Hot air balloons are the oldest and most successful human carrying flight technology. The first balloons worked on the principle that when air is heated, it rises. A hot air balloon consists of a bag called the envelope that is capable of containing heated air. Suspended beneath is a basket which carries passengers and usually, a source of heat (mostly an open flame). Most balloons today use hot air made by burning propane gas. Soon after the first hot air flights, people realized that the gas hydrogen, which is almost fifteen times lighter than air, can be used instead . Filled with hot air or gas that is lighter than the surrounding atmosphere, a balloon travels by moving wind. Since the wind direction changes with the altitude, the pilot navigates the balloon's direction by controlling the altitude, either by heating the air or by releasing ballasts.
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