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By extremely high gravity, I specifically mean around 28 Gs. I recently asked this question with regards to inhabiting the sun, and surviving in the sun's gravity seems like one of the major challenges.

I'm interested in what would be needed to allow humans to survive in such an environment, both in terms of what modifications can be made to a habitat for the humans to live in, as well as what modifications can be made to the settlers themselves.

I'd like to limit technology to 'things that could be achieved with technology we have either created or are working on', and reasonable extensions of those things. I'm assuming, for example, that we have fusion power (only way I can think of that we can power the cooling systems), and that we have in-situ genetic modification via viral DNA modification, since those are both research targets. I'd likewise consider some basic nanobots in the realm of possibility, but would like to stay away from things like large scale anitmatter production and gravity generators, since those require major breakthroughs in science that we aren't terribly close to.

Within those constraints, what can I do to allow humans to inhabit a 28 G environment for an extended period of time?

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    $\begingroup$ Are you interested in an answer that would try to reduce the 28G experienced? If the structure was orbitting the sun quick enough, the centripetal force could reduce the gravity experienced. $\endgroup$ – Twelfth Dec 9 '14 at 22:12
  • $\begingroup$ Oooh. . . Is artificial gravity against the Sun's pull allowed? $\endgroup$ – HDE 226868 Dec 9 '14 at 22:20
  • $\begingroup$ Gravity would be problem only after you survived the temperature and radiation inside sun, which you would not. So... $\endgroup$ – Peter M. Dec 9 '14 at 22:20
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    $\begingroup$ Orbiting would reduce the gravity experienced to 0G, but I'm interested in inhabiting a place with really high gravity, not orbiting around one. $\endgroup$ – ckersch Dec 9 '14 at 22:20
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    $\begingroup$ I'd prefer not to go for gravity generators, since they aren't supported by any science that I know of. $\endgroup$ – ckersch Dec 9 '14 at 22:21
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Ha! An obscure question I asked somewhere else now becomes relevant!

A while ago, I asked this question on Biology about liquid breathing. It's currently unanswered, and may stay that way forever, but it's now applicable here, which I think is cool. Anyway. . .

Liquid breathing is (as Wikipedia puts it)

a form of respiration in which a normally air-breathing organism breathes an oxygen-rich liquid (such as a perfluorocarbon), rather than breathing air.

Perfluorocarbons are strange compounds made out of carbon and fluorine. They can carry oxygen, and a lot of it. They are stable and have strong inter-atomic bonds. Almost all are liquid at room temperature. Also, they aren't flammable, which turns out to be a really good thing.

More importantly, they may be used in liquid breathing, assuming they are flooded with oxygen. There are two techniques used in liquid breathing; the one we'll have to choose is known as Total Liquid Ventilation (or TLV). It is (no surprise) the technique of completely filling lungs with oxygen-rich prefluorocarbons.

Why does this matter? Liquid breathing can help counteract G-forces. From Wikipedia,

Liquid immersion provides a way to reduce the physical stress of G forces. Forces applied to fluids are distributed as omnidirectional pressures. Because liquids cannot be practically compressed, they do not change density under high acceleration such as performed in aerial maneuvers or space travel. A person immersed in liquid of the same density as tissue has acceleration forces distributed around the body, rather than applied at a single point such as a seat or harness straps. This principle is used in a new type of G-suit called the Libelle G-suit, which allows aircraft pilots to remain conscious and functioning at more than 10 G acceleration by surrounding them with water in a rigid suit.

Unfortunately, the limit for this technique is somewhere around 20 G (and we're being generous here). Wikipedia does say that it might be possible by raising the density level of the liquid, but there's still a limit.

So all you have to do is fill the planet with an ocean of perfluorocarbons, and you're set!$^1$ Of course, most of the perfluorocarbons would evaporate at the temperatures in, on or near the Sun, but hey, you can't have everything.

$^1$ Note: The one huge downside here is that liquid breathing hasn't really been tested on humans before for long durations, so you're going to have to hope you get lucky.

