Astronauts arriving and returning from space stations carry trillions of microscopic organisms with them. For example, new species of space-adapted bacteria have been discovered on China’s Tiangong space station. Recently, 26 previously unknown species of bacteria were discovered in NASA’s cleanrooms—some of the most sterile environments ever created by humanity. Another study shows that, under stress, strains isolated from the ISS mutated and became genetically and functionally distinct from their terrestrial counterparts.