The bone density of astronauts—of both the human and rodent variety—decreases in space. Researchers report on April 19 in the journal Cell Reports that changes to the gut microbiomes of space travelers might be associated with this bone loss. Rodents that spent a month or more on the International Space Station had altered and more diverse microbiomes, and the bacterial species that bloomed in space may have contributed to the increased production of molecules that are known to influence the bone remodeling process.