A human mission to Mars may still be some time away, but scientists are already aware of the many hazards that must be overcome if the dream is to become a reality. One particular cause for concern is the potential for physiological and psychological problems that could arise from the conditions of weightlessness, isolation and confinement experienced during a journey that could last six months or more. To address these concerns ESA, in cooperation with the French space agency CNES, NASA and two Antarctic research organisations, is seeking proposals from scientists wishing to participate in two pioneering ground-based studies to simulate some of the side effects of extended periods of space flight.