds in the aerogel, but NASA took no chances. The mission was carried out under a Category 5 plane­tary protection policy, which is even more stringent than Biosafety Level 4, the protocol used to deal with hemorrhagic fevers like Ebola and Marburg.13 That means sterilization by heat, chemicals, and radiation before the spacecraft is launched, and a requirement that the returned samples are handled in a secure facility and never come into direct contact with humans.

Members of the team opened the sample return package in a clean room just down the hall from where hundreds of kilos of Moon rocks are kept, brought back by the Apollo astronauts.14 The room was a hundred times cleaner than a hospital operating theater. They were delighted to see the aerogel segments littered with particle tracks, looking like burrows left behind by micro­scopic creatures. The mission had clearly been a success.