You’d think that breaking away from the weight of gravity here on Earth would be a nice little vacation for your bones. Nope! It’s really bad, actually!

A NASA study involving mice onboard the International Space Station has shown that spending extended time in space wreaks havoc on skeletal health, leaving astronauts with bone loss that might never fully heal.

Space is about as inhospitable an environment as it can get. While astronauts have to deal with the ambient radiation and a lack of oxygen, now NASA says that the zero gravity of space turned the mice’s femurs into Swiss cheese. Though, surprisingly, their lumbar spines were mostly okay.

This suggests that weight-bearing bones, like femurs, are the most vulnerable in space. On Earth, we keep these bones active by walking, running, and even just standing, but without gravity, there’s not much for them to do, so they start wasting away.

For a while there, it was suspected that radiation was to blame for a reduction in astronaut bone density. Radiation levels on the ISS are low, and if it were truly to blame, we’d see damage in different parts of the bone. Instead, the bone loss in space happens from the inside out, particularly in the spongy marrow areas, rather than from the outer layers.

Even when compared to mice subjected to Earth-bound stress like limited movement, the bone loss in spacefarers was far more severe.

Without the regular stress of gravity, bones get soft and mushy. The human race has its eyes set on deep space exploration, but if we want to cast away our earthly shackles and live amongst the stars, NASA’s going to have to figure out a way to create some workout equipment that mimics gravity’s effects or else some of humanity’s best and brightest explorers will be reduced to the floating wheelchair-bound humans of WALL-E.