Can Spinning Habitats Solve Zero g Problem? And Answer Low g Questions?
    By Robert Walker | February 10th 2014 11:18 PM | 27 comments | Print | E-mail | Track Comments

    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

    1962 design for a hexagonal inflatable space station

    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

    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.


    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.
    • Overview of the ISS environment control system, which requires complex technology, and has many inputs and outputs and is not a closed systemHuman 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.


    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. 


    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. 

    A fountain in artificial gravity. (Illustration by Tye-Yan "George" Yeh.)

    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. 


    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.


    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.


    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. 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


    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).


    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. 


    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.


    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.


    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 impression

     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.


    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.


    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.


    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.


    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.


    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


    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.



    Be sure to add your comments if you have any thoughts about all of this, or ideas, suggestions, questions. Anything here need clarification? Any mistakes however small?


    It's something I've thought about from time to time, but started to think about a possible article as a result of a conversation on Trouble with Terraforming Mars with Hop David

    He also blogged about it himself here leading to more conversation

    Yesterday I watched a program on TV about human expeditions to Mars which was good in some ways, talked extensively about issues of zero g, but didn't mention the idea of using artificial gravity to solve these problems for interplanetary travel. 

    Also listened to  a recent space show open lines program where many of the callers wanted to know more about artificial g. It seems it is an issue whose time has come and a good time to do a survey article to encourage discussion.

    Since artificial gravity scales with the square of angular velocity, 25 rpm would make a huge difference!

    I compared terraforming Mars with volatile rich asteroidal impacts vs using the volatiles where they sit: Terraformng Mars vs Orbital Habs. My arithmetic seems to show we'd get 10 times as much real estate from asteroidal habs than using the same asteroids to build Mars atmosphere.

    If 25 rpm is possible, spin hab radius could be much smaller. With smaller radius spin hans, it would be a lot more than a factor of 10.


    Oh right, yes, good idea. You need lots of atmosphere to build O'Neil colonies. But if you can make them really small, then you can use much less atmosphere per inhabitant as the volume of atmosphere increases according to the cube of the radius but the number of inhabitants as the square.

    With my Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths - that's based on Stanford Torus habitats.

    They are smaller and easier to build but use about three times as much shielding per inhabitant. Also, the amount of atmosphere needed is smaller, 44 kt needed for a 10 Mt shielding with 10,000 population (compared with 14.6 Mt for a 23.3. Mt shielding for the cylinder with 820,000 population), so it's a small percentage of the total mass (0.44%) and there is a reasonable chance you could get it all from the asteroid belt.


    The 895 meter radius cylinder (from the Stanford torus comparison table, not using 0'Neil's designs) uses 17.8 tons of atmosphere per inhabitant, while Stanford Torus uses 4.4 metric tons per inhabitant. 


    When it comes to the shielding, the cylinder uses 330 tons per inhabitant compared with 1000 tons per inhabitant for the Stanford Torus. (NOTE, THE FIGURE OF 19.4 Mt  - CAN'T GET THAT FIGURE TO WORK, I think there might be an error in their table, perhaps out by about an order of magnitude, because it works out at 28.4 tons per inhabitant, and it can't be as little as that, if you plug in the area and multiply by 4.5 you get 271,350,000 tons, and 330 tons of shielding per inhabitant, unless I'm missing something there, also 330 tons sounds about right, about a third of the amount needed for the torus which needs extra shielding all the way around the torus not just the floor of it).


    You can use even less atmosphere if you use the innovation of a flattened torus as in the Vademecum idea suggested by a Belgian, Alexandi Birdi when he was just 16. - either a truncated ellipse cross section, or two elliptical arcs joined together, which as well as less atmosphere also has the advantage of a nearly flat floor with no change in g.


    So anyway the idea of using a really tiny cylinder could turn that all on its head. If you can get down to smaller cylinders, a few tens of meters radius - then because of that cube and square relationship, the amount of atmosphere needed is going to be tiny in comparison to the shielding.

    So for instance, a 50 meter diameter cylinder, length 250 meters. Then the volume is 49,087 cubic meters (250*π*252). Might as well just use full atmospheric pressure as it's not going to be a significant amount, so 1.225 kg/m3. So 1.225*49,087 /1000 = 60.1 metric tons.

