Humans have always dreamed of flying through the air. The dream came true in 1783 when a hot-air balloon carried its first passengers. Further experiments with balloon flight led to the development of the motor-powered airship and later the airplane. While balloons and airships were eventually overshadowed by the airplane as a means of transportation, people have continued to find useful applications for balloons.

Types of balloons

Balloons come in all shapes and sizes, from small toy balloons to large passenger balloons. They can be used for advertising purposes, for scientific experiments, for entertainment, or for recreation. Advertising balloons are usually held in place as they float high in the sky for all to see. Giant balloons of cartoon characters and other entertaining subjects are familiar sights in parades. These are controlled by people on the ground, who carry them along the parade route. Scientific balloons and those used for recreation are free to move through the air as they are guided by a pilot. Balloons used for recreation are usually round or pear-shaped. They have a basket hanging below them that serves as the passenger compartment.

How balloons fly

A balloon rises because it is filled with a gas that is lighter than air. The combined weight of the gas, the balloon bag, and the load it carries must be less than the weight of the surrounding air. When a toy balloon is blown up by mouth, it falls to the ground. This is because the weight of the rubber and the air in it make it heavier than the same volume of air. But if the air inside the balloon is heated slightly by placing the balloon on a radiator, the balloon expands. It will rise and stay on the ceiling until the air in it cools off. A toy balloon that is filled with helium will float as well. Large passenger balloons are also filled with gas to make them rise.

The lightest gas is hydrogen. Hydrogen, however, catches fire and explodes easily. Coal gas—produced when coal is heated without air—is cheaper, but it is heavier than hydrogen and also a fire hazard. Helium, almost as light as hydrogen, does not burn. Although scarce and expensive, it is the ideal balloon gas. Helium is used for tethered balloons such as the ones seen in parades. Passenger balloons are often filled with heated air, which is lighter than unheated air and has long been used to fill such balloons. Some passenger balloons are filled with helium or hydrogen, and for long-distance balloon flights a combination of helium and hot air is used.

A free passenger balloon's huge gas bag, or envelope, is made of cloth coated with rubber or some other substance to make it leakproof. It is filled through an opening at the bottom, called the neck or appendix. In a hot-air balloon, a propane burner at the neck keeps the air inside the balloon hot. The entire bag is enclosed in a strong net. The passenger basket is attached to the net. At the top of the envelope is a valve for releasing gas. This valve is connected to a cord that usually runs through the envelope and out the appendix, hanging within a balloonist's reach. Bags of sand are hung around the basket for ballast, or extra weight, so that the balloon does not rise too fast.

The balloon rises gently after the cables holding it on the ground are unhooked. The movement of the balloon is largely controlled by the wind. The main control the balloonist has is to change altitude, or move the balloon up and down, in order to catch favorable winds. As ballast is thrown overboard, the balloon rises. As gas is released through the valve, the balloon drops. The balance between gas and weight is very delicate. Only a small loss of gas or sand can cause a sharp change in altitude.

As the balloon rises, the pressure of the air around it decreases and permits the gas to expand. If the expanding bag threatens to burst, the balloonist must release gas. To come down, the balloonist lets gas escape gradually until the balloon becomes a bit heavier than the air. But the steadily increasing air pressure around a descending balloon will make it drop faster and faster unless the balloonist throws out ballast to slow it down.

History

Joseph-Michel and Jacques-Étienne Montgolfier were two brothers who lived in France. They made the first successful experiments with hot-air balloon flights in 1783. The first balloon passengers were a sheep, a rooster, and a duck, but soon afterward two men sailed for 5 ¹/₂ miles (9 kilometers) over Paris. Later that year French physicist J.A.C. Charles, accompanied by Nicolas-Louis Robert, flew a balloon filled with hydrogen on a two-hour flight.

Soon others began experimenting with balloons. Some people focused on the sport of trying to make longer and longer flights. The first successful flight across the English Channel took place in 1785. Others saw balloons as useful tools, especially during wartime. Anchored observation balloons were used by the French general Napoleon in some of his battles and by both sides in the American Civil War and in World War I.

During World War II thin steel cables held up by balloons were used to defend troops and cities against attacks by low-flying planes. In the 20th century scientists used balloons to gather data about the upper atmosphere. Today balloons carry tools to gather information about winds, air pressure, temperature, and humidity.

Free balloons are still used for sport and adventure. In 1999 Bertrand Piccard and Brian Jones became the first balloonists to complete a nonstop around-the-world voyage. Three years later U.S. adventurer Steve Fossett became the first person to complete the trip alone. In Albuquerque, New Mexico, and other locations there are annual hot-air balloon festivals featuring hundreds of colorful balloons.