My idea for a creature within my fantasy story is that of a creature which floats through the air like a living hot air balloon.

It would do this through the intake, storage and regulation of gases within its body alongside its body temperature.

Its appearance is that of a large blob varying in size from a backpack to a small car with a head in front. There would be several long appendages hanging below it to act as limbs, grabbing food to bring up to its head to eat. There are wings but they function more like a parachute to keep it in the air and for balance and directing its body. Its main body would consist of all your standard organs alongside several large sacs for storing gasses and an advanced respiratory system to manage its own breathing and the regulation of its gases.

It is an opportunistic omnivore with its main diet consisting of vegetation and carrion it finds. It doesn't hunt since it's not that hard for anything to get away from it(through running or hiding under cover).

It doesn't need to worry about predators as there is nothing capable of catching it in the air except birds of which there are no varieties capable of taking it down at full size, and even if they hunted the backpack size ones, they taste terrible.

I am undecided on which gas/es it will use to do this, any suggestions are welcome, whatever it is, it will have to be sourced through breathing from a standard earth atmosphere and/or producing it from food sources within its body. The gases will need to be able to produce enough lift for it to float in the air just above treetop level or higher. It can use its own body temperature to increase the lift provided by these gases but that is limited by how high its body temperature can get.

My question is how viable is this creature? could it actually float and survive in an earth-like world?

  • $\begingroup$ I would suggest having a look at the Overlord from Starcraft 2. It sounds similar to what you want and fits in pretty well with the game. $\endgroup$
    – Shadowzee
    Commented May 30, 2018 at 1:10
  • $\begingroup$ Very much like worldbuilding.stackexchange.com/questions/89103/… $\endgroup$
    – Willk
    Commented May 30, 2018 at 1:12
  • $\begingroup$ @L.Dutch, is that really a duplicate? The answers especially are focusing on the planet, not the creatures, and the Q seemed to be assuming the creatures and asking about the world. $\endgroup$
    – JBH
    Commented May 30, 2018 at 4:15
  • $\begingroup$ @JBH, I might have picked the wrong one. But I remember a similar question here $\endgroup$
    – L.Dutch
    Commented May 30, 2018 at 5:11
  • 2
    $\begingroup$ Remember me of that Carl Sagan hypothetic "jupiter whales" $\endgroup$
    – jean
    Commented May 30, 2018 at 12:14

6 Answers 6


As others have stated, the best lifting gas for this purpose is Hydrogen gas, as it can be produced biologically and offers the highest amount of buoyancy.

But your life form would be a very fragile thing, the skin sack holding the gas would realistically be millimeter thick at best (see following math) and could be ruptured by most anything.

I disagree with Dubukays assertion that an animal like this would have to be kilometers long though. Let's look at the math with skin 1mm thick and 1g/cm^3 dense

Volume for a spherical air sack that is 1 meter in diameter: $V = (0.5 \ m)^3 \times\pi \times 4/3 = 0.52 \ m^3 $. Fully filled with Hydrogen gas this can lift half a kilogram (rounding for convenience we say that Hydrogen provides $1 \ kg/m^3 $ lift). This is where we run into troubles. The weight of the skin is about $(0.5 \ m)^2 \times \pi \times 4 \times 1 \ mm \times 1 \ g/cm^3 = 3.14 \ kg $ ! No dice.

Volume for an air sack that is 5 meters in diameter. A lift of $(2.5 \ m)^3\times\pi\times4/3\times1 \ kg/m^3 = 65 \ kg$, while the skin masses in at $(2.5 \ m)^2 \times\pi\times 4 \times 1 \ mm \times 1 \ g/cm^3 = 79 \ kg$. We're getting closer and the square cube law is clearly working in our favor.

A 10 meter diameter air sack will give us over 550kg of lift, while the skin (still 1 millimeter thick) uses up about 320kg. This leaves a respectable 220kg or 480 pounds of usable mass.

