Space exploration will change human society, but it will also alter the human body in ways that scientists are just beginning to discover.

Changes that happen in microgravity can linger on for months after someone returns to Earth, particularly in the brain, say researchers in a study published Monday in the journal Proceedings of the National Academy of Sciences.

This new study, led by Angelique Van Ombergen, Ph.D., a postdoctoral researcher in medical sciences at the University of Antwerp, fills a gap in previous research by showing how spaceflight actually changes the way fluids shift in the brain’s ventricles, which are the hollow cavities in the brain that allow fluid to flow.

Over time, Van Ombergen found that brain fluid pushed upward by microgravity caused a slight expansion of three ventricles in the brains of 11 cosmonauts. Overall, these cavities in their brains grew by 11.6 percent over an average of 169 days. After they cosmonauts nauts returned to Earth, the expanded vessels began to shrink back to normal — but slowly. After seven months,, they they were still 6.4 percent larger than were pre-flight.

This compensatory mechanism could have two potential consequences, although mores studies are needed, says Van Ombergen.

Mark and Scott Kelly, the twins in the NASA twin experiment NASA

“No specific neurological investigations were done as part of the twin study to the best of my knowledge, but cognition was checked,” says Van Ombergen. “Our set of data is as such very complementary to the NASA Twin study.”

The effects of prolonged time in microgravity are subtle. When NASA astronaut Scott Kelly returned from the International Space Station, numerous studies were run on his genes, blood, and cells. Overall, he looked pretty good and didn’t have too many lasting changes to his body.

Van Ombergen tells Inverse that the results add important context to previous research that examined the effect of long-term spaceflight on the human body. For instance, the NASA twin study released in April on Scott and Mark Kelly, did not review anatomical changes in the brain. This new study adds to the body of work that includes the NASA twin study, she says.

What are the Side-Effects of Ventricle Enlargement in the Brain?

It’s possible that these anatomical changes could have some effect on behavior — though what they are isn’t clear from her paper.

On the other hand, it could have links to an eye condition called called Spaceflight Neuroocular Syndrome (SANS), that has been on NASA’s radar for years, and is a significant barrier to deep space habitation, according to NASA’s Human Research Roadmap.

"All astronauts suffered from a loss of visual acuity after spaceflight."

SANS manifests as slight loss of vision acuity has affected anywhere from 37.5 to 51 percent of US astronauts aboard the ISS, according to a 2017 NASA report. Previous studies propose that cerebral fluid shifts likely play a role in the development of SANS and alter pressures on the eye, leading to small changes in vision.

Van Ombergen’s study didn’t test how cerebral fluid shift is related to SANS, but she says that her study found a small correlation between deterioration in vision in the left eye, and expanded ventricle volume in the brain. While theeffect is small, it was widely seen across her study.

Each time an astronaut returns from space, we learn a little more about what will happen to future humans who spend time up there.

Though this study may not be enough to determine how to combat SANS or other physical consequences of spaceflight, it is an incremental step that comes just on the heels of the NASA twins study.