aerogel

aerogel any of a group of extremely light and porous solid materials; the lightest is less than four times as dense as dry air. Aerogels are produced from certain gels (see colloid ) by heating the gel under pressure, which causes the liquid in the gel to become supercritical (in a state between a liquid and a gas) and lose its surface tension. In this state, the liquid may be removed from the gel by applying additional heat, without disrupting the porous network formed by the gel's solid component. Silica-, melamine-, and carbon-based aerogels have been produced. Silica-based aerogels are among the lightest, and some, nicknamed "solid smoke" or "frozen smoke," are nearly transparent. Heavier aerogels were first developed in 1931 and have been used to detect high-energy particles emitted by particle accelerators . Newer, lighter aerogels with relatively high insulating properties are being tested as substitutes for the chlorofluorocarbon foams used as refrigerator insulation and as replacements for the air between the panes of double-glazed windows; other aerogels are being used as filters.

Stardust A NASA space probe launched in 1999February to collect dust from Comet 81P/Wild 2 and return it to Earth. The dust particles were collected on the surface of a paddle coated with a low-density glass foam called aerogel. On its way to the comet, Stardust collected samples of interstellar dust passing through the Solar System and in 2002November flew past asteroid (5535) Annefrank. In 2004January Stardust passed within 240 km of the nucleus of Comet Wild 2, photographing the nucleus and catching dust particles. A return capsule carrying the dust samples parachuted to Earth in 2006January. http://stardust.jpl.nasa.gov/