As a living environment, the International Space Station is hopelessly crippled. Anybody staying there for any length of time begins to suffer severe bodily disorders such as loss of muscular weight, including the all-important heart muscle, and poor sleeping patterns leading to mental issues such as tiredness and emotionality. Food has to be taken up regularly, as it cannot be grown on board. All this is down to one principal factor, the lack of gravity. Most of these symptoms are also experienced by patients who are bedridden for long periods. By contrast the problems with crop growing are unique to the weightlessness of space, where a plant's roots have no "down" to follow, nor its stem and leaves any "up". Its habitual seeking of food and sunlight is badly disrupted.
The current scientific approach is to tackle each of these issues head-on: exercise and dietary supplements for the astronauts, genetic engineering for the crops. While some small progress has been made, there is no sign yet that this can lead to long-term residence any time soon. It will take decades even before we understand the size of the problem, never mind solve it.
Take for example spaceflight-associated neuro-ocular syndrome (SANS). On Earth, fluids tend to drain down to the lower half of your body. Duing the night when you lie down they seep back up again,into your head among other places. When you get up next moring the fluid unloads back into yout body. But is zero gravity, the unloading does not happen. Throughout day and night, the fluid constinues to accumulate, eventually exerting pressure on your eyeballs and distorting them out of shape and out of focus. Astronauts who stay up for long visits suffer a marked deterioration in their vision. This can get so bad that they no longer see well enough to perform routine tasks and need to get help. Nobody knows the long-term effects, such as how much of the deterioration in vision might be permanent. One palliative being tried is a low-pressure sleeping-bag around the legs, to see if it will draw the fluid down and unload the head while the astronaut rests. Other changes take place in the brain, and we have no idea what they might lead to eventually. Would you really want to go on a far more prolonged flight, say a round trip to Mars, on that basis?
There is another way: artificial gravity. Simply spin the space station round and centrifugal force will act just like gravity does, only in reverse with the "floor" on the outside and your head pointing inwards to the central axis. It has long been studied and forms the stuff of many a space opera, not least Arthur C. Clarke's famous film, 2001: A Space Odyssey. But 2001 came and went and no such space station has appeared.
There seem to be two main reasons for this. The first is simply scale. The wheel has to be large enough not to cause dizziness and motion sickness as you whirl around, and to have room to grow crops under artificial sunlight. So it has to be much larger and more expensive than the ISS. The second is one of unique engineering challenges. How does the station keep its solar panels and communications antennae pointing in the right direction? How does an approaching spaceship dock with a spinning space station? How do you manoeuvre what is in effect a giant gyroscope to point in the direction you want it to travel? What happens if a section of the wheel gets punctured?
I would suggest that the solutions to these problems are in general easier and more practical than the current whack-a-mole symptom-based approach has turned out to be. All the technology already exists, there is no need for lengthy and exorbitant research programmes stretching decades into the future. That alone would pay for the large size of an artificial-gravity space station ten times over; it is only the rose-tinted dreams of the short-term cheapskates which have ever suggested otherwise. The remaining technical problems are all easily conquered by modern technology.
How does the station keep its solar panels and communications antennae pointing in the right direction? The simple answer is to put them in a separate module which does not spin. Physical alignment between the two units could be stabilised either via mechanical bearings or computer control. Electrical power connection between them might be by brushes similar to an electric motor, although brushless inductive or microwave power transfer would be more reliable. Whether the docking lock is stationary or rotating, discussed next, would affect the chosen solution.
How does an approaching spaceship dock with a spinning space station? There are two solutions. One is to spin the craft up under computer control until it matches the station dock. Anybody who has ever played the original version of the game Elite under manual control will know how hard this can be for a human. Modern systems can take care of the manoeuvre as easily as the old BBC Micro could, but the strange forces introduced by rotation will still be there, so it is unwise to suppose that every traveller would have no trouble with it. Dizziness and vomiting in the airlock would not be welcome, even if it didn't happen very often. Nevertheless it will probably the method of choice for the first generation of space stations, with astronauts selected for their resistance to such side effects. The better solution is to spin-down the space station dock so that it is stationary. Visitors would swing forward weightless, into a reception room where every direction is up and all the lifts and ladders are slowly rotating.
