Nobody knows. We have lots of experiments in zero gravity on the ISS. Lots of knowledge of 1 G on Earth. Lots of understanding of hypergravity also, which we can simulate on Earth and is of great importance for jet fighter pilots.
But we know almost nothing about low gravity. Can't be simulated for any length of time on the Earth. Experiments in a centrifuge are of limited value as they are always hyper gravity due to the Earth's gravity acting along the axis.
We know that it is hard to adapt back to full Earth gravity from zero g. But that also is only based on limited time in zero g. Longest period anyone spent in space is less than two years. What about say 10 years in zero g? Again nobody knows. And because of all the things that go wrong in the human body in zero g, nobody can say definitively if a human can survive two years in zero g (nobody yet has). There might also be a lot of individual variation.
I'm going to be deliberately challenging in this answer to get you thinking. So will make some suggestions of possibilities that some may find quite outrageous. Okay- but can you really disprove them? Many of these I don't think likely. But until we get experimental data - how can we know for sure? That's the question I'd like to ask in this answer.
So you can suppose almost anything here and nobody can say you are definitely wrong. Or right.
Some of those suggestions may seem a bit out on the limb and outrageous - but - though we can make educated guesses, who can flat out deny any of them?
It's not just the bone loss which is the issue everyone knows about. If it was just that then it could be reasonably thought to be a continuous and more important, monotonic (no changes of direction) curve of some sort. You would then get low gravity intermediate levels of bone loss between ordinary and zero g..
But even that's based on no observational data at all. Again can hypothesize just about anything.
And who can say you are wrong, for sure, without data to test the hypothesis?
That's just not how science works. You make hypotheses. But you need then to test them, with at least some data to make sure you are on the right track.
Human bodies are so complex and we can't get near to simulating them with computer models. All we can do is experiment and zero gravity turned up many surprises. Low gravity may also.
The problem is that biology is not linear, doesn't even follow nice smooth curves. Given two points, at zero g and at full g, and a bit of a tail of extra points in hyper gravity - you can't conclude much about what happens in between. And are many different things to take account of, in the complex system which makes upu the human body - and they probably each respond in different ways to variations in gravity levels. And then interactions between them may involve delicate balances between these different effects.
So many things are different at low g.
With so many distinct systems affected, the curve could easily be non monotonic with maxima or minima, or both, and could have chaos type discontinuities and catastrophe regions as well.
E.g. resting heart rate. It is elevated at zero g. So we have two data points there.
Without extra information, we have no way of knowing what it does in between. Does it increase steadily as you reduce the gravity from full to zero g? Or does it say decrease to start with and then increase? Or increase to a maximum then decrease again to the measured faster rate at zero g? Or several maxima and minima? Or some discontinuous curve with steps?
And - that's all experimental observation. Not that somebody predicted all these things and their predictions were confirmed. Rather they tested astronauts and that's what they found out.
We do have some data from the Apollo astronauts at lunar g for a few days for each mission, I understand. But not a lot of that, it wasn't primarily a medical mission to study the effect of lunar gravity on the human body - they had heart beat measurements but not like they were wired up with instruments all over to measure things. And what's more the data is all privately held by NASA and independent researchers can only access it after the astronauts die. And it is only for a few days so doesn't tell us much about long term missions.
I think this might be a helpful way of putting it:
I should say - I'm a mathematician not a medical researcher. Just base it on what I've read about space medicine, that they don't seem to know the answers here. And talking about what is mathematically and logically possible.
I've said in other answers that I don't think we will send humans to Mars anyway at least in the near future for planetary protection reasons - and technical reasons to do with life support - but in this answer won't go into that, just directly answering your question.
There are experiments we could do quite easily, in space, with tethered spacecraft, and with short arm centrifuges within modules of the ISS, that nobody does. Those could answer many of these questions. We could have done these experiments at any time from the late 1960s onwards.
For instance, simulate lunar gravity for a month in orbit, and monitor an astronaut in those conditions, and now you know your answer, of course all ready to return them to Earth if any problems arise.
But until we do these experiments, if we ever do, it is just informed guesswork. Which, historically, often goes wrong.
So safest thing is just to say "Nobody Knows".
But we know almost nothing about low gravity. Can't be simulated for any length of time on the Earth. Experiments in a centrifuge are of limited value as they are always hyper gravity due to the Earth's gravity acting along the axis.
We know that it is hard to adapt back to full Earth gravity from zero g. But that also is only based on limited time in zero g. Longest period anyone spent in space is less than two years. What about say 10 years in zero g? Again nobody knows. And because of all the things that go wrong in the human body in zero g, nobody can say definitively if a human can survive two years in zero g (nobody yet has). There might also be a lot of individual variation.
I'm going to be deliberately challenging in this answer to get you thinking. So will make some suggestions of possibilities that some may find quite outrageous. Okay- but can you really disprove them? Many of these I don't think likely. But until we get experimental data - how can we know for sure? That's the question I'd like to ask in this answer.
So you can suppose almost anything here and nobody can say you are definitely wrong. Or right.
- That lunar, or Mars gravity is better for our health than full Earth gravity, and that humans will live longer in low gravity, and keep in excellent health, live to 200 or whatever (in science fiction this thought leads to the idea of "retirement homes on the Moon").
