Researchers from Northwestern University, the University of Texas Southwestern Medical Center, and UT Health Houston studied the hearts of 13 astronauts who spent approximately six months aboard the ISS. Although spaceflight initially affected heart function, the astronauts gradually adapted. Daily intensive exercise also played an important role in maintaining their cardiovascular health.

One of NASA’s major concerns about sending humans to Mars is how astronauts’ hearts and circulatory systems will respond after months in weightlessness and then transition to Mars’ gravity, which is about 38% of Earth’s gravity.

Encouragingly, the astronauts showed no significant signs of cardiovascular strain when exposed to simulated Martian gravity after returning to Earth.

“This discovery gives us reason to think that we can travel across to Mars and actually work on its surface,” said study lead author Dr. James Thomas, a professor in the Division of Cardiology at Northwestern University Feinberg School of Medicine.

The study, published in the journal Circulation, investigated how the heart responds to simulated Martian gravity after an extended stay in space.

How Daily Exercise Protects Astronauts’ Hearts in Space

Living without gravity creates unusual challenges for the human body. On Earth, the heart works against gravity to circulate blood, while muscles support the body’s weight. In space, these demands are significantly reduced. Over time, muscles and the heart itself may lose mass.

Returning to a gravity environment can also be difficult. Astronauts on previous missions frequently experienced dizziness or lightheadedness when standing after landing. This happens because gravity pulls blood toward the lower body, which can cause blood pressure to drop suddenly. The condition is called orthostatic hypotension. If you’ve seen Sandra Bullock crawling after returning to Earth in the film Gravity, you’ll recognize the idea.

However, the astronauts in the new study generally coped well with the transition, possibly because they maintained strict exercise schedules during their missions.

As part of NASA’s exercise program, astronauts exercised for up to two hours each day. Their routines included 30 to 45 minutes of endurance training to strengthen the cardiovascular system and 60 to 75 minutes of resistance training to help preserve muscle mass and strength.

“When you’re in a zero-gravity environment, you have less work to do above you, so your muscle mass and heart mass decreases,” said Thomas, who is also a cardiologist at Northwestern Medicine. “You can float in the air, tap your foot, and sail away to say hello to your buddy.”

“To counter that, it’s important to exercise when you’re in space,” Thomas added.

Heart Scans Show How the Body Adapts to Weightlessness

To understand how the heart changes during long-duration spaceflight, researchers collected echocardiograms from each astronaut before, during, and after the mission. These ultrasound scans create images of the heart, allowing doctors to examine its structure and measure how effectively it pumps blood.

While aboard the ISS, astronauts were trained to perform the scans on one another every few weeks. The recordings were transmitted to Earth, allowing researchers to monitor changes in heart function throughout the mission.

Thomas worked with Dr. Benjamin Levin of UT Southwestern Medical Center and Dr. Michael Bungo of UT Health Houston’s McGovern Medical School.

“The space station operates on Greenwich Mean Time, so it’s usually around 3 a.m. for me,” Thomas said. “I was watching the recordings live as they came in and coaching the astronauts in real time to improve the images.”

The scans showed that the astronauts’ hearts initially struggled to adapt to the new environment. During the first two weeks of spaceflight, researchers observed decreases in cardiac filling, stroke volume, and several other measures of cardiovascular function.

Cardiac filling refers to the amount of blood entering the heart during each beat. Stroke volume is the amount of blood pumped out with each contraction. Both measurements decreased early in the mission, indicating that less blood was moving through the heart with each heartbeat.

However, the changes did not last.

As the astronauts spent more time in space, their cardiovascular systems adapted. Cardiac filling and stroke volume gradually improved and approached the levels recorded before flight.

These results suggest that the heart can adapt to long-term weightlessness more effectively than earlier measurements indicated, particularly when astronauts maintain regular physical activity.

Astronaut Hearts Pass the Simulated Mars Gravity Test

The researchers also examined a question that is especially important for future Mars missions: What happens when an astronaut who has spent months in zero gravity encounters gravity again?

Mars’ surface gravity is approximately 38% of Earth’s. Although it is much weaker than Earth’s gravity, it would still change how astronauts’ hearts and circulatory systems function after an extended period in space.

To recreate some of these effects, the researchers conducted simulated Mars gravity tests after the astronauts returned to Earth.

Each astronaut was tilted upward at a 22-degree angle from a lying position. This shifted blood toward the lower body, approximating some of the circulatory effects of Mars’ gravity.

The results were encouraging. After approximately six months in space, the astronauts’ hearts functioned just as well in simulated Martian gravity as they had when the astronauts were upright on Earth before their missions.

This suggests that astronauts who maintain cardiovascular health during spaceflight may be able to transition to Mars’ lower-gravity environment without experiencing significant cardiovascular problems.

The study involved a relatively small group of astronauts and used a simulation rather than an actual Mars landing. Even so, it provides valuable evidence for planning future human missions to Mars.

Combined with the benefits of daily exercise, the heart’s ability to adapt to weightlessness and respond well to simulated Martian gravity offers another reason for optimism about sending humans to Mars.

The study is titled “Effects of long-term spaceflight on cardiac structure and function: An exploratory longitudinal observational study.” Funding came from the National Space Biomedical Research Institute through grant NCC9-58, NASA through grant NNJ04HH01A, the IDP Foundation, Sandra Helton, and the National Heart Foundation of Australia.