Because when Barratt blasted off to the international space station, he needed eyeglasses for distance. When he returned to Earth, his distance vision was fine, but he needed reading glasses. That was more than two years ago. And he’s not getting better.

“We really need to understand this. This is a critical point for understanding how humans adapt to spaceflight,” he said.

In the past few years, about half of the astronauts aboard the international space station have developed an increasing pressure inside their heads, an intracranial pressure that reshapes their optic nerve, causing a significant shift in the eyesight of male astronauts. Doctors call it papilledema.

Female space travelers have not been affected.