A science Advances study aboard the International Space Station found a gravity threshold around 0.67 g, below which muscle deterioration occurs, implying Moon and Mars gravity may be insufficient to prevent atrophy on long missions.

Mice were exposed to four gravity levels—microgravity, 0.33 g, 0.67 g, and 1 g—to assess the soleus muscle, the key indicator of gravity-related atrophy, with 0.33 g maintaining size but reducing grip strength and 0.67 g fully protecting muscle function.

Historically, 24 mice were launched to the ISS in 2023, kept at these four gravity levels for about four weeks, and later analyzed to measure muscle tissue changes, focusing on the soleus as the deterioration marker.

Parallel human data from parabolic flights suggest a similar threshold range of about 0.5 g to 0.75 g, indicating the 0.67 g figure could be meaningful but needs validation in humans.

Researchers aim to refine the threshold across bones and muscles, determine how exercise shifts it, and translate findings into practical mission design and astronaut health strategies.

The study is framed within broader space health challenges—organ shifts, gut microbiome changes, and bone loss—highlighting ongoing research to enable healthier long-duration spaceflight.

Pinpointing the gravity threshold informs planning for lunar and Mars missions, including artificial gravity systems and the necessity of exercise countermeasures for extended spaceflight.

Experts note that while mice are a useful model for gravity effects, their results must be translated cautiously to humans due to differences in locomotion and muscle composition, underscoring the need for human-focused studies.

Despite the caveats, the mouse work provides a starting point for understanding gravity requirements to prevent deconditioning and guides future research on exercise in space.

The finding that 0.67 g preserved muscle function similar to 1 g suggests a clear threshold for mice, but applying this to humans remains an open question because of species differences.