Water vapour

The gaseous state of water is lighter than air (density 0.804 g/L at STP, average molecular mass 18.015 g/mol) due to water's low molar mass when compared with typical atmospheric gases such as nitrogen gas (N₂). It is non-flammable and much cheaper than helium. The concept of using steam for lifting is already 200 years old. The biggest challenge has always been to make a material that can resist it. In 2003, a university team in Berlin,