    That's for a living area of 39,270 square meters (2*π*25*250) or 0.3927 square kilometers (or 97 acres, or 39 hectares if used to those units), so at the Stanford Torus assumption of 67 square meters per person, enough living space for a hamlet of 586 people. So, only 0.1 tons of atmosphere per person. Though the hamlet is tiny, you could build lots of them and link them together in some way perhaps.

    For the shielding, need to cover entire surface with 4.5 tons per square meter. Surface area including end caps 43,196.9 square meters (2*π*25*250 + 2*π*252), so total mass for shielding 4.5 times that or 194,386 tons (presumably mined in space of course, e.g. from Near Earth Object, or from the Moon, not that big, a 56 meter diameter NEO would do).

    That makes it 332 tons per person for the shielding. Or about a third of the amount needed for the torus.

    So, if I got that all right, means three times as much living area for the same amount of mass, and a tiny percentage of only 0.03% of the total mass consisting of atmosphere. And uses only 1.03 tons per person of atmosphere.

    Of course this is a very rough "back of the envelope" type calculation to get a first idea. Do say if you notice any mistakes BTW. Just found a few, may be more.

    (edited again, fixed most of it now hopefully, one thing that confused me was what seems to be an error in the original Stanford tables, not sure, see above).


    With my original calculation (same as one done in the 1979  in the book "Colonies in Space" by T. A. Heppenheimer), then the asteroid belt at three times the mass of Ceres, had enough material for stanford toruses for 1200 times the surface area of the Earth. But with cylinders, about three times as much surface area for same mass of shielding, it's enough for perhaps 3600 times the surface are of the Earth.

    As for amount of atmosphere needed, I think you'd surely find your 1 ton in 310 of water, CO2 and nitrogen in the asteroid belt, though nitrogen would be hardest to find probably. But if nitrogen was in short supply, Ceres has a mass of 9.43 * 1017 metric tons. So your 1.5 1x 1015 tons of air at 0.16% of the mass of Ceres, would be enough for air for the entire asteroid belt at 0.03% air, turned into those tiny 50 meter diameter, hundred acre habitats (if we decided to do that not saying of course that we should turn Ceres into habitats, just if we decided to do that), so that's going to be 3600 times the surface area of Earth.

    Just a very rough calculation of course, roughly in same ballpark as yours given the smaller % of air.


    A bit of a problem for this though is that some people may be super sensitive to the spinning motions. If it's just a matter of exploration interplanetary spaceships then you could just say - that you test your astronauts before they go on an interplanetary flight or move to a space station, and only send astronauts that are not sensitive in this way. But for a habitat for colonization, may be more of an issue.

    I got a comment on this article in facebook by someone who gets sick in a spinning restaurant spinning perhaps four times an hour. So maybe that gives some pause for thought for really small habitats for general use. But on the other hand it's not the same situation. 


    Who knows, maybe he would be just fine in a habitat with spin axis perpendicular to the artificial g. Or maybe nobody can tolerate it at all, won't know for sure until we can test it for real.

    Whether or not everyone can adapt to live in a 50 meter diameter habitat, still, you'd think that some small communities could adapt to them at least. Not sure you need to go smaller than that as the amount of atmosphere needed is already tiny at 6 rpm.

    All though depending totally on whatever results are of orbital tests whenever we get a chance to test this stuff out for real to see if any of it works in practise.

    As for rotating cylinders, Arthur C Clarke beat you to that idea quite awhile ago--1972. Check out his Rama series which was about a massive cylindrical habit passing thru the solar system.

    He also was the first to think of synchronous satellites, back in the 1940's I believe.


    Sorry didn't mean to suggest it was my idea. It's an idea due to O'Neil. I think Arthur C. Clarke probably got it from him as O'Neil developed the idea in the late 60s and early 70s. Online book about his ideas here, Space Settlements.
    All Hop David suggested, and what I was doing here is to look at the idea of really tiny cylinders, based on the idea, what if we could tolerate high rpms like 12 or 18 rpm. 

    As the volume goes up as the cube and area as to the square, a lot of the mass in a big structure like RAMA is made up of atmosphere. By using a 50 meter diameter 250 meter long 100 acre cylinder, then you can get the amount of mass needed for the atmosphere down to 0.03% of the total mass if I got the numbers right, less even than for the Stanford Torus.

