Airsickness
According to former Reduced Gravity Research Program director John Yaniec, anxiety contributes most to passengers' airsickness. Yaniec gives a rough estimate of passengers, that "one third violently ill, the next third moderately ill, and the final third not at all." Vomiting is referred to as "ill".
On commercial flights offered by Zero Gravity Corporation, very few passengers experience motion discomfort. This is because the flight profile of 15 parabolas, versus NASA's 40-60 parabolas, is designed to help alleviate airsickness.
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