Gas balloons ascend because the gas inside is less dense and lighter than the air on the outside of the balloon. Heating up regular air makes its molecules expand, becoming lighter than the surrounding atmosphere. That’s what causes hot-air balloons to lift off. Gas balloons used in races such as the Balloon Fiesta’s America’s Challenge use hydrogen, a lighter-than-air gas in its natural, unheated state. But any gas that is lighter than air, including ammonia, will give a balloon lift, meaning the ability to rise.

Those flights are made in special, and rare, balloons called Rozier-type balloons, named for the Frenchman who invented them around 1783. They use double envelopes containing both hot air and gas, and they use a heater to gain more lift.

First, they are alike in some ways. The part of the balloon that holds the hot air or gas is called the envelope. The basket that carries the pilot and passengers is the gondola. With both types of lighter-than-air flight, pilots try to control their direction by taking advantage of different wind currents at different altitudes. Both kinds of balloons are classified as aircraft by the U.S. Federal Aeronautics Administration, and pilots must get separate licenses to fly each type. Gas balloon pilots typically started out flying hot-air balloons and then decided they wanted to be able to fly farther, higher, and longer. Because gas balloons cost more to fly, they usually aren’t flown as often. Their flights can last for days, unlike hot-air flights, which usually last about an hour. Gas balloon pilots may prepare for months before a competition, and when they’re racing, they sometimes fly into dangerous weather conditions or over open seas, where an emergency landing could be a disaster. They even have to be careful not to fly over certain countries, where political conditions could make them targets of hostile fire. Gas balloons usually need more people to help with their launch than hot-air balloons. It takes about ten people to launch a gas balloon, according to the Balloon Federation of America, and about half that number to launch a hot-air balloon. For a competition, the gas pilots also use the services of meteorologists who understand their needs. The pilots’ strategies are largely based on weather conditions. The only way they can “steer” a balloon is to catch the best wind currents.

The gas balloon is inflated through a tube, called an appendix, and it usually takes hours for the inflation, from setup to lift-off, to be completed. The appendix stays open during the flight to let excess gas escape and keep the balloon from bursting. Pilots make gas balloons rise by dropping weight, called ballast, from the balloon. Ballast is usually sand. The balloonists descend by letting some of the gas out of the envelope through a valve at the top of the balloon in a procedure called valving. There’s usually a cycle to a gas balloon flight. As the sun heats the gas-filled envelope, the balloon gets even more lift and can rise higher, to several thousand feet. At night, the gas inside the balloon cools off, and pilots drop bags of sand to keep from hitting objects on the ground. Then as the sun rises and heats the envelope again, the balloon gains even more lift since its load is lighter. The process usually lasts up to three cycles in a competition. When all the ballast is gone, the pilots have to land.

They compare costs, availability, and safety. Helium was the most popular gas for competition in the United States for many years because it isn’t flammable like hydrogen and balloons using helium don’t lose as much of their gas through diffusion. However, in recent years, the price of helium has skyrocketed and the availability greatly reduced.  In both Europe and the US, most pilots now use hydrogen because of the high cost and low availability. It costs roughly $10,000 to inflate a racing gas balloon with helium in the United States and these balloons usually hold about 1,000 cubic meters of gas.  It costs about $1,200 to inflate a racing balloon with hydrogen in the United States.  Ammonia costs less, but it has only about half the lifting ability of helium or hydrogen, so it’s not popular for competition.

They carry many things, but they try to keep them as light as possible, including themselves! (See equipment list for details.) Modern pilots can’t take off like the early aeronauts, who flew mainly on nerve. The basic instrument list now includes:

1. Altimeter – Measures altitude
2. Variometer – Displays the rate of climb
3. Transponder – Used to let flight control centers monitoring airspace see the altitude and speed of the balloon. It also helps other aircraft see the balloon in clouds, darkness or near airports with an electronic signal.
4. Data Recorder – Used to record the balloon’s flight length, altitude and flight track during competition. It documents the flight altitude during specific time intervals.
5. GPS – Global positioning system. This instrument helps track the balloon and give the pilots detailed information about their location.
6. Aircraft radio – Used to communicate with flight service and the chase crew.

Pilots usually carry survival gear, in case they have to make an emergency landing, but not parachutes. They also carry easy-to-eat food, warm clothes, maps, passports, and a Porta Potty-type bathroom device.