In short: my last post was about a world of flying islands. The world is like a huge gaseous sphere (with the same gases as the Earth's atmosphere, but without the effects of pollution) with several flying islands in it, there is only gravity on the islands because they are rich in minerals (which I invented, worth it highlight) and these same minerals prevent the islands from merging and forming a large rocky planet. This gravity is also very weak to the point where a jump makes you go towards another island. Going a little more to the topic of the post: this world is inhabited by several animals, many of them winged such as birds, insects and pterosaurs (in addition to fictional animals based on marine animals such as skyeels and cloudwhales), but I wonder... how would it be their flight in zero gravity?

They don't have any kind of propulsion similar to an airplane or rocket turbine(at least nothing outside the biological) and, despite the air resistance, there is no gravity outside the islands. It's like flying in space but with air to breathe without having to rely on equipment. Furthermore, animals did not arise from evolution, they were created by the magician mentioned in the previous post. Feel free to dictate the physiology of these animals if you feel it is necessary. You can use solutions that use magic as long as you explain how that magic works.

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    $\begingroup$ Larry Niven did this in The Integral Trees it is a pretty good setting, and nice set up. $\endgroup$ Oct 31, 2021 at 5:23
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    $\begingroup$ When you say "They don't have any kind of propulsion", what exactly do you mean? Given that they have, depending on their body plan, wings, arms, fins etc it seems they are perfectly capable of propulsion by swimming or flying through the air. (The air is a fluid just like water is, albeit considerably less dense.) $\endgroup$ Oct 31, 2021 at 5:37
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    $\begingroup$ Air without gravity is just thin water. Swim where you need to go. Big flippers will be an advantage. $\endgroup$
    – PcMan
    Oct 31, 2021 at 5:53
  • $\begingroup$ @KerrAvon2055, I mean something like the turbine of an airplane or that fire that comes out of the rocket for it to take off. $\endgroup$ Oct 31, 2021 at 7:17
  • $\begingroup$ These exact circumstances exist on the ISS, the Tiangong space station, and any orbiting space capsule. Of particular interest might also be the historic Skylab which was essentially an S-IVB tank converted to a space station, and thus huuuuuge. Unlike the ISS, where you are almost always within arm's reach of a wall, astronauts have reported that you could get "stuck" in the middle of Skylab. $\endgroup$ Oct 31, 2021 at 15:12

6 Answers 6


There are a few possibilities

the first is based of the answer to this question, your creatures could have large fins an "swim" through the air. The fins would have to be a bit larger than on earth sealife to get about the same accelerations. This would be the midrange method, the creature could quickly get to a good speed and also be able to slow down relatively quickly.

Another possibility is if the creature are covered in cilia which are tiny hairs that bacteria use to move them themselves around. This method is the lowrange method of propulsion it would take the creature a while to get to a high speed, and an equally long time to slow down, and it would be at the mercy of any winds in the area. One advantage it does have over wings/fins is that it would be harder to remove from the creature; so a creature using this method would be able to move even if it lost a significant proportion of it's cilia.

enter image description here

They could also use a air jet like what squid/octopi use.

A similar idea suggested by @JourneymanGeek is that the creature could have a large stomach/bladder that is filled with gas produced through digestion. It then "farts" that gas out, using it to push the creature around.

An alternative method of propulsion is that the creature could use something like Ion-propelled aircraft, which uses strong electric fields (high voltages) to ionise the air then use the same electric fields to push the ionised air over a wing to provide lift. But since you creature don't need to use such a system for lift, it could have a ion jet that uses this technique to propel the creature, somewhat similar to an ion thruster.

enter image description here

This method has the benefit of having a high top speed (about half the speed of light, if the creature is in a vacuum), assuming a powerful energy source. The creature could use this jet (or several jets) to be able to manoeuvre quickly, and be able to travel a a high speed (possibly super sonic in an atmosphere).

hopefully that helps

  • $\begingroup$ "if the creature is in a vacuum" since it's not, you can replace it with "ionic" wind (easier in the absence of gravity, unlike the MIT plane, no lift component is required) $\endgroup$ Oct 31, 2021 at 6:46
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    $\begingroup$ By airjet, we also might mean 'flight by controlled flatulence" $\endgroup$ Oct 31, 2021 at 14:36
  • $\begingroup$ @JourneymanGeek that wasn't what I had in mind (more of sucking air slowly then squirting it out) but is a cool idea to include $\endgroup$
    – Nyra
    Oct 31, 2021 at 14:53

Quite simply, they would fly the same way spacecraft navigate - reaction force. In spacecraft, reaction control systems (RCS) of different types are used. When ports on different parts of the spacecraft are used, it causes the spacecraft to roll, pitch, and yaw. Even just four or so ports pointing in varying directions can allow for some amount of control, though more are best, especially if you want more refined ability to translate up, down, left, and right.

