aerostat

An aerostat is a lighter-than-air vehicle that gains lift from buoyancy produced by a gas lighter than the surrounding air. Unlike aerodynes, which derive lift from wings or rotors, an aerostat floats because of the density difference between the lifting gas and the ambient atmosphere. Modern aerostats typically use helium; hydrogen offers greater lift but is flammable and less favored for safety reasons. The term covers balloons, airships, and other tethered or free-flying craft.

Types and design features vary. A balloon is a simple aerostat in which lifting gas is contained within a bag and the overall structure may be non-rigid, semi-rigid, or rigid. An airship is a powered, steerable aerostat that preserves its shape with a structural framework and propulsion. Many aerostats are tethered to the ground for stationary observation or meteorology, while others are free-flying and used for research, advertising, or reconnaissance. Payloads can include cameras, sensors, or communications gear.

History and use. Hot-air balloons were developed in the late 18th century, followed by gas balloons. In the 19th and early 20th centuries, airships achieved longer-range, controlled flight and were employed for reconnaissance and transport. Today, aerostats are used for weather monitoring, surveillance and signal intelligence, communications relays, and promotional purposes. Their advantages include low speed, long endurance, and large payload capacity relative to some other aircraft; drawbacks include weather sensitivity, limited maneuverability, and potential gas loss or leakage.