A recent study finds an astronaut's vision can be affected by long space flights.
Researchers have examined NASA astronauts who participated in long missions.
Several of them had flattening in the back of at least one of their eyes, causing far-sightedness.
This meant some of them reported seeing worse and some who were already near-sighted saw better than before.
Also what's unusual, female astronauts were not affected.
Researchers say the findings could place new limits on mission lengths, but that it could be years before the results are fully understood.