Hydrogen in it's liquid form has been used in space vehicles for years. Hydrogen has a high combustion energy per pound relative to any other fuel, meaning hydrogen is more efficient on a weight basis than fuels currently used in air or ground transportation. Hydrogen is the universe's most abundant element. Most of that hydrogen though, is tied up in chemical bonds. Hydrogen can exist in either a gaseous form or a liquid form. Hydrogen is The liquid form is usually used for storage while the gaseous form is used as a heat transfer, and also as a cooling agent in nuclear power plants. The name hydrogen is Greek for water