In my world ... Actually, in every hard sci-fi world with casual interplanetary travel, the G-force involved in acceleration would becomes so high that no unmodified human being could realistically survive it, leading to an authorial work around of some sort, or it simply not being addressed in the work (to my knowledge)

Now, in My fictional setting, the Administration of Mars has commissioned a series of torch ships to give Mars the means to unlock the solar system. These Torch ships have both the speed and acceleration to make a round trip from Mars to Earth in a week, and has a max speed of 0.001c . The problem is that stated above, it's transporting squishy squishy humans. But there's an upside! The people of Mars are obsessed with the concept of the artificially modifying humanity as the next step in human evolution, or "Human +". However, the Martian population is caught up in a sectarian dispute over the topic of post human evolution. One faction, the Anamists, believe that the path to human plus should be layed in biotechnology; Whereas the other faction, the Machinists, believe that humanity should move increasingly away from biotic life.

Now, due to the Torch ships being already made, inertial dampeners cannot be installed, so, the Martian administration let loose the two factions to create body modifications to survive the immense G's of the torch ships. The modifications must -

  • allow an untrained human to stay conscious at over 100 G's
  • allow a trained human to stay conscious at over 300 G's
  • allow humans to survive at over 1000 G's for extended periods of time

My Question is -

What Technological / Biological body modifications would allow humans to function and survive at the given parameters?

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    $\begingroup$ Why do you need such high accelerations? You can get from Earth to Mars in a few days without ever going over 1G of acceleration. $\endgroup$
    – Mike Scott
    Sep 12 '17 at 17:09
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    $\begingroup$ If this is hard sci-fi, then what the <insert expletive> are they using as drives? Nuclear Salt Water Rockets? You couldn't generate those sorts of accelerations even with one of those nasty things. 100-1000g acceleration is in the handwavium range, where you have science-fantasy. $\endgroup$
    – Monty Wild
    Sep 12 '17 at 22:36
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    $\begingroup$ I'm curious what this max speed is based on. Is there some resistive force that grows with increased speed? A maximum speed would seem to indicate a point at which your acceleration can no longer overcome some resistive force. $\endgroup$
    – PeterL
    Sep 12 '17 at 22:39
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    $\begingroup$ 1000G's would be plenty to separate all biological material like a centrifuge. passengers would be a sort of human lasagna, layered by density. (I can't believe I though of that. Gross) You would need ridiculous mods. everywhere. The whole body. everything. Every attribute of life fails at 1000gs (or even the other accelerations). All cellular functions would be disrupted by the centrifuge experience. Heavier atoms would sink. This causes a million problems. The best mod would be to become a uniformly dense gelatinous being. Catgut has it right. $\endgroup$
    – ryanrr
    Sep 12 '17 at 23:59
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    $\begingroup$ The administration of the Mars space program sounds like it's beyond Dilbertesque. Billions (trillions?) already spent building spaceships that would liquefy their crew and passengers, all because the people setting the specs never got any competent engineers or scientists to check them first. $\endgroup$
    – David K
    Sep 13 '17 at 11:42

11 Answers 11


In a partial article by Clare Wilson (partial because I'm not a subscription holder) we learn:

(a) Fighter pilots who train on centrifuges to learn how to control muscles and blood pressure to up their G tolerance.

(b) The maximum G-force withstood by 2010 was 31.25G which required the sufferer to be in a tank of presurized water because human muscles can't provide enough force evenly on the body to sufficiently control blood flow.

Your modifications must do these same things: "externally" presurize the body and "internally" increase blood pressure. Additionally, since the point of blood is oxygen and nutrient distribution, you could enhance the density of oxygen and nutrition in the blood so that less blood is required to feed the brain (especially oxygen).

Therefore, your modifications could include:

(a) A web of subcutaneous musculature designed to increase pressure on the body. The modification must force shut the jaw (ostensibly around an oxygen nozzle) and close the sinuses, ears, and eyes to protect them. The musculature's purpose is to force blood to the top of the body. Musculature around the skull is to increase the skull's capacity to withstand the increased inward pressure.

