Well the usual response is that the ship would be too large and expensive. But actually, the simplest form of artificial gravity in both science fiction and in real world proposals consists just of a spaceship tethered to a counterweight. This has often been proposed for missions to Mars and beyond. It hardly adds much mass and the tether can be as long as you please, tethers strong enough to do this are very lightweight nowadays. And we could test this easily, within a year if anyone wanted to. The Russian Sergey Korolev had a plan to test the physiological effects of AG on humans in 1965-6 with a Voskhod spacecraft, using its final stage as a counterweight, and the main reason it didn’t happen is that he died unexpectedly in 1966.

But there may be an even simpler approach. This is barely mentioned in science fiction, it’s the idea to have a small centrifuge as small as maybe even just 1.4 meters in radius, inside a space station or spaceship. That idea has been proposed several times, and up to the Columbia crash there was a serious plan to send a centrifuge module to the ISS, and then even after that, there was a serious proposal to add a small human powered centrifuge within one of the existing modules, but neither of these ever flew.

I think it is too early to build such a spaceship because we haven’t done any research into artificial gravity in LEO for humans. Zilch, nada. Just hasn’t been done. What data we have is promising but our research has been zero g all the way for several decades now.

Artificial gravity for plants yes. Just a hundredth of a g is enough to change expression of numerous genes in plant cells. It seems likely that agriculture under even a hundredth of a g for plants could solve many issues with zero g agriculture. Rats yes. Rats don’t get nauseous when spinning, so they have been tested in small centrifuges in space and it does indeed prevent just about all the physiological issues with zero g. But humans, for some unfathomable reason, no. We haven’t even studied the effect of a hundredth of a g on humans.

IN MORE DETAIL

The data we have so far is very promising. We have some data from experiments to study space sickness rather than artificial gravity. These experiments on Skylab involved spinning the astronauts for a few minutes only (as they weren’t meant as experiments in AG).

They showed that the astronauts did not get nauseous or even disorientated, during experiments which made them nauseous both before and after the flight. Tim Peake also demonstrated this anecdotally on the ISS during his recent visit there, though not part as a planned experiment, just in his recreation time. Here he is spinning at about 60 rpm in the ISS. for a couple of minutes, no nausea, only momentary dizziness when he stops.

Only a professional ice skater or similar could do that on Earth, probably. Or people who have a natural tolerance to fast spins (some have a defective vestibular system and are not affected in any way by spins, however fast).

He says he is pretty sure he couldn't tolerate that on Earth. So anecdotally it suggests that we can tolerate very high spin rates in zero g. Taking the radius as 0.25 meters at a guess, his head and feet will be both under full g, his torso around zero g as he spins. Could he spin like this indefinitely? As far as I know, nobody has tested to see how long astronauts on the ISS can spin like this without ill effects.

If so, it's very promising I think for the use of a short arm centrifuge to counteract health issues of humans in zero g.

The experimenters for the Skylab litter chair experiment hypothesized that the reason their subjects could tolerate back and forth spins and continuous spins without getting nauseous in space was because the otoliths aren't stimulated, because there is no gravity acting in the direction of the spin axis. (See chapter II, Chapter 11, Experiment M131. Human Vestibular Function in Biomedical results from Skylab)

Our otoliths are separate from the vestibular system. Instead of sensing turning motions, they sense linear acceleration.

In any spin on Earth you have these two things at once -

• The spin sensed by the vestibular system, which is a spin around the rotation axis
• The linear acceleration due to gravity sensed by the otolith system. For slow spins this is mainly along the rotation axis. As the spin gets faster this is at an angle to the rotation axis.

Our otoliths tell us that down is in the direction of the spin axis, offset slightly, depending on the rate of spin.

Meanwhile our eyes and our vestibular system tell us that we are spinning in a different direction. It seems to be a conflict between these.

But in space,

• Our ostoliths will sense no linear acceleration at all in the direction of the spin axis.
• The sensation from the otoliths is only a sensation of an apparent linear acceleration away from the spin axis (the “centrifugal force” due to momentum causing our body to “try” to continue in a straight line when accelerated in a circle towards the center)

So there is no conflict between the ostoliths and our eyes and vestibular system (which senses spins).

That's what the experimenters hypothesized. But they didn't do any more experiments after that to try to follow it up to see if this was correct or not.

This has major implications for artificial gravity, as it could mean that humans could spin even in short arm centrifuges.

