You've probably seen movies of orbital space habs spinning for artificial gravity. But did you know, that nobody has ever tested this to see how it works out in practise? We know that weightlessnes is bad for health, especially long term, with many potentially serious medical issues. But do we need full g, or Mars g, or lunar g to stay healthy? Nobody knows. Can we cope with a spinning hab a few meters across or do we need to think about a huge hab or tether system a couple of hundred meters across or larger? Again nobody knows. 

You would think that somebody must have tried it out by now. After all, we've had plans for spinning habs with artifical gravity going right back to the early days of the space age, and indeed before

An early 1962 design for a hexagonal rotating orbital space station - right at the start of the space age.

 
Herman Potocnik's space station, 30 meters across and spins to generate artificial gravity, illustration from his 1928 book - it is in geostationary orbit and uses llight focused from parabolic mirrors to turn water into steam for power generation.

The idea to use rotation to generate artificial gravity goes back to Konstantin Tsiolkovsky in 1903.

But the idea has never been tested at all. This is the closest we have ever got to it:

Gemini 11 tether experiment, 14 September 1966. It lasted for half an hour, and generated an imperceptible 0.00015 g

- this is the only experiment in artificial gravity ever done in space (except for some simple demos in the ISS e.g. spinning a bag of tea so you see how the bubbles collect in the centre of the bag).

It's a common theme in science fiction of course. But the details are a product of the science fiction author's imagination and of course, science fiction stories often gets future predictions wrong, so this is no guideline to whether or not it works in practise. Here is a video of the centrifuge in 2001 a space odyssey

Though a staple of movies, nobody has ever tested this idea in reality to see if it works.

Would humans be able to adapt to this, and could they live here long term? How big does the wheel need to be to prevent dizziness and medical issues? Do we need full g, or are low levels of g fine for human health? Is the difference in perceived gravity between head and feet an issue? We have no data points to help us answer these questions.

Also, we have no experience at all of closed system habitats in space. Nobody has tested a system able to circulate nearly everything. The complex life support system in the ISS depends on frequent resupply from Earth, venting waste gases to the vacuum of space, and sending wastes (including human solid wastes) back to the atmosphere for incineration.

There are a few fairly low cost experiments we could do in orbit, similar in cost perhaps to the Gemini experiment, which could settle many of these questions. They might lead to fundamental changes in our ideas about the best designs for human interplanetary exploration and human occupied space stations and settlements. Surely this research is necessary before we start expensive projects to build spacecraft based on our data for zero g and full g only?

The main focus of this article is on artificial and low g, but first let's talk a little about the closed habitat and radiation issues, as these also are significant serious issues that need to be sorted out before humans can do safe interplanetary travel.

CLOSED SYSTEM ISSUES

This repeats one of the sections from my Ten Reasons NOT To Live On Mars - Great Place To Explore

• The ISS is not at all self contained. They can't even wash their clothes, but get new clothes sent up when they need clean ones. All the dirty ones are disposed of in the supply vessels which burn up in the atmosphere.
• Human waste products can be problematic in a small habitat. Though urine can be recycled, without too much trouble, solid waste is much harder to deal with and in the ISS is again disposed of in the supply vessels. Then air, drinking water, and other waste products have to be dealt with. A hugely complex system is needed to deal with all the waste products, the diagram to the right just shows some of the main parts of the system.
• Atmosphere regulation is hard in an enclosed habitat. In the ISS there is a complex environmental regulation system which filters out many different harmful gases that can build up in an enclosed human system (that includes ammonia, hydrogen cyanide, acetone, hydrogen chloride, nitric oxide, carbon monoxide as well as carbon dioxide and many others) and keeps the oxygen levels right. If this goes wrong in the ISS (as has happened several times) you can send replacement components or emergency oxygen from Earth but on an interplanetary mission, you would be in trouble.

• Micro-organisms are problematical in an enclosed habitat. This is something the Russians found out with Soyuz. In the ISS many measures are taken to keep the numbers of micro-organisms low, including keeping the atmosphere very dry and filtering them out. Still they have occasional build ups of biofilms. (For an overview of this issue see Microbial Colonization of Space Stations)

For a closed habitat you'd have to deal with all those issues, but also,

• Need more efficient cycling of all the waste products, especially the water
• Need to recycle solid human waste (excrement), obviously also in ways not hazardous to health
• Grow your own food (though for shorter missions it might be feasible to take along most of the food for the entire mission)
• Would probably also use algae to create oxygen and absorb the CO2 from the atmosphere

Probably all of that is possible, but we haven't attempted anything like a closed habitat in space. You wouldn't want your first long term test spaceflight with an experimental closed habitat to be an interplanetary flight. Obviously it's going to need some years of testing closer to Earth first to do it safely. And our experiences with the ISS don't count, as its systems would not be suitable for interplanetary flights.

