From feelings of weightlessness to literally out-of-this-world views, the life of an astronaut is something to be envied—most of the time. Unfortunately, space is also really bad for the human body. According to a NASA study, it’s bad for the human heart, too: time in space makes astronauts’ hearts more spherical.

Gym buffs know you lose muscle mass when you don’t work out regularly. Same goes for a heart in microgravity. “The heart doesn’t work as hard in space, which can cause a loss of muscle mass,” said James Thomas, M.D., ultrasound lead at NASA, in an American College of Cardiology news release.

For the study, 12 astronauts learned how to do ultrasounds so they could image their hearts before, during and after space travel. The NASA researchers found that their hearts became 9.4 percent more spherical, which could be a sign the muscle is not working as efficiently. Luckily, their hearts returned to their normal shapes once they were back on Earth, but the effects a longer spaceflight could have are anyone’s guess.

The Good News

There’s a silver lining in all of this. The study not only tested what microgravity does to hearts in space—it also tested sophisticated mathematical models that the researchers had developed to predict what the hearts might do. The final results matched what their models had predicted, which means that they might be able to use them to predict what other, more earthly elements might do to the heart.

“It gives us confidence that we can move ahead and start using these models for more clinically important applications on Earth, such as to predict what happens to the heart under different stresses,” said Thomas, also of Northwestern Medicine, in the 2014 release. That means the astronauts’ heart images could eventually help scientists learn more about cardiac conditions that affect people on this planet.