Buoyant ships can float in the air because their fuel, usually helium these days, is lighter than the air. Any kid who has ever lost a balloon at the county fair understands, and rues, this phenomenon. The buoyant gas is counterbalanced by fuel, cargo or other ballast, allowing the ship to fly at a controlled altitude. But in order to move forward, a ship has to burn fuel, and this reduces its weight — so it would gain altitude. To balance the buoyancy, airships must vent their precious helium or let in regular air, but this wastes an expensive and diminishing resource.