Puking or bouncing (or improbably large)

If only to make the toilets work properly, some form of artificial gravity-like effect is likely to be necessary if people are to spend long periods in space in any sort of comfort. Barring new physics, the only way known to make up and down mean something is either to live on the surface of a large enough mass or on the inside of a rotating object.

Off Earth, the only large enough masses remotely hospitable (after a lot of trouble and effort) would be the Earth's moon and Mars, with gravities of 0.165 and 0.38 that of earth, respectively (that is ~1/6th and ~1/3 respectively, in old money). It might be reasonable to consider what conditions of rotating object one would need to inhabit to give conditions equivalent to the Earth, Moon or Mars. The table below shows some of the options.

Table showing the diameter to give artificial gravity, equivalent to that on the surface of the bodies shown, at the specified rates of rotation

 Rotation rate (revs. per minute) 3 2 1 0.53 0.306 Diameter (metres) 33 Moon 74 Moon 76 Mars 170 Mars 200 Earth 296 Moon 450 Earth 680 Mars 1800 Earth 6400 Earth 19200 Earth

Italics indicate the extreme possibilities of diameters of 4 miles (O'Neill Island 3 cylinder) and 12 miles (titanium sphere).

The lower the rate of rotation the better. Faster spinning increases likelihood of motion nausea. It seems to be generally accepted that most people could rapidly adjust to 1 revolution per minute (rpm) and 3rpm has been taken as a practical limit for a population other than those specially resistant to inner ear disturbance (although some would put that limit at 4rpm). Of course, these estimates are based on Earth bound experiments and are, therefore, in need of proof in space. It seem very unlikely however, that a higher rotation rate will prove easier to cope with than a lower one, so the preferred aim is clearly to have the lowest rotation rate practical to give the effect required.

The smaller the structure the cheaper. All other things being equal, the smallest structure (for the purposes of this discussion the smallest radius of rotation) that will give the desired effect is preferable, as being easier and cheaper to make and put into orbit. However, it has been suggested that for diameters less than 40 metres the differential between the effects on the upper body and the feet would be too great to be acceptable. On this basis the smallest diameter shown in the table would probably not work. However, given that we can readily manufacture structures that are strong, light (and to a degree pressurisable) of ~70 metres length (e.g. fuselage of a Boeing 747) it does not seem unreasonable to envisage rotating space structures that could give us Moon gravity at 2 rpm or Mars gravity at 3 rpm (i.e. second and third smallest diameter conditions shown on the table).

Is Earth gravity necessary? If we don't produce Earth gravity then we'll bounce, as when astronauts walked (or hopped or skipped) on the Moon, and if we stay there long enough our bones will weaken and we may find it difficult to return to Earth normal gravity. However, to spin a structure of a size that we could fairly readily produce, fast enough to give Earth gravity effect would probably result in a vomitorium.

If we add the problems of adequate radiation shielding of a small structure, it seems unlikely at this stage that people will be spending several years in such artificial gravity environments. A more likely scenario might be that of colonization on (or, perhaps better, below) the surfaces of the Moon or Mars, where radiation shielding would be a matter of depth and raw materials would be readily to hand. The role of spinning structures, of matching artificial gravities, would then make more sense in relation to these two bodies, with whatever the benefits that justify being in orbit around a body rather than on it. The lower gravities provided would still be better than zero gravity in terms of deleterious health effects, personal comfort etc..

Experience gained with these may allow the eventual manufacture of the very large diameter colonies that are physically possible, but are currently politically and financially impractical and improbable. A gradual, evolutionary approach, via lower levels of artificial gravity may eventually allow us to reach a stage of Earth normal comfort.