Spaceflight is known to take a toll on the human body, and now it seems that living in zero gravity can even reposition a person’s brain in their head. Brain scans of astronauts have revealed that long stays in space can cause the brain to shift upward in the skull and lessen the amount of protective fluid surrounding the brain. It’s unclear exactly how these brain changes affect an astronaut’s health, but the findings could have implications for how NASA keeps crew members healthy during deep-space missions.

The findings, detailed today in the New England Journal of Medicine, are from one of the biggest studies yet looking at the brains of astronauts. Scientists analyzed the MRI scans of 16 astronauts before and after spending a couple weeks on NASA’s Space Shuttle, as well as 18 astronauts before and after spending a few months on the International Space Station. The brain changes were more pronounced for those who stayed in space the longest, the researchers found.

these new findings may help explain the cause of some bizarre health problems

It’s not the first time that brain changes have been seen in astronauts, but these new findings may help explain the cause of some bizarre health problems people experience when they go to space. Astronauts often complain of increased pressure in the head during missions, as well as changes in vision. It’s possible these brain changes may be contributing to those odd symptoms. “I definitely think this has a hand in that,” study lead author Donna Roberts, a radiologist at the Medical University of South Carolina, tells The Verge.

It’s not exactly clear how the brain changes correlate to any health problems, and how long the changes last once astronauts get back to Earth. More research is needed, especially since NASA wants to send astronauts to the Moon and Mars someday — missions that could last months, or even years. The space agency may need to consider ways to mitigate these changes during deep-space trips. “The implications would be whether or not there would be a requirement on a Mars mission to provide some type of artificial gravity,” says Roberts.

NASA already knows quite a bit about the health effects of zero gravity: astronauts’ bones and muscles weaken easily in space, for instance, because people aren’t working out these systems against gravity every day. Another big change is something called fluid shift. Without the force of gravity to pull bodily fluids downward, fluids tend to shift upward — toward the head.

Roberts thinks a similar effect may be happening to the brain. “One of our theories is that because there is no longer the force of gravity pulling the brain down, the brain moves upward,” she says. This shift may also affect the fluid inside the skull — known as cerebrospinal fluid — which buffers the brain from shocks. In the brain scans of astronauts after spaceflight, the scientists saw less of this fluid around the top of the brain, and more of it inside the brain’s cavities known as ventricles. They think the brain may be pushing the fluid away from the top of the skull, possibly leading to as-of-yet unknown cognitive changes.

“because there is no longer the force of gravity pulling the brain down, the brain moves upward.”

This shift of the brain could also be pulling on the optic nerves, which could be why some astronauts experience vision changes, Roberts says. However, not everyone tested in this study had vision changes — only three of the long-term flyers did. So the brain shift may not be completely to blame. “Why aren’t the other ones demonstrating this? What’s going on?” Dorit Donoviel, the interim director of the Translational Research Institute for Space Health, who was not involved in the study, tells The Verge. “Well, there’s something else going on.”

That’s why the researchers think more study is needed, mostly to find out if the brain changes are actually linked to any known health problems. Roberts also wants NASA to do more follow-up MRIs of astronauts when they’re back on Earth, to see if these brain changes go away after a while. Plus, it’s still unknown how the brain may react to a reduced-gravity environment such as Mars. Missions to the Red Planet could last multiple years. “That’s a long time for humans to be in reduced gravity or one-third gravity,” says Roberts. “What will happen to the brain during that time?”

Donoviel says the findings aren’t too surprising given what we already know about human health in space. And she’s not worried: the human body is fairly resilient, she says. “We know the heart changes shape in space too; it becomes more spherical,” says Donoviel. “What’s more amazing is that we tolerate it pretty well.”

“Is this a showstopper for spaceflight? No it’s not.”

If necessary, there may be ways to halt these brain changes in space. There are methods to pull fluids away from the brain, such as vacuum devices that can pull on astronauts’ legs while they’re sleeping. That could potentially keep fluid pressure off the brain, says Donoviel, slowing its shift upward. “Is this a showstopper for spaceflight? No it’s not, because the changes are mild,” she says, “and I do think we’re going to be able to develop countermeasures for it.”