In 2020, NASA will launch the Mars Rover mission for robotic exploration of the red planet. But humans will not be far behind, which adds urgency to solving physiological problems of extended space flight. One of the most critical issues is vision impairment. In fact, 40 percent of astronauts return from visual impairment and intracranial pressure syndrome, or VIIP.

A new study from the New England Journal of Medicine describes the cause as microgravity shifting the brain upward. The study compared MRI brain scans of astronauts who flew long-duration missions on the International Space Station to those on shorter missions.

More than two-thirds of the astronauts with longer missions developed vision problems. They also had more flattened eyeballs and more optic nerve swelling when they returned to Earth. Researchers also found more spinal fluid in the areas of the brain where CSF fluid is produced and more fluid around the optic nerve, according to the Daily Mail.

Spaceflight-Associated Neuro-Ocular Syndrome is of special concern to NASA because of the much longer time spans astronauts will spend exposed to microgravity,  according to the BBC.

NASA has given lead study author Noam Alperin a $600,000 grant for further study of VIIP.