An airship or dirigible is a type of aerostat or lighter-than-air aircraft which can navigate through the air under its own power. Aerostats gain their lift from large gas bags filled with a lifting gas that is less dense than the surrounding air.In early dirigibles, the lifting gas used was hydrogen, due to its high lifting capacity and ready availability. Helium gas has almost the same lifting capacity and is nonflammable, unlike hydrogen, but is rare and relatively expensive.