Weightlessness and the Human Body

When astronauts spend a long time in the microgravity of space, their bodies change in ways that resemble faster aging. One of the most studied effects is bone loss. Weight-bearing bones, like those in the legs, are normally strengthened by the stress of supporting body weight against gravity. When that stress disappears, bone tissue breaks down faster than the body can rebuild it. Studies of astronauts on space stations have recorded bone loss of about one to two percent per month. This loss happens mainly in weight-bearing bones, not in bones that carry little weight even on Earth. This pattern confirms that the loss comes from a lack of mechanical stress, not from spaceflight in general. Muscles decline in a similar way. Astronauts can lose up to a fifth of their muscle mass within a few weeks if they do not follow a demanding daily exercise routine. A separate effect involves body fluids, and it happens much faster. Without gravity pulling blood toward the legs, fluids shift upward toward the head instead. This causes facial puffiness and nasal congestion within the first few days of a mission. Muscle mass and fluid balance usually recover quickly after astronauts return to Earth. Bone density, however, recovers much more slowly. Some studies suggest it may never fully return to pre-flight levels.