Elon Musk is determined to send humans to Mars within a decade, but he might want to ask his team of experts to look closely at new research on what space travel does to the human brain first.

Astronauts' brains compress and expand in space, a recent study by a research team from the University of Michigan found. Using astronaut MRIs taken before and after previous NASA space missions, the researchers were able to show that the brains underwent stress, and could have implications for future space travel. The findings could also have implications for treating other health conditions that affect brain function.

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The study, published in Nature Microgravity, is the first of its kind to look at structural changes to the human brain during spaceflight. It shows that the volume of gray matter increases or decreases slightly in space. The extent of those changes depends on the amount of time a human spends in space.

To be clear, the changes are slight and likely wouldn't affect an astronaut during travel to Mars or beyond. But they are changes in the human brain, so experts will need to determine what it means before humans spend considerable time in space.

Why does this happen? The short answer may be that the fluid in your brain shifts around while you're in space and is not subject to the pull of gravity, causing the brain to increase or decrease slightly. Essentially, gray matter volume decreases slightly as fluid is redistributed. It later returns as gravity pulls that fluid down in the body.

Though they haven't pinpointed the exact nature of the changes, the findings may lead to new ways of thinking about potentially debilitating health conditions – for instance, people on long-duration bed rest or people who have a condition where fluid accumulates in ventricles in the brain and causes pain and pressure.

Next up? Researchers are going to study possible repercussions on cognition and physical performance, as well as how long the brain changes last.

This new research adds one more problem confronting scientists and engineers trying to understand the possible impacts of space travel on humans. Several studies in recent years have looked at the health impacts of cosmic rays that will pass through ship hulls and human bodies on a mission to Mars. Earth's magnetic field shields us from cosmic rays. That protection goes away once we enter deep space. Any trip to Mars exposes human occupants to that radiation, which might have health impacts.

A study last fall, for instance, found that rodents suffered mild brain damage and cognitive problems six months after they were exposed to roughly the same levels of radiation that a human astronaut would experience during a trip to Mars. The study raises the possibility that space radiation could cause memory problems or impaired judgment. Shielding astronauts from that radiation poses a difficult problem for those trying to get human beings to and from Mars safely. The authors of that study, however, cautioned that we still don't know enough about the health effects of space radiation – or whether it has any real, lasting effects.

"This is not a deal-breaker," co-author Charles Limoli, a neurobiologist at the University of California—Irvine, said last fall. "I do not think that during the course of a trip to Mars and back the astronauts will come back with anything remotely similar to full-blown Alzheimer's. But more mild changes, more subtle changes—they would still be concerning, given the level of autonomy astronauts operate under and the amount of work they have to do."

Other research has raised similar concerns. A second study in 2015 examined mouse brains six weeks after they were exposed to space radiation, and found they experienced at least temporary problems on learning and memory tests. In both tests, the short-term effects appeared to be elevated anxiety and stress. The question is whether there are long-term impacts from exposure to space radiation.

NASA and others looking to send humans to Mars are trying to determine how to shield occupants from the radiation. It is an expensive proposition, but not impossible. This new research showing possible effects on the structure of the brain during space travel adds yet another vexing problem on top of the earlier concerns.