(b) Increased blood pressure. Blood pressure increases for many reasons. Among them are: decreased permitivity of blood vessles (blood can't get out of the distribution system), increased ridgidity of blood vessles (vessles can't contract or expand to modulate blood pressure) or worse, they shrink (contraction), the heart pumps harder (more blood per pump) or faster (more pumps per second).

(c) Increasing oxygen purity to the astronaut is a necessity, but increasing oxygen absorbtion by the lungs would help dramatically. Increasing the oxygen-carrying capacity of red blood cells would help just as dramatically.

Finally, there is the issue of the G-force directly affecting brain tissue. The nature of brain tissue doesn't lend well to intrinsic reinforcement, but we can borrow from our intrepid test subject using a presurized water tank. Increase skull capacity, but not brain volume, making space for a protective fluid, one that the body can presurize on command to hold the brain together.

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    $\begingroup$ Way to rehash my argument, though you can't alter brain physiology so you can't prevent the brain's blood vessels from exploding and hemorrhaging. Also simply putting the brain in a fluid only cushions initial acceleration. it would not handle sustained acceleration as the brain would eventually reach the containers edge and get smashed. $\endgroup$
    – anon
    Sep 12 '17 at 17:30
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    $\begingroup$ Well... I started writing this before any answers were posted and, thanks to distractions at work, didn't finish it until a few minutes ago. What this means is that the number of ways to do this are severly limited. Or, perhaps, great minds think alike? No offense was intended. $\endgroup$ Sep 12 '17 at 17:37

These Torch ships have both the speed and acceleration to make a round trip from Mars to Earth in a week

Average distance between Earth and Mars is 1.5AU, or 224,396,806,000 meters. Half a week is 3.5 days, or 302,400 seconds. Each leg of the trip is half spent accelerating and half decelerating, so we can calculate how fast we need to accelerate to do half the trip in half the time. Thus, our equation to calculate required acceleration is:

112,198,403,000m = 0.5(a)(151,200s)^2

112,198,403,000m = a * 11,430,720,000s^2

9.8m/s^2 = a

So for the flight parameters you described, you don't need to worry about G-forces at all- your required acceleration is just 1G, so your crew will feel like they're under normal Earth gravity. 3G of sustained acceleration, doable with acceleration couches, will reduce the trip time from 3.5 days to just under a day and a half.

I know this isn't directly answering your question, but it doesn't sound like you need crazy 100+G acceleration to begin with. The limiting factor on our ability to explore the solar system isn't acceleration, it's delta-V. With very high or unlimited delta-V (ie very high exhaust velocities from your drive technology, a fairly straightforward technological handwave), allowing you to accelerate indefinitely, a few Gs of acceleration is more than enough to get where you need to go at the speed of plot.

Edit: The question was modified after this was written, to now say Earth -> Mars -> Ceres 'round trip' (so presumably back to Earth after) in one week rather, than just Earth to Mars and back. This doesn't change the answer much, it just makes the math more complicated- an acceleration of 3G will complete the trip in five and a half days.

Constant acceleration gets you places fast- and every quadrupling of the distance only doubles the travel time. With 3G of acceleration and unlimited delta-V, Earth to Pluto takes less than one week.

This answer has deliberately ignored the parameter specified in the question that these ships have a 'max speed' of 0.001c, because at 100G the ship would hit its 'max speed' in five minutes, and even at a comparatively sedate 1G it only takes eight and a half hours. If we deliberately cut the engines when reaching half of maximum speed on each leg of the trip, it will take the 100G ship over eight and a half days to go from Earth to Mars, and then another eight and a half to come back, so it fails the one-week requirement. The 1G ship will accomplish the same journey in about nine days in either direction, even less of a difference than with the delta-V limit ignored.