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If you're talking about living on the Sun, (leaving aside for a second the tremendous heat-disposal problems), you have to understand that the Sun doesn't actually have a surface. At the bottom of the photosphere the density is $10^{-9} g/cm^3$, a pretty good vacuum. The reason we see a cleanish circle around the sun is a mere optical illusion (the mathematical point where a photon travels the length equal to the 2R of the Sun before it gets absorbed) so there is no clear delineation of where the Sun ends and the photosphere begins.

So any human construct (assuming the goal would not be to sink towards the center of the sun in seconds) would effectively have to be in orbit, even if that orbit is at an "altitude" of 0 km (with some extra oomph to overcome friction due to the intense solar wind). If it is in orbit, anyone on board will experience zero-G.

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This can't be done by anything resembling current humans

A serious re-work of the human body would be required.

Currently humans can survive very high g-forces for fractions of a second, but any sustained force over a few g will cause an untrained person to black out. A g-suit can help.

The resting g-tolerance of a typical person is anywhere from 3-5 g depending on the person. A g-suit will typically add 1 g of tolerance to that limit.

But not by much.

The record holder for maximum sustained g force goes to John Stapp who experienced a peak of more than 46 g, with more than 25 g for 1.1 seconds on a rocket sled. While he didn't die, his vision was damaged for the rest of his life.

Blood can't get to the brain

The big problem is keeping blood going to the brain. That's a problem very low on Maslow's hierarchy. The heart isn't strong enough to keep blood pumping against that kind of acceleration. That's the immediate problem. It might be overcome with special implants, suits, and liquid immersion. But there are more problems.

A muscle cell isn't strong enough to even lift itself alone

If cyborg humans were able to install powerful pumps and reinforce blood vessels and capillaries to withstand the pressure required to raise blood to head level, they would still have the problem of raising a fork to their mouth. Not too much because of the fork that now weighs 20 newtons, but the arm that weighs over a thousand newtons. Unfortunately adding more muscle does not help. Humans can only lift about three times their own weight, so no matter the size of the muscle, it simply can't even lift itself.

How to proceed?

You'll need some brains in vats connected to super robotic bodies. Or, ideally, human brains uploaded into supercomputers mounted on super robotic bodies. It'll be far better than the puddle that was a human body before. While there isn't any obvious paths to such the science that can allow this, I can't imagine why it wouldn't one day be possible.

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    $\begingroup$ How much would these effects be reduced by complete fluid immersion, as per HDE's answer? $\endgroup$ – ckersch Dec 9 '14 at 22:27
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    $\begingroup$ Yes, it would help, it's a good suggestion. If you can achieve neutral buoyancy, and remove or alter all compressible regions of the body, then you essentially negate the force due to gravity. This method appears in several hard science fiction stories. $\endgroup$ – Samuel Dec 9 '14 at 23:07
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    $\begingroup$ Sorry to be pedantic, but the fork does not weigh 2 kg. It weighs 20 N. Its mass is still less than 100 g. $\endgroup$ – abcde Dec 10 '14 at 3:38
  • $\begingroup$ @abcde You're not being pedantic, you're entirely correct, thanks! $\endgroup$ – Samuel Dec 10 '14 at 3:53
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Humans cannot survive in biological form such gravitation forces, as explained by others - only after singularity, consciences uploaded to computer.

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The real problem is that the heart cant deliver blood against 9G.

All fighter aircraft are red-lined at 9g (most modern fighters can do more if programmed to ignore the red-line) because there is no pilot who can survive more than 9G. And even this 9G is only possible for a short amount of time.

Overall the problem is about the force which the heart usually impinges on blood. At 9G sustained, the blood cant reach the brain. The return circulation will be prevented if the acceleration is sustained long enough, blood will concentrate on the lower parts of the body.

In WW2 Germany experimented with pilots laying prone. This allowed higher accelerations to be sustained. But practical considerations prevented the adoption of prone piloting.

Russians developed anti-G suits for their fighter pilots, those compress the lower parts of the body, to force blood up to the heart and head. But this has limitations.

All those problems are found on fighter aircraft, where aircraft and pilot are subject to intermittent maneuvering (and consequently acceleration). On a continuous situation humans would support much less than 9G.

Besides that, this 9G is rated for trained crew, common people usually pass out under much less G's.

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