We'll also have issues with cosmic radiation. Those could be dealt with using shielding, especially if you can create close to Earth equivalent shielding at the destination. For instance in an expedition to Mars orbit, you could use materials from the Mars moons, or could dig into the surface of one of the Moons for radiation shielding - probably best done with robotic precursor missions so the human habitat is already completed and shielded before the first explorers arrive to inhabit it.

HEALTH ISSUES FOR ZERO G

You might wonder, why does this matter, just use zero g. But zero g has many health issues as we have discovered. It's surprising, perhaps, quite how extensive and far reaching these issues are.

These issues include bone loss, eye problems (many astronauts have short term issues after their flight, and there's been one case of irreversible damage to sight as a result of zero g), thinner blood (reduction in blood cell count can be as much as 15% after two weeks in space), more blood in the upper body, increased resting heart rate, greatly increased levels of adrenaline, reduced digestion, issues in liver and kidney function, changes in function of immune system, reduced thirst leading to dehydration, increased core temperatures, can only get rid of heat by sweating, not by convection so increased sweating leading to magnesium deficiency, increased iron, can't take most medicines orally, only subcutaneously because of the stomach, liver and kidney issues, the list goes on and on.  

For some of the main issues see The body in space and Health in space, and some chapters from Laboratory Science with Space Data: Accessing and Using Space-Experiment Data, and Wikipedia on effects of weightlessness. 

It's not known if humans can live long term in zero g. The record is 437 days but the Russian cosmonaut who survived that long in space might just be extremely lucky.

In a recent space show a doctor William Rowe, a specialist in human physiology in space. gave as his opinion that because of all these complications, most people would die within two years of exposure to zero g. It's also likely to be unsafe to carry a fetus or give birth in zero g, though it is unethical to do the experiment to find out for sure. Pregnant women are not permitted to fly to the ISS for this reason. William Rowe has also turned up possible evidence of a risk of sudden heart failure after moderate exercise such as a space walk, due partly to adapatation to zero g conditions. Researches in this area are somewhat restricted because NASA has a policy that they don't release individual medical data about their astronauts until after they die, and until then only release aggregate data.

Nobody knows whether or not the same issues apply to low g such as lunar or Martian levels of gravity. With only two data points, zero g, and full g, we can't interpolate to find out what happens with low g.

The obvious solution is to use artificial gravity in a spinning habitat or using a tether system. But again, we run into this issue that we have no experimental data to go on. Nobody knows if humans can survive long term in a small spinning habitat or if it needs to be hundreds of meters in diameter. 

DIFFERENCES BETWEEN ARTIFICIAL GRAVITY AND NORMAL GRAVITY

Einstein with his thought experiments showed that there is no way you can distinguish between uniform acceleration in a straight line and a uniform gravitational field. However artificial gravity in a rotating habitat is not the same as uniform acceleration in a straight line. 

There are several effects, not just the Coriolis effects, though they are all less noticeable as the habitat gets larger. The effects you might notice right away are:

1. Your weight will change slightly as you move around the habitat, depending on the direction and speed of motion. In a very small spinning habitat, if you run fast enough (or cycle) in the direction opposite to the direction of spin, then you could become weightless. Cycle even faster and you gain weight again. 

2. You also have the spinning motion itself. This is different from the Coriolis effect. For instance if you took a perfectly balanced gyroscope, you'd notice that from your point of view, it turns around on the spot once every revolution. 

3. In a small spinning habitat you'd notice that the gravitational effect on your head is different from the effect on your feet. For instance if you bend to pick something up and then stand up again you might notice that your ears and tongue suddenly feel lighter. 

4. Coriolis effect. In a spinning hab then this makes a difference to vertical motion. For instance if you throw a ball vertically upwards, it will curve away from you in the direction the habitat is spinning. Similarly if you stand up suddenly, then you'll find you seem to get pushed over in the direction of spin. 

