The above-stated requirements for conditions of temperature, light, gravity, atmospheric composition and pressure, and water are probably the major human requisites; yet there are many others. These will be stated briefly.
As indicated earlier, other life forms must be present, since ultimately all human food supplies (and probably the very existence of free oxygen in the atmosphere) depend on photosynthesis in green plants. There must also be an absence of inimical intelligent beings in prior possession, for the phrase "habitable planets for man" is meant to indicate planets that are not already "taken." Man, presumably, can always cope with nonintelligent life forms.
Commonly experienced wind velocities in otherwise habitable regions must be of tolerable levels. Regions in which wind velocities consistently reach strong gale force (about 50 miles per hour) or higher would not be considered habitable.
Similarly, dust normally encountered should be below certain specified levels. Air Force Pamphlet 160-6-1 suggests that total dust (containing less than 5 per cent free silica) should not exceed 50 million particles per cubic foot of air and that high-silica dust (containing more than 50 per cent free silica) should not exceed 5 million particles per cubic foot of air. Dust concentrations exceeding these threshold limits are considered harmful.
The bodies of water of a planet are the primary receptacles for airborne dust; and water droplets forming on dust nuclei provide the primary means of removing dust from the atmosphere. Thus, it is to be expected that a planet possessing oceans would not have a particularly dusty atmosphere, while a planet with a turbulent atmosphere but without oceans would be a very dusty place indeed.
Ambient levels of radioactivity or ionizing radiation, whether caused by radioactive materials in the crust or by high-energy particles coming through the atmosphere from stellar sources, must be of acceptable intensity. For genetic reasons it would probably be desirable to specify dosages from natural background radiation of less than 1 roentgen per year on a steady-state basis, or approximately 0.02 rem (roentgens-equivalent, man) per week. (The Atomic Energy Commission's steady-state tolerance level is 0.3 rem per week for workers in atomic energy plants; the average natural background radiation on the Earth's surface is about 0.003 rem per week.) Not much is known at present about the effects of prolonged steady-state exposures to very low levels of dose rate. The "mutation-doubling rate," however, is generally taken as 30 to 50 roentgens; and if
Table 4. Summary of Characteristics of Habitable Portions of Planets
Temperature mean annual (°F) mean daily (°F) Light at surface, peak values (lu/cm2) Gravity (relative to Earth normal) Atmospheric composition, inspired partial pressure (mm of Hg) oxygen carbon dioxide helium™ neon® nitrogen" argon" krypton" xenon" toxic gases Water vapor, partial pressure (mm of Hg)
Other characteristics open bodies of water indigenous life forms tolerable wind velocities, dust levels, natural radioactivity, meteorite-infall rates, volcanic activity, and electrical activity
" If more than one inert ingredient is present, maximal values should be in proportion to the relative concentrations.
b Amount depends on specific gas. c A function of temperature.
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