Lifting gas

A lifting gas, or lighter-than-air gas, is a special kind of gas that is lighter than the air around us. Because it is lighter, it rises above normal air, which makes it very useful for flying machines that do not need wings to stay in the sky, called lighter-than-air aircraft.

Only certain gases can work as lifting gases. For example, dry air weighs about 1.29 grams per liter under normal conditions, and lighter-than-air gases must weigh less than this to float upward. These special gases help airships and other flying machines stay aloft without needing engines to push them up continuously.

Gases used for lifting

Hot air is often used in balloons for fun. When air is heated, it becomes lighter and rises above cooler air. This is how many recreational balloons work.

Hydrogen is the lightest gas and is great for lifting things up. However, it can catch fire easily, which is why some places don’t allow it for certain uses. Helium is safer because it won’t catch fire, but it is more expensive and doesn’t come from many places on Earth. Other gases like coal gas, ammonia, and methane have been used too, but each has its own problems, like being heavy or harmful. Some special balloons even mix helium and hot air to get the best lifting power.

Gases theoretically suitable for lifting

Water vapour

The gaseous state of water is lighter than air because it has a lower molar mass compared to typical atmospheric gases like nitrogen. It is not flammable and much cheaper than helium. Using steam for lifting has been an idea for over 200 years. In 2003, a team in Berlin made a balloon lifted by steam, but this is not practical because of the high temperature and condensation issues.

Hydrogen fluoride

Hydrogen fluoride is lighter than air and could theoretically be used as a lifting gas. However, it is very corrosive, toxic, expensive, and heavier than other lifting gases, making it impractical for use.

Acetylene

Acetylene is lighter than air and could be used as a lifting gas. But it is very flammable and does not lift well, so it is not a good choice.

Hydrogen cyanide

Hydrogen cyanide is lighter than air and could technically be used as a lifting gas at certain temperatures. However, it is very toxic, does not lift well, and has a low boiling point, so it is not used.

Neon

Neon is lighter than air and could lift a balloon slowly. It is not flammable like helium, but it is rare and expensive, and it is heavier than other lifting gases.

Nitrogen

Pure nitrogen is safe and easy to find because it is a major part of air. But it is only slightly lighter than air, so it is not a good lifting gas.

Ethylene

Ethylene is less dense than air. However, it is highly flammable and expensive, making it impractical as a lifting gas.

Diborane

Diborane is slightly lighter than nitrogen. But it is very dangerous, even more so than hydrogen, making it unsafe to use.

Vacuum

Theoretically, a vehicle could use a vacuum or partial vacuum to lift. An Italian monk imagined such a ship long before the first hot-air balloon flight. In perfect conditions, a "vacuum balloon" could lift better than hydrogen or helium balloons. But making it with materials that stay strong without collapsing is not possible with today’s technology.

Aerogel

Though not a gas, researchers made an ultralight aerogel that is less dense than air. However, it does not float normally because air fills its pores. In 2021, researchers made a carbon aerogel float by heating it, which lowered the density of the trapped air.

Hydrogen versus helium

Hydrogen and helium are the most commonly used gases for making things float in the air. Even though helium is a bit heavier than hydrogen, both are much lighter than the air around us, which helps them lift objects.

The ability of these gases to lift things depends on how much lighter they are compared to air. Scientists use a special formula to figure this out, which looks at the difference in weight between the air and the gas. This difference tells us how much weight the gas can help lift. At sea level, hydrogen can lift a little more weight than helium—about 8% more. However, as you go higher up or the temperature changes, the lifting power of both gases changes, but hydrogen will still be about the same amount better than helium.

High-altitude ballooning

When balloons fly very high, the air gets thinner and the pressure inside the balloon also drops. This makes the balloon grow much bigger to lift the same weight. Balloons meant for the very high sky, like the stratosphere, need to stretch a lot to push enough air aside. That’s why they look almost empty when they start their journey.

Another way to fly high for a long time is to use a superpressure balloon. This special balloon keeps more air inside than the air around it, helping it stay strong and steady during its flight.

Submerged balloons

Because water is much heavier than gases—about 1,000 times heavier—gases underwater can lift things very well. It doesn't matter much which gas is used because water's weight makes all gases seem almost the same. But some gases can turn into liquid under high pressure, which can make a balloon stop floating suddenly.

When a balloon rises from underwater, it can get bigger or even burst because the pressure gets lower. To avoid this, the balloon needs to let gas out as it rises or be strong enough to handle the pressure change.

Divers use special bags filled with air, called lifting bags, to help lift heavy things like old cannons or even whole ships from the ocean floor. They fill these bags with air from tanks they carry or from pumps on their ships above water.

Submarines have special tanks they fill with air to help them rise or sink in the water, acting like underwater airships. Some deep-sea vehicles called bathyscaphes use gasoline to help them stay up underwater.

Balloons on other celestial bodies

A balloon can only float if the air around it is thicker than the balloon itself.

• Balloons cannot work on the Moon because it has almost no air.
• Mars has very thin air, so a very big balloon would be needed to lift even a tiny weight. Some ideas have been made to explore Mars with balloons.
• Venus has thick air made of a special kind of gas. Because this gas is much thicker than Earth air, normal Earth air could help a balloon float on Venus. This has led to proposals for a special floating home in Venus's sky. In 1985, the Soviet Vega program sent two helium balloons high up in Venus's air.
• Titan, one of Saturn's moons, has thick, cold air that could support balloons. Using special air vehicles on Titan was proposed. The Titan Saturn System Mission plan included a balloon to travel around Titan.

Solids

In 2002, aerogel was recognized by the Guinness World Record as the lightest solid. Aerogel looks and feels like a very empty sponge because it is mostly air. This makes it very light, not because of what it is made from, but because of all the air inside it. A similar material called SEAgel, made from agar, can be filled with helium gas. When you put this special solid into a container filled with a heavier gas, it can float.