Can Astronauts Spend Years In Space - And How Quickly Can They Recover On Return To Earth?

The longest time anyone has spent in space is just over fourteen months. So far, astronauts have recovered surprisingly quickly, even after the longest duration spaceflights. Within a few weeks they are almost back to normal health. There are some longer term effects, on load bearing bones, which may take a couple of years to clear up, and very rarely, permanent effects on the eyes.

But what about longer periods in space than that? Their condition continually deteriorates for as long as they are in space and so far nobody has spent as long as two years in space; the record is fourteen months. So the question is still wide open for longer duration flights.

First let's look at what we know already.

REMARKABLY QUICK RECOVERY CONSIDERING THAT HUMANS DIDN'T EVOLVE IN ZERO G

An astronaut in space is actually in a rather poor way, would be considered quite ill, as if suffering from some degenerative disease, if they exhibited the same symptoms on Earth. Their muscles get weaker and weaker for as long as they are in zero g, they lose bone mass, and the way their body functions changes in various ways, not for the better.

But you can manage fine in zero g even if your muscles are very weak, so they only notice the effects when they get back to Earth.

Chris Hadfield, when he returned after 5 months in space, was not permitted to drive for 21 days. He also had to wear a G suit to keep blood in his head region. In zero g, your blood rushes to your head at first, with no gravity to pull it down. Your body adjusts to produce less blood, about a fifth less than it produces in full g, so when you get back to Earth you don't have enough blood supplied to your head.

At first he couldn't stand up in a shower. Things like that.

Chris Hadfield, Canadian astronaut, who talked about his experiences adjusting back to Earth gravity after his mission to the ISS

His return to Earth.

For his experiences on return to Earth see for instance: Astronauts Deal With Some Tough After Effects When Returning to Earth and Back on Earth, Chris Hadfield ‘tottering around like an old man'

But most of these bad effects go away quickly, within a few weeks or months.

It takes some years to recover from the bone loss. So, for a while your bones are a little more likely to break than usual. (You lose 1 or 2% of your bone mass a month, in zero g, from load bearing regions).

Also, many astronauts get eye damage, though usually minor and it clears up when they return to Earth. Occasionally the damage is permanent Spaceflight Bad for Astronauts' Vision, Study Suggests

EVOLUTION NEVER ENCOUNTERED ZERO G CONDITIONS

The human body never evolved to cope with zero g conditions. For the entire history of the Earth all animals, and even all microbes, have been living in full gravity and are adapted to it. Even microbes (surprising everyone) have differences in the ways they behave in zero g Bacteria In Space Grows in Strange Ways

We can only simulate zero gravity on Earth for a few minutes at a time (in Reduced gravity aircraft).

So, before the first human missions to space, in the 1960s, they sent monkeys into space. Before then, for all they knew, it was possible that humans would not survive even an hour in zero g.

When we found that humans had no trouble living in zero g for several days, this was encouraging, and meant a medical "go" for astronauts to visit the Moon.

Then later, when we found that humans can survive for months in zero g, with no major issues, this was even more encouraging.

LONGER DURATION FLIGHTS OF YEARS

However - that doesn't necessarily mean astronauts can spend years on end in space. Some longer duration spaceflights have been promising, others less so.

The longest duration flight of all was also perhaps the most encouraging. That's the flight by Valeri Polyakov who spent just over fourteen months in the earlier Russian space station MIR.

March 22, 1995: Longest Human Space Adventure Ends | WIRED

Valeri Polyakov in MIR at the end of his fourteen month (437 days) mission, the longest single space flight by a human to date.

He is a medical doctor. He was strongly motivated to keep himself healthy in space, as he wanted to prove that humans could survive a zero gravity flight to Mars.

He exercised for two hours every day throughout his flight. He was in fine condition right to the end, and he wanted to spend 24 months in space. And when he returned to Earth he walked a short distance straight away from the capsule to a nearby chair, as he wanted to prove that humans would be healthy enough to walk on the surface of Mars after they got there on a mission.

He did continue to lose bone mass for as long as he was in zero g. But his bone loss was remarkably low compared with other astronauts, only 0.5% a month for a total bone loss of 7%.

That figure is for loss in the most vulnerable area of your skeleton, the load bearing bones (you don't lose any bone at all in non load bearing regions of the skeleton such as the skull). Book Review: 'Leaving Earth'

Typical responses to zero g,. Valeri Polyakov experienced far less bone loss than this, possibly due to his 2 hours a day exercise regime. Effects of Long-Duration Spaceflight, Microgravity, and Radiation on the Neuromuscular, Sensorimotor, and Skeletal Systems

Valeri Polyakov on return to Earth. See 5 Astronauts More Badass Than Any Action Movie Hero

Was Valeri Polyakov someone with a remarkable physique for zero g? Or was his two hours of exercise a day the key? Would anyone find it okay so long as they exercise for that long every day?

