This critical compilation involves a review of a very broad and complex range of subjects involved in many different disciplines with widely scattered literature. Since the effects of many of the specific environmental factors are harmful, it is difficult to select a point on a scale from no effect to death and use some criteria to say that normal or even minimal growth and reproduction are occurring. The effects of environmental factors are dependent on (1) the specific factor, times, (2) the concentration or energy, times, (3) the time of exposure or application of the factor. Many reports, especially older ones, do not give all of the necessary data to permit proper evaluation. A complicating factor is that the effect of each factor depends on the other factors before, during, and after its application. The condition of the organism itself is a great variable. Proper evaluation requires the critical review by a variety of biological specialists, physicists, and chemists. To determine the potential of Earth organisms to survive or grow under other planetary environmental conditions, a number of experiments have been carried out attempting to simulate planetary environments, especially of Mars, as reviewed previously. While the results are of real interest, they do not provide much basic information. Further, as the Martian environment is more accurately defined, the experimental conditions are changed. In addition, some experimenters have altered certain factors, such as water content, to allow for potential microhabitats or for areas which might contain more water at certain times. Table III.—_Extreme Physical Environmental Factors_ ----------------------------------------------------------------- Physical Minimum Organism factors ----------------------------------------------------------------- Temperature -30° C Algae (photosynthesis), pink yeast (growth) ----------------------------------------------------------------- Magnetism 0-50 gamma (=×10⁻⁵ Human gauss) ----------------------------------------------------------------- Gravity 0 g Human, plants, animals ----------------------------------------------------------------- Pressure 10⁻⁹ mm Hg (5 days) _Mycobacterium_ _smegmatis_ ----------------------------------------------------------------- Microwave 0 W/cm² ----------------------------------------------------------------- Visible 0 ft-c Animals, fungi, bacteria ----------------------------------------------------------------- Ultraviolet 0 erg/cm²