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What Is the Lightest Solid?

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Aerogel cube held by Jet Propulsion Lab scientist Peter Tsou.

Aerogel cube held by Jet Propulsion Lab scientist Peter Tsou.

NASA JPL
Question: What Is the Lightest Solid?
Answer: The lightest or least dense solid is the aerogel. The winning composition, produced by Lawrence Livermore National Laboratory, consists of 99.8% air and 0.2% silicon dioxide. This aerogel has a density of 1.9 mg/cm3. Although it can be colored, aerogel starts out looking like solid smoke -- translucent, with a bluish tinge. Aerogel is an incredibly good thermal insulator. A famous photo of the material shows a blow torch flaming one side of a thin slab of aerogel, with unmelted Crayola crayons on the other side! NASA has used black-tinted aerogel (black was used to cut down on infrared transmission) on its Mars Rovers. The Stardust spacecraft used aerogel to capture particles from a comet and from interstellar space. According to NASA, a block of aerogel the size of a human being would only weigh 455 g (about one pound), yet would be capable of supporting an object weighing half of a ton. Aerogel is expensive to produce, so you won't be able to pick some up at your local store to examine... yet.

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