In aeronautics, a balloon is an unpowered aerostat, which remains aloft or floats due to its buoyancy. A balloon may be free, moving with the wind, or tethered to a fixed point. It is distinct from an airship, which is a powered aerostat that can propel itself through the air in a controlled manner. Many balloons have a basket, gondola, or capsule suspended beneath the main envelope for carrying people or equipment (including cameras and telescopes, and flight-control mechanisms). Principles A balloon is conceptually the simplest of all flying machines. The balloon is a fabric envelope filled with a gas that is lighter than the surrounding atmosphere. As the entire balloon is less dense than its surroundings, it rises, taking along with it a basket, attached underneath, which carries passengers or payload. Although a balloon has no propulsion system, a degree of directional control is possible by making the balloon rise or sink in altitude to find favorable wind directions. There are three main types of balloons: The hot air balloon or Montgolfière obtains its buoyancy by heating the air inside the balloon; it has become the most common type. The gas balloon or Charlière is inflated with a gas of lower molecular weight than the ambient atmosphere; most gas balloons operate with the internal pressure of the gas the same as the pressure of the surrounding atmosphere; a superpressure balloon can operate with the lifting gas at pressure that exceeds that of the surrounding air, with the objective of limiting or eliminating the loss of gas from day-time heating; gas balloons are filled with gases such as: hydrogen – originally used extensively but, since the Hindenburg disaster, is now seldom used due to its high flammability; coal gas – although giving around half the lift of hydrogen, extensively used during the nineteenth and early twentieth century, since it was cheaper than hydrogen and readily available; helium – used today for all airships and most manned gas balloons; other gases have included ammonia and methane, but these have poor lifting capacity and other safety defects and have never been widely used. The Rozière type has both heated and unheated lifting gases in separate gasbags. This type of balloon is sometimes used for long-distance record flights, such as the recent circumnavigations, but is not otherwise in use. Both the hot air, or Montgolfière, balloon and the gas balloon are still in