24 mice launched into orbit in 2023. What happened to their bodies could help humans better survive in space

24 mice launched into orbit in 2023. What happened to their bodies could help humans better survive in space

By Jackie Flynn Mogensen edited by Claire Cameron

Humans did not evolve for space. Ironically, because we insist on going there anyway, scientists now know from studies of astronauts that the lack of Earth’s gravity can wreak havoc on our health, such as by displacing our soft, watery organs, throwing off gut bacteria, weakening our bones, and more.

However, none of that seems to dampen the spirits of the astronauts heading 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 of mice sent to the International Space Station (ISS) suggests that living in different gravity causes changes in our muscles – and it identifies a critical threshold when problems can start to set in.

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If you are reading this on Earth, you are experiencing approximately 1 g gravity. On Mars, you’ll feel about 38 percent of Earth’s gravity, or 0.38 g. The Moon’s gravity is even smaller, at about 0.17 g, or one-sixth of the earth’s. But at what gravity does our body lose its normal function?

The study was published Friday in the journal The progress of science, shows that mice in space that were kept at 0.67 g maintained most of the muscle function and muscle fiber composition. But when the mice lived in an environment below this gravity, their muscles began 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 very interesting information about long-duration missions to Mars and beyond,” says Mary Bouxsein, a co-author of the study and professor of orthopedic surgery at Harvard Medical School, adding that it is somewhat reassuring that mice can maintain their muscle function without a full 1. g.

In 2023, an international group of scientists 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, scientists on the ground analyzed their muscle tissue for signs of deterioration.

In particular, 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 grip strength. At 0.67 g, However, the mice saw “full protection of muscle function,” says Bouxsein, meaning their grip was about the same as it was at Day 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 effects of space travel on health, but who was not involved in the new study.

“A key question will be the extent to which these findings will translate to humans during spaceflight, particularly with regard 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 also different, says Bouxsein. But the thesis provides an important starting point for future research into the health effects of space travel, 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 can do one day. On the Red Planet, humans will be exposed to gravity below 0.67-g threshold.

“It suggests that Martian gravity alone would not be enough to preserve muscle function,” says Bouxsein. On the other hand, she notes, we might not need as much power on a planet with less gravity anyway.

“Maybe on the way back, when you come back to Earth, you have to build it up so you’re ready to go when you come back,” she says.

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