Loss of bone density during spaceflight may arise from a developmental ‘switch’ that reduces production of new bone-building cells. Astronauts face many perils while in orbit, including steady degeneration of the skeleton in microgravity conditions. Researchers led by Russell Turner at Oregon State University in the USA recently examined this phenomenon by sending rats into space for two weeks. In keeping with previous findings, these animals showed increased breakdown of bone and accumulation of fat within vertebral samples. Osteoblasts, the bone-building cells that normally balance this breakdown process, arise from the same stem cells as fat cells. Although time spent in orbit did not affect osteoblast function, Turner and colleagues found evidence that low-gravity conditions somehow induce these stem cells to preferentially form fat instead of generating osteoblasts to replace lost bone.