# How would this astronaut-in-training know she was in a pool on the Moon and not on the Earth?

Many are familiar with the scenes of astronauts-in-training practicing weightless activities while submerged in a pool with neutral buoyancy.

In this scenario, an astronaut is in such a pool, undergoing such training. This pool is completely contained, essentially a large cube completely filled with water. There is a hatch for egress at the top, usually closed. There is no deck around the pool, no grating or stairs. The water in the cube, however, is not pressurized. It is open to the 'atmosphere' through vents at the top. There are windows around the top, bottom, and sides, for viewing - both from the inside out, and the outside in. Air and communications are continually supplied to her through an umbilical chord. She has a complete astronaut's suit on, fully sealed, pressurized, and self-contained.

So, during one session of her training, this astronaut 'blacks out'.

When she regains consciousness, she looks around and absolutely everything appears to be the same. Same pool, same support swimmers in full gear (she can not see their faces) around her, same environment beyond the pool through the windows that she can see. It appears from her suit chronometer that she was unconscious for only a few seconds.

At what point could she recognize that she was no longer on Earth, but had been transported to an exactly identical facility on the Moon, while unconscious?

EDIT

There seems to be some confusion about 'neutral buoyancy'. She is the same density as the water. Her pressurized suit, her apparatus, and the density of the water have all been manipulated so that the mass of the water she displaces is exactly equal to her total mass. She neither floats nor sinks. The idea is, that while she is in the water, she is essentially experiencing weightlessness. Any buoyant force pushing her up is exactly countered by the force of gravity pulling her down. When she moves, F=ma is gravity independent. If she moves, the mass of the water she displaces or 'moves aside' is exactly the same mass as the part of her body that is replacing it. Inertia is exactly the same, Moon vs Earth. It requires the same force for her to move in the water on Earth as on the Moon.

The buoyancy force is zero. Be very careful if using formulas and equations, you do not fall into the 'divide by zero' trap. 8-8=0, and so does 5-5=0. If both sides are equal, then no matter how big or small both sides are, the result is still zero. One answer is not bigger or smaller than the other.

Thus, for an acceptable answer, although the tags do not require it, any use of formulas used to 'prove' discrepancies between the situation on Earth and the Moon should include realistic numbers, and a numeric result for Earth and Moon calculations that can be compared. The numbers used have to reflect neutral buoyancy.

Also note, she can not leave the water. There is no 'head room' above the water. She is always 'in' the water.

Note also that she is in a pressurized suit. The suit pressure is automatically adjusted, so it is the same pressure in the water on the Moon as on the Earth.

SECOND CLARIFYING EDIT

She has no particular reason to suspect that she is no longer on Earth. This is story-dependent, and therefore was not specifically mentioned. She was sedated on the trip. Subtle differences would not make it to her conscious level. It has to be an 'in your face' difference for her to become aware of it. WHY and HOW she has been moved is story-dependent and beyond the scope of the question. However, what is perhaps relevant to the question (in retrospect) is that there is no motivation for her to interpret any subtle differences as being the result of her being on the Moon vs on the Earth. They have to be very noticeable differences that can not be attributed to anything else, in order for her to become aware of them.

THIRD CLARIFYING EDIT

Although this is also plot-dependent, and part of the story line (i.e. not normally relevant), I should clarify that those who brought her to the Moon definitely do not want her to know she is on the Moon. That was the purpose of causing her to 'black out'. However, that is not necessarily relevant to an answer. It is the 'neutrally buoyant' part that is the plot-independent, story-line-independent factor, not anything else that may or may not happen. I am not after an answer that can be 'contrived' by the story line. I am after an answer that can stand independently of any story line.

FOURTH CLARIFYING EDIT

I have found this article as a potential resource. It is a study regarding neutral buoyancy tanks and virtual reality headsets to simulate the 'real thing'. The research was funded/sponsored by NASA.

There are many advantages of training at the NBL. For example, astronauts become accustomed to being confined in the bulky spacesuits. Perhaps more importantly, they experience the sensation of floating as they would in zero-gravity. This is achieved by maintaining neutral buoyancy, meaning that the astronauts do not float to the surface or sink to the bottom. Neutral buoyancy is a good analog for zero-gravity because common sensory cues to body orientation are rendered uninformative. These include somatosensory cues that provide information about pressure on the skin as well as proprioceptive cues that provide information about joint articulation and muscle tension. Both types of cues normally provide information about how the weight of the body is supported. While underwater, only vestibular cues from the inner ear remain to provide reliable non-visual information about the direction of gravity. In this altered sensory environment, astronauts gain valuable experience not only maneuvering in the spacesuits, but also practicing novel locomotion methods.

