Balloon (aeronautics)

A balloon is an unpowered aerostat that stays aloft through buoyancy.

Buoyancy is the force that allows an object to float in a fluid, like air or water. A balloon consists of a large fabric envelope filled with a lifting gas. This gas must be lighter than the surrounding atmosphere. Because the entire balloon is less dense than the air around it, it rises. Most balloons feature a basket, gondola, or capsule suspended underneath. This structure carries passengers or a payload, such as telescopes or cameras.

Unlike an airship, a balloon has no propulsion system to move itself. It moves entirely with the wind. However, pilots can achieve a degree of directional control. They do this by changing altitude to find wind currents moving in different directions.

There are three primary types of balloons based on how they achieve lift. The first is the hot air balloon, also known as a Montgolfière. This type obtains buoyancy by heating the air inside the envelope. As air is heated, it expands, meaning a specific volume contains less air and becomes lighter. The second type is the gas balloon, or Charlière. These are inflated with gases that have a lower molecular weight than the ambient atmosphere.

The third type is the Rozière balloon. This combination type uses two separate gasbags. One bag holds a heated gas, while the other holds an unheated lifting gas. This design allows for better altitude control during long-distance flights.

Gas balloons function using specific lifting gases. Historically, hydrogen was used extensively because it provides great lift. However, hydrogen is highly flammable, so it is seldom used today after the Hindenburg disaster. In the nineteenth and early twentieth centuries, many used coal gas. Coal gas was cheaper and more available, though it provided only about half the lift of hydrogen. Today, helium is the standard for most manned gas balloons and airships because it is safe. Other gases like ammonia or methane have been tested, but they have poor lifting capacity.

Scientific missions often use unmanned gas balloons to reach much higher altitudes.

Scientists use two distinct sub-types of light-gas balloons. Zero-pressure balloons are the traditional version. They are partially inflated so the internal gas pressure matches the external atmospheric pressure. As the balloon rises, the gas expands and the envelope swells. At night, the gas cools and contracts, causing the balloon to sink. To manage this, pilots must release gas when too high or drop ballast weights when too low. This process limits their flight to a few days. In contrast, superpressure balloons use a tough, inelastic envelope. They are filled to a pressure higher than the outside air. This allows them to maintain a constant volume and altitude. Superpressure balloons can stay aloft for months, often lasting until ground control commands them to open the envelope.

High-altitude balloons serve critical roles in science and observation. Some can fly over 100,000 feet, which is about 30.5 kilometers, into the air. These are often designed to burst at a specific altitude. When they burst, a parachute deploys to bring the payload safely back to Earth. Other specialized versions include cluster ballooning, which uses many small gas-filled balloons for flight.

Some balloons are also tethered to a fixed point on the ground. Tethered balloons are used for military observation, meteorology, or commercial purposes. In windy conditions, these are often shaped aerodynamically and called kite balloons to keep them stable.

Ballooning has a long and fascinating history. Unmanned versions, like Kongming lanterns, were used in China for military signaling during the Three Kingdoms era. In 1709, Bartolomeu de Gusmão demonstrated a small hot air balloon in Lisbon. The modern era of ballooning began in France in 1783. Joseph and Etienne Montgolfier built the first hot air balloon to carry passengers. Their first flight on September 19, 1783, carried a sheep, a duck, and a rooster.

Later that year, on December 1, Professor Jacques Charles flew the first gas balloon. His hydrogen flight covered 27 miles and lasted over two hours. These early pioneers paved the way for all modern aeronautics.

One notable figure in balloon history was Pilâtre de Rozier. He realized that long-distance flight required better altitude control. He developed the Rozière design to use both hot air and gas. However, his attempt to cross the English Channel in 1785 ended in tragedy when his hydrogen bag caught fire. He became known as "The First to Fly and the First to Die." It took until the 1980s for technology to make the Rozière type safe by using non-flammable helium. Today, balloons connect to many fields, from weather tracking to recreational sport, continuing to help us explore the atmosphere.