The International Space Station is an orbiting laboratory and construction site that synthesizes the scientific expertise of 16 nations to maintain a permanent human outpost in space. While floating some 240 miles (390 kilometers) above Earth’s surface, the space station has hosted a rotating international crew since November 2000. Astronauts and supplies are ferried by the U.S. space shuttles and the Russian Soyuz and Progress spacecraft. Astronauts who reach the facility aboard one of these missions typically live and work in orbit for about six months. Simply by spending time in orbit, astronauts reveal much more about how humans can live and work in space. Crews have learned the difficulties of diet, in a world in which their sense of taste is decreased, and of getting a good night’s sleep while secured to a non-floating object. But the crew is also occupied with a full suite of scientific experiments, the ongoing improvement and construction of the station, and a rigorous regime of physical training. Astronauts must exercise for two hours each day to counteract the detrimental effects of low gravity on the body’s skeleton and circulatory system.