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Extravehicular activity (EVA) is an essential part of space missions. When International Space Station ALPHA (ISSA) is fully operational, EVA assembly, installation, maintenance and repair operations will become everyday repetitive work activity in space. Due to the high cost of EVA sorties, a logical step is to try to improve the productivity of the EVA astronaut in order to increase the amount of labor per EVA hour and to optimize the work/rest regime. This is a main goal of decompression protocol development. In order to allow the astronauts sufficient productivity within the EVA space suit it is necessary to operate the suit with an internal pressure lower than 1 atmosphere. The suit pressure of approximately 0.3-0.4 atmosphere (30-40 kPa or 220-300 mm Hg ) induces the risk of decompression sickness (DCS) by the formation of gas bubbles (GB) from excess nitrogen dissolved in the organism by breathing air at normal pressure in the space cabin atmosphere.

The human thermoregulation study during space operation has such practical result as scientific justification of automatic thermal control system (ATCS). A primary goal is a maintenance of the thermal comfort during all possible types of human activity. It is an optimal control aim to ATCS. The main purpose of ATCS development is an identification of physiologically valuable requirements and validation of optimal laws to control human thermal status in the space suit conditions. An identification is possible only if we have a good knowledge of the control aim and current thermal status. It needs to have a criterion or assessment of interface between thermal status, actions and disturbances. The model of human thermoregulation and metabolism in space suit with ventilation loop and water cooling loop is a theoretical basis for experimental tests of ATCS. Our model has two interdependent parts: 1) anatomical/physiological compartment and 2) technical compartment.