[Mechanism of development of space motion sickness]

Astronauts and scientists are scheduled to stay aboard the International Space Station (ISS) for three-month periods, and various problems relating to the human ability to live in space for such long periods of time, as well as medical problems, are being studied. For the crews that have operated and worked on the US space shuttle, the longest time spent in the gravity-free (accurately speaking, microgravity) environment of space has been about two weeks. Even such short periods have generated reports of various physical changes and symptoms caused by the absence of gravity. Particularly striking effects are circulatory system changes, muscular atrophy and decalcification of bones. In addition, serious problems arise during the time in space due to the symptoms of space motion sickness, caused by neuro-otological changes. Space motion sickness is experienced by nearly 70% of the first-time astronauts, in the form of such symptoms as nausea, and vomiting. It is said that space motion sickness develops at the time of adaptation of the body to the gravity-free environment. Unlike motion sickness on Earth, space motion sickness is characterized by a feeling of fullness of the head, a hot face and reduced sweating. The provocative factors include movement of the head in the pitch and roll directions and the unusual visual field. This paper reviews the research that has been carried out to date on space motion sickness, and its counter measures comprehensively, in terms of the changes in the otolith organ as well as asynchronization of information between the movements of the head and eyeballs, the deep sensations and body fluid shifts. Other problems, which may be encountered as a result of yet longer stays in space, are also discussed.