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A balloon is a flexible and often expandable bag that can be filled with air or another substance (generally balloons are filled with a gas). Early balloons were made of dried bladders of animals.

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Balloons as flying machines

A balloon is conceptually the simplest of all flying machines. The balloon is a cloth "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 "gondola" attached underneath that carries passengers or payload.

The first balloon, launched in 1783 by two Parisian brothers named Montgolfier, used hot air to obtain buoyancy. Balloons using the light gas hydrogen for buoyancy were also developed at the same time. Jean Pierre Blanchard[?] made the first piloted balloon flight in North America on January 9, 1793. Although a balloon has no propulsion system, as balloonists became more experienced they learned a degree of directional control through the measure of rising or sinking in altitude to find favorable winds.

Both the hot-air, or Montgolfière, balloon and the light gas balloon are still in common use for Earthly activities. Montgolfière balloons are relatively cheap as they do not require high-grade materials for their envelopes, and they are popular as a balloonist sport activity.

Light gas balloons are predominant in Earth-based scientific applications, as they are capable of reaching much higher altitudes for much longer periods of time. They are generally filled with helium. Although hydrogen has more lifting power, it is explosive in an atmosphere full of oxygen. With a few exceptions, scientific balloon missions are unmanned.

There are two types of light-gas balloons: zero-pressure and superpressure. Zero-pressure balloons are the traditional form of light-gas balloon. They are partially inflated with the light gas before launch, with the gas pressure the same both inside and outside the balloon. As the zero-pressure balloon rises, its gas expands to maintain the zero pressure difference, and the balloon's envelope swells.

At night, the gas in a zero-pressure balloon cools and contracts, causing the balloon to sink. A zero-pressure balloon can only maintain altitude by releasing gas when it goes too high, where the expanding gas can threaten to rupture the envelope, or releasing ballast when it sinks too low. Loss of gas and ballast limits the endurance of zero-pressure balloons to a few days.

A superpressure balloon, in contrast, has a tough and inelastic envelope that is filled with light gas to pressure higher than that of the external atmosphere, and then sealed. The superpressure balloon cannot change size greatly, and so maintains a generally constant volume. The superpressure balloon maintains an altitude of constant density in the atmosphere, and can maintain flight until gas leakage gradually brings it down.

Superpressure balloons offer flight endurance of months, rather than days. In fact, in typical operation a Earth-based superpressure balloon mission is ended by a command from ground control to open the envelope, rather than by natural leakage of gas.

For air transport balloons must contain a gas lighter than the surrounding air. We can distinguish:

  • hot air balloons: filled simply with air, which by heating becomes lighter than the surrounding air; they have been used to carry human passengers since the 1790s;
  • balloons filled with:

Large helium balloons are used as high flying vessels to carry scientific instruments (as do weather balloons), or even human passengers. See: Montgolfiere, Zeppelin, Airship.

Balloons in the military

Some military use of balloons is discussed in hot air balloons.

In World War II, gas-filled barrage balloons with cables hanging from them were used to interdict low-flying aircraft in the Battle of Britain. Also, the Japanese attempted to send bombs to the US via balloons carried in the jet stream; see fire balloons.

Balloons as decoration or entertainment

Party balloons are mostly made of artificial polymer rubber and can be filled with air, helium, water, or any other suitable liquid or gas. When rubber balloons are filled with air, their shapes can last for weeks. When rubber balloons are filled with helium so that they float (restrained by ribbons or strings) they seldom can hold their shape for more than a few hours. The enclosed air or helium escapes through small pores, and helium atoms being much smaller than the nitrogen and oxygen molecules in air, it escapes much quicker. Even a perfect rubber membrane eventually loses helium to the outside, and its contents are contaminated by oxygen and nitrogen migrating inward from the outside. The gases in question actually dissolve in the rubber on one side and are released from solution on the other. The process by which a substance migrates from a region of high concentration, through a barrier to a region of lower concentration is called osmosis.

Partygoers sometimes entertain each other by untying a balloon and inhaling the helium. Because the speed of sound in helium is about twice that in air, the helium causes the vocal tract to become more responsive to high-pitched sounds and less responsive to lower ones. The result is a voice that sounds high-pitched (and usually very funny).

Beginning in the early 1990s, some more expensive (and longer-lasting) helium balloons have been made of thin, unstretchable, impermeable Mylar films. These mylar balloons have attractive shiny reflective surfaces and are often printed with color pictures and patterns. The most important attributes of Mylar for balloons are its light weight, increasing buoyancy and its ability to keep the helium gas from escaping for several weeks. However, there has been some environmental concern, since the Mylar does not biodegrade[?] or shred as a rubber balloon does, and a helium balloon released into the atmosphere can travel a long way before finally bursting or deflating.

Balloon artists are entertainers who twist and tie inflated tubular balloons into sculptures. The balloons used for balloon sculpture are made of extra-stretchy rubber so that they can be twisted and tied without bursting. Since the pressure required to inflate a balloon is inversely proportional to the diameter of the balloon, these tiny tubular balloons are extremely hard to inflate initially. A pump is usually used to pre-stretch the balloons because inflating these balloons by one's mouth alone is close to impossible.

Decorators may use dozens of helium balloons to create balloon sculpture. Usually the round shape of the balloon restricts these to simple arches or walls, but on occasion more ambitious "sculptures" have been attempted.

Water balloons are thin, small rubber balloons intended to be easily broken. They are usually used by children, who throw them at (or to) each other, trying to get each other wet. (See practical joke.) A popular game with this idea is a water balloon toss, where two lines of people stand opposite each other and throw balloons back and forth until each and bursts, showering whomever failed to carefully catch it.

Balloons in medicine

Angioplasty is a surgical procedure in which very small balloons made of a special material are inserted into blocked or partially blocked blood vessels near the heart. Once in place, the balloon can be inflated to clear or compress arterial plaque[?], and to stretch the walls of the vein. A small surgical stent[?] can be inserted in its place to keep the vessel open after the balloon's removal. See heart attack.

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