Crew members will be stationed for three months or more on orbit aboard the International Space Station (ISS), an international project which will take 18 years to complete. After obtaining good experience from the ISS activities, the duration crew member stays in the space environment should progressively increase as may be required for the manned moon base and Mars flights in the next space generation. As crew members stay longer in the space environment, "habitability " for crew members will become more important in manned space systems. How can we prepare comfortable living areas and working areas for crew members? One answer is to fully accommodate the unique factors of the space environment and crew members. In this context, we cannot establish habitability for manned space systems without clarifying the influences of the space environment on crew members and providing the countermeasures for crew members. In this paper, the author proposes a "habitable model" which is composed of four factors: "Physiological factor", "Psychological factor", "Habitable environment factor", and "Operational factor". The paper also clarifies guidelines for establishing and maintaining "habitability" for the manned space systems.
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