Spending a long time in a weightless environment has an impact on the human body, and evidence exists of changes to the bone and muscles of astronauts who have participated in long-duration missions in space. To cope with the requirements of long-stay missions aboard the International Space Station, the European Space Agency, together with the French space agency CNES and the Japanese space agency NASDA, are validating countermeasures through a preventive-medicine ground-based simulation that makes use of the bed rest model.