If you adapted this to a biological framework, you see similarities to things like squids moving by ejecting water. Perhaps you could eliminate some control ports by giving the creature enhanced flexibility, as to bend their control ports in different directions. Reducing the number of ports would increase efficiency maybe, but decrease flying ability, which could be quite important for predators.

I'd imagine the creature would need a specialized organ for storing the reaction propellant - this might double as a source of lift if the creature flies over the islands.

  • $\begingroup$ squids move by sucking in water then squirting it out. Ink is only sometimes added in small quantities to the water to provide a cloud in the water. $\endgroup$
    – Nyra
    Oct 31, 2021 at 6:07
  • $\begingroup$ @Nyra thank you for the catch, I was mistaken. $\endgroup$ Oct 31, 2021 at 7:13

For "natural" forms, this problem is already solved on earth. Fish (and other swimming creatures) move around in a fluid where they generally do not depend on dynamics to keep from falling. Instead they have swim bladders that keep them neutrally bouyant, and use fluid dynamics for propulsion only.

This is pretty much exactly the situation for your sphere, except the fluid is air, not water. And the solutions are the same, except for volumetrics. The most common method will be a main rear fin that is either moved side-to-side (like fish) or up-and-down (like dolphins and whales) for propulsion. Smaller side fins act like ailerons to translate some of the forward propulsion of the main fin into up-down and roll motions. Vertical fins above and below provide yaw stability, and by twisting the main body, these fins also allow the creature to turn side-to-side.

Other creatures may use respiration to propulsion, inhaling large quantities of air, then expelling it through small orifices in a form of jet propulsion. But these will generally be less efficient than the fish-like forms, and thus will likely only be used by creatures that stick close to the islands for protection.

The big difference from water dwellers will be size. Because air is much less massive than water, the fins will need to be much larger with respect to the body to give the same level of motion control. And since this will also make the fins much more unwieldly, there will be compromises that water dwellers do not have to make. Still any "natural" forms (ones that might have arisen through evolution, not creation) will more closely resemble fish than they do birds, as much of the shape of birds comes from fighting gravity.


Pretty much all flight on Earth relies - unsurprisingly - on gravity, so there aren't a lot of examples we can go with from Earth's flying species. We can get a few ideas from aquatic species that might be more useful, but air and water are quite different environments in terms of viscosity, so 'swimming' through air is unlikely to be useful. Still, there are probably analogs depending on the ecology of your world.

For this I'm going to assume that there are air currents of some sort in the gas envelope/atmosphere. This seems to be necessary to allow a proper respiration cycle and prevent atmospheric stagnation. I'll leave it to you to explain how those air currents work, I'm just interested in their effect on life forms.

Given those air currents we can start with microscopic airborne algae, similar to the phytoplankton you find in Earth's oceans, that forms the basis of your ecology. Drifts of algae will be carried along by the air currents, possibly forming visible dust clouds, probably limiting visual range in high density areas.

Along with the algae swarms you could add other lightweight plant forms, structured to be carried along on the breeze. They'd be fluffy, spherical masses with leaves (green if chlorophyll is dominant, depending on ambient light conditions) on the outside and root mats on the inside that capture particulates and moisture from the atmosphere. I imagine there's at least one variant of this plant that grows to be a large hollow sphere that floats along, possibly carrying small animals and other things inside and out.

Simple animal life can use the air currents as well, simply floating along with very little actual motion. Larger, denser bodies will be less affected by the currents, so in algae-rich areas the food will be passing by on the breeze. Passive capture of nutrients in this way could support fairly large animal forms, since their energy use is very low.

More active animal forms will need to develop from more mobile precursors. Amoeba with cilia and flagella for motion are fairly common on Earth, and there's no reason they couldn't develop in your world. Simple forms grow and become more interesting.

Now that we have the basics, we can start theorizing on the developments.


Algae, plants and drifting collectors just need to float in the breeze. They don't need any motive power since their movements are controlled by the air currents. Some drifting collector-class creatures may have fins or other aerodynamic features to better orient themselves, but these can be purely structural with little or no control.