It does seem that astronauts and cosmonauts who do most exercise have less issues with bone loss in space than others. Astronauts who spend six months or more in space are required to exercise for at least two hours a day. Exercising in Space

Also does the bone loss just continue, so that the body loses more and more bone indefinitely, or does it stop at some point?

Nobody knows. It might be that the bad effects level off to some extent, but it is also entirely possible that the human body just continues to deteriorate further and further.

If the effects are cumulative, it is possible that a sufficiently long period, of several years in zero g would be enough to kill any human, because of the many changes to our metabolism in space (not just bone loss). We just don't know. As with the early spaceflights that showed we can survive for days, then weeks, then months in zero g, we need to find out if humans can also survive for years in zero g.

We have started a new experiment, the first long duration spaceflights for some time. NASA astronaut Scott Kelly and Russian Federal Space Agency cosmonaut Mikhail Kornienko, are spending one year in the ISS, returning to Earth in 2016.

Mikhail Kornienko (left) and Scott Kelly (right) who is spending one year in the ISS - started in 2015.

Interestingly, Scott Kelly has a twin, Mark Kelly (a retired astronaut). This will help with the studies - Mark will stay on the Earth and the responses of his body will be compared with Scott.

Mark Kelly is the one with the moustache on the left. On the launch day he pranked the NASA admins by turning up to watch the launch with his moustache shaved off - the only way the admins could tell the two of them apart Astronaut's twin tricked NASA by showing up at space launch

Actually, this was not a planned twin experiment originally. They chose Scott Kelly for other reasons, they needed someone already trained, who had enough radiation exposure left for a one year mission. (Like radiation workers, astronauts are limited in the total amount of cosmic radiation they can be exposed to over a career lifetime). But afterwards, once he was selected, when they talked about genetic information, he came to them and said "what about doing twin experiments". One if by Land, Two if by Space.

It involves making the astronauts' data public, and these are bound by privacy restrictions because of laws such as the genetic non disclosure act, so it never crossed their minds to do a twin experiment. But when he suggested it, and said that they were both happy with sharing genetic data for the experiment - NASA were delighted to go ahead with it. (For more on this background, see fifteen minutes into this talk by Dr. Julie Robinson, ISS Chief Scientist on David Livingston's "The Space Show").

Anyway to summarize what we know so far, exercise in space helps - but you need to do a lot more exercise than you would on the Earth. And that's just in an attempt to maintain roughly the level of fitness you might have if you were a bed bound patient on the Earth. Of course you don't need to be super fit if you are floating in zero g all the time.

ZERO G BED REST SIMULATIONS

But it's actually worse for health than a normal bed bound Earth patient. Many things go wrong in your body. To try to simulate the effects of zero g, volunteers stay in beds for months on end, tilted with feet raised, and head lowered.

One of the "Pillownauts" exercising with head tilted downwards to simulate some of the effects on the body of zero g. See The 'pillownauts' helping man get to Mars by lying down for nine WEEKS (and they even have their own 'bed spacesuits')

- but even that isn't really quite as bad for you as zero g.

SOME OF THE ZERO G MEDICAL ISSUES

Exercise in zero g just slows down the deterioration, doesn't fix it. For instance, as well as the bone loss, and muscle loss and peak muscle power loss, which exercise can help with, you still have

lower resting heart rate in zero g
Soon after you enter zero g, your resting heart rate decreases. It gradually increases during flight and after a long duration flight you have a faster heart rate than normal for 15 days after return to Earth..
fewer red blood cells
and more blood in your head,
loss of appetite
, so it is hard to eat as much as you need
similarly, don't get so thirsty
and it is hard to drink enough
magnesium deficiency
because you sweat more than usual because you can't lose heat by convection, only by radiation (no gravity means hot air doesn't rise but just creates a blanket around your body).
Immune system not working optimally
stomach and internal organs not working quite right

Over long periods of time that takes its toll on the human body.

SURGERY IN SPACE

In a long duration flight there is also a chance that one of the members needs surgery. If there is no possibility of an emergency return to Earth, it has to be done on the spot.

It's probably only possible to do minor surgery. Surgery has been carried out successfully in zero g conditions in tests on Earth in zero g aircraft flights, using magnets to keep their instruments in place and operating only during the few seconds of zero g each time.

