# Could there be an atmosphere in which I could both be alive and float?

There an atmospheres in which I can live, like Earth's. There are atmospheres in which I can float, like if it were a really heavy gas. Is there some atmospheric composition where I could both be alive and float, unaided?

By be alive, I mean I could survive until I run out of food or water. By float, I mean I can be above the ground with no effort (indeed, I probably couldn't control it.).

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

• There are atmospheres in which I can float, like if it were a really have gas. What atmospheres are you thinking of? – HDE 226868 Jul 13 '15 at 18:59
• @HDE226868 Sorry, typo. I fixed it. – PyRulez Jul 13 '15 at 18:59
• I didn't mean the typo; I meant that particular atmospheres. – HDE 226868 Jul 13 '15 at 19:00
• I wonder if you might fiddle with gravity, instead of atmosphere per se, but anything with that light of gravity couldn't hold an atmosphere. Hmm.... – Mikey Jul 13 '15 at 22:48
• YES! Believe it or not you simply need to hunt for a gas planet without liquid/solid core but massive enough you won't be killed by its crushing pressure. Make sure to bring lots of oxygen tanks and food supply since most gas planet is made of hydrogen gas. – user6760 Jul 14 '15 at 4:22

## 9 Answers

No.

First off, there are no gasses which are denser than a human. To float, you need the medium to be as dense or denser than you are. You might be thinking of thick atmospheres on low gravity planets, which might allow you to fly by flapping some attached wings, like on Titan. That atmosphere is not breathable though.

So, in order to be floating in something you can breathe, it would need to be far denser, that means it'll have to be a liquid. There are some proposed liquids that a human can breathe, but an atmosphere usually means the gases surrounding a planet.

This is possible through semantics. You can get loose with the definition of an atmosphere and have a planet covered in a breathable liquid which you call the atmosphere.

Alternatively, you can look into making a gas torus. This is really only an atmosphere (there is no ground), but does allow you to float around in what you're breathing. Check out Larry Niven's Integral Trees for more inspiration.

• While the setting of Integral Trees is cool, I'm pretty sure that it's not really that plausible, either. In reality, I'd expect such a gas torus to dissipate on fairly short timescales; even if actively replenished from a nearby gas giant, as in the books, there's only so much gas available there, too. I could buy it remaining relatively stable for, say, thousands of years, but not over astronomical / evolutionary timescales. – Ilmari Karonen Jul 13 '15 at 22:14
• @IlmariKaronen Uh, dissipate to where? If the gas is orbiting the neutron star fast enough it would stay in orbit the same as any planet does. Also, existence is the only requirement for this question, not existance for evolutionary timescales. – Samuel Jul 13 '15 at 22:32
• I'd guess, dissipate to "the vacuum of space"? Atoms attempt to achieve equilibrium according to the laws of thermodynamics, moving from areas of high pressure (the atmosphere) to areas of lower pressure (space). In order for the gasses to remain a part of the torus, they would need some motivation to stick around. Scientists call that motivation 'gravity'. However, once you get into the gravity required to sustain atmospheres, you'll quickly see that floating becomes impossible. – Ayelis Jul 14 '15 at 15:27
• @IlmariKaronen The Cloud Ring has been analysed a fair amount and for the most part it does add up (the revolution speed might be too low). It's not a simple ring around the neutron star, there is what remains of the gas giant (and maybe other stuff too, I don't remember) that acts to shepherd the atmosphere back into position. – Tim B Jul 14 '15 at 16:25
• Ahh, so a spiral of trapped gas that's being slowly and inexorably pulled into the star. Sure then. That. So long as it's within the Goldilocks zone of the neutron star, you won't be frozen solid or baked alive, and so long as something closer to the star is deflecting the ionizing stellar wind (the way the Earth's magnetic field does for us) you won't be nuked. – Ayelis Jul 14 '15 at 17:11

Yes, you can float in air at standard pressure which is moving upwards at around 200kph. It's generally known as indoor skydiving, but you could imagine a world where wind creates this situation.

