Scientists now know that spending a long time in space changes astronauts' bodies all the way down to their DNA. Scott Kelly spent 340 days on the International Space Station while his twin brother, retired astronaut Mark Kelly, stayed on Earth.
DNA samples taken from both brothers after Scott returned to Earth showed distinct genetic changes for Scott that began as soon as he went into space. Some changes in Scott's gene expression also continued for a time after he returned to Earth.
Scott Kelly also showed what doctors called a "meltdown" in his muscle mass. One researcher described it as the equivalent of a 20-year-old turning into a 60-year-old in three months. NASA will publish a detailed report on the Twins Study next year.
Source: Space.com