Doctors remove tumour in first zero-g surgery

POTENTIAL HEART CONDITIONS

The numbers of astronauts and cosmonauts are few so far, of course, so it's a small statistical sample. However a fair number of heart beat irregularities have been detected, enough to raise the question whether zero g increase tendencies for heart rhythm irregularities, which could lead to potentially fatal heart conditions. For details, see MEDICAL EMERGENCIES IN SPACE (Mars Society summary) and Cardiac rhythm problems during space flight (wikipedia)

There's also William Rowe's suggestion that Neil Armstrong had a potentially life threatening heart condition towards the end of his space walk - when he had a high heart rate accompanied by shortness of breath (dyspnea). Possible space flight-induced catecholamine cardiomyopathy: Neil Armstrong - and listen to William Rowe talking about this hypothesis on David Livingston's Space Show - he also talks about the steps that should be taken immediately if these symptoms are spotted, for instance during a space walk.

So far no astronaut has had a heart attack in space, but it is something that needs to be researched and monitored carefully.

GRAVITY AS A MEDICINE

The transition from zero g back to full g on the other hand is much easier on the astronaut's body. For the most part, it seems that gravity is more like a medicine than a problem on return to Earth. The body, weakened by the astronaut's time in zero g, immediately starts to recover, at a remarkable rate, over just the first couple of days.

So the main question might be, how long can a human continue in zero g before they have to return to full g in order to stay healthy, or indeed, stay alive?

Indefinitely? Two years? Is even 437 days an endurance test that only some could survive?

We don't know the answer to that yet. But - what if we were to find a way to supply that medicine of gravity in space conditions? Then we might be able to bypass zero g health issues.

GRAVITY AS THE WONDER CURE

From the speed with which astronauts recover from some of the worst effects of zero g when they return, it seems like zero gravity is like a disease for humans with gravity as a "wonder cure".

So, that suggests another approach which might be worth investigating. Rather than extensive exercise, two hours a day, in zero g, just to keep your health at worse than the health levels of a bed bound patient on Earth - what happens if you have just a moderate level of exercise, as you do on Earth - and create gravity in space, artificially?

ARTIFICIAL GRAVITY

The solution to that might well be some form of artificial gravity e.g. two habitats spinning around a tether, or a large doughnut shaped sleeping centrifuge, or perhaps a small personal centrifuge inside of a space station.

The physics of the situation of course is well understood - what spin rates lead to what levels of artificial gravity. So for instance, to give a few examples, you can get full Earth gravity using 30 rpm with a 1 meter radius, or 1 rpm with a 900 meter radius.

For lunar gravity you can use 12 rpm with a 1 meter radius, or 0.4 rpm with a 900 meter radius.

But the effects of all this on human health have never been studied in space.

It all depends on what gravity prescription you need for health, and also, on what spin rates humans can tolerate in zero g, and whether you need gravity all day 24/7 or just for, say, an hour or less a day.

Experiments using centrifuges on the ground are of somewhat limited value because they need to be backed up with data from space conditions to help calibrate them. Space conditions differ from spins on the ground in many ways. Here are some of the differences:

On the Earth you always have full Earth gravity operating.

Any spinning motion increases that so all artificial gravity experiments on Earth involve hypergravity.
Generally have full Earth gravity along the rotation axis for the spin

- in space there will be no gravity along the spin axis
Can't experiment with true partial gravity.

It might be that the optimal gravity level for health is less than full g. We have no way of finding this out on the Earth.
We can't experiment with temporary gravity - i.e. gravity for only part of the day

- in space you could get into a centrifuge and experience artificial gravity for just a few minutes (say while using the toilet) or half an hour or so (meal times say, and to help with digesting the meals) or several hours (for exercise and while resting) or eight hours (while asleep). Even if you can only tolerate a few minutes of full g a day, that might be beneficial. We can't test for this on Earth.
Gravity gradients differ.

On the ground, in any centrifuge experiment, the gradients shade from full gravity to hyper gravity. In space they shade from partial gravity to full gravity
Coriolis effect acts in a different direction.

It's awkward to walk in straight lines in centrifuges on the ground, or to move your hand horizontally. This would not be an issue in space, because the axis of rotation is above your head. Instead you'd feel the Coriolis effect when you stand up suddenly or sit down suddenly or move your hand vertically.
Felt gravity levels in space depend on the direction you walk

- heavier when you walk with the spin, and lighter when you walk against the spin. On Earth then you get pushed sideways when you walk with the spin or against it.

That is - except in cases where you are suspended on your side to walk around with your body parallel to the ground - rarely tested. In that case you do get heavier if you walk with the spin and lighter if you walk against it - but you are already in hypergravity because of the full Earth gravity pulling you sideways. Can't really simulate the effect of getting heavier or lighter as you walk around and change direction while walking in low gravity.
Spin reversal.

