§3The body

Bone that does not fully come back, muscle that weakens under daily exercise, eyes that change shape, and a surgeon up to 22 light-minutes away.

Microgravity is the best-studied bad environment humans have ever lived in. Decades of ISS data describe what six months of weightlessness does to bone, muscle, and eyes. The findings are established; what makes them a Mars problem is that the damage is only partly reversible and the treatment options shrink with distance.

Bone

§3.1 Bone mineral density in weight-bearing bone falls by about 1–1.5% per month in microgravity (NASA Human Research Program evidence report; ESA figures put the proximal femur at ~1.5%/month, about 10% over six months). A meta-analysis (Stavnichuk et al., npj Microgravity, 2020) found lumbar spine and pelvis −6.2%, lower limbs −5.4%, with a lower-limb rate of −0.8% per month.

The critical finding is what happens after landing. Gabel et al. (Scientific Reports, 2022) found tibia strength and density still 0.9–2.1% below preflight a full year after return, with 9 of 17 astronauts not fully recovering, and longer missions recovering worse. NASA’s Lifetime Surveillance of Astronaut Health data agrees: at one year post-flight, only 34% of astronauts had regained preflight hip density and 46.8% spine density. established

Muscle

§3.2 Losses of roughly 12–20% in specific muscle groups over six months, and 25–30% loss of calf twitch force (Comfort et al., npj Microgravity, 2021); recent syntheses cite 20–30% loss of cross-sectional area and a 10–15% decline in strength. The countermeasure is ARED resistive exercise, about 2.5 hours a day. It attenuates the loss but does not prevent it; higher intensity is most protective. That is the ISS answer, with a full gym and no gravity at all. What 0.38 g does instead is the subject of §4.

SANS

§3.3 Spaceflight-Associated Neuro-ocular Syndrome. Optic disc edema has been reported in approximately 70% of astronauts completing extended ISS missions, as quantified by optical coherence tomography (Laurie et al., cited in Frontiers in Ophthalmology, 2024); the earliest signs (disc edema, globe flattening, choroidal folds, a hyperopic shift) affect roughly two-thirds of long-duration crew. The cause is still not fully understood; the leading candidates are the headward fluid shift and altered cerebrospinal-fluid dynamics. No permanent vision loss has been reported to date (the longest mission is about 14 months), but some structural and refractive changes persist indefinitely, and NASA rates SANS as requiring mitigation for missions of one to three years. established as a syndrome; unknown as a multi-year trajectory.

Medicine at distance

§3.4 One-way communication delay to Mars runs up to about 22 minutes, roughly 44 minutes round trip. Evacuation in medically meaningful time is impossible; all three medical evacuations in spaceflight history were from low Earth orbit. NASA’s Integrated Medical Model (Antonsen et al., npj Microgravity, 2022) model output concludes that “the current operating paradigm for medical support will need to change,” and surgical capability is deemed mandatory for a ~30-month mission.

Immune function and isolation

§3.5 Spaceflight causes immune dysregulation (T-cell downregulation, reactivation of latent viruses), and the combined effect of radiation and microgravity on the immune system is a formally acknowledged knowledge gap. Isolation and confinement are studied on the ground: NASA’s CHAPEA Mission 1 ran 378 days (June 2023–July 2024), Mission 2 began October 19, 2025, alongside HI-SEAS and Antarctic analogs.

Expedition or settlement?

§3.6 For an expedition (a small crew, ~30 months, Earth-supplied) every item above is a serious but bounded risk with a partial countermeasure. For a settlement, each is open-ended: no data on recovery after years, no data on eyes past about 14 months, no surgeon who did not also make the trip.