6.5 decided that a population of 1000 was an adequate compromise; this figure was also negotiable since the ES group would first design for configuration to guarantee the quality of the prototype. If the hoped-for population figure was accurate, this would permit a picked, trained, adult crew (though the presence of children would be essential to verify the environment's effect on their growth). And since anyone having trouble adapting could be returned to Earth, the environment could spin up to 3 RPM as allowed in Section V.2.7. It was, in fact, essential to be able to return those who would adjust well to the colony environment as well as those who would not. The expected 30-year design life (or, in any case, the finite design life) would eventually force the entire population to move to Earth or to another colony. The ES group also had to consider the unfortunate possibility of a slow system failure such as a gradual accumulation of toxic substances in the human consumption cycles or an excessive leakage of breathing mixture. These problems could require moving the inhabitants to Earth in a short time since maintaining the existence of that many people in transit would be difficult. Thus permanent adaptations or adaptations requiring intermediate readaptation periods before return to Earth were unacceptable in a prototype colony. Unfortunately, as stated in Section V.3.1, there was no data on the permanence or semipermanence of such gravitational adaptations. The ES group decided that pseudogravity levels in the .Sg-lg range would be acceptable. The only ways to provide pseudogravity are linear acceleration and centrifugal force. Since linear acceleration was hardly compatible with staying near LS, the ES group decided to spin the colony. VI.3.4: Candidate Hulls: Since the colony would provide a breathable atmosphere within the vacuum of space, it should have an enveloping skin of some sort. Its function as a pressure vessel suggested a continuous structure without abrupt bends in the skin shape (which would lead to stress concentrations). The ES group considered two spinning-pressure-vessel hull shapes. Shape 1 was a cylindrical section with hemispherical endcaps; shape 2 was a torus. Both shapes are shown in Figure 6.2.
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