So yes, the animal will be large, but not infeasibly so. There's still a few problems left, but nothing that can't be solved. For one, the animal can't become airborne from birth, because it will need a few hundred kilograms of loose skin for its balloon. Supporting this excess weight is very problematic for a terrestrial animal.

I propose a life cycle split into two parts: The creature is born in lakes, rivers or oceans and grows until it can sustain enough skin for an air sack that will let it fly. It rises to the surface and begins filling the air sacks with hydrogen derived from the surrounding sea water. It is obviously very vulnerable during this period, though this can be alleviated by living in bodies of water without large predators or blowing up in large colonies.

It gently rises from the water once it has accumulated enough hydrogen gas and floats away. It will likely stay above water for its entire life - should its air sack get popped it will gently fall into the water below and heal the damaged skin - but may forage inland above rivers. Mating happens in the air and females drop a large number of eggs above promising bodies of water. Food can be captured with tentacles, as other users have suggested. Hydrogen gas will leak quickly through the thin skin, so it constantly replenishes it.

It's probably unlikely for an animal like this to evolve naturally, but I don't think it's physically impossible.

  • 1
    $\begingroup$ for dimensional calculation I would have preferred consistent units... not $m \times \ mm \times g/cm^3$ $\endgroup$
    – L.Dutch
    Commented May 31, 2018 at 2:54
  • $\begingroup$ The units are consistent, just not all converted to base units. 1 m^2 x 1 mm x 1 g/cm^3 = 1000 g $\endgroup$
    – Sunny
    Commented May 31, 2018 at 2:59
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    $\begingroup$ 'blowing up in large colonies' is an excellent slip of the keyboard.. $\endgroup$ Commented May 31, 2018 at 14:54
  • $\begingroup$ The real problem is surface area, skin that thin will lose water like a sponge in blast furnace. $\endgroup$
    – John
    Commented Feb 7, 2019 at 20:28

Sounds very much like the oafan people from Schlock Mercenary's current storyline

They use hydrogen because it can be generated from water by using a simple electrical current which can be generated biologically. Altitude is adjusted compressing the body slightly to decrease volume but not mass and vice versa

Of course it makes them very scared of fire....

Body temp isn't a good method of controlling altitude as it's slow to respond and requires a continuous fuel source to maintain the heat.

If you treat your creatures like a man-o-war jellyfish, they would be the predators and the birds, the prey. They could drift down to above tree height and catch birds or at night birds and bats fly into their tenticles.

Eating carrion isn't very likely. Carrion will be on the ground and unless open plains, wind would be very dangerous to a living balloon.

The biggest risk is storms and strong winds. Your creatures would need to rise above the storms or completely deflate and ride it out on the ground. You don't want to be a hydrogen balloon in a thunder storm.


Your animal can use body cavities filled with hydrogen to provide buoyancy.

Hydrogen can be obtained by catalytic breaking of methane. Methane on the other side is produced in large quantities in the bowels of vegetation eating creatures (ask cows for info) like yours.

By venting/contracting/relaxing the bags the creature can control the buoyancy and therefore the flight level.

Let's assume your creature is the size of a peregrine falcon, weighing 1600 grams. The buoyancy force can be calculated as $F_b = g(d_a - d_h)$. Placing the correct numbers in the formula you get that with 1.5 cubic meter of hydrogen you can lift 1600 gram.

That would mean a bubble about 5 meters diameter. Not exactly a nice looking animal, but fitting in your description:

Its appearance is that of a large blob

Only caveat: stay away from lighting and electric eels.

