Scientists have finally gotten to the bottom of why astronauts struggle to poop in space, thanks to a new blood test. The findings also reveal an unexpected side effect of this cosmic constipation that may need to be addressed before humans take long-haul trips to Mars and beyond, the study authors say.

As soon as people started living in space, it became apparent that it was harder to go "number two" among the stars than down on Earth — and not just because of how complicated it is to use a space toilet. For most astronauts, this gastrointestinal change is just an inconvenience, but some have to take laxatives to overcome the problem. The drugs are routinely stocked on most space missions, including the recent Artemis II mission.