this amount is spread out over a human lifetime up to the end of the reproductive period, such a dose would be accumulated at about 1 rem per year. Much higher dose rates are tolerable, of course, and probably the present mutation rate in human beings could be multiplied by a factor much higher than two before it would become painfully evident.

Other conditions that might render a planet uninhabitable would be an excessively high meteorite-infall rate, an excessive degree of volcanism, a high frequency of earthquakes, and possibly an excessive degree of electrical activity (lightning).

In summary, habitable portions of planets have the characteristics shown in Table 4.

Having now summarized the principal requirements of human beings with respect to the environmental conditions provided by a planet, we must next consider the varieties of planets that are known to exist or that can be postulated. Once the ranges of environments that prevail on planetary surfaces are Circumscribed, we can determine how well these correspond to human requirements.

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