  • 1
    $\begingroup$ Electric heels? Are they a fashion accessory I’m missing out on? $\endgroup$
    – Joe Bloggs
    Commented May 30, 2018 at 6:52
  • 1
    $\begingroup$ @JoeBloggs, they are fishy shoes... or typos... pick the one you like more $\endgroup$
    – L.Dutch
    Commented May 30, 2018 at 7:09
  • $\begingroup$ Some calculations of body volume & mass would be helpful. Hydrogen is too heavy for small backpack with arms ;) $\endgroup$
    – Mołot
    Commented May 30, 2018 at 7:53
  • $\begingroup$ @Mołot, done. see edit $\endgroup$
    – L.Dutch
    Commented May 30, 2018 at 8:11
  • $\begingroup$ Sadly I don’t think you can spread the mass of a peregrine falcon across a sphere 5 meters wide and still have something living- see my answer below for some more math $\endgroup$
    – Dubukay
    Commented May 30, 2018 at 14:44

Sadly, no. Tl;dr: the minimum size of such a creature is on the scale of kilometers and thus pretty infeasible. Instead, try making the creature some kind of colonial organism and boosting your planet.

Let's assume a spherical creature.

What we're trying to figure out here is the minimum size of a biological gasbag. We model that as a sphere of $H_2$ gas surrounded by a thin shell of skin.

Beware, physics below

Our initial equation starts out pretty simply:

$V_{hyd}*F_{buoy} = M_{skin\ shell} = V_{shell}*\rho_{shell}$

where $\rho$ is the density of our shell.

This is then expanded to give us some actual formulas. We're trying to solve for the radius of this biological gasbag, so we're hoping to end up with $r$ alone on one side set equal to a bunch of numbers.

$\frac{4}{3}\pi r^3*F_{buoy} = 4\pi r^2t*\rho_{shell}$

Where $t$ is the thickness of the shell- I'm going to assume it ends up being 1 meter thick. Sounds approximately right to me. We can simplify a bit with that information and some quick algebra:

$r^3 * F_{buoy} = 3r^2*\rho_{shell}$

Which immediately simplifies to exactly what we were hoping for!

$r * F_{buoy} = 3*\rho_{shell}$

Let's deal with those other two variables. The $F_{buoy}$ is the force of buoyancy due to our lifting gas, in this case hydrogen. There's a lot to it, but Wikipedia has a shortcut: $1\ m^3$ of hydrogen can lift $\approx 1.1kg$. Cool! We can also deal with the other variable, $\rho_{shell}$. Here, a quick google search tells us that the density of skin is about $800\frac{kg}{m^3}$ (ew). Let's plug those numbers in.

$r*1.1 = 800*3 = 2400$

Note: I fudge my units for simplicity's sake here. The $F_{buoy}$ term is a good bit more complex.

So our minimal radius for our idealized gasbag is $\approx 2200m$, or 2 kilometers.

We can also reality-check our skin thickness estimate of ~1m. We’re about 2 meters tall and our skin is about 2mm thick, so I’m pretty happy with it. (Actually, there were a lot of bad guesses before I zoned in on 1m but you don’t need to know about all those)

spherical cow, from http://abstrusegoose.com/406

Biological assessment:

Totally infeasible. A creature 4 kilometers long is nowhere near plausible, and that's the absolute minimum. You'd have to add things beside skin, and that all adds weight, and every time you add something you increase the radius that much further. With some back of the envelope calculations, I get a minimum size of 8 kilometers; including water and muscle mass as well as a tubular body. What really sunk this, however, was the circulation system. Even though the volume scales as the cube of the radius, the amount of liquid needed to provide circulation throughout the body scales even faster. Sad.

Possible solution: modify the environment

I fudged the buoyancy term in my derivation above, but it's based on essentially two things- the force of gravity and the density of the atmosphere. Here in Worldbuilding, we're free to modify both of those! What we want is a small planet (low gravity) and a dense atmosphere. If we have a atmosphere like Venus, which is some 60 times denser than Earth's, and a planet about the size of Titan, which has a gravity about 1/8th of ours, we can get a much larger buoyancy force. On this planet, every cubic meter of hydrogen is going to be able to lift around 250 kg- a massive increase from the 1.1 we used on Earth. This cuts our minimum radius down to just 10 meters! That's much more reasonable for an organism, and quite manageable in any fiction novel.