And these don’t need to be massive constructions, like they are on Earth. Remember Tim Peake was spun around by another astronaut even with no centrifuge at all. Try doing that on Earth. We could do preliminary experiments in AG right now, without any centrifuges at all. Ask the astronauts to spin each other as for Tim Peake and measure things like the effects on their heart rate, blood count etc. Also get some preliminary data on how long they can spin for, without ill effects. Two minutes is fine. What about ten minutes? An hour? A day?

When an astronaut goes into space their blood count goes down and their heart rate goes up almost right away. So why not, for starters, see what happens to those two parameters when they are spun like Tim Peake for a few minutes at a time? Does that have any effect? They are not even doing rudimentary experiments like that. No data at all since Skylab.

But - better than that, why not send up a light weight centrifuge to the ISS. The problem of course is finding a space for it now. The interior is packed full of other things. We need something like this ideally

Just a crude 3D drawing I did - shows two possible orientations of a hammock like device - fastened with cables to a central pivot. You’d need some way to set the astronaut spinning, but at a pinch you can just have another astronaut give them occasional pushes, as with Tim Peake. So you could do this experiment, if you can find the space, with just two pivots like that, some cable, and a hammock for the astronaut to lie in during the experiment.

It wouldn’t take much work though to design some kind of a motor. Remember it only has to keep them spinning in zero g. Just gentle tugs from time to time. Even some kind of a motor in the center with an arm that extends a meter along the cable and gently pushes against it. If the astronaut wants to stop, they are moving only at meters per second relative to the walls. They just need to reach out and gently touch something as they spin, to slow down and it should be easy to design in some kind of railing for them to grab, or some kind of braking system to make that easier.

The cable only needs to be strong enough to hold the astronaut’s weight under full g. So we are not talking about any kind of super materials there. A cable as strong as the cable for a child’s swing would be fine.

MOMENTUM CONTROL

Ideally you need a counterweight in the floor or ceiling spinning the opposite direction so that when you set it spinning there is no net effect of the ISS spinning the opposite direction. But note, if you start an astronaut spinning, it causes the ISS to start to spin very gently in the opposite direction. Nothing more happens during the spin. When you stop the spin of the astronaut, then this annuls the effect of the previous angular momentum change so the ISS would then stop spinning.

So the ISS momentum control gyroscopes should be able to handle this fine, a small change of angular momentum in one direction at the start of the experiment which they would automatically annul, then a small change in the opposite direction, which once more they would annul returning to their previous state.

TOO SOON TO DESIGN A SPINNING SPACE STATION

So, it is far too soon to design a large spinning space station. For all we know, it might be that all you need to keep the astronauts healthy is enough space to spin a hammock like this inside the space ship. If so, all those big spinning designs are only a convenience for walking and agriculture and not needed for health.

We also don’t know what level of artificial gravity humans need. It might be that lunar gravity is just fine to keep us healthy, it could even be better for us than ordinary gravity. Or it could be better for some people. We only have a few days of data per astronaut from the Apollo astronauts with 1960s and 1970s technology on the effects of lunar gravity on human health from missions that were not primarily focused on this.

Maybe even a hundredth of gravity has significant health benefits for humans. Or maybe we need nearly full g. We just have no data on this yet. Experiments on Earth with people lying head down to simulate just some of the zero g health effects, then spinning them - they have very limited relevance at present. It might be that this does simulate some of the health effects of artificial gravity in space. But again, they might not. We don’t have any data from space to use to assess whether it is a good model of it. After all the body is always under full gravity indeed, somewhat hyper g during those experiments.

And we don’t know what spin rate humans can tolerate in space long term. From Tim Peake’s anecdotal experiment, clearly we can tolerate a spin rate large enough to cause full gravity for a few minutes. Depending on how long we can tolerate this for, we could have

• A centrifuge toilet. This would eliminate many of the inconveniences of using a toilet in space, and give the astronauts a few minutes of gravity every day
• A centrifuge for eating. Eating will be much easier under gravity, even low levels.
• A centrifuge for exercising. They won’t need to have elaborate counter pressure approaches. Just do normal exercises on top of a spinning hammock. Also one of the main problems with exercise in the ISS is that under zero g, there is no convection. The warm air from your exercise just collects around you like a blanket, meaning you can only lose heat by sweating. To stay healthy in zero g they have to do two hours of vigours exercise every day to prevent bone loss. But that is two hours of sweating, and all that sweating during the day has health effects too.
• A centrifuge for sleeping. Just sleep in a hammock. Sleeping for eight hours a day in full gravity might have major health benefits for the astronauts.