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– L.Dutch
Apr 7, 2021 at 15:02

The astronaut would know right away.

The buoyant force can be expressed as $$F_B = \rho_f g V$$, where $$\rho_f$$ is the density of water (about 1000 kg/$$m^3$$) this quantity would not change on the moon. V is the volume of the astronaut, like density, her volume would not change from simply being transported to the moon. Since the gravitational acceleration on the moon is about 1/6th that of Earth's, the buoyant force she would experience would be about one sixth the buoyant force she previously experienced.

The total force she experiences in the pool is

$$F_{T}$$ = mg - $$F_B$$ = [m - $$\rho_{f} V$$] g

Which also differs from Earth's by a factor of 1/6th due to the factor of gravitational acceleration, "g". This would correspond to a very noticeable change of gravitational acceleration when moving around in the pool of water. Everything would sink much slower and surfacing would become much easier.

After reading through the comments on both the OP and my post it would appear that there are some common misconception being applied here so I'd like to address them here.

[Neutral Buoyancy]

The simple equation I showed above is valid, both physically and mathematically in the case of neutral buoyance which simply is when $$\rho_f V = m$$. There is no "divide by zero error" this is a simple relationship which states that the net external force on a body is zero and thus the body (the astronaut) will not experience acceleration. Of course, in the case of permanent neutral buoyancy (as required in the OPs edits) the difference due to gravity will not be observed kinimatically strictly for motion within the water. If there are no objects to move around, and the astronaut cannot surface it will be much more difficult to observe the effects of gravity.

Neutral buoyancy, however, does not mean that the buoyancy force is zero, it means that the buoyancy force is exactly equal to the force of gravity. Nor does it mean that there are no forces on the astronaut, rather it means that the sum of the forces is zero (more on this below).

[Neutral Buoyance as a simulation of zero-g or low-g environment]

As stated in the attached article excerpt in the OP, neutral buoyancy can be an important tool to mimic certain aspects of low-g/zero-g environments. It must of course be understood in context, however. For instance, the article (bolding my own) states:

...Neutral buoyancy is a good analog for zero-gravity because common sensory cues to body orientation are rendered uninformative. These include somatosensory cues that provide information about pressure on the skin as well as proprioceptive cues that provide information about joint articulation and muscle tension. Both types of cues normally provide information about how the weight of the body is supported...

Drawing attention to what the article is not saying, neutral buoyancy does not mean that you do not feel your own weight.

In the OP, in the edits, it is stated:

She neither floats nor sinks. The idea is, that while she is in the water, she is essentially experiencing weightlessness

This is not entirely correct, you do in fact experience internal forces which support your own weight, this is because your body has to support its structure against the force of the water supporting you against accelerating under gravity. Of course since water distributes that force over your body, you have less sensation of the direction gravity is acting as the article excerpt states. However, since the astronaut will still (even in neutral buoyancy) feel 1/6th the total force as on Earth, this apparently sudden difference would be noticed. EDL does an excellent job of discussing how this weight difference would play on equilibrioception. To add to this aspect of weightlessness (or near weightlessness) would include some version of Space adaptation syndrome which has numerous physiological effects including the distribution of fluids in the body and disorientation.

