The crew needs either a small unit inside the ship or a vehicle design that rotates around a central pivot point (think 2001: A Space Odyssey). Hargens said a rotating arm of one-kilometer diameter will produce the equivalent of the gravity felt on the Earth at sea-level.
Smaller centrifuges have produced nausea among astronauts, according to Hargens, and take up a lot of space inside a cramped vehicle.
Another possibility is a special compression suit that forces body fluids into the legs, and helps maintain fitness.
Cabin Fever Put six or seven people in a confined space for 18 months, send them to a place nobody's been before, with no way to escape, is likely to produce stress, tension and perhaps even severe psychiatric problems, according to NASA's 2009 Human Research Program report.
Based on studies in Antarctica and other isolated environments underwater, the report cited the risk of "increased human performance errors due to sleep loss, fatigue, work overload, and circadian desynchronization; and, increased errors due to poor team cohesion and performance, inadequate selection/team composition, inadequate training, and poor psychosocial adaptation."