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This character has a set of dragonfly-like wings. These are not normal for her species, and would normally be something of a birth defect requiring amputation. But through liberal application of Clarketech and handwavium, and lots of training, she's kept them and kept in shape to use them.

However, they're nowhere near big enough for unassisted flight in Earth-like conditions. They cover an area between 1 and 2m2 (I'm leaning toward 1.25m2, but 2.5 is not out of the question). Even if she runs twice as fast as the average Earthling, that's still not enough lift to fly. Could be useful for roof-hopping or cliff-to-valley shenanigans, but that's about it.

But eventually, the stars align such that she winds up on a not-so-Earth-like planet (more of a moon, really), needing to grab another person and fly to safety. Or away from immediate danger, anyway. It's not a very safe planet/moon in general, what with all the vulcanism.

The planet is something like a larger version of Io, with a thick atmosphere. The setting contains handwavium that allows the characters to carry around a breathable air supply without adding significantly to their weight. If there is no way to get reasonable temperatures, protection can be handwaved up as well, though that might add more weight than the air supply.

According to my shady calculations, she should be able to pull this off if she can get up to around 15m/s, if the gravity is 20-30% that of Earth, and if the air density is a Titan-like 1.5× Earth.

My question is: can a world like this exist, and if so, are there limitations, instabilities, etc that must be accounted for?

The planet or moon should be geologically active (preferably through Io-like tidal forces, but this is not mandatory), with gravity and atmosphere suitable for this character to fly with a human-sized payload. Combined mass of chars should be somewhere between 110 and 130kg. Bonus points if the temperature is in a tolerable 0-45°C, but this is not required.

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    $\begingroup$ An interesting question, but I assume that she has some sort of propulsion system? The world record for fastest human runner is only 12.4 m/s. Rollerblades can get you up to 13 m/s, and a bicycle can get up to 77.7 m/s. (Changing from "fly away" to "extended glide" might be easier, if this proves unfeasible?) $\endgroup$ – Chronocidal Nov 29 '19 at 16:54
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    $\begingroup$ @Chronocidal the initial directoin of the 15m/s need not be horizontal. There are a selection of tall buildings around here where I could readily achieve a 15m/s velocity, though my glide ratio is likely to be rather poor... $\endgroup$ – Starfish Prime Nov 29 '19 at 17:16
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    $\begingroup$ @StarfishPrime That's a very good point, which I shouldn't have overlooked. $\endgroup$ – Chronocidal Nov 29 '19 at 18:07
  • $\begingroup$ That area isn't anything like large enough for cliff-to-valley shenanigans -- a hang glider for even a lightweight human runs between three and five times that lifting area. $\endgroup$ – Zeiss Ikon Dec 13 '19 at 19:17
  • $\begingroup$ @Chronocidal Might mention that it takes a very special bicycle, and a "peak human" rider, to get to more than 150 mph. Common bikes, not drafting behind cars or trains, and not ridden by superbeings, are good for just about 15 m/s sustained speed on level ground. $\endgroup$ – Zeiss Ikon Dec 13 '19 at 19:19
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I don't think that dragonfly like wings could be structurally strong enough to support a human size being in flight. The largest prehistoric dragonflies weren't nearly as large as a human.

I think I remember that it was calculated that humans should be able to fly with artificial wings on Titan due to the low gravity and dense atmosphere. Because of the unbreathable air and cold temperatures the humans would have to wear thermal clothing and oxygen tanks and masks.

But I don't remember what size the artificial wings would be and how they could compare to the size of your character's wings.

Another place where humans would be able to fly by flapping artificial wings like birds would be inside a large enough enclosed space in a Moon colony.

https://scifi.stackexchange.com/questions/221660/what-was-the-first-science-backed-example-of-human-powered-flapping-wing-flight/221686#2216861

So if your were writing another type of story you might make the setting a moon base with a vast interior space with an Earth like atmosphere, or maybe for some reason an atmosphere even denser than Earth's.

The escape velocity of the Moon is so low that if it acquired a dense atmosphere like Earth's it would lose it fast by geological or astronomical standards. I once read that the Moon would lose an Earth like atmosphere in about a thousand years.

Titan, with its slightly higher escape velocity than the Moon, and much lower atmospheric temperatures than Earth, can hold on to a dense atmosphere many, many times longer than the Moon, and even so it is calculated that its atmosphere has to be slowly and steadily renewed from various sources.

You would want your world to have as low a surface gravity as possible and plausible, to make flying easier, and also to have as high an escape velocity as possible, in order to make acquiring, and retaining, a dense atmosphere for eons until the characters arrive there, more plausible.

Fortunately, the formulas for calculating surface gravity and escape velocity are different.

https://en.wikipedia.org/wiki/Surface_gravity2

https://en.wikipedia.org/wiki/Escape_velocity3

And I think that the surface gravity and escape velocity of worlds do not change in exact unison, and therefore it is possible to design a world with a somewhat more favorable ratio between surface gravity and escape velocity than might be expected a first sight, and every little bit helps.

The flight in the story should happen at as low an altitude as possible. With the lower surface gravity of the world, the atmospheric density will vary with height much less than on Earth. But even so, on Mars the atmospheric density is about twice as high in the lowest basin as on top of the highest mountain, and every little bit helps.

And it may be a good idea to have the local atmospheric pressure where the character makes their flight be unusually high. There might be a slow and gradual or even sudden and catastrophic release of volcanic gases from a volcanic source, and that may raise the air pressure high enough for the character to be able to fly as well as providing winds away from the sources of the gases. The volcanic gases may also be very hot compared to the (unspecified by me) air temperature, and may thus rise in a thermal updraft which the character may use to gain height and then slowly glide to their destination. And every little bit helps.

Or they might be at the shore when gases are released from the seabed or from a lake.

https://www.sciencealert.com/massive-underwater-domes-of-methane-look-set-to-blow-at-any-moment4

https://en.wikipedia.org/wiki/Lake_Nyos_disaster5

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The problem with Io is that it has no atmosphere. It was blown away long ago by the heat of the planet and eroded by the Jupiter's magnetic field pulling particles. Io's dust is the main source of the accelerated particles that are responsible for Jupiter's orbital space radioactivity, because the magnetic field accelerates them to dangerous speeds and they hit and they act as ionizing radiation. Io lost it's gas because it is small. Maybe if it was as big or bigger then Earth it would hold it but then it would be much hotter due to more mass being affected by the tidal forces.

The end result is that your being won't fly because there is no atmosphere in Io as Io is and in an alternate Io as big as Earth the gravity would forbid your flight (that's assuming the greater gravity is enough to counter the heat and the magnetic erosion).

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