Space flight is hard on the human body. Astronauts lose muscle mass and bone density. They’re exposed to radiation that increases the risk of cancer and other problems. And that’s just the beginning. Long flights aboard the International Space Station have led to changes in the eyes, blood, and brain.

Most astronauts who’ve spent at least six months in orbit have reported vision problems. The eye can change shape, and connections between the eye and the optic nerve can be frayed. Researchers aren’t sure what causes the problems. It could be simply the lack of gravity, but other factors could be at work as well.

Astronauts also suffer a loss of red blood cells. A recent study found that the body is actually destroying the cells. And it continues to do so as long as the person is in space. Cell production improves after landing, but levels remain low even a year later. Again, there’s no confirmed explanation.

Another study found that weightlessness also “rewires” parts of the brain. A dozen space travelers were scanned before and shortly after their trips, and again a few months later. The scans revealed changes to the connections between parts of the brain that control motor functions. And the changes persisted after the return to Earth. Researchers say the brain was adapting to weightlessness, where moving is quite different than it is on Earth.

So a long trip to space is no vacation for the human body.

Script by Damond Benningfield