NASA astronaut Scott Kelly spent almost 1 year on the International Space Station (ISS). His twin brother, Mark, spent that year on Earth. Now scientists are delivering their full report on the relative effects of living in space (Science 2019, DOI: 10.1126/science.eaau8650). Researchers found that Scott’s telomeres—molecular caps on chromosomes known to get shorter over an organism’s life span—became longer while he was aboard the ISS, but they quickly returned to preflight lengths on his return. On some chromosomes, they eroded completely. Scott also experienced changes in DNA methylation—which can change a gene’s expression—in immune cells, patterns the researchers say are associated with stress. Interestingly, his twin saw different methylation changes but of greater magnitude. In Scott, they also saw increased chromosome aberrations consistent with radiation exposure. The researchers acknowledge the study’s imperfections. It had a tiny sample size and Mark’s activities and diet were not controlled. And because Earth’s magnetic field shields the ISS from some radiation, Scott’s experience may not translate to deep space missions.