In space, the spinning sensation in your ear will reverse direction if you turn your head around (spin reversal). So for instance depending on which direction you are facing compared with the direction of the spin axis - your head tumbles in a forwards vertical tumble, a backwards tumble, a sideways tumble to the left or sideways to the right, or intermediate tumbles. The direction continually changes. The anterior and posterior canals in the vestibular system in your ears should be able to detect this. This effect never happens in the usual experiments on the ground if you keep your head vertical and parallel to the spin axis, though they can happen if you lean your head sideways, or if you turn your head upside down, as you spin, or in experiments with reclining volunteers with head towards the spin axis.
The direction of the spinning motion affects a different part of the ear .

On the ground, in the usual experiments, your horizontal canals (in your vestibular system in your ear) get stimulated. With artificial gravity in space, the spin axis is overhead, rather than to one side. So it's a tumbling sensation and your vertical canals get stimulated (depending on orientation)
The otolithic organs (the utricle and saccule) will respond differently in space conditions without the full Earth gravity along the axis.

These help us sense linear accelerations. They are implicated in space sickness as the body adjusts to different ways of interpreting the sensations from our otoliths. They would surely make a difference to the sensations we feel in artificial gravity. Experiments with the Skylab litter chair suggested that we may tolerate back and forth spinning motions more easily in space. And the otoliths were thought to be responsible for this increased tolerance (even though they don't detect spinning motions at all, still, they are stimulated differently without the gravity along the spin axis, which seems to improve tolerance of spinning motions for some reason)

Spinning motions for artificial gravity would stimulate the posterior and anterior canals instead of the horizontal canal because the axis of rotation is above your head. The Utricle and Saccule otolithic organs are stimulated differently as well.

(For background on structure of the ear and the motion sensitive organs see Otoliths.

The limited data we have from space is based on the Skylab litter chair experiments, discussed in papers such as: The relative roles of the otolith organs and semicircular canals in producing space motion sickness. These experiments were designed to study motion sickness rather than artificial gravity).

However at present there are no plans at all for experiments in artificial gravity in space with humans, nor has anyone ever done these experiments in the past.

Joe Carroll particularly has been trying to get NASA to do some simple tether experiments using the final stage of a crewed rocket launch (which also goes into orbit) as the counterweight. He has been suggesting this ever since the Space Shuttle - but with no success. There just doesn't seem to be any interest in flying these experiments at present.

Joe Carroll's main motivation is to explore effects of low and intermediate levels of gravity on the human body, for instance, lunar or Mars gravity.

This shows the Soyuz TMA together with its final stage at top left - just after separation from final stage (not a photograph, this is a screen shot of a simulation in Orbiter )

The final stage also goes into orbit for several days - eventually returning to Earth.

Joe Carroll's idea is to connect them together with a lightweight but strong tether and start them spinning around the common centre of gravity. At this point the Soyuz TMA is well away from the ISS, in a lower orbit, and there is no chance of collision.

Remarkably, his suggestion uses hardly any fuel. The fuel used to spin them up gets recovered as an extra delta v boost when the tether is cut at the end of the experiment. It adds almost no payload weight either, as the tether is lightweight.

We could do this experiment as a routine part of every crewed mission to the ISS, and get data about the effects of different levels of artificial gravity on the human body in space (e.g. Moon, Mars and Earth levels), as well as learn about human tolerances for the spinning motions of artificial gravity in space conditions.

For details: Crew Tether Spin - With Final Stage - On Routine Mission To ISS - First Human Test Of Artificial Gravity?

This is one of several videos I made with orbiter, to show Joe Carroll's idea. The Soyuz would actually be attached sideways on to the tether, not like this, but for techy reasons this was easier to simulate. The white cube is just a symbol to show the center of gravity, and the tether is shown a bit wider than in real life, to make it easier to see. For more of these videos see: Crew Tether Spin For Artificial Gravity On Way To ISS - Stunning New Videos - Space Show Webinar - Sunday

RUSSIAN EXPERIMENTS WITH RATS

Russia did promising experiments with rats in a centrifuge, which suggested that artificial gravity does help with zero gravity health issues - but have never scaled those experiments up to try them on humans.

Cosmos 936 experiments with rats in a small centrifuge. The results showed that artificial gravity helped combat some of the medical effects of zero g.

