Lifting gas
Encyclopedia
Because of the Archimedes' principle
Archimedes' principle
Archimedes' principle relates buoyancy to displacement. It is named after its discoverer, Archimedes of Syracuse.-Principle:Archimedes' treatise On floating bodies, proposition 5, states that...

, a lifting gas is required for aerostats to create buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

. Its density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

is lower than that of air
Density of air
The density of air, ρ , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity...

(about 1.29 kg/m3, 1.29 g/L). Only certain lighter than air
Lighter than air
Lighter than air refers to gases that are buoyant in air because they have densities lower than that of air .Some of these gases are used as lifting gases in lighter-than-air aircraft, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air...

gases are suitable as lifting gases.

### Hot Air

Hot air
Hot air balloon
The hot air balloon is the oldest successful human-carrying flight technology. It is in a class of aircraft known as balloon aircraft. On November 21, 1783, in Paris, France, the first untethered manned flight was made by Jean-François Pilâtre de Rozier and François Laurent d'Arlandes in a hot air...

is frequently used in recreational ballooning. According to the Ideal gas law
Ideal gas law
The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...

, an amount of gas (and also a mixture of gases such as air) expands as it is heated. As a result, a certain volume of gas has a lower mass as the temperature is higher. The average temperature of air in a hot air balloon is about 212 °F (100 °C).

### Hydrogen

Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, being the lightest existing gas (14 times less dense than air), seems to be the most appropriate gas for lifting. Moreover, it is very cheap and easy to produce, e.g., by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

of water or by displacement from an acid or water by a reactive metal. But hydrogen has several disadvantages:
• Hydrogen is extremely flammable. Some countries have banned the use of hydrogen as a lift gas for commercial vehicles but it is allowed for recreational free ballooning in the US and Germany. The Hindenburg disaster
Hindenburg disaster
The Hindenburg disaster took place on Thursday, May 6, 1937, as the German passenger airship LZ 129 Hindenburg caught fire and was destroyed during its attempt to dock with its mooring mast at the Lakehurst Naval Air Station, which is located adjacent to the borough of Lakehurst, New Jersey...

is frequently cited as an example of the hydrogen safety
Hydrogen safety
Hydrogen safety covers the safe use and handling of hydrogen. Hydrogen poses unique challenges due to its ease of leaking, low-energy ignition, wide range of combustible fuel-air mixtures, buoyancy, and its ability to embrittle metals that must be accounted for to ensure safe operation...

risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to reinvestigate the safety issues of using hydrogen as a lift gas: with good engineering and good handling practices, the risks can be significantly reduced.
• Because the hydrogen molecule is very small, it can easily diffuse through many materials, so that the balloon will deflate quickly. (That's the reason why most hydrogen or helium filled balloons are coated by a thin layer of aluminium.)

### Helium

Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

is the second lightest gas. For that reason, it is an attractive gas for lifting as well.
A major advantage is that this gas is noncombustible. But the use of helium has some disadvantages, too:
• The same diffusion problem as above described with hydrogen;
• Helium is very expensive.
• Although abundant in the universe, helium is very scarce on earth. The only commercially viable reserves are a few natural gas wells, mostly in the US, that trapped it from the slow alpha decay
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...

of radioactive materials within the earth. By human standards helium is a non-renewable resource
Non-renewable resource
A non-renewable resource is a natural resource which cannot be produced, grown, generated, or used on a scale which can sustain its consumption rate, once depleted there is no more available for future needs. Also considered non-renewable are resources that are consumed much faster than nature...

that cannot be practically manufactured from other materials. When released into the atmosphere, e.g., when a helium-filled balloon leaks or bursts, it eventually escapes into space and is permanently lost.

### Steam / Water Vapor

The gaseous state of water
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

is lighter than air, incombustible and much cheaper than helium. The concept of using steam for lifting is therefore already 200 years old. The biggest challenge has always been to make a material that can resist it. In 2003, a university team in Berlin, Germany, has successfully made a 150 °C steam lifted balloon. But it is generally impractical due to high boiling point and condensation.

### Ammonia

Ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

is sometimes used to fill weather balloon
Weather balloon
A weather or sounding balloon is a balloon which carries instruments aloft to send back information on atmospheric pressure, temperature, humidity and wind speed by means of a small, expendable measuring device called a radiosonde...

s. Due to its high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast). Ammonia is relatively heavy (0.6 kg/m3), poisonous and irritant.

### Methane

Methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, the main component of natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

, is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the smaller molecules of hydrogen and helium. Of course, methane is highly flammable and therefore, like hydrogen, not appropriate for use in passenger-carrying airships. It is also relatively dense and a potent greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

.

### Coal gas

In the past, coal gas
Coal gas
Coal gas is a flammable gaseous fuel made by the destructive distillation of coal containing a variety of calorific gases including hydrogen, carbon monoxide, methane and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen...

, a mixture of hydrogen, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

and other gases, was also used in balloons. It was widely available and cheap; the down side was a higher density (reducing lift) and the high toxicity of the carbon monoxide.