    BTW the first to suggest a spacecraft in geostationary orbit seems to be Herman Potočnik in the 1928 book where he describes a space station at a geostationary location.

    Arthur C. Clarke's innovation was the idea that geostationary satellites would be used for communication.
    You're replying to me? I hope it doesn't seem like I was trying to take credit for the idea. I got it from Gerard O'Neill and my article linked to a Wikipedia article on O'Neill cylinders.

    Rendezvous with Rama is one of my favorite books. I got this Pattern of Primes from that book. I hadn't realized it was written as earlier as 1972. If I remember that book also foretold a Spaceguard, an effort to search for hazardous near earth asteroids. Very prophetic! The man was a giant.

    Michael Martinez
    In the 1970 edition of the World Book Encyclopedia there was an article about a possible future trip to Mars which included a series of diagrams showing a 6-human habitat (sort of triangle attached to a rectangle).  To generate artificial gravity (after completing a constant acceleration phase) they proposed that the habitat would extend a boom (somewhat similar to your tube) and just spin with a counterweight attached to the far end of the boom.  When they needed to start braking for Mars orbit they would retract the boom, stop the spin, and turn the habitat the other way to begin the deceleration phase.
    It was all very cool at the time but I guess by now everyone is a bit more realistic in their expectations.  You're trying to conserve mass with your examples but if I were going on a long space trip I would want room and comfort.
    Yes, actually another thing about this, I don't know where to find it now, but someone worked out ways you can accelerate and decelerate habitats linked by a tether without needing to retract the tether. It's just a matter of firing the rockets on both habitats at the right times and directions and they keep spinning around as before and decelerate (or accelerate) at the same time, and preserves the same tension in the tether and amount of artificial g.
    We actually do have 12 sets of data on humans living at 1/6th G, though only for a short period, and all of those periods bracketed by periods almost as long in freefall, so it's of limited value.

    There is also video of Pete Conrad hamster-caging during the Skylab 2 mission (Oh, now there is a missed opportunity! 3 Skylab modules would be as much pressurized volume as the ISS - and we flew one and had one ready to launch 20 years before we even started building the ISS in orbit!) ( - fascinating! Actually, all three astronauts on that flight!)

    *walks away muttering about conspiracy theories about NASA administration tasked to Man as a whole out of space...*

    Oh that's a good find with Skylab 1. Here is the same video as an embedded video.Jogging at 3.38 into the video

    It takes them 5 seconds roughly to jog around the inside, so 12 rpm. I'm not sure of interior radius, exterior is 6.7 meters diameter, suppose internal radius is 3 meters, then  that's 2*pi*3/5 = 3.8 meters per second they are jogging at, so then they could achieve 0.49 g using the online centrifuge calculator. So about half a g. 

    They are having difficulties coping with the Coriolis effect tending to fall over. But they haven't had the chance to get used to it in the way you would if the habitat was spinning constantly.

    Also, running for a while without getting dizzy immediately, which is promising, and the issue of the head at nearly zero g and feet at half g don't seem to be a problem short term. Seems moderately promising :).

    Interior of skylab

    . Here is another video of athletics inside the "exercise wheel"

    There's some more 18 minutes into this video

    Pity we don't have Skylab any more to do some proper experiments. You could fit a lightweight carousel inside the jogging track and set it spinning at a constant rate and get some easy experiments in true artificial gravity with a really tiny radius.


     For that matter could experiment with setting up a carousel there for the astronauts to sleep in - sort of like hammocks. Have a central pivot, line length ways for them to hang from, hang the hammocks from it, and set them swinging around it, until they reach full g (or whatever is needed). Need some small motor to make sure they keep swinging around and don't gradually stop due to air resistance.

    That would be a light and easy experiment to do if we had a big living area 6 meters in diameter in space to try it out in, and see if sleeping under full g helps to prevent zero g medical issues.


    The Bigelow inflatable BA330 has an interior diameter of 6.7 meters so larger than Skylab
    Due for launch perhaps in 2015 (depending on Falcon Heavy progress I expect)
    Animation of launch and deployment video for BA 330

    And the proposed Bigelow inflatable BA 2100 has diameter of 12.6 meters. That's easily large enough so you could have an interior carousel with artificial g somewhere in the station. 

