Where it comes from
The inner ear determines the position of your head by tracking how tiny crystals shift under gravity. In weightlessness, they stop shifting, and this source of information disappears completely.
The other sources keep working. Your eyes see the station's interior, with its floor and ceiling, while your muscles and joints provide their own information, and the resulting picture does not match the vestibular system's silence.
The main explanation for nausea during sensory conflict is this: the body interprets the contradiction as a sign of poisoning. Some poisons disrupt coordination first, so the body triggers the same response as it does to poisoning, just in case.
That's what space sickness has in common with seasickness. The mechanism is the same, but the source of the contradiction is different: on a ship, the vestibular system detects the rocking, while your eyes see a motionless wall in the cabin.
How it's managed
The main remedy is time. For most people, the condition passes within 2-4 days: the brain readjusts and starts relying on vision and sensations from the body, while giving vestibular signals less weight.
What share of crews experience this condition in the first days of flight? It's more the rule than the exception
Organizing the space also helps. On a station, people try to maintain a consistent orientation: where the ceiling is, where the floor is, and where the labels are, so that vision provides a stable reference point instead of confusion.
The crew is usually not scheduled for complex tasks during the first 24 hours, especially spacewalks. The reason is not only how they feel: nausea is directly dangerous in a spacesuit, making it one of the serious limitations.
After returning, everything happens in reverse. The vestibular system starts working again, but the brain has learned to ignore it, so the same condition occurs during the first days back on Earth.
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Sources
- Space motion sickness develops in a significant share of crews during the first days of flight and is linked to a mismatch between vestibular, visual, and proprioceptive information in weightlessness
- adaptation usually takes several days, and the opposite adjustment occurs after returning