### Neon

Neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...

is lighter than air and could lift a balloon. Like helium, it is incombustible. However, it is rare on Earth and expensive, and is among the heavier lifting gases.

### Nitrogen

Pure nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

has the advantage that it is inert
Inert
-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...

and abundantly available, because it is the major component of air. But it is only 3% lighter than air, and for that reason it seems useless. Nevertheless, an aerogel
Aerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...

has been produced, named SEAgel
SEAgel
SEAgel is one of a class of high-tech foam materials known as aerogels. It is an excellent thermal insulator and among the least dense solids known. SEAgel was invented by Robert Morrison at the Lawrence Livermore National Laboratory in 1992...

(Safe Emulsion agar gel) that floats in air if it is filled with pure nitrogen.

### Vacuum

Theoretically, an aerostatic vehicle could be made to use a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

.
As early as 1670 - that is long before any balloon had sailed - the Italian monk Francesco Lana de Terzi
Francesco Lana de Terzi
Francesco Lana de Terzi was an Italian Jesuit, mathematician, naturalist and aeronautics pioneer...

postulated that concept: He described a ship with four vacuum spheres.

But, even in a theoretically perfect situation (with weightless spheres), a 'vacuum balloon' is only 7% lighter than a hydrogen filled balloon, and 16% lighter than a helium filled one.
Technically seen, it seems impossible to make such thing, because the walls of that vessel would have to be so heavy that it would amply compensate the profit of the vacuum.

### Plasma

Another medium that in theory could be used, is a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

: Ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s repelling each other could give a pressure that counteracts the atmospheric pressure.
But this seems to be extremely impractical, so that it may only be interesting for science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

.

### Combinations

Of course, it is possible to combine some of the above solutions. A well known example is the Rozière balloon
Rozière balloon
The Rozière balloon is a type of hybrid balloon that has separate chambers for a non-heated lifting gas as well as a heated lifting gas...

, that combines a core of helium with an outer shell of hot air.

## Hydrogen versus helium

Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. Both provide about 9.8 N of lift (1 Newton is the force required to accelerate 1 kg at 1 m/sec2) per cubic meter of gas at STP.

The lifting power in air of hydrogen and helium can be calculated using the theory of buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

as follows:

The density at sea-level and 0 °C for air and each of the gases is:
• Air
Density of air
The density of air, ρ , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity...

(ρair) = 1.292 kg/m3.
• Hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

(ρH2) = 0.090 kg/m3
• Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

(ρHe) = 0.178 kg/m3

Thus helium is almost twice as dense as hydrogen. However, buoyancy depends upon the difference of the densities (ρgas) - (ρair) rather than upon their ratios. Thus the difference in buoyancies is about 8%, as seen from the buoyancy equation:
• Buoyant mass (or effective mass) = mass × (1 - ρairgas)
• Therefore the buoyant mass for one m3 of hydrogen in air is:
• 0.090 kg * (1 - (1.292 / 0.090) ) = -1.202 kg
• And the buoyant mass for one m3 of helium in air is:
• 0.178 kg * (1 - (1.292 / 0.178) ) = -1.113 kg

The negative signs indicate that these gases tend to rise in air.

Thus hydrogen's additional buoyancy compared to helium is:
• 1.202 / 1.113 = 1.080, or approximately 8.0%

## High altitude ballooning

At high altitude, the air pressure is lower than on the earth surface. To get the same lift, the same amount of lifting gas is needed, but because the pressure is much lower, that amount of gas needs much more volume.

Balloons that are designed to lift to extreme heights (stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

), must be able to expand enormously. That's the reason why they seem almost empty at launch, as can be seen on the photo.

## Submerged balloons

Because of the enormous density difference between water and gases (water is about 1,000 more dense than most gases), the lifting power of underwater gases is very strong. It doesn't matter which gas is used, because the relative differences between the gases are neglegible in relation to the density of water. Of course it has to be taken into account that some gases can liquefy under high pressure, and then they will abruptly lose their buoyancy.

A submerged balloon that rises will expand or even explode because of the strong pressure reduction, unless gas is able to escape continuously during the ascent.

## Balloons on other celestial bodies

A balloon can only have buoyancy if there is a medium that has a higher average density than the balloon itself.
• Balloon cannot work on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

because it has no atmosphere.
• Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

has a very thin atmosphere - the pressure is only 1/160th of earth atmospheric pressure - so a huge balloon would be needed even for a tiny lifting effect. Overcoming the weight of such a balloon would be difficult, but several proposals to explore Mars with balloons have been made.
• On Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, the density of the CO2 atmosphere at the surface is (65 kg/m3, fifty times that of Earth, and gravity is comparable. A small balloon could lift substantial weights. In 1985, the Soviet Vega program sent two balloons to float in Venus' atmosphere at 54km altitude. Because CO2 is so dense, ordinary (Earth) air would be a lifting gas on Venus. This has led to proposals for a human habitat that would float in the atmosphere of Venus at an altitude where both the pressure and the temperature are earthlike.
• Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's biggest moon, has a dense atmosphere of mostly nitrogen appropriate for ballooning. There are concrete plans for scouting this moon with balloons.