    It could be launched on a Falcon heavy according to this blog post, so may well get something like this in the future (Falcon heavy well on track to commercial operation in near future).

    Assuming a carousel with 12 meters radius inside of the station, if it rotates at 9 rpm then you get full g. That's linear speed 11.3 meters per second, or 25 miles per hour. Which seems not too hard as an engineering feat, get on it, speed it up and you have an area of artificial full gravity where you can do exercises or sleep etc.
    You are, of course, going to need either a counter-rotating carousel, or a fairly energetic flywheel, to manage the angular momentum of astronauts getting on and off of it. Said flywheel being tap-able for emergency power, I do believe...

    And then there's the idea of 2 BA-2100s joined by a tether/tunnel, with floors parallel to the hemispherical floors. Decent length tunnel, and you can have rotation slow enough that the vast majority of people would have no trouble adapting. Eventually, you put a multi-way adapter at the center point, and spread 6 or 8 arms out... I would suspect, though I haven't done the math, that you wouldn't need particularly stiff arms, or strong inter-arm separators, to allow for spin-up and spin-down.

    Someone discussed at a meeting I attended a couple of weeks ago (Astronaut Memorial Week) that a manned mission to Mars would start out at $2B and go up... And it struck me that that's *40* Falcon 9 launches, at published prices for launches on SpaceX's website. (25 or so Falcon Heavy launches, but I've not done the math to see which turns out to be cheaper, and Falcon Heavy hasn't flown yet, anyway...) I'm reasonably sure we could put together modules using that to make a Mars mission quite possible, especially using the ion engines that the Dawn Probe has already demonstrated. The thing I like about Bigelow and Habs is - there's two of them, in orbit right now, maintaining pressure and temperature, and they have been doing so for a combined total of around 15 years now!

    Yes I wondered about that. In Skylab, the three astronauts were able to jog fine. It's total mass was 76 metric tons in that case, and assuming 60 kg for a human, three astronauts would be 180 kg, so that's 0.2% of its mass. So their 12 rpm run would translate into 0.02 rpm of the entire space station approximately, spins around once every 50 minutes so I wonder if they noticed any effect like that of their runs.
    However would only make a difference for as long as the carousel is running. When you stop the carousel you stop the rotation. So for instance the astronauts jogging around the inside of Skylab, at 0.2% of its mass, if they run around five times, they turn the Skylab around by a hundredth of a full turn. A single astronaut, by running around the track five times, would change the orientation of Skylab by, very roughly, one degree, and when he stops running, the skylab would also stop in its new position, shifted around by one degree. 

    But you might well care about the orientation of the station even to a few degrees, and then if you are running your carousel for hours on end, it's going to be a noticeable significant effect.
    So, yes that sounds good, to have a flywheel. Yes could use for attitude control of the station generally and emergency supply. Just when vehicles dock, when people move around, they may accidentally set it spinning and you might then want to stop that and flywheel sounds ideal.

    Or - could be some functional thing e.g. a water tank that dual functions as a carousel counter mass. Maybe you use water anyway as internal shielding of the habitat - if so, then the section behind the carousel have that designed to counter rotate in opposite spin direction to the carousel. Just ideas here, obviously would need to look into engineering details.

    With the connecting column, yes I was thinking in terms of flexible material, with the cable stays in it. Could be as wide as the habitats themselves. Indeed, you could also do it so that it inflates to somewhere between a column and a sphere, as an equipotential surface anchored by the two weights of the habitats, a bit like ideas for domes on the Moon, but here is between the two habitats - just for plants and things that are not bothered by cosmic radiation, similar idea again to the lunar greenhouse type habitats. 
    So - to save on weight etc - and is going to be perhaps the largest part of the external surface area of the space station if it is say 50 meters tether - you are talking about a lightweight flexible transparent material, shielding for UV, similar to materials for lunar and Mars greenhouse concepts. So probably transparent plastics. You'd have to replace them from time to time  as they degraded in the UV from the sun - same problem as for anywhere else in space, how to replace the plastic without losing the air inside and damaging the plants growing inside it. 

    Anyway if that idea worked could then have maybe part of it more shielded as recreation area for humans - once it inflates you could then e.g. pack inside of parts of it with stores and wastes to use as extra solar flare and cosmic radiation shielding.

