In microgravity, without the continuous load of Earth’s gravity, the tissues that make up bones reshape themselves. Bone cells readjust their behaviors—the cells that build new bone slow down, while the cells that break down old or damaged bone tissue keep operating at their normal pace so that breakdown outpaces growth, producing weaker and more brittle bones. For every month in space, astronauts’ weight-bearing bones become roughly 1% less dense if they don’t take precautions to counter this loss. Muscles, usually activated by simply moving around on Earth, also weaken…
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