Artist's sketch of a fountain in artificial gravity, showing how the Coriolis effect transforms vertical motions in the habitat. Illustration by Tye-Yan "George" Yeh from The Architecture of Artificial Gravity (Chapter in Theodore Hall's Dissertation)

There's a nice java applet here, which you can use to explore the Coriolis effect for yourself

Ringworld Physics Simulator

Try it with Lock Viewpoint. Double click to make a ball, then hold down Shift key, click on the ball and with mouse still held down, move it rapidly in the direction you want to throw it and release, and you can see how the ball curves in its trajectory due to the Coriolis effect.

One fun thing you can try is to throw the ball upwards in the counter spin direction. If you get the speed and direction right you can catch it coming back to you from the spin direction.

All those effects are subtle in a larger habitat, but in a small rotating habitat would be perceptible, and all of them might also have health effects. For instance lighter gravity at your head level might lead to more blood pooling in the head, similar to zero g. 

There are other differences of course. On Earth then you can tell that the gravitational field is non uniform because the stars directly overhead change depend on your exact geographical location, but only if you travel a long way.

On the spinning habitat this is more noticeable. You'd notice that the upwards direction is different for you and for your friend, say a quarter of the way around the habitat. 

OTHER UNIMPORTANT DIFFERENCES

There are a few other differences, as the Earth's gravitational field is not uniform and the Earth's spin introduces Coriolis effects. But these are negligible.

Then there is an effect a bit like the gyroscope effect of the spinning motion itself on Earth also, but of course more subtle. Anywhere except at the equator, the Foucault's pendulum changes direction gradually as it swings. Anywhere on the Earth, you could notice that a perfectly balanced gyroscope changes the direction of its spin axis in a 24 hour cycle.

We have minute Coriolis effects here as well, but also too subtle to be important except at the scale of large scale weather effects such as hurricanes.

TESTING IN A CAROUSEL

You can test this to some extent in a Carousel. But sadly it's not the same as artificial gravity. The axis of rotation is parallel to the direction of the perceived g force (or at an angle if you spin it fast enough to go above 1g) while the axis of rotation in a habitat in space under artificial g is perpendicular to the perceived g force. 

So, in a carousel, you get Coriolis effects as a result of horizontal motions rather than horizontal motions. For instance if you try to walk in a straight line you get pushed sideways and if you stand up suddenly you won't get pushed in the spin direction.

Also you don't have the difference in gravity between head and feet you get in a small rapidly rotating habitat under artificial g.

Then you can't test the combination of the rotation effects with low g, lower than Earth gravity. The effects of the spin might be worse under low g, or might be not so noticeable. There is no way to tell which way that goes with carousel experiments on Earth.

That's enough of a difference that I think, so that it's not really possible to draw any firm conclusions about artificial gravity from ground studies. Still, let's see what is known.

RESULTS OF CAROUSEL TESTS ON EARTH

First, humans can adapt to spinning in carousel type motion. Figure skaters particularly learn to spin many times on the spot, very rapid spins that would make anyone else sick. They start with just one or two rotations and gradually build up to eight, so that shows you get some adaptation to spinning motion. 

Here is Julia Lipnitskaia in the 2014  Winter Olympics.

She spins so fast at times it's like a blur, especially in the spin at the end. Anyone not accustomed to ice skating would get sick, but ice skaters get used to the spins, and adapt so they can spin without any problems at all.

Note, that unlike ballroom dancers, she doesn't keep her head faced in one direction and whip it around. Her head spins around constantly, at the same rate as her body.

Then, there is this story of someone who spent over  52 hours on a circus carousel Carousel rider breaks record after 52+ hours

And someone else who spent 25 hours in a Ferris wheel

TESTS IN CAROUSEL TYPE ROTATING ROOMS

This is a recent series of experiments by NASA with people who lived in a rotating room for hours at a time. They did find that people adapted to it and that they no longer perceived the Coriolis effect but learnt to compensate for it. It helped if they did repetitive tasks so that they got used to the Coriolis effect more quickly.

They found that their subjects could adapt to 25 rpm in their rotating room. So that's pretty fast, much faster than the often quoted 3 rpm. (More about their research).

Then there's the 1960s NASA carousel experiment with the subject in slings. This is perhaps the closest we can get to simulating artificial g on the Earth, for instance the Coriolis effect acts in the right direction though without the other more subtle effects.

Here is the video

But none of this answers the question really, how humans would react to artificial gravity in a small habitat.

