Living in a cramped, tightly controlled space, surrounded by a near-vacuum environment, is only one of the many inconveniences for humans dwelling in space stations. Much more serious is the effect of weightlessness. In that state, many familiar daily activities such as moving, sitting, sleeping, eating, washing, and other bodily functions become cumbersome. Moreover, there are severe physiological reactions by our body. Even on Earth, many types of degenerative change occur when prolonged bed rest takes the load of gravity off our bodies. It is therefore clear that extended stays in zero-gravity or low-gravity environments on the Moon and Mars will not be enjoyable. For the successful colonization of the solar system, it is therefore necessary to find ways of restoring gravity. This can be achieved by employing centrifugal forces. The use of rotating habitats was proposed almost half a century ago (see Billingham et al. 1979). Figures 9.5-9.7 show plans for large space colonies, constructed like wheels, rotating around a central axis. Humans would live inside the hollow pressurized tube, forced to the inner surface of the torus by centrifugal forces. Such a wheel, with a spoke radius of 10 km, for example, could provide a habitat of 63 km length. Rotating with a speed
of 1100 km/h, which would be unnoticed just like travel in an airplane, the inhabitants would experience a weight similar to that felt on Earth.
Figure 9.6 shows a multi-level torus which has recreational, residential, and industrial areas, with trains servicing the length of the colony and elevators the different levels. By importing soil from the Moon, the landscape could be modeled in an Earth-like manner, with individual housing designs. Intensive and well controlled agricultural production (Fig. 9.7) could supply food for a population of many thousands of inhabitants.
Other designs of such space colonies, envisioned by O'Neill (1974, 1989), would have the shape of long cylinders with a diameter of 6 km and a length of 30 km, rotating around their axis once every 110 s. The axis of the habitat would be pointed toward the Sun. As seen in Figs. 9.8 and 9.9, the cylinder walls of such a colony would have three transparent sections where sunlight would illuminate three interior "valleys" via mirrors. By manipulating the mirror surfaces, the Sun could be made to rise and set in a natural 24-hour day-and-night cycle, and even the seasons could be mimicked. A problem with this type of space habitat is, however, that by orbiting around the Sun, the station's rotation axis will lose its orientation to the Sun. O'Neill therefore envisioned two identical cylinders rotating in opposite directions, connected by thin support structures (Fig. 9.10). In such twin systems, the rotation axes could be forced to always point to the Sun.
It was found that inside an O'Neill station, clouds would form naturally at a height of 1 km above the inner surface. Soil provided from either the Moon or near-Earth asteroids would be the basis for landscaping and agriculture, generating environments similar to those on Earth. With a five times larger usable area, compared to a torus-type colony, the O'Neill station could have a population of from 200 000 to 20 million people. Moreover, such colonies would be surrounded by small service stations (Fig. 9.10), in which every environmental condition for optimal agricultural production could be realized. Because of the limited volume of such agricultural stations, control of the environment, pests, and plant diseases could be easily achieved.
In this most advanced stage, space colonies would become fully self-sufficient. With their own fleet of space vehicles and extensive asteroid mining operations, they could evolve into small independent worlds of their own, possessing a fully developed industry capable of manufacturing the whole range of commercial products (foodstuff's, household machines, tools, building materials, electronic devices, telecommunication equipment, motor vehicles and spacecraft). As a consequence of this industrial activity, they would become major trading partners of the Earth, by exporting metals, minerals, energy, equipment, and food in return for specialized high-tech products. By man ufacturing reentry vehicles and heat shields in space, it will be possible to transport these goods cheaply to the terrestrial surface.
In addition, the O'Neill-type habitats would become major construction centers for all kinds of projects to build other habitats and outposts on planets, moons, and asteroids. They would possess workshops to build power plants and have dockyards for assembling space vehicles. One can safely envisage that, eventually, large numbers of such O'Neill-type space colonies with millions of inhabitants will float in the vast expanse of the solar system, producing every commercial product conceivable, while at the same time being fully self-sufficient and independent.
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