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 supplied regularly, as it cannot be grown on board. All this is down to one single 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 hopelessly disrupted.
The current scientific approach is to tackle each of these issues head-on: exercise and food 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 practical interplanetary travel any time soon. It will take decades even before we understand the size of the problem, never mind solve it.
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. A wheel large enough not to cause dizziness and motion sickness as you whirl around has to be larger and more complicated 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?
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. 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 mistaken 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 might be via a mechanical bearing or digitally via separate stabilisation systems. Electrical power transfer might be by brushes similar to an electric motor, although a 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 will know how hard this can be for a human. However, though the computer can take care of the manoeuvre itself, 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 other solution is to spin-down the space station dock so that it is stationary. Visitors would step forward into a reception room in which every direction is up and all the lifts and ladders are slowly rotating. The technical issue is one of air seals where the moving surfaces meet and the vacuum of space lies beyond them, but as sealing against leakage has long been a feature of vacuum pumps of all descriptions, not least rotary pumps, it can at worst require only careful engineering.
How do you 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. Several thrusters distributed around the rim of the wheel offers a more elegant solution, but they need complex and counter-intuitive synchronisation as the wheel turns. Nowadays 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 larger 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. Now, the smaller the space station the faster it has to spin to maintain Earth gravity and the greater these effects become. 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. These effects has never been properly put to the test, but I estimate that a viable space habitat needs to be at least 40 metres (130 ft) across, with a radius of 20 metres (65 ft) and an overall circumference around the rim of 125 metres (410 ft). This reduces the variations to less than ten 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 (520 ft). While the ISS checks in at just over 100 metres long, the habitable space clusters along its shorter axis but various sideways extensions bring it to about the same as its overall length. So adding artificial gravity would increase the size of the habitable zone by some 60%. The uninhabited equipment bays can be left in a weightless central module. Some recent concepts for a Mars trip have proposed a much smaller wheel of half 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 it is unlikely to be a viable long-term solution.
While a truly habitable wheel would not necessarily be that much larger than the ISS, the current station 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. The habitable wheel is inherently braced against itself and hence is stronger and stiffer than the branching tree structure of the ISS. Is there any way to build a such a wheel in a similar bit-by-bit fashion and then power it to a higher orbit?
A step by step 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 length 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. 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.
The modules at each end may now 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. If desired, an additional arm could be spun up and synchronised at right angles to the first, then slid over the existing hub to provide structural strengthening and a couple of extra access routes to the hub. As the rim grows towards completion, the position of the hub will neeed 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.
As a first step in the programme, it will be important to establish the minimum size necessary for people of varying fitness. A manned research station would need to be built. The arm would need to be extendable, or the relative weights of the two end modules progressively altered, to adjust the Coriolis forces in the living module for each experimental period. 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 4 Dec 2018