Space Travel Poses No Threat to Heart Function, Mouse Study Suggests

Astronauts who spend months in orbit lose muscle mass. Their legs weaken. Their bones thin. But their hearts—the one organ that cannot afford to fail—have remained something of a mystery. Does zero gravity damage the cardiac muscle the way it ravages skeletal tissue? A new study from researchers at the University of Chicago and University of Nebraska suggests the answer is no, offering cautious reassurance as NASA plans longer missions to the Moon and eventually Mars.

The research centered on five mice that spent 38.5 days aboard the International Space Station. Scientists extracted heart tissue from these animals and examined the sarcomeres—the molecular machinery inside cardiac cells responsible for contraction. When they measured the force these sarcomeres could generate, they found something unexpected: the space-exposed hearts were just as strong as hearts from mice that never left Earth. Protein analysis revealed no meaningful degradation either. The heart, it seemed, had weathered the weightlessness without complaint.

This finding runs counter to what researchers initially expected. Cardiac muscle and skeletal muscle share enough similarities that scientists thought microgravity might damage both in comparable ways. Jonathan Kirk, a co-senior author of the study published in npj Microgravity, described the relief of discovering otherwise. "There are a lot of things in common between cardiac and skeletal muscle, so we thought that we would see some decrease in heart function from space travel," he explained. "But in the end, we're pretty happy that this is the result we found. It's obviously wonderful news for astronauts in the space program that the heart is going to be okay in space."

The path to this research was serendipitous. In August 2023, Kirk was presenting his work at the University of Nebraska, sprinkling Star Wars references into his talk for effect. Afterward, Pooneh Bagher, a co-senior author of the new paper, approached him with an offer: she had access to frozen heart tissue from mice that had flown on the space station, left over from a Baylor University project. Kirk accepted immediately. "It was a perfect fit, and that's exactly why we go to seminars and have in-person conversations with our peers," he recalled.

What makes this study particularly valuable is the biology of mice themselves. A mouse heart beats roughly 600 times per minute—six to ten times faster than a human heart. This accelerated metabolism means that 38.5 days in space for a mouse approximates 7.5 to 10 months of cardiac stress for a human. Kirk emphasized the significance: "If there's going to be a problem with the heart, you're going to see it in the sarcomeres first." By examining these molecular motors directly, researchers can detect trouble at the cellular level long before an astronaut would feel sick or show outward symptoms.

The study was not without limitations. The sample size was small, and the mice spent a relatively brief time in orbit by human standards. Researchers did observe traces of inflammation in the cardiac tissue, which warrants further investigation. Kirk and his team plan to study mice exposed to longer durations of microgravity to determine whether inflammation increases over time. They also want to analyze tissue collected while still in orbit, rather than from animals that returned to Earth, to rule out whether the stress of re-entry itself might have affected the results.

For Kirk, the appeal of space biology research transcends the practical questions about astronaut health. "Space science is fascinating because it lets you look at science from a totally new angle," he said. "Everything is different, and therefore it's a tool to understand our biological system under entirely new conditions from anything else we can do in the lab." Sometimes such studies reveal entirely new research frontiers. Sometimes they simply confirm what we hope to be true: that the human body, given the right conditions and preparation, can survive the journey outward.