That would require air seals where the moving surfaces meet and the vacuum of space lies beyond them. Air is a precious commodity in a space station and once it's gone, it's gone. The rotating seals must be made airtight; how can this be done? Luckily, sealing against leakage has long been a feature of vacuum pumps of all descriptions, not least rotary pumps, and is an old and well established technology. Many hi-tech industrial processes and science experiments operate in a high vacuum close to that of space. They rely on high-vacuum pumps to suck out the air and to scavenge any that tries to leak back in. All such pumps have (or did in my day) parts which slide or rotate across each other, with high vacuum on one side and atmospheric pressure on the other. Scavenging the air as it leaks through the high-pressure seal, but before it reaches the low-pressure seal, is an effective solution. Cryogenic type pumps, or cryopumps for short, even employ ultra-cold surfaces, comparable to the cold of outer space, to condense the air and catch the liquid runoff. For a space station the outer surface of the scavenging chamber would be rotating. The artificial gravity this close to the hub would be minimal, but it would be enough to hold the liquid air condensate so that it can be drawn off, warmed up and recycled.
Another problem is how to manoeuvre what is in effect a giant gyroscope to point in the direction you want it to travel? Back in the day, that would have been a real problem. Manoeuvring thrusters need to be out from the centre so that they get good leverage and their size can be kept down. The ends of a long axle-like hub would be one solution but to realign the wheel would need a strong and heavy bearing. Distributing several thrusters around the rim of the wheel offers a more elegant solution, but they need complex and counter-intuitive synchronisation as the wheel turns. Again, this can easily be put under computer control. The thrusters could alternatively be put on gimbals so that the direction of flight could be changed without having to realign the wheel at all. Hybrid techniques would be possible, whereby a fast response is obtained by steering and/or pulsing the thrusters, while for acceleration and deceleration at either end of a longer journey, the axis would be realigned to point in the direction of travel, so that the gimballing system could be locked down to save energy.
But why the large size? The problem arises in what is called the Coriolis effect. For example if you are standing inside such a wheel and you throw a ball backwards against the direction of rotation, it will be rotating more slowly and so will fall more slowly. In fact, if you were to throw it just hard enough, it would not sink but would remain above the floor and presently sail right round to club you in the back. Well, actually it would have stopped rotating entirely and you would be the one who sailed round and clubbed the ball with your back. It will no doubt be a fun but highly illegal game to see how far you can throw one along the corridor. But throw the ball forwards in the direction of spin and it will be pulled hard downwards, thudding to the floor like a lump of lead. The same happens to different bits of you when you move. Even something as simple as turning around while you walk can create significant differences between one ear and the other, upsetting your sense of balance. At the same time, the "gravity" falls off towards the spin axis, so your feet are pulled down more than your head. If this effect is too strong then bending down to pick up something off the floor will feel very strange. Combining these various effects will do nothing for the spacefarer with a poor sense of balance or a weak stomach. The smaller the space station is, the faster it has to spin to maintain Earth gravity and the greater these effects become. Many of these effects have been tested on Earth by putting people in centrifuges. They suggest that a viable space habitat needs to be at least 60 metres (200 ft) across, with a radius of 30 metres (100 ft) and an overall circumference around the rim of 100 metres (300 ft). This reduces the variations (gravity gradient) to around five percent of gravity over the height of the human body. Total habitable length of the rim plus say two spokes and the hub would be around 160 metres (500 ft). By comparison the ISS checks in at just over 100 metres long, with the habitable space clustering along its shorter axis. Various habitable sideways extensions add up to about the same as its overall length, making 200 metres in all. So adding a full artificial gravity wheel would make the size of the habitable zone some 12 1/2 times bigger.
Finally, there is the question as to whether we really need Earth normal gravity to remain healthy. Might say half or three-quarters Earth gravity be enough? No direct technical issue arises from this, but it could ease some othe others. For example it might be that a 0.75 gee environment would be enough, and that it would ease the dizziness problems so that a smaller diameter could still be comfortable.
While a full wheel would be much larger than the habitable parts of the ISS, the uninhabited equipment bays can also be included, substantially reducing the overall size increase. Some recent concepts for a Mars trip have proposed a much smaller wheel of a quarter that size or less. This might work for a small cadre of screened and medicated astronauts, but there has to be a big question mark over how many people could function normally under such a regime for long periods, and such small wheels are unlikely to be a viable long-term solution.
The ISS began life much smaller and has grown through several cycles over many years. It is also in a low Earth orbit and in no position to go anywhere else - except back down. It flies so low that it brushes the halo of air extending beyond the atmosphere proper and over time that is slowly dragging it down. It is so fragile that even the small thrusters needed to stabilise its orbit and extend its lifetime present a significant engineering challenge. By contrast the habitable wheel is inherently braced against itself and hence is stronger and stiffer than the branching tree structure of the ISS. In practice the first such wheel would be built in a similar bit-by-bit fashion and then powered to a higher orbit.
