Astronauts lose 1 to 2 percent of their bone mass for each month they spend in space. As far back as the Gemini missions, conditioning exercise regimes have been used to slow the rate of bone loss, but a 2001-2004 NASA-sponsored study showed that crew members aboard the International Space Station (ISS) were still losing up to 2.7 percent of their interior bone material and 1.7 percent of outer hipbone material for each month they spent in space. If ISS crew members lose this much bone density after 4 to 6 months in space, astronauts on long missions to Mars—voyages that could take years—could lose enough bone mass that they suffer fractures while carrying out tasks on the Martian surface.