NOT DIRECTLY APPLICABLE TO HUMANS

These results are not directly applicable to humans as rats don't experience nausea. They rely instead on a superb sense of taste and learning, so that if they eat bad food they won't eat it again. As a result they can withstand high spin rates with no ill effects. (This is also true of some humans with defective vestibular systems - more on this later)

Human experiments would need to test tolerance of spin rates in space as well as the (expected to be beneficial) health effects of artificial gravity on humans.

CENTRIFUGE ACCOMODATIONS MODULE

This was a module that would be used to study effects of variable levels of artificial gravity on small animals such as rats, and micro-organisms and plants, on the ISS. So basically, it would continue and expand on the early work by the Russians on rats:

Centrifuge Accommodations Module

However it was cancelled in 2004 because of cost overruns and scheduling issues.

SHORT ARM CENTRIFUGE EXPERIMENTS WITH HUMANS

So, as well as the large scale tether spins suggested by Joe Carrol, we could also do human experiments in short arm centrifuges in space. There have been designs for instance for doughnut shaped centrifuge sleeping quarters which could be attached to the ISS or to interplanetary spacecraft.

Nautilus-X - plan for an interplanetary spacecraft with a centrifuge sleeping compartment. A similar idea has also been suggested for a sleeping module for the ISS.
The idea was it would be mainly a sleeping centrifuge, so you get artificial gravity at night. It is just wide enough to fit into it with a spacesuit on - for safety reasons for the early tests of it. It's inflatable and would fit on an Atlas V or Delta IV rocket. It would have, optionally, a food prep and dining area and a partial g toilet facility. See Nautilus-X--Holderman_1-26-11 and they projected the cost as between 83 and 147 million dollars, and development time, to launch, as three years. So if they had started in 2011 when they did the study, we would have this module in space by now.

I think myself it is far too soon to finalize such a design. Because we know so little about the effects on human health of artificial gravity or what human spin tolerances are. And any design like that requires you to make dozens of very particular engineering decisions based on assumptions about what is good or not for human health.

The physics of how these could work is well understood. But there have been no tests so far to tell us what gravity levels are needed for health, and for how long. Or whether the gravity should be intermittent, and which activities it is most beneficial for, or if it needs to be 24/7.

Nor do we know what spin rates humans can tolerate, in sleep, while exercising, eating, work, or recreation in artificial gravity conditions in zero g, and we can't necessarily apply ground based results directly, as the Skylab litter chair experiments showed.

So it is hard to know at present how effective artificial gravity would be for ameliorating health issues, or how well humans would tolerate them, or the best design for human health (level of artificial gravity, radius, spin rate, how much a day) .

Some researchers in a study at MIT and another study there found that most people can adapt to spin motions as fast as 30 rpm with training over as few as five sessions of an hour each. Which leads to the question, is such adaptation also possible for artificial gravity spins in space?

In their conclusion they say

"If, as has been suggested by previous flight research, microgravity actually provides an even less nauseating environment for centrifugation, then vestibular problems should certainly no longer remain an excuse that stands in the way of flight-testing an SRC [Short Radius Centrifuge] countermeasure. An orbiting test platform would allow not only definitive answers to the integration of otoliths and canals in the process of vestibular adaptation, but would also provide the first solid data beyond bed rest analogues about the efficiency of AG [Artificial Gravity] against musculoskeletal and cardiovascular losses. Furthermore, only in microgravity does the opportunity arise to examine the physiological effects of partial-g load, those between microgravity and Earth-normal 1-g."
"In order to truly address the operational aspects of short-radius AG, a centrifuge must be made available on orbit. It's time to start truly answering the questions of "how long", "how strong", "how often", and "under what limitations" artificial gravity can be provided by a short radius device.
2002 ESASP.501..151H Page 155

WHAT WE COULD DO RIGHT NOW

We could get the first human artificial gravity experiments in space up and running soon, easily, if there was the political will to do it.

We could do tether system type experiments using Joe Caroll's methods right away, possibly within a year, as space tethers are well understood (he has been responsible for a number of long tethers that have flown in space for other purposes, so has a lot of experience with methods of deploying them and the best materials to use, longevity, and so on).

As for a short arm centrifuge, that's not so easy.

It would have been easy to try them when the ISS was first built. at least temporarily. Especially for instance a very small, two meter diameter centrifuge, which you could test lying down or sitting (like resting in a hammock), which would give you the first ever data points:

Tranquility module when it was first installed, with plenty of space for a very short arm centrifuge inside, at least until it was filled with other equipment

But it is now filled with equipment and lined, and it would be hard to find space to fit in a short arm centrifuge.

Zero gravity exercise machine. This is in the Tranquility module, and there would be enough space here for a 2 meter diameter centrifuge but of course the space is already in use.

