An abridged French edition of The Greatest Challenge (E. P. Dutton, 1965), this book surveys the medical, physiological, and psychological challenges of human spaceflight as they stood midway through the Mercury and Gemini programs. Martin Caidin examines the effects of prolonged weightlessness on the cardiovascular system (including the two-week safety limit set by Soviet physicians), visual perception disturbances in orbit, the psychology of isolation, and the logistical constraints of future interplanetary travel. The book also argues for the irreplaceable role of human judgment over automated systems in space exploration.