  • $\begingroup$ I should note that this assumes the use of hydrogen gas, not hot air, because the question wasn’t perfectly clear which one we’re working with. Using hot air makes the creature much larger. $\endgroup$
    – Dubukay
    Commented May 30, 2018 at 14:38
  • $\begingroup$ question for you, when you start at a point of a 1m shell that means your starting point with no lifting gas is a 1m in radius sphere of skin with no lifting gas right? Because, when I plug in the thickness of human skin (about 1.3mm) into your equation I get a sphere of about 3m to lift the skin shell which seems much more reasonable $\endgroup$ Commented May 31, 2018 at 8:02

The problem with hot air balloons is they require a lot of energy to become buoyant, and this would require an improbable metabolism. A typical hot air balloon uses a powerful set of propane burners or equivalents, meaning a creature would have to produce a similar amount of heat (and likely using far lower energy density fuels, unless you want to postulate something really crazy like it eats coal...)

The alternative is even more bizarre. Buckminister Fuller speculated that geodesic domes, expanded to extreme sizes, could eventually take off like hot air balloons since the entrained air inside would outmass the structure by an enormous amount. The mass of the structure grows with the square of the volume, while the mass of the air inside grows with the cube.

The following extract from a paper posted to GEODESIC by Robert T. Bowers explains the idea.] ``When considering a geodesic sphere, the weight of the sphere is a function of the surface of the sphere. The amount the sphere is lifted by warm air is a function of the volume of the sphere. In mathematical terms, weight is a function of the radius squared, while volume is a function of the radius cubed. This is very significant. Even as the radius of a sphere increases, thus increasing the sphere's weight, the lift of the sphere increases more. If you image a sphere that could grow larger, as the sphere gained a little weight, it would gain much lift.

``Buckminster Fuller proposed that as spheres of great size are considered, the amount of air enclosed grows huge compared to the weight of the sphere. Of a sphere with a radius of 1320 feet, the weight of the enclosed air is 1000 times greater than the weight of the sphere's structure. If that volume of air was heated only one degree, the sphere would begin to float!


I know it sounds like science-fiction, but here’s how Bucky proposed a Cloud Nine would work. A half mile (0.8 kilometer) diameter geodesic sphere would weigh only one-thousandth of the weight of the air inside of it. If the internal air were heated by either solar energy or even just the average human activity inside, it would only take a 1 degree shift in Fahrenheit over the external temperature to make the sphere float. Since the internal air would get denser when it cooled, Bucky imagined using polyethylene curtains to slow the rate that air entered the sphere.

So a creature which was essentially a hollow sphere over a kilometre in diameter could theoretically become a hot air ballon with a combination of solar heat and metabolic process, but imagining what sort of life cycle that would be required to reach a kilometre diameter size before taking to the skies is a bit mind bending.

  • 1
    $\begingroup$ Even if all this stuff can be done in a engineering way the problem lie in the heat lost for the air and with a great surface comes a great heat (energy) loss even if the temperature differential is low $\endgroup$
    – jean
    Commented May 30, 2018 at 12:23

This sounds similar to the Affront in Ian Banks' Excession. They are described as being:

A bulbous mass about two metres in diameter, which hangs from a frilled gas sac one to five metres in diameter. Six to eleven tentacles of varying length and thickness grow from the central mass, of which at least four end in leaf shaped paddles.

However they cannot survive in an Earth-like atmosphere.

Affronters require a high pressure, low temperature environment, and breathe an atmosphere composed mostly of nitrogen and methane, plus other trace hydrocarbons.

These creatures either 'walk' on their lower limbs most of the time, but with most of their mass supported by their own bouyancy, or they lazily paddle through the air at a low altitude. When they are in a hurry they have a gas vent (anus) on the bottom of their bodies for propulsion.


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