Then there's also the issue that vibrations from the centrifuge could be transferred to the rest of the ISS - but - that was an issue with the exercise machine also, and it doesn't seem insurmountable, for instance for a cycle powered centrifuge.

If the will was there, I wonder if it could perhaps be flown, for instance, in the Bigelow inflatable habitat experiment (that's my own suggestion).

Could this be fitted with a short arm centrifuge, with the human volunteer reclining parallel to the rotation axis? It's going to be more or less empty inside apart from some monitoring equipment. And the astronauts will enter it from time to time to check the instruments. What about doing a very short arm centrifuge test at the same time, just a suggestion?

With very short arm centrifuges, astronauts travel at slow relative velocities. E.g. with a 1 meter radius centrifuge (astronaut reclining, hammock style, parallel or perpendicular to the spin axis), the fastest spin rate for full gravity is 30 rpm, or two seconds per turn. That makes their relative velocity PI meters per second, or a bit over three meters per second, or a brisk walking speed. That's similar to the speeds at which astronauts move around inside the ISS. So there are no significant safety issues there.

This is not particularly with the idea that it would solve all your problems. The am is just to get at least a few data points, the first ever, at minimal cost.

If not with this one, maybe for future larger inflatable habitats.

Bigelow BA 330. at 6 meters diameter as planned, would have plenty of space for even large centrifuges. It's large enough even to fit in a jogging track as for Skylab.

NASA has actually patented a human powered centrifuge, where the astronauts cycle and in the process set the centrifuge rotating for artificial gravity, at the same time that they do their workout. Human Powered Centrifuge - NASA patent, 1997

In this 1997 NASA patent, the invention can be powered in two ways, either by a stationary astronaut pedalling the bicycle next to the centrifuge, or by one or two astronauts cycling as they spin in the centrifuge. This gives the astronauts an aerobic workout, with or without artificial gravity. The spin rate, and so the amount of artificial gravity can also be varied for the same amount of effort cycling. It also generates electricity as a byproduct, which can be stored in batteries and used for the spacecraft.

And work is continuing on this, here are a couple of pictures of a human powered centrifuge in testing, two people use it and one of them pedals while the other does exercises - in this case squats:

Space Cycle tests artificial gravity as solution to muscle loss, see also Working out in artificial gravity

Also medical tests have been done with humans in bed rest experiments on a short arm centrifuge:

The results were encouraging Human Centrifuge Preserves Muscle at Zero-G

For some reason, despite many suggestions to fly such experiments, none of the countries with manned spaceflight programs have ever done any experiments at all of this nature in space conditions.

As we start to think in terms of longer term missions in the Earth Moon system, and interplanetary missions for humans, maybe there's a chance of stimulating interest in these ideas again? And get at least a few data points from space to complement the many ground based experiments.

And then, the various discussions and mission planning involving whether or not to use artificial gravity could be based on some data from space, and not just ground based data with centrifuges in hyper g, and bed patients in reclined with head downwards.

QUESTIONS WE DON'T KNOW THE ANSWERS TO

What spin rates can humans tolerate in space before they get dizzy or nauseous?
Do the gradients from low gravity to higher gravity in small centrifuges in space affect human health, and if so in what way?
Does it help to use very short arm centrifuges with the astronauts in reclining positions?

If so, is this best with the body parallel to the axis, or perpendicular to it?
How well do humans tolerate the spin reversals as they turn their heads, and vertical Coriolis effects as they rise and fall, or lift their hands and arms up and down, in Artificial Gravity?
What level of gravity is optimal for human health?
We know that the optimal amount of gravity must be somewhere between zero g (unhealthy) and full g (because hyper g is unhealthy) - but which level is best?
How do humans respond to intermittent gravity,

for instance gravity only when asleep, eating meals, exercising and going to the toilet? Is this enough to stay healthy, or might this even be better for health than gravity 24/7, or is it perhaps, for some reason even worse for health than zero g?

Surely when we get tourist habitats in space (such as the proposed Bigelow inflatable habitats) they will at least have AG for the toilets and probably also at meal times and maybe for recreational low g and jogging tracks. So, if we don't find out before them, maybe that may give us a few answers or at least some data points to reason on a firmer basis.

Or, maybe some of the newer space capable countries, once they have their own astronauts in orbit, might explore this.

