In the first several days of a mission, about half of all space travelers suffer from persistent nausea, sometimes accompanied by vomiting. Drugs to prevent motion sickness can provide some relief for the symptoms of space adaptation syndrome, and the condition generally passes in a few days. After a few days in space, the vestibular system disregards all directional signals. Soon after an astronaut returns to the earth, the organs of balance resume normal operation. Over a period of days or weeks, an astronaut's body experiences deconditioning .
Strenuous exercise helps prevent deconditioning.
Space travelers ride exercise bikes, use treadmills, and perform other types of physical activity. After many months in space, a process called demineralization weakens the bones. Most physicians believe that demineralization results from the absence of stress on the bones in a weightless environment. The experiences of Soviet cosmonauts who spent long periods in orbit showed that vigorous exercise and a special diet can minimize demineralization. In addition, astronauts can carry portable life-support systems in backpacks when they work outside the main spacecraft.