A hot air balloon is an aircraft lighter than air, with its upper part resembling a giant balloon and a lower part consisting of a basket for passengers and a heat source, typically an open flame.

The balloon operates by heating the air inside, resulting in a lower density than the surrounding cold air, generating lift and causing the balloon to ascend.

The basket is designed to carry passengers and the heat source, usually an open flame. Modern hot air balloons are commonly made of nylon fabric, and the opening is constructed with fire-resistant materials.

A hot air balloon is an early form of aerospace vehicle, and its fundamental principle of ascending relies on using heated air or gases with lower density than the external air. When the air inside the balloon is heated, it becomes less dense and lighter than an equivalent volume of cold air, causing the balloon to rise, akin to how ice floats in water.

The gases commonly used inside hot air balloons include heated air, hydrogen, and helium. Hydrogen, due to its flammable and explosive nature, has limited applicability. Helium, on the other hand, is an inert gas, and its lesser-known properties make it a suitable choice for hot air balloons. So, why is helium used as the gas for hot air balloons?

1. Lower Density than Air: The critical factor allowing a gas to be used in a hot air balloon is its density, with the gas needing a lower density than air. Helium has a significantly lower density than air (air density is about 1.29 kg/cubic meter, while nitrogen density is about 0.1786 kg/cubic meter). This property aligns with the requirements for the gas in a hot air balloon, enabling it to generate upward lift within the balloon and elevate the entire apparatus.

2. Chemical Stability and Safety: Helium is an inert gas exhibiting very low chemical reactivity. It rarely undergoes reactions with other substances under normal conditions. Additionally, helium is among the most challenging gases to liquefy and cannot solidify under standard atmospheric pressure. Its stable chemical properties ensure that helium is safe for hot air balloons, minimizing the risk of accidents during its application.

3. Abundant Reserves: Helium is the second most abundant element in the universe, following hydrogen, constituting approximately 24% of the mass of celestial bodies. Helium is widely present in natural gas fields, radioactive veins, and geological formations.

Helium-containing natural gas is sourced from gas reservoirs, oil and gas fields, and gas trapped in oil fields. Industrial-grade helium-bearing natural gas is found in geological strata from different eras, concentrating in the Paleozoic and Mesozoic eras, particularly in the Paleozoic reserves. Helium is also found in hot spring areas, mineralized brines, and the atmosphere.

With abundant reserves and extensive extraction capabilities, helium can meet various industrial needs, making it suitable for inflating hot air balloons.

In summary, helium is utilized in hot air balloons due to its lower density than air, chemical stability and safety, and abundant reserves. These properties make helium a suitable and safe gas for creating the buoyancy required for the ascent of hot air balloons.