    Yes, I wonder if with the lower costs, and private companies like Space X and Bigelow, that missions to Mars by humans (I hope, of course, that it's to Mars orbit or its moons rather than surface at present) might suddenly become much easier than before - once we have closed habitats working, as well as artificial gravity. Maybe we don't need to push for it as a big project, maybe some time in the future when its time comes it will be possible as a result of all the other things we are doing.

    Yes, Bigelow have two engineering demonstration third scale versions of their modules in orbit. 

    Here is there Genesis artist''s impression (currently in orbit as Genesis I and II):
    Genesis artist's impression

    Here is Robert Bigelow talking about them and other things.

    Just had an idea, to use transparent water tanks for the cosmic radiation shielding. Have two of those tubes and fill the space in between with water tanks.
    Say 1 meter on the side water tanks, so each one is one metric ton. Eventually want a thickness of 4.5 meters for full cosmic radiation shielding, but could make a good start with 1 meter of water shielding. Add this as extra diameter beyond the diameter of the habitat itself.

    So - suppose it is the 6.7 meter diameter Bigelow, two of those habs, then perimeter about 21 meters, 50 meter tether that gives you 1050 tons approximately - more than that because it bulges in the middle and outer perimeter is 24 meters, not trying to be precise. Is a lot compared with weight of the ISS, double its weight, but is only three Falcon Heavies (payload to LEO 53 metric tons). Though we don't have it yet I think reasonable to assume Falcon Heavy for near future projects, with first commercial mission penciled in for 2015.

    Further into the future then it's a 15 by 10 by 10 meter cube of ice from NEOs, or since it's just water would be ideal for the ideas for future low cost non rocket space launch e.g. various ideas for firing water and propellant into orbit.

    That 1 meter shielding is a lot better than the ISS, enough so you could stay there reasonably safely for months or even years without significant risk of getting cancer/

    For full shielding same as for Stanford Torus, want 4.5 times as much, so that's more like 13 or 14 Falcon Heavies. But - if it is frequently visited from Earth - you could build up to that gradually as the project develops. Of course at same time build up protection for the habs at both ends too.


    You'd have an outer Whipple shield of course to slow down the micrometeorites, made of some type of clear plastic though in this case. Then the water itself would heal micrometeorite punctures by freezing as it expands into the vacuum. On the inside, if anything gets all the way through, then would seal with patches, on very inside have waterproof layer of plastic covering entire inside. 


    You could have a swimming pool at both ends where the tube meets the habitats. Because swimming is great for full body exercise, and if you have say a couple of meters thickness of water, would help as additional radiation protection for the habitat, maybe indeed could be one side of a storm shelter too for part of the habitat.


    Easily arranged by shielding inside the tube.


    Assuming again that we use the 6.7 meter diameter habitats, then have layers crossing the tube every 2 meters, at 50 meters long, then that's 35 square meters for each layer (more in the middle as it bulges out under atmospheric pressure but forget that for now) so total of 35*26 = 910 square meters. That's enough to provide all the food you need for a vegetarian diet for about 3 people by the Jeaveson intensive gardening method (same as for Biosphere 2). (See also Biointensive agriculture) As for oxygen, then you can make enough with a small area of algae as found in some Russian experiments long ago, so I don't think we need to worry about that with this much area available.

    For enough for say 6 people, double the length of the tube and so on. It could be a first step towards experiments in sustainable living in space.

    So anyway the main new idea here was just to use water as a transparent shielding. Expect surely others have thought of the idea but don't remember seeing it.
    For your whipple shield, use some form of aerogel. Place in blocks easily removed and replaced, with blocks that have been exposed for some time brought in, collapsed, collect the micrometeoroids and other debris for recycling, and refoam the material to make a new block.

    Thanks, nice idea :). Since most aerogels are transparent would let the light through - and already used for micro-meteorite capture in space for study for Stardust.
    Did a google search and found this page suggesting the idea, of an aerogel to combine as Whipple shield + thermal and radiation protection. In their version, you have a metal layer as well for the first layer of protection of the Whipple shield - but surely can find a transparent polymer with similar properties to the usual metal layers for a transparent Whipple shield.
    Just come across this, a science fiction author who suggests the idea of building a large sphere in space, fill it with air, make it as large as you like - and don't bother rotating it. Instead you have spinning habitats inside it, using the volume of the air inside.
    The science behind the story - Sun of Suns.

