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 with motion sickness (nausea), visual illusions and disorientation." one understanding of motion sickness is that nausea is a pro-survival evolutionary adaptation. this is because the sensory stimulation of a maladapted high acceleration environment that the body is not accustomed to is recognized by the brain as being similar to the sensory conflict from eating a poisonous plant, in which case vomiting is a reaction.