Before moving on to discuss how the present atmosphere of Venus might be made breathable for humans, there is one additional topic, related to the collisional impacts issue discussed earlier, that should be addressed. This concerns the incredibly slow rotation rate of Venus, equivalent to a slothful 224.7 Earth days (see Table 7.1).
To a certain extent, the slow rotation may be a nonissue, but many researchers have suggested it might be desirable to increase the Venusian spin to something like that of the Earth's. Here the idea is that many terrestrial plants and crops do not grow well in permanent daylight conditions. This is likely a problem that can presumably be solved by genetic modification, and we should also note that initially all the food crops will be grown in artificial environments where the outside light can be easily controlled. If one does wish to produce a more rapidly spinning Venus, however, then it had better be done early on in the terraforming stage, before human colonization has begun.
Perhaps the simplest way to both cool Venus and induce an artificial day/night cycle shorter than the natural one (a period equivalent to 116.75 Earth days) is to place a louvered sunshade or variable transparency parasol at the Venusian L1 point. A circular parasol located at the Venusian L1 point would need to be about 25,000 km in diameter in order to completely obscure the Sun. This is a colossal size, over twice that of the planet itself, and a poignant reminder of just how complex the engineering and material resource requirements for terraforming Venus will be. By introducing a variable transparency or louvered system the sunlight levels on the daylight hemisphere of Venus could be turned on and off as required. On the night-side hemisphere of Venus, however, one would have to use either a series of orbital mirrors to reflect sunlight onto inhabited regions, or rely solely on artificial lighting.
The most basic spin-up mechanism would be one resulting from glancing impacts. This is not a mechanism that is active now, but when the Solar System was newly formed (4.56 billion years ago) and the planets themselves were still growing through accretion, there were many large, multithousand kilometer-sized objects moving along dynamically unstable orbits around the Sun. Indeed, the origin of Earth's Moon, the large obliquity of Venus along with its slow rotation rate, the relatively large iron core of Mercury, and the high obliquity of Uranus are all attributed to offcenter impacts from large proto-planetary bodies that occurred late in the planetary formation stage.
Arranging collisions from large, several hundred kilometers in diameter KBOs is probably the most straightforward way to increase the spin rate of Venus.8 Indeed, there is much to recommend the collisional method for partially denuding the Venusian atmosphere, spinning up the planet, and potentially generating Venusian moons and/or an equatorial debris shade. Certainly, the directed-impacts method smacks of a rather Neanderthal approach, but it is nonetheless a highly practical way of achieving some of the desired initial goals in the terraforming of Venus. There is a ready supply of large KBOs in the outer Solar System to perform the task at hand, and the essential technical means of altering and guiding an impactor's orbit are already known to us in principle. The practical ability to realize the required engineering, however, is still likely to be many centuries away—a further indication, if one was actually still needed, that the terraforming of Venus will not be a quick or easy task.
Directed collisions are by no means the only ways by which our descendants might spin-up Venus or for that matter an asteroid, satellite, or other planet. Freeman Dyson (Institute for Advanced Studies, Princeton), who is never afraid of thinking both big and bold, suggested in the mid-1960s that an electric motor arrangement could be engineered to increase the spin rate of a planet. His starting point for the idea came about from thinking about how the existence of a technically advanced extraterrestrial society might be recognized. This line of thinking eventually led Dyson to the idea of what are now called Dyson spheres and the concept of what are known as Kardashev Type II civilizations.9
Since an advanced civilization would undoubtedly require a vast resource of raw material, it seemed reasonable to conclude that it would develop the means of disassembling large asteroids and possibly even planets (planets, presumably, that is, not required for terraforming). Rather than disassemble such objects by direct mining, Dyson reasoned that it would be simpler for the asteroid or planet to disassemble itself. This self-destructive step could be achieved, he argues, by inducing rapid spin. Indeed, if the centrifugal force due to rotation exceeds the tensile strength of the material body, then the body will literally fly apart, and this is what Dyson had in mind. In a somewhat medieval sense, it might be said that this mechanism literally flogs itself to death.
The full physical details of the Dyson motor need not concern us here,10 but the idea is to turn the planet (or asteroid) into a giant electric motor. Indeed, by generating very specific magnetic field topologies around the object to be spun up and by placing numerous electrical generators in orbit around it, the planet/asteroid will behave like a massive armature. At this stage, suffice it to say, the end result of the engineering is that angular momentum is transferred to the entrapped body, and it will begin to spin faster.
Eventually the spin limit, the point at which the object flies apart, will be achieved, and the assorted pieces can then be captured and further processed into building material.
Dyson's idea is certainly elegant, but it seems overly complicated. Although the directed-collision approach can achieve the same end goals more simply than the Dyson motor in the asteroid or small moon disruption cases, the physical destruction of a planetary-sized object, should this ever be desired, may well have to proceed by a method such as that proposed by Dyson.
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