    His is a far future science fiction idea so has artificial suns and so on.

    But seems to me, in near future also, it might be a great way to bring down the mass to population ratio from the 330 tons per person of any of the ideas using radiation shielding over the entire living area.

    In a big cylinder or sphere, filled with air, just need cosmic radiation shielding at the 4.5 tons per square meter on the outside of it. Could spin it really slowly like 1/100th g or something so you can use it for plants. Inside it just have a network of smaller habitats, or compartments, which spin around on their own axes inside it at the right rate to generate 1 g or whatever g is needed. Including some at zero g. Some large, some small, and because you don't need to do any more radiation shielding, then they can be really lightweight in construction.

    The Stanford Torus had the idea of a lightweight inner torus spinning inside the radiation shielding so you don't need to support that 4.5 tons per square meter, but since it's moving at over 200 mph relative to the shielding always seemed a bit dangerous somehow - though not really any more so than say a high speed train. Anyway - so later designs have the shielding rotating with the habitat. But if it's rotating slowly enough for 1/100 g no problem rotating the shielding - this is something I talked about before with idea of spinning habitats on the surface of a Stanford Torus. The only new thing here is the idea to have a cylinder instead of the torus and fill the inside with those spinning habitats. Need to have ways to direct the sunlight inside to the habitats using mirrors, but have mirrors already for the Stanford Torus.

    Is just an idea to think about, I wonder if it would work and is useful?
    Another idea here - if you are mainly interested in the interior of the O'Neil cylinder rather than maximizing the full g surface area to live on - then it doesn't need to be ten times as long as it is wide.
    Instead - slowly spinning so 1/100 g, only 45 kg per square meter for full radiation shielding instead of 45 metric tons - make it about as long as it is wide, or even shorter. So, for the very largest cylinders, 8 km in diameter as before but say, only a couple of kilometers in length

    The idea there is to maximize the amount of sunlight inside so you can fill the interior with lots of spinning habitats. And could eventually, because so slowly rotating, even say 1/1000 g, just to make sure things on the outside surface can be put there and stay there instead of floating off - so no problem with strength of materials don't need the titanium and steel of the original O'Niel, and have it tens of kilometers in diameter.

    Then for end walls, use 4.5 meters thickness of water so it lets the sunlight in. So interior is bathed with sunlight from both end walls, but with no issues of cosmic radiation (and easy to remove the UV also).

    As with the O'Neil idea, could have orientation control, have two of them counter rotating perhaps - and orientated so always once side faces the sun - or perhaps you could have them just stable in space, with seasons and with mirrors at either end so that in "winter" which happens twice a year when it is edge on to the sun (so maybe do in orbit with 2 Earth years orbital period) - then some light still gets reflected into the habitat for winter levels of lighting.

    Just ideas to think about, obviously rather further into the future. But with this method the materials in the asteroid belt would be enough, not just for a thousand times the surface area of the Earth, but tens or hundreds of thousands of times the surface area of the Earth - so long as you can find a source for all the atmosphere. The atmosphere would be the limiting thing then rather than the cosmic shielding and eventually you'd get that from comets, Kuiper belt, or wherever, outer solar system.
    But not so far future. O Neil pointed out for his smaller "island one"
    The removal of half a million tons of material from the surface of the moon sounds like a large-scale mining operation, but it is not. The excavation left on the moon would be only 5 yards deep, and 200 yards long and wide: not even enough to keep one small bulldozer occupied for a five-year period.
    It's also about the same amount of material as you get in a small NEO.

    His idea was to get the oxygen for the water for his habitat from the Moon so you only need to supply the hydrogen from Earth so can lift one ton of hydrogen from Earth for 9 tons of water in orbit. =

