It’s no secret that sending humans to Mars poses many risks for the health of astronauts, whose bodies must endure months and years outside the normal gravity of Earth. But exploring beyond orbit entails a less obvious danger to space travelers, according to a report from the National Research Council due for release soon. The same microgravity which affects the way an astronaut’s blood flows can also alter — and potentially disable — mechanical systems crucial to a long-duration crew’s survival.