• But can this not be compensated for? The density of water can be chemically altered. The 'weight' (mass) of her and the displaced water would be equal, on Earth and on the Moon, regardless of the gravitational force. The 1/6 gravity would be the same on both her and the water. I see nothing in your equations that, given g is the same for both astronaut and water, would make a difference ratio wise. The masses remain in the same ratio. It is neutral buoyancy, so she neither sinks nor rises. Apr 6, 2021 at 3:14
• @JustinThymetheSecond If just sitting there floating, yes, no difference could be detected, but moving is a different story. Thus, just ratio-wise, no, but kinematically, yes. The low acceleration applies everywhere, to everything, on the Moon as on the Earth. Earth's acceleration is a very innate thing to us because we have lived all our lives under the same acceleration. For example, when we drop an object, we instinctively try to account for gravitational acceleration when going to catch it. Thus the second the astronaut moves the illusion will disappear. Apr 6, 2021 at 3:55
• @JustinThymetheSecond Replacing water with a denser fluid would be noticeable when moving as well since swimming would mean moving more mass and thus the reactive force difference would be recognized. Apr 6, 2021 at 3:56
• @JustinThymetheSecond, because making the water denser would increase its viscosity. You might be able to make it look like objects don't rise as fast because the liquid is denser, but that also means that sinking objects are moving even slower because of a combination of the lower gravity and increased density. You could fake the sinking by lowering the density of the fluid (somehow), but then rising doesn't work like it should, and you can't obviously raise and lower the density at the same time. And, of course, as soon as she moves against the resistance of the fluid, the game is up. Apr 6, 2021 at 6:34
• @Frostfyre No argument there. That is essentially 'by definition'. It does not follow that viscosity has to change noticeably. Raising or lowering the temperature can change viscosity. Would someone be able to determine the viscosity was changed by temperature, density, or other factor? 'Hey, I can move my hand millimeters per hour faster!!'? Or 'Hey, it takes half a calorie more to move my hand!!!!" Apr 6, 2021 at 13:57

On Earth, her inner ear, assists in feeling her balance since it partially aligns with gravity.

But in a significantly different gravity, one where humanity didn't evolve, that sense will be significantly altered. I expect if she stood perfectly still, she'd feel that familiar sense of vertical we get being on Earth since its the fluid motion around and through the hair follicle like 'sensors' of our inner ear. But when she moved, the lower gravity means the fluid is more free to slosh around, and generate less forceful stimulus, and takes longer to dissipate its energy and return to rest. That would be a very noticeable sensation and feel disorientating.

Astronauts on the International Space Station -- zero g -- experience this effect reported in this article from NASA.

And, yes, for all the middle children out there, this is reporting on zero-g not low g. We haven't been to the moon in a good long while so there isn't a lot of research on the topic, so this is a case of reasoning by extension. But it is supported by some still open research questions in these journal articles

Similarly, the weight of your face and fingers, would feel different in low-g v. 1 g. You can feel gravity if you let the muscles of your face go slack, or let your fingers or wrists go loose. The natural tension of ligaments and tendons and muscle tissue will find a new balance point against the pull of 1/6th g and that will feel different -- a different position -- than in a 1 g field.

Another thought is to image how water in low gravity behaves, as is surface tension can start dominate its shape rather than gravity. N.B. I can't find any info on the surface tension of endolymph so this is speculative suggestion

• Can you provide references to this actually happening in low (vs. no) gravity? I don't recall astronauts mentioning that their balance was affected while on the Moon; further, it's my understanding that the semicircular canals are mainly inertia-driven gyroscopic organs, rather than gravitationally (or straight-line acceleration) driven. Apr 6, 2021 at 15:08
• It is an interesting suggestion. Standing 'perfectly still' vs 'standing perfectly vertically still' bit. I do not know offhand about any studies done on astronauts as to whether these inner ear changes are sufficiently noticeable as to be attributable to changes in gravity. They do need to accommodate their balance to weightlessness, and they often experience seasickness or nausea. I suspect divers would experience some discrepancy between buoyancy and balance. Is the sensation great enough and different enough for her to attribute it to being on the Moon, vs just being in water, floating? Apr 6, 2021 at 15:09
• Experiments have shown that it requires a minimum of 0.15g for the vestibular system to be able to perceive up and down correctly (newscientist.com/article/…). At 0.17g, the moon is very close to that line, which is why you'd see astronauts suddenly seem to lose their balance while moving around: their body could literally not perceive its orientation. Even in neutral buoyancy humans in a pool would still know which way was up. If she shut her eyes and couldn't tell, she'd know something was wrong, even if only with her. Apr 6, 2021 at 17:41
• @JustinThymetheSecond I think you should view this answer as screaming that you're not on the Earth anymore, but not that it screams about where you are now. Once you've determined that something is up, it's easier to do some experimenting to figure out what that something is. Apr 6, 2021 at 18:24
• @JustinThymetheSecond, you're treating neutral buoyancy as if it were the absence of gravity. It is not. Gravity still affects your inner ear, and if you aren't being tossed around, you will know which way it up. And there's another factor you are overlooking: the suit she's in may be neutrally buoyant in the pool, but she's not neutrally buoyant in the suit. If she's wearing one like is currently used, there's room between the suit and herself, and she'll note that she's not being pressed downward nearly as strongly as she had been just a minute before. Apr 6, 2021 at 21:03

The pool is open to air at Earth-ambient pressure at the top, even though there is not headroom there to allow exiting the pool other than via the hatch.