Remember also that we can always do any of these with levels of AG less than full g which may well be beneficial too, even possibly, more beneficial than full g (we just don’t know). The first one, the AG toilet, particularly seems something that surely we’d be able to tolerate. AG for eating food also, it seems reasonably likely that they could tolerate that much. Eat their food in a spinning hammock. It’s never been tried in space. If they simply don’t get nauseous in zero g while spinning, then why not?

The current space agencies sending humans into space NASA, ESA, ROSCOSMOS, CNSA (China) are just not interested in doing these tests and experiments even though researchers in artificial gravity on Earth have often called for them. They seem to think it involves expensive equipment, that they have to build a human centrifuge module to do it.

But as I said, you don’t need any extra equipment at all (except medical monitoring devices which they have already) to make the first steps in studying effects on humans of AG, after all the astronauts are already testing it anecdotally. They could start on that today. It is just that nobody is doing any measurements to find out what the physiological effects are of spinning in the ISS, or testing to see, not just anecdotally, but through hard data, to follow up on those early Skylab experiments that showed that we can tolerate spins more readily n space than on the ground.

So far all we have are a few experiments with Skylab which were done rigorously, but not with a focus on AG, rather with a focus on understanding space sickness - and Tim Peake saying he is pretty sure that he wouldn’t have been able to tolerate this on Earth, but no control experiments before and after, no monitoring, nothing.

We also have the early out of control Gemini where a spacecraft span faster and faster for a few minutes until they got it back into control which was not a planned experiment and there was no physiological testing.

But it has huge potential benefits for astronaut health and in other ways too, if we can tolerate even minutes at high spin rate. I think maybe when we get commercial space, if not before, we may get the first tests of this idea. Think what a difference it will make to space tourists if they have a centrifuge for eating and for the toilet, and for sleeping (if desired). Surely we won’t get these ideas ignored for ever.

WHAT WE DON’T KNOW

We don’t know

• The optimal level of artificial gravity for health - could be anything from a hundredth of a g to full g.
• How much the optimal g varies from person to person - do old people, young children, people with bone conditions, heart problems etc perhaps have different optimal levels? Might it be that it is so individual, depending on genetics, that everyone has a slightly different or maybe hugely different optimal g level for health?
• How long humans can tolerate a spin rate fast enough for that optimal g - whatever it is - in a short arm centrifuge
• How much spin tolerance varies from person to person. We know on Earth that ice skaters can tolerate much faster spins than anyone else, apart from some people with a defective vestibular system who don’t get nauseous from spinning at any rate however fast. So there must be some variety in tolerance in space also - unless everyone tolerates it fine and doesn’t notice it.
• Whether we need artificial gravity full time for health or for a few minutes a day - or indeed, whether cycling back and forth between artificial g and zero g might be beneficial for health
• If it turns out we need a slower spin rate to tolerate artificial gravity - we don’t know how slow it has to be.

I don’t know why they aren’t doing this. It seems to be some kind of a blind spot. To do it you would have to go through the procedure of proposing an experiment and it getting accepted which is quite a cumbersome thing for the ISS. Their schedule is so full already.

PLANNED EXPERIMENTS NEVER SENT INTO SPACE

Now, it hasn’t been completely ignored, but it’s not been a priority enough to send these experiments into space. Artificial gravity was a priority for the ISS up until the loss of Columbia in 2003, It was proposed first, in NASA Ames, then later on the project was passed on to the Japanese space agency, then called NASDA, now called JAXA, who built a Centrifuge Accommodations Module - an entire new module designed to study effects of artificial gravity - which however never flew because the Space Shuttle was needed to get it into orbit. See page 55 of this paper.

Then in 2010 there were proposals to send a smaller centrifuge to the ISS, to be located within one of the existing modules - but it never happened. Here is a 2011 idea for a 1.4 meter radius centrifuge to be located in the permanent multipurpose module:

Sketch of the AGREE centrifuge for the ISS. From page 15 of Design and Validation of a Compact Radius Centrifuge Artificial Gravity Test Platform. It would have replaced the four racks at the end of the Permanent Multipurpose Module. Astronauts would cycle in a seated position. This is one exercise excellent for health which you can do with an extremely compact radius centrifuge like this. Chris Trigg concludes: "Given the compact design, subject positioning, available sensors, tested accuracies, and validated operations, the MIT Compact Radius Centrifuge represents one of the most unique yet realistic centrifuges currently in available for artificial gravity research. It is hoped that through these future studies the MIT CRC will provide a better understanding of the effects and capabilities of an inflight-centrifuge, and perhaps contribute in some small way to progressing towards the inevitable trip to Mars. "

a sketch for a human powered artificial gravity in the ISS .