Perhaps this gets a bit closer, but it was just done for fun, not as a scientific study, an old NASA video of astronauts jogging inside Skylab on its jogging track. They certainly don't seem to be under any distress jogging around at about 12 rpm, and generating 0.5 g approximately. The Coriolis effect probably makes them more clumsy than they would be otherwise, but you'd adjust to that if you were spinning all the time. It seems moderately promising, but far too short to settle anything.

Video of Skylab astronauts on its jogging track.

(more videos of this below in the comments section).

EXPERIMENTS WE COULD DO TO SETTLE THE QUESTION

First, you could fly some small mammals to the ISS together with a lightweight carousel for them to live in for a few months of artificial gravity - but would it be conclusive for human adaptation? For instance since they are smaller than humans, the variation in gravity between feet and head would be far less.

There doesn't seem to be much risk in using humans as test subjects here. After all zero g is bad for human health long term anyway.

You could start off with micro g as for Gemini 11. Then once you see what effect that has, gradually increase the artificial g, and try lunar, then Mars gravity and then finally full gravity, of course monitoring health all along. 

Joseph Carrol suggested that as an experiment that astronauts could do before they dock with the ISS, is to keep the spent booster of the Soyuz tethered to the passenger module and use the booster stage as the counterweight for the artificial gravity.

Or, alternatively, follow the example of Gemini and send up two separate spacecraft and tether them together with humans in each one. 

WHAT WILL WE DISCOVER IF WE DO THESE EXPERIMENTS?

Nobody knows, so this can only be a guess. Perhaps humans would adapt to artifical gravity even in a small habitat, of a few meters across, spinning fast enough for artificial gravity, like the sailors who adapt to motions at sea and ice skaters who adapt to spinning motions. But it's not an exact analogy because of the different direction of the spin axis and the effect of the differences of artificial gravity between head and feet.

SMALL HABITATS

Suppose for instance that humans can adapt to a habitat of twelve meters across.and 12 rpm - that's enough for full g (well 96% of full g). Certainly the experiments so far would seem to suggest it's possible. But if you stand up in your habitat, then your head will be at 64% of full g. This is something we can't simulate at all on the Earth so who knows if that's an issue or not.

You could rotate the entire habitat. Or you might keep the outside stationary and rotate an inner shell. The speed of motion of the outside of the carousel is about 7.6 m/second in this example (17 miles per hour).

Who knows, maybe we could adapt to a habitat of only six meters diameter and 18 rpm, again enough for full g? Your head now is at roughly a third g while your feet are at full g. Does that matter? The 18 rpm might well be tolerable from the Earth experiments. Speed 5.7 m/second, or 13 mph.

TETHERED HABITATS

For another example, suppose that the tether is 50 meters long, and the rotation rate is 6 rpm which from the ground experiments would seem an easy rate to adjust to. Then that gives full g once again. Now you travel at a rather faster 16 meters / second, or  36 miles per hour. Your head is at 88% of full g.

You can try out other values for the radius and rpm with this online Centrifugal Force Calculator

 1989 NASA artist's concept of a vehicle which could provide an artificial-gravity environment of Mars exploration crews. The piloted vehicle rotates around the axis that contains the solar panels. Levels of artificial gravity vary according to the tether length and the rate at which the vehicle spans.

STRENGTH OF TETHER

The tether doesn't need to be extraordinarily strong. It only needs to support the habitats under Earth normal gravity. For instance a cable strong enough to suspend the habitat on the Earth is strong enough.

WHAT IF THE TETHER BREAKS

It's not a big deal if the tether gets severed. Take our example of a 50 meter tether, 6 rpm, and full g spin, then the two habitats would move apart at a relative velocity of 72 miles an hour. This is tiny compared with the many kilometers per second delta v needed for interplanetary travel or to get into space from Earth. So long as one of the habitats has some fuel left for maneuvres, it's easy to get back together and attach a new tether.

If the tether broke while close to the Earth, you might wonder if there is a risk of one of the habitats hitting the Earth. It's natural to think that, as many things about orbital dynamics are unintuitive to us. Even the brightest of astronomers get caught out by this sort of thing at times, if not used to working with orbital dynamics.

But that wouldn't happen. For instance, if you stand outside the ISS and throw a ball towards the Earth, it just goes into a slightly different orbit from the ISS. Yes, it travels towards the Earth to start with. But as the ball orbits around to the other side of the Earth, the extra momentum from your original throw sends it away from the Earth again. The two effects cancel out putting it into a similar orbit to the ISS.

