Astronauts grow up to 2 inches taller while living in space.

On Earth, gravity pulls on the body and squeezes 23 soft discs located between the bones of the spine. These discs act like cushions or shock absorbers. In the microgravity of the International Space Station, this downward squeeze stops. This process is called spinal unloading.During the first few days in orbit, the discs soak up more fluid and get bigger. This makes the entire spine longer. NASA studied this during the 2015 Twin Study with astronauts Scott and Mark Kelly. Scott Kelly grew about 2 inches taller during his year in space.This height change is not permanent. When astronauts return to Earth, gravity pushes their spine back to its normal size within a few months. Being taller in space can also be painful. The stretching of muscles and nerves often causes back aches for the crew.Space travel also affects bones. Astronauts can lose over 1% of their bone density every month. To stay healthy, they exercise for two hours every day. They use a machine called the ARED to lift weights and keep their bones and muscles strong.