It’s not that different from the spinning hammock idea. I am sure this will happen eventually, it is just very very low on their list of priorities. The topmost priority for the ISS for human factors research is research into effects of zero gravity on humans and into ways of ameliorating those effects during long duration missions in zero gravity.

If just for convenience of tourists of a centrifuge toilet, eating areas and sleeping areas, if nothing else, I expect it to be rather higher in the list of priorities for commercial flight when those start to happen, especially once we get longer duration commercial flight and space hotels in LEO.

LONG TETHER CENTRIFUGE

There is another way also that we can explore artificial gravity in space. This is another thing we could test, pretty much right away. We could have the experiment running probably within a year if there was the political will to do it. I'd follow Joe Carroll's idea of an artificial gravity research gravity in LEO. It can start off as simple as just one space module with a counterweight. And indeed the first experiments are even simpler than that. He has been advocating it for years. He's an expert on space tethers and several of his tethers have flown in space.

The idea actually dates back to the 1960s. We now know that Sergey Korolev had a plan to tether a Voskhod with its spent final stage which he put forward in 1965-6. It was going to be a 20 day flight to upstage the Americans. It would have included a pilot, and a physician and the artificial gravity experiments would have lasted for 3=4 days during the flight. He died unexpectedly in January 1966 and the mission was postponed to February 1966 then cancelled outright. So we came very close to doing this experiment way back in 1966. (See page 17 of this thesis).

Apart from that we only have the Gemini tether experiment which generated a level of micro gravity too low for the astronauts to sense it.

Joe Caroll's idea similarly is to start with a tether spin experiment with a module tethered to its final stage, which goes into orbit anyway. The way he does it, all the delta v put into spinning up the assembly get released at the end of the experiment. This boosts the spacecraft when the tether is released as well as achieving a controlled re-entry of the final stage into the Pacific (at present its reentry is uncontrolled). It uses no extra fuel unless the tether is severed by space debris, which from his experience in improving tether design is now a very low probability event. He would only use the excess fuel carried by the Soyuz in event that more is needed than expected during the launch, and use it only if not needed (usual situation|). This means that the Soyuz would still get to the ISS even in that worst case scenario.

It can be designed to be safe and could be done right away, as quickly as the Gemini tether mission was put together, for a near future crew mission to the ISS. They'd use the longer phasing approach of several days, so you could test several days of artificial gravity. The Soyuz TMA or any other crewed spacecraft can do Joe Carroll's tether spin on the way to the ISS, deliberately use the longer two day phasing approach to get to the ISS and do your first experiments on the way. The cost wouldn't be much as human spaceflight experiments go, just to add a tether to a Soyuz TMA mission that is going to the ISS anyway. They still have the older two day approach as a fallback option so that should be no problem either. They would be able to cut the tether at any time and continue in zero g if there were any issues arising during the experiments.

Though this would be a short experiment, there are many things you can test in a short mission. It would of course test things such as tether dynamics and tether spin up. Also radio communications during tether spins, and orientation of the panels to achieve adequate solar power throughout the orbit.

Also, in particular, it would give us the first real data on spin tolerances of humans in artificial gravity long term. It's a different experiment from the short arm centrifuges in the ISS, because these can be much longer tethers, far too long to fit inside the ISS, so with slower spin rates. Also the artificial gravity would be continuous rather than intermittent. I think we need to explore both, as we have no idea which is going to be most effective for keeping humans healthy. Again there is no way we can test the differing effects of continuous versus intermittent artificial gravity on Earth, except as hyper-gravity with the Earth's gravity acting along the spin axis on top of the artificial gravity.

This video shows a 600 meter tether at 1 rpm joining a Soyuz TMA to its final stage to achieve lunar gravity. Even the most highly susceptible people have no problems with 1 rpm in rotating room experiments on the Earth long term. So probably this would be fine for everyone in space also - that is if the Earth experiments are a reasonable guideline, which nobody knows of course (that's why we need to do the experiment). There are some indications that in space, with spins around a horizontal axis (above your head) and no gravity pulling sideways along the rotation axis, that we can tolerate spins better than on Earth. Though the data is very limited so far.

This video is done in Orbiter, a remarkable space mission simulator by Dr. Martin Schweiger with lots of add ons contributed by enthusiasts.

Thanks to Gattispilot, for making the tethers, and for techy advice about how to attach everything together.

