Keeping astronauts healthy and strong in the harsh conditions of space has long been one of NASA’s most pressing challenges. While the spectacle of astronauts bouncing across the Moon’s surface might inspire awe and curiosity, the reality is far more complex: microgravity—essentially the near-weightlessness experienced in orbit—takes a toll on the human body, especially on muscle and bone. For decades, scientists have wrestled with the question: just how much gravity do we need to keep our muscles from wasting away? Now, a groundbreaking study published on March 13, 2026, in Science Advances has brought us a step closer to answering that question, with findings that could shape the future of lunar and Martian exploration.
The research, conducted aboard the International Space Station (ISS), exposed 24 male mice to varying levels of artificial gravity—0.33 g, 0.67 g, and 1 g, with 1 g being Earth’s normal gravity—for up to 28 days. The mice were launched in March 2023, and after their stint in orbit, 23 returned alive to Earth in April 2023, where scientists meticulously examined their grip strength and looked for signs of muscle atrophy. According to Gizmodo, this is the first time such a controlled experiment has been carried out in space, targeting the precise gravity threshold at which muscles begin to deteriorate.
What did the study reveal? The results were striking: any gravity level below 0.67 g led to muscle deterioration in the mice, while at 0.67 g and above, the animals showed no muscle loss, no decrease in strength, and no significant changes in muscle fiber composition. Even at 0.33 g, muscle deterioration wasn’t complete, but the myofibers—the muscle fibers that make up the tissue—underwent noticeable changes. The researchers also found that 11 metabolites varied with changing gravity levels, hinting at complex molecular mechanisms at play.
These findings are more than just a scientific curiosity. They have immediate and profound implications for human spaceflight. The Moon’s gravity is about 0.17 g—just one-sixth that of Earth—while Mars clocks in at around 0.38 g, or 38 percent of Earth’s gravity. Both of these environments fall well below the 0.67 g threshold identified in the mouse study. As Gizmodo notes, this raises a red flag for NASA’s ambitions to establish a sustained human presence on the Moon through its Artemis program and, eventually, to send astronauts to Mars in the 2030s.
“With NASA’s goal of sending humans to Mars in the 2030s, a comprehensive understanding of the molecular mechanisms underlying these gravity-induced changes is urgently required, as is the development of appropriate countermeasures to prevent deleterious effects on skeletal muscles,” the study authors wrote in Science Advances. The mechanisms by which gravity regulates muscle health are still not fully understood, largely because of the difficulty in studying these effects in mammals during spaceflight.
Mouse models have become invaluable in this research, offering a practical and controlled way to observe physiological changes over time. Mark Shelhamer, a professor at Johns Hopkins University and former chief scientist for NASA’s Human Research Program, told Gizmodo that experiments like this are essential because “we are able to do experiments in rodents that are more difficult or impossible with humans.” The ISS, equipped with the Japan Aerospace Exploration Agency’s MARS centrifuge system, allowed researchers to simulate different gravity levels for the mice, something simply not feasible with human astronauts in orbit.
But what about 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, has conducted her own research using parabolic flights to simulate brief periods of microgravity in humans. Her studies suggest a similar gravity threshold for muscle maintenance in people, falling between 0.5 g and 0.75 g. “You have to start somewhere, and this is an exciting development,” she told Gizmodo in an email, adding that the mouse study “is a solid starting point for future studies.”
Shelhamer echoed this sentiment: “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. So this study is certainly helping to define that.”
The stakes are high. Astronauts aboard the ISS experience microgravity, which, despite the station’s proximity to Earth, feels like zero gravity because they are in a constant state of free-fall. In this environment, the human body undergoes rapid changes: without exercise, astronauts can lose up to 20 percent of their muscle mass in just five to 11 days. Even with rigorous exercise—two hours daily on specialized devices like the Advanced Resistive Exercise Device (ARED), the T2 treadmill, and the Cycle Ergometer with Vibration Isolation and Stabilization System (CEVIS)—astronauts can still lose up to 40 percent of their muscle mass after five months, according to Science Advances.
Why does this matter for future missions? If the gravity on the Moon and Mars is insufficient to prevent muscle atrophy, astronauts could face serious health risks during extended stays. This means NASA and other space agencies will need to develop robust countermeasures, such as artificial gravity systems or even more advanced exercise protocols, to keep crews fit for duty. As Shelhamer put it, “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. And we really hope that that is enough. Otherwise, you’ve got to take exercise equipment when you’re planning longer missions.”
Ploutz-Snyder hopes that future studies will not only help validate the 0.67 g threshold in humans but also examine how this threshold affects bone health, how exercise might shift it, and what practical steps can be taken to mitigate muscle loss. “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,” she explained to Gizmodo.
Of course, while mice are not humans, the parallels are striking. Both Ploutz-Snyder and Shelhamer agree that these results are a crucial first step toward defining the requirements for a sustainable human presence beyond Earth. As we look to the Moon, Mars, and perhaps even further afield, the lessons learned from a handful of mice spinning in a centrifuge high above our planet may be the key to unlocking the future of space exploration—and keeping the next generation of astronauts strong, healthy, and ready for whatever challenges await them.