Space Adaptation Syndrome

Space adaptation syndrome (SAS) or space sickness is a condition experienced by around half of space travelers during adaptation to weightlessness. It is related to motion sickness, as the vestibular system adapts to weightlessness.

Space motion sickness is caused by changes in g-forces, which affect spatial orientation in humans. According to Science Daily, "Gravity plays a major role in our spatial orientation. Changes in gravitational forces, such as the transition to weightlessness during a space voyage, influence our spatial orientation and require adaptation by many of the physiological processes in which our balance system plays a part. As long as this adaptation is incomplete, this can be coupled to motion sickness (nausea), visual illusions and disorientation."

Modern motion-sickness medications can counter space sickness but are rarely used because it is considered better to allow space travelers to adapt naturally over the first day or two than to suffer the drowsiness and other side effects of medication. However, transdermal dimenhydrinate anti-nausea patches are typically used whenever space suits are worn because vomiting into a space suit could be fatal, as it could obscure vision or block airflow. Space suits are generally worn during launch and landing by NASA crew members and always for extra-vehicular activities (EVAs). EVAs are consequently not usually scheduled for the first days of a mission to allow the crew to adapt, and transdermal dimenhydrinate patches are typically used as an additional backup measure.

Space motion sickness was effectively unknown during the earliest spaceflights as these were undertaken in very cramped conditions; it seems to be aggravated by being able to freely move around and so is more common in larger spacecraft. After the Apollo 8 and Apollo 9 flights, where astronauts reported space motion sickness to Mission Control and then were subsequently removed from the flight list, astronauts (e.g. the Skylab 4 crew) attempted to prevent Mission Control from learning about their own SAS experience, apparently out of concern for their future flight assignment potential.

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Wikipedia