Essay 12.3 Space Motion Sickness

Astronaut David A. Wolf, STS-112 mission specialist (with his feet secured to a foot restraint on the end of the Space Station Remote Manipulator System or Canadarm2) participates in a six-hour and four-minute session of extravehicular activity on which he was joined by astronaut Piers J. Sellers (out of frame). (Photo courtesy of NASA)

When astronauts experience weightlessness (see photo above), their otolith organs no longer sense gravity. This produces a major change in the signal sent from the otolith organs to the brain and the brain needs time to adapt to this change. Until the brain can adapt, the incoming sensory cues from the otolith organs do not match those typically experienced on Earth. This leads to sensory conflict, which, in a general sense, we define as a mismatch between how various sensory cues correlate with one another. Typically, astronauts adapt to the absence of measurable gravity in just a few days. However, this can lead to problems when the astronauts return to Earth because now the mechanisms used by astronaut’s brain are tuned to the sensory inflow that occurs when the astronauts experience weightlessness. Hence, Earth sickness can occur until an astronaut’s brain readapts back to the pull of gravity and he or she learns to use their otolith organs normally again.

Space sickness is a form of motion sickness suffered by about half of astronauts the first time they experience weightlessness. Some astronauts even experience the opposite and have short-term Earth sickness when they return to Earth! What causes these unique manifestations of motion sickness?