As soon as she looks up, she'll seem something isn't right -- because the surface of the pool will have taller and slower-moving waves than she's used to seeing from underwater. As set forth here, wave propagation speed depends on the wave period, and lower gravity will give waves longer period, hence slowing them down.

This is because waves are influenced by gravity; the lower the gravity, the higher a given amount of kinetic energy (water movement) can lift the surface, and the slower that lifted water comes back down. Whether she can feel the difference in gravity or not (possible, but I'm uncertain of it in a neutral buoyancy setup like this, especially if she's been in the pool for a while), the difference will be very readily visible as long as the water has a surface that's visible to the trainee.

Every move the trainee (or divers) make will create waves and ripples on the water surface. Even if not consciously noticed, the (to the trainee) sudden change in the character of the waves, their shadows and refraction will be instantly noticed as "something changed." Safety sense (trained in to pilots long before they become astronaut trainees) will then result in a careful check around, likely in talking to "mission control" as well.

• The wave idea is interesting. But first there has to be something creating the wave, something she already has had experience with, and knows how high and fast the wave should be. Normally, in such training, there is no reason to 'make waves'. In fact, I suspect it would be discouraged. Light ripples reflecting on the bottom of the tank from the surface waves, perhaps, like in a ripple tank? Will the wave travel 'shadow' be slower or faster moving along the bottom? This has possibilities. Apr 6, 2021 at 14:55
• Every move the trainee (or divers) make will create waves and ripples on the water surface. Even if not consciously noticed, the (to the trainee) sudden change in the character of the waves, their shadows and refraction will be instantly noticed as "something changed." Safety sense will then result in a careful check around, likely in talking to "mission control" as well. Apr 6, 2021 at 15:03
• So, do the wave ripple shadows propagate slower or faster? I agree with the 'back of the mind' safety consciousness thing. They are trained to not disregard 'intuition', but they are trained to manage it to prevent paranoia. Apr 6, 2021 at 15:11
• The waves themselves propagate slower under lower gravity. The same amount of energy lifts the water higher, and it falls back more slowly. Apr 6, 2021 at 15:47
• That part I understand. The height would be the wave 'amplitude' and the rate of the rising and falling determines the 'frequency'. But does frequency translate into wave speed? One blip per second and ten blips per second can still propagate at the same speed in a ripple tank. I suppose the waves would be higher amplitude, lower frequency, longer wavelength, and that would appear that they are traveling slower. Like the speed of the record was slowed way down and volume was turned up. I-I-I-I----b-b-b-u-u-u-r-r-r-r-i-i-i-i-e-e-e-e-e-d-d-d-------P-P-P-P-a-a-a-u-u-u-u-l-l-l-l- type thing. Apr 6, 2021 at 16:12

The astronaut should immediately be suspicious that something has happened.

If she's transported to the moon too quickly, then she's immediately going to notice that the pressures on her body from gravity are significantly different. However, unless she's teleported instantaneously then she'll already have some of the side effects of low gravity - https://www.businessinsider.com/how-body-changes-outer-space-2015-10#9-it-messes-with-your-senses-9. When she wakes up she'll likely have a stuffy nose, which will conflict with the idea that only a few seconds have passed. Also, #8 in the above list mentions that a person's vestibular system is affected by changes in gravity.

Together, the effects of unexpected low gravity would likely feel like vertigo. Her first thoughts would likely be related to having blacked out, and a logical conclusion would be that she might be having some sort of health emergency. If she starts to panic and her heart races, more blood will flow to her head than she's used to. Again, this could be misinterpreted as a health emergency.

The key point is that there is no real way for her to be tricked into believing that nothing has happened. How long she panics about her health vs looking for other explanations is going to be heavily dependent on her personality.

Also note that any significant changes to the density of the water will be noticeable due to inertia. If you have to make the water twice as dense then you will need to make the astronaut plus her suit be twice as dense as well, and all of that extra mass will need to be added to her suit. That will result in a significant change in how hard it is for her to move around, though once again this could be attributed to muscle weakness due to a health emergency.