The difference between single-digit-G acceleration and 100+G only becomes relevant with interstellar distances and high-fraction-of-c delta-V. For anything within the Solar System there's simply no need for such high acceleration.

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    $\begingroup$ @anon - sure, but "You can't do that" is also not a useful answer, whereas if travel times are as stated, this is all the OP would need to make his world consistent. $\endgroup$
    – jdunlop
    Sep 12 '17 at 17:44
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    $\begingroup$ @anon Higher accelerations don't cut much time off the trip time to Alpha Centauri from the point of view of the rest of the universe, though they do cut down a lot on the shipboard time. $\endgroup$
    – Mike Scott
    Sep 12 '17 at 18:14
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    $\begingroup$ @TheoclesofSaturn - My takeaway from this answer is that the premise behind your question is mistaken. Travelling at "top speed" the entire time, the trip from Earth to mars will take 15 days. But space doesn't have a top speed - you can keep accelerating and accelerating. So throw that limit out. Then, you can have a relatively gentle but sustained acceleration, make it in plenty of time, and you don't need heavy Gs. $\endgroup$
    – Bobson
    Sep 12 '17 at 22:54
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    $\begingroup$ @jdunlop: On the contrary, when asking for science-based answers, "you can't do that", or in this case "you don't need to do that" are sometimes the best answers of all. $\endgroup$
    – jamesqf
    Sep 13 '17 at 3:20
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    $\begingroup$ @jdunlop: The OP asked a question about a (quote) "hard sci-fi world", but with premises that are not possible for a hard sci-fi setting. Any answer that accepts those premises is therefore automatically wrong (because it can't apply to the OP's setting) and inferior to this answer. $\endgroup$
    – ruakh
    Sep 13 '17 at 5:42

You can't

Anon already answered this, to a degree, but it's worse than they stated, largely because of the stresses involved in your description. It isn't just blood pooling that's the problem - the actual tissues would suffer damage.

100Gs is the approximate acceleration experienced by your brain when you smash your head at 40km/h into a wall. In a momentary experience, it can cause a concussion, and definitely does damage. A sustained 100G force applied to an unmodified brain inside a skull, whether or not the skull is reinforced to survive the pressure, will crush it into salsa. In fact, while I don't know the outer bounds, I strongly suspect that no biological brain, as we currently understand them, could survive 100Gs sustained for any reasonable length of time.

But it's not just the brain - your eyes would be crushed, too. As would literally all your internals, especially the extra-sensitive gamete factories - testicles and ovaries. By the time you removed/reinforced all of those (even notwithstanding the brain restriction), what you're talking about isn't going to resemble a human to any real degree.

And even that wouldn't be sufficient. Bear in mind that in something like a car crash, a person experiences 100Gs. Even assuming a really-well-padded acceleration couch, sustained 100G forces are going to crush bones. 300-1000Gs will start crushing structural supports, so it's hard to imagine any biological matter not being turned into a fine marmalade.

The best answer is Catgut's - you don't need accelerations like these.

  • $\begingroup$ So I did mention this when compared it to the brains impact against the skull when falling off the empire state building. Though what is usually the damaging factor in concussions is blood on the brain is toxic and can kill nerves. The unknown ambiguity is how much strain the nerves and brain matter can take before severing neural connections. Which is something you too did not clarify $\endgroup$
    – anon
    Sep 12 '17 at 17:46
  • $\begingroup$ The Empire state building comparison is hyperbole. At terminal velocity, the acceleration you'd experience on impact would be approximately 200 000 m/s (twenty thousand Gs), assuming a 7mm meningeal layer between the brain and a skull reinforced so that it doesn't engage in non-elastic deformation (splattering) removing some of the velocity. $\endgroup$
    – jdunlop
    Sep 12 '17 at 17:55
  • $\begingroup$ regardless of the math, what is normally damaging the brain is the blood being where it shouldnt which is something that could potentially be fixed. What we both dont address is how much stress the brain matter itself can withstand. I currently dont know how much force brain matter can resist, there isnt any medical study that i know that does answer this. $\endgroup$
    – anon
    Sep 12 '17 at 18:01
  • $\begingroup$ I suspect that any such study would probably fail on its way past the ethics board! ;) $\endgroup$
    – jdunlop
    Sep 12 '17 at 18:05
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    $\begingroup$ @Aaron - no, my calculation was deceleration from terminal velocity in atmosphere (53 m/s) over a distance of 7mm (the meningeal cushion between brain and skull). Solving for (small) t in d(7mm) = 1/2at^2 and 53 m/s = at gave me the value. $\endgroup$
    – jdunlop
    Sep 13 '17 at 3:37

