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Adaptation to weightlessness is not without physiological concerns.
About half of the US astronauts experience space motion sickness.
Crewmembers have served as test subjects to study this problem.
At present, there is no sure method to prevent the problem or even to predict who would be susceptible but NASA continues to study this problem using a number of techniques.
Typically, nausea gradually leads to vomiting episodes. In some instances, without warning, sudden projectile vomiting
Fortunately, this sickness is a problem in only the first three days of space flight and most persons adapt even sooner.
After adaptation, crewmembers are virtually immune to motion sickness.