Imagine you’re an astronaut on a spacewalk out there in the velvet-black, profound universe. In the middle of powering in some screws on a piece of robotic equipment, your left eye stings so badly that you can’t keep it open. Then the same thing happens to your right eye. That’s what happened to the Canadian astronaut Chris Hadfield. “In just a few minutes, I’d gone from 20/20 vision to blind. In space. Holding a drill,” he relates in his book, “An Astronaut’s Guide to Life on Earth.”