Machinists win - just convert brains to digital and download to solid state hardware. As a bonus, computer brains could require far less accommodation than biological brains and their support systems (i.e. their bodies), thus reducing both complexity and mass of your torch ship drastically. Furthermore, with the increasing prevalence and capability of AI, the next phase of human evolution must be able to compete with the vast processing power computer hardware can afford, so this is a smart direction to move in even in general.

The Anamists could take a similar route and convert the human body into a sentient amorphous gel composed of intricately interconnected microstructures. Remove the importance of macrostructure integrity (and presumably make those microstructures far more resilient to high sustained forces, which is a lot more manageable) and your survivability problem is much simpler to solve.

  • $\begingroup$ Digital download of one contiousness is liable to be impossible. Also, assuming continuity existence is correct, brain downloads kill you $\endgroup$
    – user15036
    Sep 12 '17 at 19:10
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    $\begingroup$ Sounds like a bleak future for the machinists then. What keeps them going, knowing they can never fully overcome their biological limitations? $\endgroup$
    – talrnu
    Sep 12 '17 at 19:16
  • $\begingroup$ I'm not sure a solid state drive would survive those Gee loads either. $\endgroup$
    – Ash
    Sep 13 '17 at 15:10
  • $\begingroup$ @Ash Perhaps not conventional, modern SSDs. I assume some significant advances in memory and material technologies would be necessary to accommodate even a single human mind. I was more emphasizing the importance of minimizing the number of moving parts anyway - less movement means less of an impact on function due to changing external forces. Try shaking a running HDD violently or putting it in a high-G centrifuge and see what happens ;) $\endgroup$
    – talrnu
    Sep 13 '17 at 17:00
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    $\begingroup$ @talrnu Yeah maybe, I can't think of a material that's going to stand up to a 1000G shock loading but maybe. No thanks I have enough HHD coasters already. $\endgroup$
    – Ash
    Sep 13 '17 at 17:12

The only semi plausible way for humans to survive being subjected to high accelerations (and this applies to being shot out of a mass driver or being aboard a torch ship) would be to immerse the individual in an incompressible fluid medium which fills all the open spaces and cavities in the body. Being suspended in an oxygenated fluid like this would allow the person to survive high accelerations since the entire body structure would be supported and there are no voids or empty spaces for the acceleration forces to exploit as weak points in the structure of the human body.

For obvious reasons, you could not simply stick a person immersed in fluid in a bottle and then hit the throttles of the spaceship. The fluid would have to be monitored and properly oxygenated at all times, and any waste products removed and filtered out, so the chamber would need to have some very elaborate high pressure pumps and fittings to operate in a high "G" environment.

This would seem to be true given the OPs conditions regardless if the person inside the container is genetically engineered, a cyborg or even just a normal person. Only a fully realized upload living in a solid state VR optimized to survive high G environments is likely to not need additional life support systems

However, even total immersion in fluid has limits:

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.

Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G.[55] Extending acceleration protection beyond 20 G requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists.

Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application.[2]

So long as we are limited to an organic brain, it seems that the upper limit to acceleration would be @ 20 g.

