Astronauts' hearts handle simulated Mars gravity after months in orbit

At about 3 a.m. for him, James Thomas watched heart scans arrive live from the International Space Station. The astronauts performed the ultrasound scans on one another every few weeks, and Thomas, back on Earth, coached them in real time to improve the images. The recordings tracked the heart of each crew member before, during and after a mission of roughly six months.

The question behind this work is simple to state and hard to answer. NASA plans to send astronauts to Mars, a planet where gravity is about one-third as strong as on Earth. Astronauts who spend months in microgravity lose muscle and cardiac mass, and many have struggled with dizziness on standing up after landing, because blood pressure can fall suddenly. Whether a heart that has been floating for months could cope with a return to partial weight was an open question for the mission planners.

The answer depends largely on exercise. The astronauts followed a NASA program called Exercise Countermeasures and worked out for up to two hours a day, including 30 to 45 minutes of endurance exercise and 60 to 75 minutes of resistance training. The scans show a clear pattern. During the first two weeks in orbit, cardiac filling and stroke volume fell, meaning each heartbeat moved less blood. That change was temporary: as the missions went on, both measures gradually returned toward their levels before launch.

To recreate Mars on Earth, the team tilted the astronauts 22 degrees upward from a lying position after they returned, which pushes blood toward the lower body. The hearts handled this simulated environment about as well as they had handled standing upright before launch. The study is described by its authors as exploratory, and it covers only 13 people. The tilt imitates the blood shift of a lower gravity; it does not reproduce the planet itself.

For the people planning a crewed Mars mission, the finding removes one obstacle from the list, without clearing the path. The cardiovascular question is now answered with encouraging data, and the exercise routine that protects the heart is already in use on the station. The study does not set a date for any mission, and the next step is to test the same approach on more crews and under more conditions.