Floaters also includes baloon-type creatures, like the Portugese Man-O-War jellyfish. A lot of variation is available there, from large single bags to hundreds of tiny gas cells. Gas can be generated biologically, through symbiotic bacteria or drawn directly from the surrounding atmosphere using a bellows arrangement.


Extend the aerodynamic surfaces on a floater and add more control musculature and you can better control movement through the air streams. Being able to tack against the wind makes a sailer more able to maneuver to greener pastures.


Sails become active surfaces that use rippling motion to push against the air, allowing the creature to move against the airflow or in areas with low air movement. Think of how cuttlefish fins work, but much bigger. Also imagine ribbons and carpets that undulate in the air, using cilia on their surfaces to increase air resistance. Might look something like how snakes do when they're moving through water or over sand.


Simple balloons move well with the breeze, but with a little effort the internal gasses can be pressurized and used to maneuver. Over time a variety of animals of all sizes can develop from here, from simple mobile spheres to fast hunters that draw in air, pressurize it, then push it through directional nozzles. With the right sort of multi-stage pressurization this could even be a sustained flight model.

(Juvenile jet-based creatures would have to be strongly familial since they'd be completely helpless until they learn how to control their jets. Packs of baby jets would probably be terribly cute.)

Opposed Wings

While Earth-formed wings use gravity, wings in this environment would need to have some other opposed force. Rather than having pairs of wings pushing 'down' and providing aerodynamic lift, a set of opposed pairs of wings pushing against each other would work as well. This could work well for insect-like forms, but other large creatures could benefit from it as well. A bird with four wings and a pair of tail surfaces would work well.

Instead of simple pairs you could have a ring of wings where each wing opposes both of its neighbours during the full cycle. This would be quite maneuverable but potentially not as fast.

Flapping Umbrella

I don't know what else to call this. I looked, but I just can't find a good way to say it. Take an umbrella with open flaps along the surface. Open it up, letting air pass throught those flaps. Now close it, having the flaps seal against the surface. Net result: weird, slightly ugly propulsion along the shaft of the umbrella.

(I don't know, it just popped into my head. Probably thinking of octopuses. Octopodes. Octopi? You know, Sea Cthulhus.)

Those are all very basic concepts with plenty of variation and development possibility. Some creatures will undoubtedly use hybrid motion forms, combining a relaxed drifting option with high-energy movement capability when hunting or hunted.


A propeller on an airplane is just a little wing that sweeps through the air and propels it backward. By reaction force, this propels the aircraft forward.

Even in a jet engine on a present-day airliner, most of the propulsion force of each engine is exerted by the fan, which is again just a lot of little wings traveling around a circular path and propelling air backward.

Flying animals could use wings in the same way, except that instead of traveling around a circular path in one direction like an airplane's propeller, you may find it preferable for the wings to flap through an arc, flap back along the same arc, and repeat.

Since the wing reverses the direction of travel along the arc, it will have to reverse its angle to the air in order to propel the air backward on each stroke in each direction. A wing that has the bones and muscles along the front edge, with long flexible feathers attached to this structure and pointing backward -- much like the wings of the birds we have on Earth -- might work.

The wings of our birds are asymmetrical (the top is different from the bottom) because they need to deflect air mostly downward. Your birds' wings could be symmetric. Or perhaps they become asymmetric during a stroke in one direction, but then reverse the asymmetry during the reverse stroke.



You did say that I can use magic, there is a magic one. Just make the animals use telekinesis to flight.

Magic swim

They can use magic to move like they were swiming in a fluid with high viscosity, like this insect.

  • $\begingroup$ Magic/handwaving is always an option, of course, but if realism is desired, then "air swimming" does not work very well, at least if the air is relatively still. Astronauts have discovered that really the only way to move around in space is planning your movements with how you propel yourself off of surfaces, and catch handles and such at your destination. $\endgroup$ Oct 31, 2021 at 7:12
  • $\begingroup$ @WasatchWind But you can if is Honey instead of Water. $\endgroup$ Oct 31, 2021 at 7:25
  • $\begingroup$ Of course - magic allows any matter of choices to be made - but high viscosity swimming would also have other difficulties. It would be extremely tiring. $\endgroup$ Oct 31, 2021 at 7:26
  • $\begingroup$ @WasatchWind This insect basically does it, and it don't see it tired. $\endgroup$ Oct 31, 2021 at 7:30

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