  • $\begingroup$ Current record holder is 31.25G. $\endgroup$ Sep 12 '17 at 17:51
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    $\begingroup$ Is that instantaneous or sustained? Since there are some issues with fluid immersion no actual experiments have been done with humans to date to validate the 20G estimate. $\endgroup$
    – Thucydides
    Sep 13 '17 at 4:25
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    $\begingroup$ From another article, "In a milestone experiment in 1958, researcher R. Flanagan Gray climbed into the "iron maiden"; a human shaped capsule that was then filled with water, and withstood 31.25 Gs for a full 5 seconds, a human endurance record that stands to this day." It appears it was not sustained (although what "sustained" means is debatable, you can subject human flesh to a few pounds of force and, given enough time, destroy it....) $\endgroup$ Sep 13 '17 at 5:04

Enter - The Humble Woodpecker

The Woodpecker, of the Picidae family, is one of the few complex organisms whom surpass the requirements of this question by all parameters, with the ability to stay conscious while under a G-force of up to 1200 G's, yes, as in twelve followed by two zeros. This is impressive and all, but how can it stay conscious under 25 times the number of necessary to kill a human?

  1. The Woodpecker's skull is vary thick and spongy, and concentrated around the rear and forehead
  2. The Hyloid Bones are many times larger , and rap around the skull to for an single, sling shaped bone which holds the skull in place
  3. There is less space for the brain to rattle around , and the brain is positioned against the skull

These Modifications on a human being may allow the riders of these torch ships to operate at the given parameters

  • $\begingroup$ But for how long can they sustain it? $\endgroup$
    – Erik
    Sep 13 '17 at 12:37
  • $\begingroup$ I think you'll find the skull, not the brain, suffers 1200G. The point of the design of the woodpecker's skull is that the brain doesn't experience 1200G. $\endgroup$
    – Dan W
    Sep 13 '17 at 13:57

This cant work

If unallowed to alter brain physiology then this is impossible.

What causes people to faint at high G's?

-Blood flow becomes trapped in the legs and lower extremities causing the brain to become oxygen deprived. Or inversely, the same effect happens when too much blood goes to the brain and pools which at high enough Gs' can cause aneurysms.

To solve this you would need to reinforce the circular system to resist pressure and maintain circulation. As you cannot change the brain physiology what would happen is: you would have blood pooling and eventually bursting in rear of force facing regions of the brain.

Personally, you could justify using nano or micro machines to provide the reinforcement of the brain's circulatory structure as minimally invasive and easily reversible alteration to the brain's physiology. Although, at a 100G without some kind of structural improvement it's being squished against the skull comparable to hitting the ground after falling off the empire state building. No idea how the nerves can handle this, usually its blood in the brain that does the worst damage.

For giggles: I thought about using a strong enough magnet to control the water molecules. But the fidelity needed to make that work would in itself be insane let alone iron particles would be stripped from the blood causing again oxygen deprivation if not cellular shredding.

Aside from the brain problem, Micromachines could be used to reinforce the circulatory system as well as the other bones and organs so they dont explode or collapse beyond safe tolerances.

  • $\begingroup$ It appears that this answer caused the OP to revise his strict guidelines. In which case I go defer to micro machines reinforcing the body $\endgroup$
    – anon
    Sep 12 '17 at 17:41

I concur that 1 G will be perfect for not only travel in the solar system, but it can get you close to the speed of light in a year. The immersion solution is interesting and it would act like a fight pilot's G suit, but I can't see it solving bone breaking forces similar to a crash at 100 Gs. In addition, it would be equivalent pressures to being thousands of feet under water.

I think the anon poster's magnetic field isn't for "giggles" only. It should be a practical approach for short duration's of high G. It makes use of the fact that many atoms are weakly repelled by a magnetic field (diamagnetic).

