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?
When astronauts experience weightlessness (as in the 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 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 an astronaut returns to Earth because now the mechanisms used by the astronaut’s brain are tuned to the sensory inflow that occured when the astronaut experienced weightlessness. Hence, Earth sickness can occur until the astronaut’s brain readapts back to the pull of gravity and he or she learns to use their otolith organs normally again.