After the astronaut is calm enough to think things through carefully, one thing she can do is to experiment with various objects in the pool. Neutral-buoyancy pools are often used to prepare for spacewalks, so they have mock-ups of the outside of the shuttle or space station for them to work on. She may be neutrally buoyant, but all she needs to do is find one object that isn't. Once she has found that object, she can swing it around a little to get a feel for how much mass it has, and then drop it to see if it behaves as expected. When she sees that it falls significantly slower than expected, she will be able to confirm that she is somehow no longer experience normal Earth gravity.

It will be quite hard for her to determine exactly where she is. Using the densest objects she can find she can do a ballpark estimate of how strong gravity is (i.e it takes a heavy object about 6x as long to hit the bottom means gravity is about 1/6th normal). However, I can think of only one way to tell the difference between being on the moon and being in a spaceship that is accelerating at 1/6 g - unless she was abducted by aliens the spaceship has a limited amount of fuel and so will eventually stop accelerating.

• Astronauts-in-training do not panic, or they do not remain astronauts-in-training. The trip from Earth to Moon is story dependent, and out of scope. The reason she was given by 'capcom' to explain the blackout is beyond the scope. Again, I emphasize, she has absolutely no reason to suspect anything. it is a normal training exercise. She has no need to attribute any particular anomaly to being 'on the moon', that can not be reasonably explained by some perturbation of the Earth-based training exercise. Astronauts-in-training routinely get vertigo. It is intentionally part of their training. Apr 6, 2021 at 17:29
• You do not need to make ANYTHING 'twice as dense'. The masses remain the same, Earth vs the Moon. Any necessary manipulation for necessary equipment changes would be undetectable. Perhaps a calorie or two of muscle expenditure. Neutral buoyancy is the same, no matter what the gravity. Neutral on Earth, neutral on the Moon, for the same conditions. Mass displaced equals mass doing the displacing. Inertia is F=ma, no gravity necessary. A body of a given mass has the same inertia in deep space as it does on Saturn. Apr 6, 2021 at 17:38
• @JustinThymetheSecond I'm giving you reasons why the trip from Earth to Moon is actually relevant for being able to give you a good answer, along with the reason given to explain her blackout. If you feel like these aren't significant concerns then it's because you have information that you have not yet shared with us. She is feeling immediate physical side effects which, while possibly something she's felt as part of her training, wouldn't be a part of the training taking place in the neutral buoyancy exercise. Apr 6, 2021 at 17:41
• @JustinThymetheSecond regarding the density change, I think I misunderstood "... and density of the water have all been manipulated" to mean that there could be a significant change in the density rather than just tweaks to maintain neutral buoyancy. Apr 6, 2021 at 17:43
• A lot of those details are back story, related to plot. This is taking two points in the plot, ignoring everything in between as irrelevant. I have been criticized in the past for giving out too MUCH back story. Given just her experience in the tank on Earth, and her experience in the tank on the Moon, is all that is relevant to her. It is all she knows. She knows nothing about what happened in between. As far as she is concerned, nothing happened in between. Any effects can be passed off by capcom as artifacts of her black out in the first place and 'just get on with the training'. Apr 7, 2021 at 3:27

There is a sense called proprioception that tell you where you body is located relative to itself. Multiple sensory detectors are involved in forming this sense, one of which would be completely unaffected by external forces by definition!

Normally functioning proprioception contributes to balance by providing kinesthetic feedback of the extent of head and limb movements through somatosensory signals from musculo-tendinous receptors in the neck and joints.

Musculotendinous receptors-

Golgi tendon organs- sensitive to stretch of the tendon due to muscular contraction and causes the muscle to relax.

Muscle spindle- responsive to active or passive stretch. as well as the rate (phasic) and length (tonic) of stretch.

If the position she wakes up in is different from the her position when she is knocked out by even millimeters, she will notice. When a car stops, there is the smallest little jolt no matter how slowly you decelerate. My guess is that the change in proprioception would be interpreted similarly.

P.S. That's a pretty neat premise for a story. It is hard to think of a case with less evidence available.

