Aerostat

An aerostat (From Greek ἀήρ aer (air) + στατός statos (standing) through French) is a lighter than air craft that gains its lift through the use of a buoyant gas. Aerostats include unpowered balloons and powered airships. A balloon may be free-flying or tethered. The average density of the craft is lower than the density of atmospheric air, because its main component is one or more gasbags, a lightweight skin containing a lifting gas to provide buoyancy, to which other components such as a gondola containing equipment or people are attached. Especially with airships, the gasbags are often protected by an outer envelope.

Aerostats are so named because they use aerostatic lift which is a buoyant force that does not require movement through the surrounding air mass. This contrasts with the heavy aerodynes that primarily use aerodynamic lift which requires the movement of a wing surface through the surrounding air mass. The term has also been used in a narrower sense, to refer to the statically tethered balloon in contrast to the free-flying airship. This article uses the term in its broader sense.

In conventional usage, the term aerostat refers to any aircraft that remains aloft primarily using aerostatic buoyancy.

Historically, all aerostats were called balloons. Powered types capable of horizontal flight were referred to as dirigible balloons or simply dirigibles (from the French dirigible meaning steerable). These powered aerostats later came to be called airships, with the term "balloon" reserved for unpowered types, whether tethered or free-floating.

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