Aerogel is the least-dense solid material ever created, consisting of about 99.8% air and made up of silica. The revolutionary substance is hard enough to stop a bullet in its tracks, and has been used on NASA's Stardust spacecraft, launched in 1999 to comet Wild-2, to capture super-high-speed particles in its wake for return study on Earth. In addition, Aerogel has very good thermal protection properties and was used to insulate the Mars Pathfinder and Mars Exploration Rovers in 1997 and 2004, respectively. Aerogel may be a widely used material in future households, but the cost of making it is currently in the hundreds of dollars per cubic inch. MORE INFORMATION CAN BE FOUND HERE      &     HERE.