Humans did not evolve for space. Ironically, because we insist on going there anyway, scientists now know from studying astronauts that the lack of Earth’s gravity can wreak health havoc, such as by shifting our soft, watery organs, throwing off gut bacteria, weakening our bones, and more.
None of that, however, seems to dampen the spirits of the astronauts who go to space. And understanding more about how gravity affects our health could help humans live better off Earth.
Now a first-of-its-kind study in mice that were sent to the International Space Station (ISS) suggests that living with different gravity causes changes to our muscles—and it identifies a critical threshold when problems may start to set in.
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If you’re reading this on Earth, you’re experiencing about 1 g worth of gravity. On Mars, you’d feel about 38 percent of Earth’s gravity, or 0.38 g. The moon’s gravity is even less, at just about 0.17 g, or a sixth of Earth’s. But at what gravity does our body lose its regular function?
The study, published on Friday in the journal Science Advances, shows that mice in space that were kept at 0.67 g maintained most of their muscular function and muscle fiber composition. But when the mice lived in an environment below that gravity, their muscles started to change for the worse. The results could help keep humans healthy in space, including on possible future trips to explore other planets.
“I think this provides some really interesting information about long-duration missions to Mars and beyond,” says Mary Bouxsein, a co-author of the study and a professor of orthopedic surgery at Harvard Medical School, adding that it’s somewhat reassuring that mice could maintain their muscular function without a full 1 g.
In 2023 an international group of researchers sponsored by NASA and the Japan Aerospace Exploration Agency (JAXA) launched 24 mice on a SpaceX Falcon 9 rocket for a visit to the ISS. There the mice were kept in one of four gravity levels—microgravity, 0.33 g, 0.67 g and 1 g—for around four weeks. When the mice returned to Earth, researchers on the ground analyzed their muscle tissue for signs of deterioration.
Specifically, the team looked at the leg’s soleus muscle, which is known to be sensitive to gravity. At 0.33 g, the mice’s muscle size was about the same as it was under full gravity, but the animals were weaker, as measured by their grip strength. At 0.67 g, however, the mice saw “full protection of muscle function,” Bouxsein says, meaning that their grip was about the same as it was at 1 g.
The results clearly show the effect of different levels of gravity on muscle structure and function, says Se-Jin Lee, a geneticist at the University of Connecticut, who also studies the effect of spaceflight on health but was not involved in the new study.
“A key question will be the extent to which these findings will translate to humans during space travel, specifically with respect to the threshold for seeing significant effects on muscle health in humans,” he says.
To Lee’s point, mice are obviously different from humans. We use our muscles differently—mice scurry on four legs while humans evolved to walk on two—and our muscle composition is different, too, Bouxsein says. But the paper provides an important starting point for future research on the health effects of spaceflight, such as studies of how different degrees of gravity affect other tissues and what effect exercise has on muscle loss.
The study also raises questions about whether it is possible to live on Mars, as SpaceX CEO and tech billionaire Elon Musk hopes humans might do one day. On the Red Planet, humans would be subject to gravity below the 0.67-g threshold.
“It does suggest that Mars gravity alone would not be enough to preserve muscle function,” Bouxsein says. On the other hand, she notes, perhaps we won’t need as much strength on a planet with less gravity anyway.
“Maybe on the way back, when you’re coming back to Earth, you need to build it up so that you’re ready to go when you get back,” she says.