For nearly two decades, the International Space Station has circled high above us with a crew on board. These astronauts and cosmonauts from a variety of different countries come together to conduct experiments aimed at improving life not just for the humans down below, but for future travelers that might one day make similar journeys into the unforgiving vastness of space. The science tied to that endeavor, in fact, rarely stops when these men and women return to earth after their tenure in the heavens comes to a close. Scientists have long striven to better understand the effect space can have on the human body — and there’s no better way to learn than to study the very humans who have lived in that environment.

One such study, conducted under the purview of Russia’s NASA equivalent, Roscosmos, has spent years collecting data on the brains of active cosmonauts both immediately after their return from rotations aboard the ISS and again seven months later. In order to qualify for the study, each cosmonaut needed to spend 189 days in space (about six months), allowing the human body enough time to acclimate to its environment in space and then allowing seven more months to see how well the body rebounds from that shift.

The results of the study were troubling for anyone that hopes to one day see the human race venturing deeper into space: it seems that some deformation of the brain, caused by exposure to microgravity, may be permanent.

The study included MRIs of ten cosmonauts in pre-flight checks, immediately post-flight, and seven months after returning to earth. In particular, it tracked the volumes of grey matter, white matter, and cerebrospinal fluid (CSF) found in the brain. Previous studies had already confirmed that space flight tends to lead to a sharp increase in cerebrospinal fluid (CSF) found in the brain thanks to the human body’s reliance on gravity to keep fluid levels balanced. This new study, however, seems to indicate that those increased levels of CSF during space flight leads to a slight reduction in white matter during recovery, as well as a compression of grey matter in the brain. While most of the grey matter compression goes away after extended periods of time back on earth, the reduction in white matter may be permanent. The issue seems to be related to the body’s ability to regulate those CSF levels even long after a cosmonaut’s return to earth.

The study explains:

The findings from an average of 7 months after a return to Earth can be summarized as showing that most of the loss in the gray-matter volume that was seen immediately postflight had recovered to preflight levels, while CSF volume continued to increase in the subarachnoid compartment. The expansion of CSF spaces in light of postflight decreases in the gray-matter volume and a reduction in the white-matter volume at follow-up suggests a persistent disturbance in CSF circulation even many months after a return to Earth.

Currently, scientists aren’t entirely sure what this change in cosmonaut body chemistry may lead to, in terms of long-term medical effects. One medical ailment already tied to irregular CSF levels in space-fairing humans is blurred vision. The pressure from the increase in fluid tends to flatten the back of astronaut’s eyeballs. Most astronauts see a full recovery after spending some time at home, but some have had to seek corrective treatment as a result of their time in orbit.

There’s still a great deal of science left to do regarding how prolonged exposure to microgravity may affect the human mind, but it’s integral work if people hope to one day populate far-off worlds. Even a trip to Mars, which would take about as long as each cosmonaut spent aboard the ISS for this study (one way) will need to consider how the voyage will affect the brain chemistry of those making the trip.