- That lunar or Mars gravity is worse for our health than full Earth gravity and people will live a few decades there before they die, and children not be able to grow up to adulthood.
- Worse even than zero g, e.g. that you wouldn't survive even a month at low gravity (I know astronauts manage for months, even nearly two years on the ISS, but this is low gravity, not zero g. Does the same apply?). Personally I think this is very unlikely. But can you rule it out?
- That it is not possible to give birth in low gravity, babies always die before birth or are deformed or have serious physiological problems (we have no data from zero g here to use)
- That humans can give birth in low gravity no problem at all
- That birth in low gravity is easier than birth in full gravity and leads to healthier babies on average.
- That you can't adapt back from low gravity to Earth gravity after a year or two there, at all (worse than zero g)
- That you don't notice anything and could just step into Earth gravity and after a moment or two of disorientation just walk normally (better than zero g)
- That it takes months to adapt but you do eventually (same as zero g)
Some of those suggestions may seem a bit out on the limb and outrageous - but - though we can make educated guesses, who can flat out deny any of them?
It's not just the bone loss which is the issue everyone knows about. If it was just that then it could be reasonably thought to be a continuous and more important, monotonic (no changes of direction) curve of some sort. You would then get low gravity intermediate levels of bone loss between ordinary and zero g..
But even that's based on no observational data at all. Again can hypothesize just about anything.
- Bone loss intermediate between zero and full g
- Bone loss worse than zero g.
- Bone loss negligible, same as full g.
And who can say you are wrong, for sure, without data to test the hypothesis?
That's just not how science works. You make hypotheses. But you need then to test them, with at least some data to make sure you are on the right track.
Human bodies are so complex and we can't get near to simulating them with computer models. All we can do is experiment and zero gravity turned up many surprises. Low gravity may also.
The problem is that biology is not linear, doesn't even follow nice smooth curves. Given two points, at zero g and at full g, and a bit of a tail of extra points in hyper gravity - you can't conclude much about what happens in between. And are many different things to take account of, in the complex system which makes upu the human body - and they probably each respond in different ways to variations in gravity levels. And then interactions between them may involve delicate balances between these different effects.
So many things are different at low g.
- Heart beats faster - resting heart
- Possibly greater risk of heart attack due to faster resting heart rate
- Blood pools in the head and upper body
- Red blood cell count goes down
- Can't lose heat by convection so sweat more
- If you exercise to stay healthy you sweat even more
- Magnesium loss (on-going) due to sweating
- Eyes affected, for many (not all) astronauts
- Internal organs function differently.
- Immune system affected
- It's hard for astronauts to get enough food.because they don't feel so hungry
- They also don't feel so thirsty and get dehydrated.
- Medicine doesn't work the same way, has to be injected rather than taken as pills because of the changes of function of internal organs
- It's now known that even some cells behave differently in zero g at the cellular level (a surprising result as they were thought to be too small to be affected by gravity)
With so many distinct systems affected, the curve could easily be non monotonic with maxima or minima, or both, and could have chaos type discontinuities and catastrophe regions as well.
E.g. resting heart rate. It is elevated at zero g. So we have two data points there.
Without extra information, we have no way of knowing what it does in between. Does it increase steadily as you reduce the gravity from full to zero g? Or does it say decrease to start with and then increase? Or increase to a maximum then decrease again to the measured faster rate at zero g? Or several maxima and minima? Or some discontinuous curve with steps?
And - that's all experimental observation. Not that somebody predicted all these things and their predictions were confirmed. Rather they tested astronauts and that's what they found out.
We do have some data from the Apollo astronauts at lunar g for a few days for each mission, I understand. But not a lot of that, it wasn't primarily a medical mission to study the effect of lunar gravity on the human body - they had heart beat measurements but not like they were wired up with instruments all over to measure things. And what's more the data is all privately held by NASA and independent researchers can only access it after the astronauts die. And it is only for a few days so doesn't tell us much about long term missions.
I think this might be a helpful way of putting it:
- What is the optimal gravity level for human health? Is it full g, or slightly less, or a lot less?
- And if less than full g, how much healthier are humans at this gravity level? Does it extend our lifespan? Is it healthier for old people as some science fiction writers have optimistically suggested? E.g. because heart doesn't have to work so hard. (Or is it worse for old people, for very same reason e.g. that heart needs to be exercised for health ?)
- What is the worst level for human health below 1 g (hyper g of course kills you if it is high enough)
- Is it zero g, or something higher than zero g? If higher than zero g, could it be lunar or Mars g?
I should say - I'm a mathematician not a medical researcher. Just base it on what I've read about space medicine, that they don't seem to know the answers here. And talking about what is mathematically and logically possible.
I've said in other answers that I don't think we will send humans to Mars anyway at least in the near future for planetary protection reasons - and technical reasons to do with life support - but in this answer won't go into that, just directly answering your question.
There are experiments we could do quite easily, in space, with tethered spacecraft, and with short arm centrifuges within modules of the ISS, that nobody does. Those could answer many of these questions. We could have done these experiments at any time from the late 1960s onwards.
For instance, simulate lunar gravity for a month in orbit, and monitor an astronaut in those conditions, and now you know your answer, of course all ready to return them to Earth if any problems arise.
But until we do these experiments, if we ever do, it is just informed guesswork. Which, historically, often goes wrong.
So safest thing is just to say "Nobody Knows".
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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