The sterilization goal established for Mars landers is a probability of less than 1 in 10 000 (10⁻⁴) that a single viable organism will be present on the spacecraft. Laboratory studies of the kinetics of dry-heat kill of resistant organisms show that at 135° C the number of bacterial spores can be reduced 1 logarithm (90 percent) for every 2 hours of exposure (refs. [ref.58] and [ref.62]). The reduction in microbial count needed is the logarithm of the maximum number on the spacecraft (10⁶) plus the logarithm of the reciprocal of the probability of a survivor (10⁴), or a total of 10 logarithms of reduction in microbial count. Thus, with an additional 2 logarithms added as a safety factor, a total of 12 logarithms of reduction in count has been accepted as a safe value which can be achieved by a dry-heat treatment of 135° C for 24 hours. This is the heat cycle that is currently under study and being developed for use in spacecraft sterilization ([ref.60]). However, other heat treatments at temperatures as low as 105° C for periods of 300 hours or longer are under study ([ref.63]). Based on results to date, it is reasonable to believe that a full complement of heat-sterilizable hardware will be available when needed for planetary exploration. Every effort is being made to improve the state of the art to a point where spacecraft can not only withstand sterilization temperatures, but will be even more reliable than the present state-of-the-art hardware that is not heated. chapter 3 _Environmental Biology_ BIOLOGICAL EFFECTS OF WEIGHTLESSNESS AND ZERO GRAVITY High priority has been given to studies of weightlessness. Gravity is one of the most fundamental forces that acts on living organisms, and all life on Earth except the smallest appears to be oriented with respect to gravity, although certain organisms are more responsive to it than others. The gravity force on Earth is 1 g, but this force may be experimentally varied from zero g, or weightlessness, to many thousands of g’s. Zero gravity or decreased gravity occurs during freefall, in parabolic trajectory, or during orbit around the Earth. Gravitational force decreases by the square of the distance away from the Earth’s center. It is reduced about 5 percent at about 200 nautical miles’ altitude. Gravitational force greater than 1 g can be obtained by acceleration, deceleration, or impact. It also can be increased by using a centrifuge which adds a radial acceleration vector to the 1 g of Earth.