Water and carbon are both repelled, constituting a significant part of the of all carbon based life forms. A frog has been levitated with a strong magnetic, which corresponded to 1 G only:


Now anon also mentioned iron particles being stripped from the body. They are mostly in the blood's hemoglobin. How long could it handle 10 Gs? I don't know. Perhaps a 100 Gs wouldn't be too problematic for the time period needed to get up to speed. The description mentioned that max speed is 0.001 c. As CatGut mentioned, at 100 Gs, it will only take 5 minutes to get up to speed. That kind of acceleration would be good for running away from something.

A different point to ponder. Cars and aircraft have max speed limits. Why do you want to say there is a max speed limit? Spacecraft under Newtonian mechanics will be limited to how much energy they have access to, and how much mass they can fling behind them (depending on your definition of torch ship). You can always go faster by performing a gravity assist around a planet.

A strong magnetic beneath a crew member would encounter the inverse square law where doubling a given distance from the source would result in one quarter of the force. The crew members would need to be inside the magnetic coil of some sort. A spaceship accelerating at 100 Gs could be compensated with 99 Gs of magnetic force, giving the crew members a comfortable 1 G of gravity like acceleration.


Assuming that you have a science fantasy setting where you have ridiculously high accelerations, the most obvious solution is to digitize the passengers and crew.

However, if you must accelerate human bodies, but allow modifications, then the solution to this problem is a variety of fluid immersion where the body is immersed and ports are placed in otherwise sealed or low-fluid-transfer-rate bodily cavity walls to allow rapid pressure equalization. The immersion fluid would have a density as close as possible to the average density of soft tissues or a bit more. Soft tissues with a lower density could be artificially made denser by adding mass at a nanoscopic level of detail in order to distribute it as evenly as possible.

Where we have significantly denser hard tissues such as bone, it should be interwoven with and partially replaced by a strong light substance such as carbon fiber at a microscopic level. Numerous sockets would be built into the bones and ports placed in the skin so that under acceleration, the bones can be mechanically anchored to an external structure to prevent differential movement within the body.

In order to further equalize the density difference between the body's fluids and solid structures, a significant amount of the oxygen-16 in the water in the body could be replaced by oxygen-18, and if necessary Hydrogen could be replaced by deuterium.

With bones strengthened and anchored, and density equalized as far as possible, the possibility exists that humans could endure very high accelerations in fluid immersion tanks, potentially on the order of hundreds of g.

Blood would have to be artificially oxygenated and nutrients supplied intravenously, and the brain would have to be hooked up to a direct neural link VR rig in order to prevent psychological trauma from being locked inside an acceleration tank, as well as to control the ship, since having all bones anchored and immobilized would prevent manual control.

Body modifications such as piercings or surgical implants like joint replacements, gastric bands and pacemakers would have to be removed or anchored along with the subject's bones - imagine the consequences of applying a 500g+ acceleration to an unanchored earring with a mass differential of several grams - a 5g gold earring might effectively weigh at least 2.3 kg under such conditions and cause quite a bit of damage.


Human jelly:

Basically you would need to make the human body denser, and more homogeneous, less the modern bag of dirty water and hard structural members and more of a firm, thick jell that is less compressible and contains less cavities. That's the altering humans side, you then put that new and improved human into a high viscosity, extremely dense fluid balloon. The liquid should match the new body for stiffness while having a density just low enough to give buoyancy, this will give you the maximum possible protection from G loads. I don't know how far you could go with this kind of system, these days we're limited to near-water density and viscosity and can provide about 20Gs of load protection The Forever War suggests a similar system that, from memory, uses only a slightly elevated viscosity and density and a lot of pressure and protects from up to several hundred Gs of acceleration during computer controlled combats.


Our squishiness is the obvious problem.

We are completely dependant on fluid transfer and our bodies would probably undergo centrigual water-oil separation at the cellular level! We would have to rectify our squishiness completely and properly solidify ourselves. This very likely removes the option of remaining conscious. A fateful option would be to explore instantaneous flash-freezing, (emphasis on the 'instantaneous') - an enhanced form of cryogenesis which probably carries a few risks on its own.


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