Well here I was in January asking what would make an astronaut retire, and it turns out Gary knew the answer all along.
“Radiation,” he says. “There’s a certain amount of radiation they can absorb, and when they hit their limit they’re grounded. So if anyone goes to Mars that’s all they can do. They max out on radiation.”
“They’re going to die on Mars anyway, though.” I said.
“WHY WOULD YOU SAY THAT?”
“I don’t know. Aren’t they going to be settlers?”
“NO! YOU CAN GET TO MARS IN MONTHS.”
So, that’s what I learned. You can come back from Mars but you can’t go out again because you’ll hit your radiation max.