Get the angle of your throw exactly right and it returns to your habitat after around half an orbit. (This is hard to do, you have to make the longest diameter of its new elliptical orbit exactly the same as for the ISS or it will have a different orbital period).

If you don't catch it, then it crosses your orbit, and eventually comes back again from the opposite direction after the orbit is completed. Then it keeps retracing this new orbit (slowly decaying due to atmospheric drag of course, similarly to the ISS itself).

To hit the Earth you need to throw the ball fast enough to counteract the effect of the orbital speed of the ISS. Indeed, counter intuitively, , the easiest way to hit the Earth is to throw the ball paralllel to the Earth's surface, in opposite direction of the motion of the ISS at exactly its orbital speed of 17,100 mph. It will then lose its orbital velocity and immediately start a fall towards Earth accelerating under gravity until it hits the atmosphere.

IDEA FOR A TUBE INSTEAD OF A TETHER

One idea I've had myself, not seen it elsewhere yet, is that you could use a tube instead of a tether. So, cylindrical, with many strong stays running the length of the cylinder (like the stays of a suspension bridge), and wide enough for the astronauts to use as a quick way to get from one of the habitats to the other.

This would have the advantage that it couldn't be severed by micro-meteorites; at most a few of the stays would be cut. It could be repaired easily from the inside using spare stays, and could be fitted with ladders and compartments to make it easy to travel from one habitat to the other. Make it wide enough to create rooms, and it could be inflated with atmosphere, and used for growing plants for food and used for extra storage space,. You could have a zero g room at the hub as well, perhaps an extra inflatable hub region, and use that for docking.

But all this can be light weight, easy to roll up for transport to space, and inflate. It's not designed for permanent living qualities for humans, just with enough shielding to use as an egress tube from one habitat to the other and to keep in the atmosphere for plants. Perhaps it could be transparent as well, as a cylindrical 50 meters high transparent greenhouse connecting the two habitats.

CENTRIFUGE SLEEPING QUARTERS

Also seems worth exploring the idea of centrifuge type sleeping quarters. 'With this idea, since the astronauts just lie down in their quarters rather than walk and move about and spend most of their time there asleep, probably they could be even smaller, just a few meters across. 

Here is the idea for Nautilus X - a NASA concept for a spaceship for interplanetary travel with a centrifuge module meant to be used for sleeping. The radius is quite large but the tube itself is narrow,just wide enough for astronauts in sleeping postions inside it length-ways, plus a bit more space to allow them to wear a full spacesuit inside (required for safety reasons for initial tests in space). There is a proposal to fly a module like this on the ISS but so far it remains just an idea.

FIND OUT MORE ABOUT ARTIFICIAL GRAVITY ISSUES

To find out  more see Joseph Carrol's Design Concepts for a Manned Artificial Gravity Research Facility and his powerpoint slides presentation

Many more papers and resources here by various authors

RELEVANCE TO MISSIONS TO MARS

If you've read my other articles here, you'll know that I'm interested in interplanetary missions as far as Mars orbit. We don't absolutely need them, as we could explore Mars using autonomous robots, which are increasing in capabilities all the time. But humans in close orbit telepresence contact could greatly speed up exploration of Mars and new discoveries about origins of life on the planet and present day life. Whether it is worth the extra expense (compared with unmanned missions to Mars) I don't know but it certainly is of great value and would hugely speed up the process of scientific discovery on Mars.

However, I see no value at all in a mission to the Mars surface at the present time, as this runs a greatly increased risk of contaminating the  planet with Earth life. It seems to bring no scientific benefits at all, only risks of contamination, as it seems you can explore Mars better by telepresence than on the surface (bearing in mind how clumsy astronauts are in spacesuits). Orbital missions would achieve this for far less cost and greater safety - and with the ability to explore several areas of Mars at once in a single expedition, even on opposite sides of Mars, with direct close up telepresence. 

For more about all this, if you haven't read it yet see Why Mars is NOT a Great Place to Live - Amazing to Explore From Orbit - with RC Rovers, and Nature Inspired Avatars

It's still important to sort out these issues, whether  your aim is to land humans on another planet, or to keep them in orbit. Do you need full g for inteprlanetary travel or is low g enough, and how should either be achieved? Then, the plan for HERRO was to live in Martian gravity so you feel the same g effects as your telerobotic avatars on the surface - but is that okay for long term health? Or do you need extra spin and full g? We need to know the answers before making detailed plans.