Step by step assembly of a wheel requires a little more ingenuity, but is entirely possible. Indeed, for a small crew there should be no need for a complete wheel at all.
The minimum essentials for any space station include a habitable module, a docking airlock, an equipment and storage bay and a solar array. The minimum configuration for a rotating structure of any size is a long stick which rotates around a point half way along, like the baton of a drum majorette. Set the dock at the point of rotation, the habitable module at one end and the equipment bay at the other and you need only adjust the two arm lengths thus created to achieve a smooth rotation. If the station were off-balance then its hub would slowly gyrate around a point off-centre. Although software would let a spacecraft dock, this would introduce unwelcome mechanical stresses on the two craft, and perhaps induce seasickness. It would be necessary to provide some mechanism for aligning the port with the centre of rotation, either by giving the port some freedom of movement or by distributing the payload more evenly around the main structure. Like an aircraft, the station would have a permissible range for its centre of gravity. If the central dock is able to slide along a fixed superstructure, with the habitable tubing along each arm flexible in length, then only one adjustment system is needed. A space station of this type stars in my novella Nobody Steals My Air.
This design has a potential problem in that it is not dynamically stable. Over time the arm will wobble off the axis of rotation. In the short term this can be managed with active controls, but for extended use this would consume too much energy and a more stable geometry would be needed.
To grow the space station, the modules at each end may be extended around the rim and the new sections held in place by lightweight tension wires, exactly like the spokes of a traditional bicycle wheel. The more you add, the more dynamically stable the station becomes. If desired, a second cross-arm could be spun up and synchronised at right angles to the first, then slid over the existing hub to provide structural rigidity and a couple of extra access routes. The position of the hub would then need to be correct in two dimensions. While this means that it would no longer be so easily adjustable, a small range of adjustment would still be feasible. Luckily the station's increased mass would also reduce the effect of any off-centre loading, but care in distributing that load must always be a constant operational necessity, just as it is with all aircraft.
What would the structure be made of? Most designs have assumed light alloy metals like any other spacecraft, including the ISS. Metal provides the necessary strength to hold the air pressure and to stiffen the structure. But that creates a lot of weight and bulk to lug up there. Carbon composites might help a little, but would be more difficult to make compatible with the extreme environment of space. Inflatable fabrics offer a much more compact and lightweight launch package. Once inflated, the air pressure is enough to hold them rigid, just like the tyre of a road vehicle. The ISS is already trialling the approach, with the Bigelow Expandable Activity Module (BEAM) having been in place since 2016. It has been a resounding success, having exceeded expectations and been given a major life extension.
Radiation, high-speed objects and extremes of heat are significant hazards. Most space radiation comprises massive particles such as protons and atoms. Multiple thin layers, separated by a short distance, are extremely effective against high-speed objects. On hitting the outermost layer the object and the layer both melt or vapourise. The atoms or droplets scatter and hit the next layer as a diffuse cloud, each particle having much lower energy. The cascade soon dissipates. Aerogel can be thought of as multiple layers and is very effective at stopping small, high-speed particles. A sandwich of protective outer metal foil over a layer of aerogel would shield the structural fabric from extremes of hot and cold, from small meteors and from all radiation except gamma rays. The structure would be manufactured with the foil attached by short threads. Following inflation in space, a foaming liquid compound would then be injected into the cavity; the gas generated would both expand the cavity and fill it with an open foam which would quickly dry to become the aerogel.
As a first step in the programme, it will be important to confirm the minimum diameter and rotation speed necessary for people of varying fitness. Technologies for aligning the hub and shuttle craft will also need to be tried out and evaluated. A research station will need to be built, initially unmanned.
Once the vacuum seal and the basics of getting on and off are sorted out, the astronauts can move in. The arm would be extendable, and the relative weights of the two end modules progressively altered, to establish the best way to adjust the Coriolis forces in the living module, while maintaining constant gravity throughout.
If the research prototype proves viable, it could be extended to a full wheel with greater onboard facilities. This would be useful in proving the construction methods for larger permanent stations.
Nothing requires a major technological breakthrough, just careful use of what we can do already. Nor do I see any real point in packing the production model off to Mars at the first available opportunity, there is plenty to get done nearer home - and nearer the Moon - first. But that is another game altogether.
Updated 10 Dec 2021