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 consists of a lightweight skin filled with a lifting gas to create buoyancy.
These aircraft take advantage of a lighter than air gas to become fully or partially buoyant. In addition to typical aircraft control methods such as elevator, rudder and engine throttle, control of ballast and gas volume are available.