The word aerostat was originally French and is derived from the greek aer (air) + statos (standing). An aerostat is a lighter than air object that can stay stationary in the air. Aerostats include free balloons, airships, moored balloons and tethered Helikites. Such a vehicle is lifted by buoyancy, containing a gas less dense than air within an envelope. The gas is generally helium or hydrogen as these are lightest. Helium is safest as it's inert and so doesn't burn, but hydrogen has slightly mo… (More on Aerostat)