Let's look at Tungsten hexaflouride, "one of the heaviest known gases under standard conditions." This gas is rather toxic so it won't work, but as you'll see the toxicity isn't the only problem. Tungsten hexaflouride has a density of 13g/L. The density of the human body is around 1g/mL, about 100x more dense.

Using the ideal gas law (pV=NRT), in order to increase the density by 100x, we need the amount N to increase 100x relative to the volume V. R is a constant, so we need to decrease T and/or increase p.

Decreasing the temperature T isn't going to help us much. Going for 30 degrees Celsius (86 F) to 10 degrees (50 F) only increases the density by 7%. Too much colder and we it isn't really survivable long-term.

So basically all of the increase needs to come from increasing the pressure p. However, 100x pressure isn't going to work for you. The pressure itself might be survivable, but there's nothing you could breathe that wouldn't kill you:

It's unclear that there's any gas out there that we can breathe at, say, 100 times the atmospheric pressure.

Unaided, you're out of luck. You'd need a pressure suit and an oxygen supply or a tank of breathable handwavium air in order to make this work.

As Samuel and Rob Watts have explained, there's no atmosphere in which an unaided, unmodified human being can be lighter than or as light as the surrounding “air”. So what does this leave us with?

You could be a modified human being. Given how far humans are from being able to fly, let alone float, the modifications would be extensive.

You could of course be an aided human being. All it takes is a large enough balloon containing helium, hot air or some other gas that's lighter than air.

If neither of these cheats are acceptable, here's another one. If you get rid of gravity, it doesn't matter that you're heavier than air!

All you need to set this up is a confined space with no or very little gravity. The space needs to be confined, otherwise the air will escape. The space will need to contain no masses large enough to create significant gravity. One way to create such an environment is at the center of a large sphere — a hollow planet. No matter how heavy the sphere is, as long as it has spherical symmetry (or close enough), there's no gravity in the central hollow part.

Like all science fiction ideas, it's been done before. One hard-ish example is Karl Schroeder's Virga series. Virga is a world with no gravity (people live in rotating cylinders) but full of air. Due to the lack of metals, technology is mostly pre-19th century, with wooden ships — pirates in space. Humans can “fly” through the world, with or without equipment — but Newton's third law applies, so you need rocket propulsion or some other mass to throw or you'll be stranded in space.

• Just a note, a hollow sphere has zero gravity everywhere inside (physics.stackexchange.com/questions/150238/…) – justhalf Jul 14 '15 at 5:54
• @justhalf That statement is slightly misleading, perhaps only due to its brevity. The gravity from the sphere itself cancels inside, but there is still non-zeroed gravity from any nearby massive bodies. Obviously being inside a hollow sphere on Earth, one would still experience Earth's gravity. – Samuel Jul 14 '15 at 16:36
• @Samuel Hamsters being the prime example. – PyRulez Jul 20 '15 at 22:41
• @PyRulez That example really really makes me wish spheres actually were antigravity devices. – Samuel Jul 21 '15 at 16:40

Yes, if you create a universe filled entirely with air instead of relatively empty vacuum. Getting around would be a bit more difficult than swimming, since air has about 1/50th the viscosity that water does, but the same principles should apply in general. Of course, some method of aerodynamic propulsion would certainly be preferable...

Possibly, but instead of modifying the atmosphere change the planets spin.

Here's the idea: the ISS is 'floating' in space, but in reality it is simply falling forward so fast that it does not have time to hit the ground before it crosses the horizon.

See this for the idea.

If the earth rotates so fast that its centripetal force was the same as its gravitational force then everything on that height would effectively float.

Precisely, it would be as if we were on orbit even at ground level.

I think it would be hard to keep the atmosphere attatched to the earth at that point though, like Mars cant keep its own because of its own gravity, but if you had a dome around the atmosphere it could still work.

Most planets tend to slow down though, not speed up their spin.

