Keeping astronauts strong and healthy in microgravity is no easy feat. Those serving six-month missions aboard the International Space Station must exercise for two hours per day to stave off muscle loss. Pinpointing the level of microgravity at which atrophy begins would therefore be hugely helpful for establishing a sustained presence on the Moon and Mars.
That’s exactly what a study published today in Science Advances sought to do. The researchers exposed mice aboard the ISS to various levels of artificial gravity, then examined how their muscles responded. They found that 0.67 g (67% of Earth’s gravity) is a key threshold—any gravity level lower than this caused their muscles to deteriorate.
While mice are not humans, this is an “exciting study nonetheless because we are able to do experiments in rodents that are more difficult or impossible with humans,” Lori Ploutz-Snyder, dean of the University of Michigan’s school of kinesiology and former lead scientist for NASA’s Exercise Physiology and Countermeasures Project, told Gizmodo in an email. She was not involved in the study.
Muscles need gravity to stay strong
Mouse models are currently one of the best ways to study the long-term muscular impacts of microgravity, Mark Shelhamer, a professor of otolaryngology-head and neck surgery at Johns Hopkins University and former chief scientist of NASA’s Human Research Program, told Gizmodo. They provide a practical and controlled way to study the physiological effects of microgravity over time.
The 24 mice involved in this study launched to the ISS in March 2023. The crew aboard the station used the Japan Aerospace Exploration Agency’s MARS centrifuge system to expose the rodents to 0.33 g, 0.67 g, and 1 g conditions for up to 28 days. In April 2023, 23 of the mice returned to Earth alive, and the researchers dissected them to look for changes to their grip strength and signs of atrophy.
This revealed that even a low gravity level of 0.33 g was enough to stop their muscles from deteriorating completely, although the rodents’ muscle fibers did change in composition. At 0.67 g, the mice showed no muscle deterioration, loss of strength, or fiber changes.
While these results may not directly transfer to humans, they are—at the very least—a warning sign that gravity levels on the Moon and Mars likely aren’t sufficient for preventing atrophy in astronauts over extended periods of time, as they are well below the 0.67g threshold. Specifically, gravity on the Moon is roughly 0.17 g and 0.38 g on Mars.
Defining how much gravity humans need
Interestingly, Ploutz-Snyder’s research—which exposed humans to short periods of microgravity using parabolic flights—identified a similar threshold of 0.5 g to 0.75 g. Further research will help determine whether this is just a coincidence or an indication that the 0.67 g threshold is meaningful for humans. She believes, however, that it is a solid starting point for future studies.
“You have to start somewhere, and this is an exciting development,” she said.
Shelhamer agrees. “Before I saw this study, I would [have said] we know nothing about how much gravity exposure is necessary to halt or slow down the deconditioning that goes on when you send people into space,” he said. “So this study is certainly helping to define that.”
Both experts said that understanding this threshold will help scientists understand whether gravity levels on the Moon and Mars would be sufficient enough to sustain astronaut health. This is critical, as NASA eventually hopes to establish a lunar base through the Artemis program and send humans to the Red Planet.
“We have no idea if being on the Moon in one-sixth [Earth] gravity or being on Mars in three-eighths [Earth gravity], is enough to stop the deconditioning of bones, muscles—all the other things,” Shelhamer explained. “And we really hope that that is enough. Otherwise, you’ve got to take exercise equipment when you’re planning longer missions.”
Additionally, understanding this threshold would help scientists determine what levels of artificial gravity might be most helpful and efficient for long-duration spaceflights and if NASA can scale back exercise countermeasures for astronauts exposed to some level of microgravity—whether it be natural or artificial, Ploutz-Snyder explained.
She hopes future studies will not only help validate and refine the 0.67 g threshold in humans but also investigate how this threshold differs for bone deterioration, how exercise shifts the threshold, and come up with practical implications for this information. Such work could bring us closer than ever to a sustainable human presence on the Moon, Mars, and beyond.
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