Aerogel is sometimes referred to as “blue smoke” or “solid smoke”. It is as delicate as a flower, yet durable enough to withstand extreme environments.
STARDUST used an extraordinary substance called silica aerogel—a silicon-based solid with a porous, sponge-like structure in which more than 99 percent of the volume is open space. Aerogel is 1,000 times less dense than glass, another silicon-based solid. This exotic material has many unusual properties, such as uniquely low thermal conductivity, refractive index, and sound speed, in addition to its exceptional ability to capture hypervelocity dust. Aerogel is made by high temperature and pressure critical point drying of a gel composed of colloidal silica structural units filled with solvents. Over the past three years, aerogel has been made and flight qualified at the Jet Propulsion Laboratory. We used the JPL facility because it allowed us to have full control over the media properties and purity. Silica aerogel produced at JPL is a water clear, high purity silica glass-like material that can be made with bulk density approaching the density of air. We have repeatedly demonstrated it's strength in launch and space environments.
Cone-shaped track made by particle captured by silica aerogel.
When a hypervelocity particle hits the aerogel, it buries itself in the material, creating a narrow cone-shaped track, as is slows down and comes to a stop. Since aerogel is mostly transparent the tracks can easily be seen by using a stereo microscope. The cone is largest at the point of entry, and the particle is collected at the point of the cone. This provides a directionality detector and is the basis of our approach of using single slabs of aerogel to collect both cometary and interstellar dust, and being able to differentiate between them because the one side of the collector is exposed in the comet dust impact direction and the opposite side is positioned toward the interstellar dust stream. The captured particle is seen optically just beyond the tip of the cone, and it can be recovered by a variety of techniques, ranging from extraction with a needle, to microtoming, and focused ion beam etching. Recovered samples are then treated by sequential analysis techniques that have been developed for the analysis of small meteoritic samples and interplanetary dust particles. We have already developed techniques for the removal and analysis of captured grains from silica aerogel.