Aerogel - lightest solid
Aerogel is 99.8% air and 1,000 times less dense than glass yet it can withstand high temperature, is robust enough to survive a space launch and delivers 39 times more insulation than the best fibreglass. This exotic substance was invented in the 1930s but recently refined by NASA for the purpose of catching space-dust, Aerogel was used on the Mars Pathfinder rover and its latest assignment is to capture both cometary samples and interstellar particles aboard the Stardust mission. Although much less dense than glass, Aerogel is another silicon-based solid but is composed of individual features only a few nanometers in size linked in a highly porous structure. Its unusual properties include low thermal conductivity, refractive index and sound speed - but it's the ability to slow down and capture fast moving dust with minimal heating or other effects that would cause their physical alteration that grabbed NASA's attention.
Source: stardust.jpl.nasa.govAdded: 12 June 2006