    Herne Webber
    Forgive me if this is an ignorant idea, but for shielding, instead of complex and expensive aerogels, why could one not simply spray whatever thickness of water is needed *outside* of the spacecraft?  You could have both micrometeorite and cosmic radiation protection, and it would be easily mended as it wore or if it ever cracked, and it's clear.  The water could be sourced from near Earth objects; it would be rather like living inside of a comet.  It has the added benefit of being able to be shaped into solid structural components, which could be used for support and/or as counterweights.  The optical properties of ice could also be wielded to create beauty.  Of course, one could add things to the water before spraying to increase its strength, or fix the optical properties such that the UV would be just right for both plants and for human vitamin D production.
    Yes, that's an interesting idea, thanks, a thin layer of optically almost clear ice to act as a Whipple shield. Of course ice normally forms cloudy. Doesn't matter over opaque areas but where you want the light to get through, need to devise a way to make optically pure ice in a vacuum. Ice sculptors use special clear ice block makers. Seems the cloudiness is to do with impurities in the water, dissolved gases, and how quickly or slowly you cool it. Maybe the vacuum would help by removing the bubbles of gas that make the ice cloudy?
    Another thought about the short low g slowly rotating O'Niel cylinder idea - make it rotate slowly enough and the terminal velocity is so low that you could fall from the centre of the cylinder and still be moving slowly when you hit the sides.
    Using the online terminal velocity calculator - not sure of cross sectional area of a human, would depend on orientation also but put in 0.1 m2 as that gave 200 mph as this site says that speeds over 200 mph require significant skill for a sky diver to achieve, so someone just falling at random is not likely to go over that. Then 1/1000 g gives the terminal velocity 6 mph pretty safe, like bumping into something when you are running.

    You could build gardens as platforms suspended in mid air, and get around by jumping from one to another. And would have no worries about people falling off them. And have spinning habitats in the middle for the humans to live on - all assuming we can tolerate high rpm artificial gravity - which nobody knows yet of course.
    There is a simple reason why a ship with an artificial gravity generating wheel-habitat will not be built any time soon.  We still stage missions to deep space from the surface of the Earth.   As long as that is the case, we will use rockets, with modules stacked on top of them, to go to space.  
    Leaving aside pipe dreams like inflatable habitats, we will not see an artificial gravity environment in space for say.... half a century at least.  If anyone would do it it would be NASA and we don't even have a human space flight vehicle of our own.  

    Until we have established the infrastructure in space to build space craft from materials found in space (mined from the moon or asteroids) we will not build a craft with artificial gravity.  If we try, then it'll be like constructing the ISS and more likely will only be half finished the abandoned as soon as it's politically expedient to do so. 
    Science advances as much by mistakes as by plans.
    Hontas, the inflatable habitats already do exist, Bigelow have a couple of demo projects, third scale, in orbit already. By 2015, Falcon Heavy will probably be ready for commercial flights, and able to launch something like that.
    The tether experiments don't need anything new at all, just use the Russian Soyuz launcher to launch a mission to the ISS and use a tether with the cable attached to the empty booster - or two Soyuz launched at the same time, to do Gemini type tether experiments.

    This is all on conventional rockets, no new technology.

    Also the Nautilus X centrifuge habitat was just a centrifuge sleeping module for the ISS. It could have been built and flown but never was. Not an engineering issue.

    Then, we don't need to build a big wheel type habitat right away. We might not even need it to be big at all. For instance, if 6 rpm is tolerable for artificial g, the spinning space station only needs to be 50 meters in diameter and it can be even smaller if higher rpms are tolerable such as the 12 rpm of the Skylab astronauts on their jogging track.

    Then if you go for the tether or tube system, you only need two habitats joined by a tether. With two of the Bigelow inflated habitats, a 50 meter tether between them, spin at 6 rpm and you have full g artificial gravity in both habitats. 

    Turn the 50 meter tether into a 50 meter air filled tube with multiple cables as stays as in my tube idea, and you also have an easy way to get from one habitat to another. Then with my idea to make that tube wider, and more substantial, if that worked out, you have a hugely increased living area in the middle between the two habs for growing plants etc.

    That idea could be built using two Falcon heavy launches for the original two habs plus the tether / tube - and then maybe another 3 for more substantial living area between the two habs.

    But all depends on the experiments. And those we could do right away. Simple tether experiments like Gemini. And if it worked, could have full g space stations by tether system easily within a few years using the Falcon Heavy.

    Inflatable habitats exist as test items and experiments.  So did nuclear powered rocket engines.  I'll just be very surprised if we see anything more daring than a redux of the Saturn V in the next 20-50 years. 
    Science advances as much by mistakes as by plans.
    Okay - I don't know about that, maybe NASA and Russia just won't explore this. Certainly haven't shown any interest in it for the last several decades.

    But there are lots more than just Russia and NASA will soon be able to send human spaceships into space, with the development of the Falcon Heavy.

