A bioregenerative life-support system will be required in long manned space flight, especially with several astronauts such as would be required for a manned mission to Mars in the 1980 time period. While almost 15 years is a long leadtime, the biological research and engineering problems are formidable, and a system would have to be developed at least 5 years before the mission. The power and weight requirements for both chemical and biological regenerative life-support systems were presented in table VIII. These should be considered tentative best estimates based on present data. The use of bioregenerative systems in spacecraft systems has been studied by Bongers and Kok ([ref.175]) who put the electrolysis-_Hydrogenomonas_ system in proper perspective with the following statement: The bioregenerative systems are more or less in a transitory phase between research and development. The power data can be considered fairly accurate, at least within ±20 percent. The postulated weight data, however, represent approximations, particularly with respect to auxiliary equipment and construction materials. Also omitted are the weight penalties most probably involved in the processing of the solid output of the exchangers, elegantly defined as potential food. Further research is required in this area to evaluate the regenerative systems, especially the bacteria, with respect to this potential. Furthermore, as yet there is no experimental proof that the growth rates of the heavy bacterial suspensions can be realized in a large design, determined on a relatively small scale with fairly precise control of physiological conditions and gas exchange. This aspect may affect considerably the weight involved in a chemosynthetic balanced system. Nevertheless, at present, this approach still seems most promising. CABIN ATMOSPHERES⁹ ⁹ Includes part of [ref.196]. In the first U.S. manned space flight program, Project Mercury, and in the face of very severe weight limitations, a cabin atmosphere of pure oxygen at one-third atmospheric pressure was adopted. This choice probably represented the greatest simplification which could be achieved readily and, at the same time, provide protection against some of the risks of rapid decompression. Although breathing pure oxygen at higher pressures was known to be attended by some undesirable physiological effects, the short duration of the flights to be undertaken, and the low pressure employed, suggested that no harmful results would result in this case. That these expectations were generally borne out is now history. Preparations for space flights of longer duration—many weeks or months—present similar problems and require special attention to phenomena which may be either undetectable or of trivial significance on a time scale of a few days. Physiological Criteria in the Choice of Cabin Atmosphere