I don't think it necessarily follows that just because we evolved in full g 24/7 that this is either needed for human health, or even, optimal for human health. As it is now, you can hypothesize almost anything and nobody can say you are wrong. E.g.:If one person says:

"We have evolved under full g, so full g is absolutely essential 24/7 and most humans can only tolerate small spin rates of a fraction of an rpm for long term stays in space. So we have to build tether systems with multiple kilometer long tethers between the habitat and a counterweight (e.g. use a discarded third stage) - this is the only way we'll ever do long duration spaceflight"

And someone else says:

"Optimal health is for lunar gravity, for one hour a day, with the rest of the time at zero g. Alternating between the two regimes benefits health, and with a bit of training, most humans can tolerate high spin rates. So you just need a two meter diameter individual centrifuge, which you use in a reclining position, and do a little light exercise in it every day to remain healthy, even healthier than in full g 24/7."

And someone else says

"Artificial gravity is totally impractical at present. We need to keep astronauts in zero g and use medicines and several hours of exercise a day to keep them healthy, and develop ways to live in zero g for longer and longer periods of time."

If you are writing a science fiction story, or a movie script you can go with any of those, take it as the "future history" and run with it.

But in real life, how can we know who is right there? Maybe all are wrong? The only way to know for sure is to do some experiments in artificial gravity (as well as continuing zero g experiments) to test these hypotheses and others.

USING MEDICINES TO STAY HEALTHY IN SPACE

We might also be able to stay healthy indefinitely in space using drugs to treat the various things that go wrong with the human body, combined with several hours of exercise a day. This approach is what the main space agencies favour. Preventing Bone Loss in Space Flight.

They say that solving zero g health problems with artificial gravity is impractical with present day technology. They give what seem cogent reasons for this view. Listen for instance to what Dr. Julie Robinson, ISS Chief Scientist says as guest on the Space Show.

Maybe they are right, but as we saw with the MIT quote (Page 155), some of the researchers who research into artificial gravity effects on humans are of the opinion that it is high time that we had data from centrifuge experiments in space conditions in order to have definitive answers to these questions.

HUMANS WITH DEFECTIVE VESTIBULAR SYSTEMS

On page 95 of Packing for Mars, by Mary Roach she mentions that NASA Ames researcher Bill Toscano has a defective vestibular system. He only realised this when they put him on the spinning chair and he experienced no nauseous effects at all from the spinning. So, he at least, could spend 24/7 in a short arm centrifuge type one meter radius spinning hammock at 30 rpm for full gravity with no ill effects. The same is also true for some deaf people.

Bill Toscano from NASA Ames doesn't get dizzy or nauseous when he spins because of a defective vestibular system. He only realized this when he tried out a spinning chair and had no nauseous effects at all from the spinning.

That leads to another thought. Depending on the results of experiments on short arm centrifuges in space - should we use people with defective vestibular systems for long duration spaceflights? Whatever the results for everyone else, they at least could easily tolerate full gravity 24/7 in a small spinning habitat or in a small two meter diameter centrifuge, or whatever is needed.

That could be a huge reduction in cost of the mission, and the mass of the spacecraft, with improvements in the performance of the astronauts, especially for multi year and even decades long missions. Perhaps in the future it might be a major factor for choosing astronauts for the very longest duration spaceflights.

MIGHT SOME ASTRONAUTS TOLERATE HIGH SPIN RATES FOR ARTIFICIAL GRAVITY?

We might also choose astronauts who happen to tolerate very fast spin rates even with normal vestibular systems. For instance ice skaters can tolerate rapid spins that make everyone else nauseous quickly.

Ice skaters don't feel nauseous or dizzy at all during these rapid spins, due to their many years of training. They may feel dizzy momentarily as they come out of the spin.

That doesn't necessarily mean they can tolerate the tumbling type motions of artificial gravity, because that's in a different direction.

However, because of the out of control Gemini 8 spin, astronauts are put through 3 axis spin training during astronaut training. That's just in case they get into a situation like that where a rocket motor gets stuck or misfires and they find their spacecraft rapidly spinning and tumbling out of control. So they are likely to have more tolerance for tumbles and spins than most, just because they train to be able to tolerate it.

The experiences of the Skylab astronauts also suggest astronauts can tolerate very rapid ice skater type spins in zero g for at least a while. If only they had tested the medical effects of these motions, and also done tests to see how long they can keep up these spins and tumbles and jogging around the track?

Skylab - 1970s US space station that was wide enough in diameter to have a jogging track around the interior. It was based on a modified final stage of a Saturn V launcher. It flew from 1973 to 1979.

Jogging starts at 3.30 and they jog at around 10 rpm, so probably experienced around 1/3 g, taking the radius as 3 meters. The longest jog is for one minute 50 seconds (including various gymnastic tumbles in the middle). So though that is not a medical experiment, it does show that humans can tolerate at least 1/3 g, and 10 rpm for just short of two minutes with no signs at all of discomfort.