• Proprioception is interesting. There would be small, subtle differences. but would it pass the 'noticeable' test, or would it just be 'contrived'? She did black out (in the plot, due to 'bad guy' drugs, but 'explained' by Moon capcom' as 'oooops excessive anti-nausea meds'), she is predisposed to think 'space', has no reason to believe 'not in simulation', and capcom on the Moon has the standby 'artifact of the blackout, all our systems read normal, continue training' prepared response. If I need 'contrived' I can go with it, but I was hoping for something more scientifically substantive. Apr 9, 2021 at 14:01
• The entire premise of the neutral buoyancy tank is to confuse and nullify the normal human proprioceptive feedback mechanism, to make it easier for the trainee to think 'weightless space'. The deception methods would be the same equally effective simulated techniques on the Moon as on Earth. It's all about 'Think space, not Earth (or Moon)' in both cases. Apr 9, 2021 at 14:06
• I fell like the upshot here is... that she will notice that she blacked out. That seemed implicit in the question, and doesn't seem related to what is actually being asked. Apr 9, 2021 at 15:04
• From her perspective, there was a change in sensory inputs that was not continuous. If you move your arm from the front of you to behind you, you still feel all the parts in between. She would not feel any in between parts when she wakes up. The blackout itself will give her a clue that something happened. She would probably contact capcom seconds after waking up, rather than after looking around. This is the first clue she could possible have. Apr 9, 2021 at 15:36
• Having the first clue being something the character, even the reader, ignore is just good writing. The 'jolt' caused by the non-gradual change to proprioception can be one such clue. "When she hit with the hammer, a sudden jolt travel though her whole body. 'Whoa, I must have so much more training to due. I wasn't prepared for that at all.'" One short story later "The kidnapped me! I knew that hammer didn't hit that hard!" Apr 9, 2021 at 15:46

## Forces from the suit would be hard to control for.

Forget the water for a moment. She's in a pressure suit. Outside could be guacamole or hard vacuum, but inside, she rests on the lining of the suit. If the suit is actually inflated - if it is like a bubble she could rattle around in - then you can imagine that she is down at the bottom feeling Earth or Moon gravity, and if she flipped it over, she would fall with Earth or Moon acceleration to the other side of the suit. Now to be sure, the suit can be cleverly designed so her body is under pressure from all sides and there are no voids to test and the pressure is more than the pressure of gravity. But it's hard to picture making that so perfect that there is no hint of indication. You have to rely on the idea that unconsciousness and disorientation during cave diving will cause a person to doubt their own senses.

## Diving is different.

Going down ten meters in the tank should double the pressure on the suit. If it were not pressurized, she would feel tremendous pressure in her ears and need to adjust frequently, while on Earth. (six times less so on the moon) Since the suit is pressurized, that won't happen - however, the tension on the suit will change, because it is under more pressure differential when she is higher in the tank. If she is experienced with this, and giving it the sort of thorough attention you'd expect from a trained diver and astronaut in training, you would think she could hardly fail to notice. This effect gets less the higher the pressure is in the suit - but the effect above should then be much more.

• Here is the concept of the space suit I had in mind. space.com/4074-slimming-future-spacesuits.html Nothing like the current suits, I mentioned this in a comment to another answer, in case you missed it. The pressure inside the suit is continually adjusted to maintain a constant pressure, A routine procedure in neutral buoyancy tank simulations. In space, the pressure does not change. Apr 10, 2021 at 0:14
• I have found nothing that explains how pressure is maintained and equalized in the astronaut training tanks. However, these astronauts do not seem to have any particular system to prevent the 'bends', although the recommended ascent rate is 60 feet per minute, so not a big issue in a 40 foot tank. I suspect they may slowly pressurize (and depressurize) the suit to the highest pressure expected. Apr 10, 2021 at 0:39
• However, the suits are specifically designed to have sufficient constriction so as to maintain integrity at one atmosphere difference between inside and outside (in the case of space, one atmosphere inside, zero outside). It is an interesting question - I have never read any sci-fi that describes a problem or change in sensation such as you suggest when the 'air lock' is pressurized and de-pressurized. The suit would experience much the same thig you describe going from 10M under water to the surface (one atmosphere). But there is nothing I have found in a search. Apr 10, 2021 at 0:46
• The things they do not tell you in a sci-fi novel. science.howstuffworks.com/spacewalk3.htm Apr 10, 2021 at 1:25