• The problem of being in orbit at "ground level" is that the atmosphere, which allow me to breath, would probably slow me down and forcing me in one direction or another... – bilbo_pingouin Jul 14 '15 at 18:06
• Inside buildings with their windows closed the atmosphere would move with the ground and the walls, and you presumably could float like the astronauts on the ISS. I think you're right that there would be some serious winds outside. – bdsl Jul 14 '15 at 19:13
• Hydrostatic equilibrium will do weird things to planets that approach "ground-level orbit". Basically, all of the planet's guts will leak out at the place where there's zero g. – mic_e Jul 15 '15 at 5:03
• The atmosphere would be orbiting along with you as well, at least to some level since moving away from the planet would change the orbit speed. If the planet was sufficiently big though, the difference in gravity might be negligible. @mic_E Hence why I said you might need a dome around the atmosphere – GettnDer Jul 15 '15 at 16:07
• @GettnDer The planet would stop being a Spheroid a long time before gravity would be zero; if constraint, it would violently attempt to change its shape. Your dome wouldn't just have to constrain the atmosphere, it would have to stop trillions of km³ of rock from spilling out, with the sheer force that's currently pressing Earth into its shape. Even if your dome could do that (think Ringworld-strength materials), I don't think the atmosphere would remain at the "outside" of the violent mix within. – mic_e Jul 15 '15 at 23:03

Another idea that wasn't covered by the previous excellent answers but was mentioned by @Mikey in the comments.

If you built artificial habitats on some of the moons of the solar system and included large domed areas with STP atmospheres, a human body should be capable of direct flight. Just add wings and start flapping.

Another thought: if you engineered an atmosphere for the Moon, it would not remain for geologic timescales but it might be fine for habitat over human timescales (100,000 - 1,000,000 years). My theoretical + empirical spreadsheet of Solar System bodies indicates a half-life for gaseous $H_2O$ of around 100,000 years - longer for the heavier $O_2 + N_2$ molecules.

Could be a fun world to play in.

• So if aliens dumped the right atmosphere on the Moon once and then left us to it, it should be habitable, or close, for thousands or hundreds of thousands of years? Who'da thunk! – Steve Jessop Jul 14 '15 at 15:22

Similar to the Integral Trees, you also achieve this by having a hollow planet (or space station). If gravity is low enough, or you are near the center of gravity this can lead to being effectively weightless allowing for suspension in atmosphere.

Setting aside for a moment whether or not it's possible to find a gas that can be compressed to the density of a human without undergoing a phase change, suppose you are neutrally or nearly-neutrally bouyant in some fluid. The fluid has your density or higher.

Firstly, your lungs will struggle to move the fluid in order to "breath", since more force is required to shift a larger mass of fluid. That's a problem but it's not necessarily instantly fatal. Secondly, any significant "depth" of such fluid will have very high pressure on a planet, since there's a lot of weight of this stuff above you. Divers in water (approx same density as a human) can go to to a few hundred meters maximum in Earth gravity.

Hence, you need to look for micro-gravity situations (where everything "floats" irrespective of density), or else for environments where there isn't miles of fluid above you (so, not really an "atmosphere" worth the name). If we're not allowed a roof, then perhaps a small body with low gravity, and some kind of atmosphere-generator, would be sufficiently "floaty" even though really you are much denser than the atmosphere. You're sinking, just at very low acceleration. You'd have to think this is inefficient, though, since without a roof the atmosphere on a low-gravity body will be leaking into space.

As for whether a fluid you float in can even be a gas rather than a liquid: it doesn't seem likely that any gas could have a density close to that of a human, without being at such high pressure that a human can't survive.

Of course hand-wavium is good for a lot of engineering applications, but it doesn't qualify as "hard science". If you just assert the existence of a non-toxic gas that's 20% denser than water at standard temperature and pressure, and mix it with 20% oxygen, then you'd float in it and breathe it (albeit with increased difficulty compared to air). You'd also have to explain why the oxygen doesn't all float to the top. And for the reasons above to do with weight of fluid above you, the atmosphere would have to be a lot shallower than Earth's, or the gravity far weaker, or some combination of the two.

Now, if you're willing to consider a "liquid atmosphere", then you're in business. Just grow gills, use water (which will need to be oxygenated somehow) and don't go too deep into the "atmosphere" (or as it's more typically called, "ocean") ;-)