Various spins and turns in zero g, demonstrated by astronauts on Skylab.
Perhaps there will be astronauts who, like the ice skaters for spins on Earth, can tolerate far faster spins (in the tumbling direction) in short arm centrifuges. If so, again they'd be obvious choices for long duration interplanetary missions if artificial gravity is beneficial for health - as seems likely.

It's also worth noting, that if temporary artificial gravity spins are sufficient for health, for instance if it is enough to have gravity only for exercise, going to the toilet, eating and digesting your food, resting, and maybe for sleep - these are all activities where you don't need to do frequent turns of your head.

You may be able to keep your head more or less still relative to the spin, which can greatly reduce tendencies for nausea and giddiness. Also, they are all activities that don't need fine hand / eye co-ordination - so Coriolis effects might not matter either, again potentially permitting fast spin rates during these activities.

MAIN PROBLEM, ON EVIDENCE SO FAR IS KEEPING HEALTHY IN SPACE

Anyway whatever approach is used, whether it is artificial gravity, or zero g with medicines and lots of exercise, the evidence so far seems to suggest that the main problem is keeping healthy in space. If we can keep our astronauts healthy, they can probably spend many years, even decades in space and recover quickly on return to Earth.

But until we find a way to make sure they stay healthy in space, we can't be sure that they can survive long multi-year missions in space. NASA's current policy is to explore ways to keep humans healthy in zero gravity for as long as possible.

Though that's obviously of interest and nobody is saying that we should stop that research, some experimenters think that we should also have a more active program to research into the possibilities of keeping healthy through use of artificial gravity as well.

With artificial gravity, we've reached the point where we need experiments in orbit to validate ground experiments. Animal models are of limited value since they respond differently to artificial gravity spins (such as the rats that never experience nausea in a very short arm rapidly spinning centrifuge).

There are two main directions we can explore here. First is the use of a tether spin - which with Joe Carroll's proposal could be investigated in the near future during a routine crew mission to the ISS. Then for the short term temporary gravity tests we need a centrifuge within the ISS, or an extra module with a centrifuge on board.

Either of these could give us our very first data points on the effects of different levels of artificial gravity on human health in orbital conditions, and on human tolerances of spin rates for artificial gravity.

The tether spin experiments could also give us first data on effects of gravity 24/7, at least for a few days at a relatively slow spin rate impossible to achieve in a short arm centrifuge for the same gravity levels. And the centrifuge experiments could help us understand effects of temporary gravity for a few minutes, or hours a day at faster spin rates, impossible to do with the tether spins. So both approaches have their strong points and they complement each other.

FIND OUT MORE

For more details of Joe Carroll's experiment, and more about the separate idea of experiments using a short arm centrifuge in space, see also my answer to this quora question: Robert Walker's answer to Can we create a spaceship with centrifugal artificial gravity with today's technology?

And for details of the medical effects of zero g, there are answers by several Quora contributors here, including myself: How long can humans live in space and what is the worst case scenario for someone who lives too long in space?

See also The Most Unusual Laboratory (Not) on Earth for a podcast which discusses some of the latest results from the ISS and the new Kelly twins experiment.

You might also be interested in my guest appearance on David Livingston's Space Show on these topics: Robert Walker, Friday, 3-14-14

And my Science20 articles on these topics:

This article originated as my answer to a question on Quora: What is the longest time an astronaut can spend in space before it is too hard to re-acclimatize to Earth?

REASON FOR OPTIMISM

I think there is a good chance that we find a solution to these problems, but I don't think we have enough information yet to know for sure what is the best way to do it. They've only explored one approach - controlling the harmful effects of zero g.

Will that work for multi-year missions even with use of medicines and 2 hours a day exercise?

And whether it will work or not, is it the best approach? What if a short arm centrifuge would solve all the health problems, just used while sleeping or eating meals, for instance? That's low cost, and would free up astronaut's time if they don't have to do two hours of exercise every day - and if it worked could mean they are healthier than they are in zero g with all that exercise.

Or what if the solution is to tether a spacecraft to its final stage or another spacecraft and use a 24/7 slow tether spin?

So far we've only explored one approach to keeping humans healthy in space - zero g with a lot of exercise each day - for the last 45 years. It's surely time that we did some research into the other two main approaches as well - tether spins and short arm centrifuges.

WHAT DO YOU THINK?

What are your thoughts on all this? Do you think we'll solve these problems in the near future and be able to send humans on long duration spaceflights for many years at a time, even interplanetary missions?

Especially, has anyone got any thoughts about whether and how it might be possible to do artificial gravity experiments in space in the near future?

As usual, if you have any questions do say, and don't hesitate to point out any typos or correct anything I've said if you have better information about anything at all.