Note that the video shows an "eyeballs out" configuration. The tests would only go from low g up to full g, but still, this is not the most comfortable orientation for the crew. Joe Carroll's plan is for an "eyeballs in" configuration. It's just that for techy reasons I found it much easier to position the Soyuz in the simulator in this "eyeballs out" orientation . The tether would be brighter than this, and you may notice a cube at the center of gravity of the tether - this is just to indicate where the center of gravity is and would not be there in reality.

Based on these very early tether spin experiments, we can answer basic questions such as, can humans tolerate spinning for two days, and if so what tether length and spin rate is tolerated? (The experiment is designed so it is easy to abort from it at any time - you just cut the tether, and then continue to the ISS). And what are the immediate effects on the human body of artificial gravity? What is the gravity prescription for health (what g level, how many hours a day or do we need it full time) and how easy is it to apply the desired levels using AG?

Another experiment you could do in the near future is a tether experiment launched from the ISS. The crew would take the crew module to the ISS with the final stage still attached. Then to do an experiment, they leave the ISS, fly far enough away from the ISS so that a severed tether won't endanger the station, spin up, do the experiment for several days, then spin down and return to the ISS. This idea was suggested by Tim Cole in 2012.

Based on those preliminary results from the Soyuz TMA, or any other crewed capsule that goes to orbit with a third stage which you can use as a counterweight, you'd work towards designing a larger AG research lab in the future for longer duration studies. It might be based around using the newer modules from the ISS when it is decommissioned, for a hub for spacecraft to dock to and for zero g research, and then tethered habitats for the crew going round it. If it gets more elaborate, perhaps it would also use spent final stages, fitted out in advance as "wet workshops" like the early ideas for Skylab.

This then would create a small facility in orbit. It doesn't need to be a big hundred billion dollar facility like the ISS, just a small space station to start with, which can also be a basis for a staging post in LEO later on. It could also be a facility for research into closed systems, growing plants and so on. It would have a science component of course, like the ISS, but the main objective would be human factors. It would be forward looking, helping us to find out what role humans can play in space in the future. Which of course would have science benefits in turn. Once we know more about what humans can do and how best to support them, we can then send them on science expeditions further and further afield into our solar system.

It could have a zero gravity module attached to the hub. So, there's no reason why you can't combine zero gravity with artificial gravity in the same station.

It would start small, based on this idea that we are still experimenting, and are not yet very experienced in space travel. At this stage, I think we need to try out ideas, and lots of them, to see what works. This could lead to advances that we would never get if we proceed in a linear planned out way with some grand plan for the future, based only on the knowledge we have so far.

The same small space station in low Earth orbit would be an ideal place to test methods for life support for longer duration missions on the Moon or further afield. Again if it is in LEO you have easy access to it from Earth. Because it is a small space station, you could do this at the same time as human missions further afield, say, to the Moon. It could even be useful right away as a place for astronauts to go to first, to disembark from rockets launched from Earth and to transfer to spaceships traveling from LEO to the Mon.

It doesn't have to have a continual human presence. That is one of the things that makes the ISS so expensive. Make it largely automated and controlled from Earth, so that it can be left in orbit unoccupied for months on end in between experiments. If astronauts stay healthy in some level of artificial gravity, and if we can perfect biologically closed systems so that they produce all their own food and oxygen, then you could send them supplies only once a year, perhaps, or less, and then you could start to occupy it continuously. So then the costs would go right down, and typically at least some of the astronauts would stay up there for several years at a time. It is not a commitment to billions of dollars a year, but rather, you occupy it and use it as needed as you find out more.

One interesting idea for interplanetary missions, the delta v in an interplanetary tether spin can be used as an abort scenario for a mission to Mars orbit. If you set up the tether spin right, you can boost straight back to Earth - in an emergency - by cutting the tether at the right moment in its tether spin when it reaches Mars. Here is a paper about that idea: Artificial gravity and abort scenarios via tethers for human missions to Mars.

You can also maintain artificial gravity with a tether spin like that even through insertion burns, so you don’t have to think in terms of cutting the tether or reeling in the counterweight when you get to Venus, Mars, Jupiter, Mercury etc. You can just continue spinning for AG all the way through the insertion burn and again also for the transfer burn to get back to Earth. See "Method to Maintain Artificial Gravity during Transfer Maneuvers for Tethered Spacecraft"

This is from the section Need for adventurous experiments in life support and artificial gravity in LEO first from my new book OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps? which I am working on at present. It is nearly finished :). I have rewritten the first part of that section here, with formatting, copy editing, some new points, and will include this rewrite back into the book when I next work on it.