    Once someone does the experiment - if this does turn out to solve all the serious medical issues of zero g - do you think that perhaps NASA might then think about adding a Nautilus X style centrifuge sleeping quarters to the ISS? Or do other artificial gravity experiments of their own including sending up paired tethered habitats?

    Research into nuclear powered rocket engines, if you mean the likes of the Orion nuclear pulse rocket, that's a bit different. It was stopped when the scientists realised that the fallout from the rocket exhaust made them too dangerous because they'd increase the number of cancer deaths in the general population. 

    But here with artificial gravity, there is no danger involved in the experiment, indeed it's solving a serious medical issue in zero g environments like the ISS.

    As for inflatable habitats, is really a separate thing, would be convenient for artificial gravity experiments but you just need human rated spacecraft and a tether to do your first artificial g experiments. I think that Bigelow might well do it though. The difference is, it's a private company so they don't need to depend on US government policy decisions. If someone else, and that could include e.g. any other government in the world, they had interest from Japan for instance - if any of them want to send a Bigelow habitat into space, and can afford to do it, then we might well see our first full scale inflated habitat in space.
    BTW NASA have actually awarded a 17 million contract to Bigelow for an inflatable habitat to attach to the ISS in 2015. It's still a test module, but pretty large, 4 meters by 3.2 meters diameter. The astronauts will probably not use it, just enter it occasionally to get data. But it shows interest of NASA in inflatable spaceship technology. Plus also means Bigelow has finance to continue working on their ideas.
    Small fast rotating sleeping centrifuges are sometimes proposed. But would they help against loss of bone an muscle? Wouldn't the astronaut need to stand up and being awake, not lying flat on the floor, in order to get the resistance exercise which helps prevent those health problems? Would a rotating cylinder of a bed in the ISS really be medically helpful? If so, how?

    I indeed agree that centrifugal force in human space travel is an idea which is finally about to come. Microgravity might be good for medical and materials research, but it's not for human space exploration.

    Yes that's a big question. But one thing, when they try to simulate the bone loss of zero g on Earth the volunteers don't just sleep horizontally, but with their heads lower than their feet. Also there's a lot more to the zero g health issues than just bone loss. And patients with long term bed rest don't lose 1 to 2% of their bone mass a month as astronauts do in zero g.So - I think at least it's possible that it would slow down the bone loss. But either way - the main thing is - that it would give us some data which we don't have. It's far too soon to think about planning small rapidly spinning centrifuges as a solution to zero g problems. Humans might not be able to tolerate them - and they might not have any effect, who knows.

    But it's not too soon to send a small centrifuge like that into space as an experiment - to get our first ever data points on the effects of artificial gravity on human health in zero g, and tolerance of humans for spinning in zero g.

    The astronaut could also sit up, even in a small centrifuge, and could do various exercises - plus also eating in artificial gravity (might help with the chronic malnutrition, and dehydration issues of astronauts in zero g).

    I have another article on this here:

    Could Spinning Hammocks Keep Astronauts Healthy in Zero g?

    Also for anyone interested in the tether ideas, then I have an article on that also:

    Ingeious Idea: Soyuz Crew in Tether Spin On Way to ISS - For Artificial Gravity - Almost No Extra Fuel

    BTW someone pointed out to me recently - that we do have one data point on rapidly rotating centrifuge type motion in space. 

    Gemini 8 did an unintended centrifuge in space experiment, actually hypergravity centrifiuge in zero g when it went into an uncontrolled spin.

    Spinning at 60 rpm at one point. Assuming  as rough guess, say half a meter radius, that's 2 g.

    Anyway not sure how long the fastest spin was but they were spinning for a total of 25 minutes at rates going up to 60 rpm, though I think that was just a short time, anyway total spinning time, from 5.41 to 6.06.

    Says here neither of them experienced any loss of orientation (spin is at about 19.30 into the video)

    Gemini VIII Neil Armstrong Agena Docking&Abort: 

    Both were nauseus afterwards and sick (but not during it). 

    It's not too surprising that they were sick as  they'd not had any chance to adjust to it over a period of several days as in the MIT experiments, was at double the 30 rpm, and with hypergravity as well.

    But Armstrong was able to act during the spin - they kept their co-ordination - the main problem he had was the artificial g.

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