While we’re on the topic:

Astronauts need precise, reliable vision, so its deterioration during spaceflight is hardly a minor problem. And it’s a particularly humbling one. NASA has known about the eyesight issue for decades. “We saw this on Skylab”—the first U.S. space station, which intermittently housed astronauts for up to three months at a time from 1973 to 1974—“and on the shuttle,” Charles says. The importance of it just wasn’t clear until astronauts were regularly spending months in orbit. And at the moment, NASA doesn’t know how to fix it back on Earth. Bone mass, muscle mass, blood volume, aerobic fitness all return to normal, for the most part. But astronauts’ eyes do not completely recover. Nor do doctors know exactly what would happen to eyesight over the course of a mission four or five times longer than those of today.

Clearly our bodies are not made for the extreme environments of space. Raise your hand if you thought space-ready humans would need calloused toes and superhuman eyes.