If nothing else, I would expect things like increased radiation exposure would be a factor. Archive: https://archive.today/gU8q9 From the post: >>“Space is the ultimate stress test for the human body,” said Catriona Jamieson, a stem cell biologist at the University of California, San Diego, in a statement. Recently, Jamieson’s team discovered that spaceflight accelerates aging in human blood-forming stem cells.1 The researchers found that stem cells that spent about a month in space had a reduced self-renewal capacity and showed signs of molecular aging. Their results, published in Cell Stem Cell, demonstrate the possible dangers of spending extended time in space. In 2015, NASA recruited twin astronauts Scott and Mark Kelly for an experiment to investigate the effects of spaceflight stress on humans.2 The researchers found significant differences in molecular profiles between Scott, who spent nearly a year in space, and his identical twin Mark, who stayed on Earth. Some of the differences, such as telomere shortening and signs of DNA damage, persisted until the endpoint of the study, six months after Scott returned to Earth.