As you heard from my interviews or previous blogs, on average, astronauts lose about 1-2% of their bone mass per month in spaceflight, which is similar to what someone with osteoporosis loses in a year. What we don’t have a good sense of is whether the bone loss plateaus or continues with time. This is important to understand as a mission to Mars could take about 3 years in total length. Fortunately, with US astronaut Scott Kelly’s upcoming return to Earth (March 2nd), we should learn much more about long-term bone loss in spaceflight. Scott Kelly will have been on the International Space Station for 340 days. Interestingly, he has an identical twin, Mark Kelly, which has also been undergoing bone testing. For those not familiar, Mark Kelly is a retired astronaut. Examining the bones of these brothers will provide important information regarding spaceflight induced bone loss for physicians and scientists alike.