What is known comes from the two endpoints. At 1 g, bodies work. At microgravity, decades of ISS data document what happens: bone mineral density falls by around 1–2% per month in load-bearing bone, muscle atrophies, fluid shifts toward the head and cause the vision changes known as spaceflight-associated neuro-ocular syndrome, and the cardiovascular system deconditions. Aggressive exercise regimes slow all of this without stopping it.
The unanswered question is whether the curve between those endpoints is linear, threshold-shaped or something else. If a third of a gravity is enough to trigger normal bone loading responses, Mars is straightforward. If the response is proportional, settlers accumulate deficits indefinitely. Nobody knows, because the experiment requires either a partial-gravity centrifuge in orbit or people living on the Moon or Mars.
The generational questions are harder still. No mammal has gestated, been born and developed in reduced gravity. Skeletal development, inner-ear and balance development, cardiovascular formation — all of these are shaped by load during growth. A child raised at 0.38 g may simply be a person adapted to Mars and unable to visit Earth, which is a civilisational fact rather than a medical one.
The mitigations are unattractive. Centrifuges can produce artificial gravity but building one large enough for continuous living on a planetary surface is a megaproject in its own right, and intermittent short-radius centrifuge sessions have never been shown to substitute for constant loading. In practice, Martian gravity is a risk that gets accepted rather than solved.
Unlike orbit, Mars provides continuous mechanical loading. Fluid distribution, sedimentation, convection and simply putting objects down all work, which removes a large class of microgravity problems.
Structures, excavation, lifting and launch all cost less at 38% weight. Escape velocity from Mars is 5.0 km/s against Earth's 11.2, which is why local propellant production closes.
A sol is 24 hours 39 minutes. Circadian rhythm, one of the harder problems on the Moon or in orbit, is essentially a non-issue on Mars.
Every physiological number comes from 0 g or 1 g. The behaviour of the human body at 0.38 g is entirely unmeasured, and cannot be measured without going.
If bone and muscle response scales with load rather than switching on above a threshold, settlers lose density continuously with no equilibrium point.
No mammal has gestated or developed in reduced gravity. Whether a child raised on Mars could ever visit Earth is an open question with no experimental path that is ethically simple.
A planet-surface centrifuge large enough to live in continuously is a megaproject, and intermittent sessions have never been shown to substitute for constant loading.
| Research needed | Partial-g centrifuge | In orbit, or lunar surface data |
| Countermeasures | Exercise, unproven at 0.38 g | Extrapolated from ISS |
| Generational data | None exists | No mammal gestated in low g |
| Escape velocity | 5.0 km/s | vs. 11.2 from Earth |
3.72 m/s², about 38% of Earth's. A 70 kg person would weigh the equivalent of 26.6 kg on Mars.
Nobody knows. All physiological data comes either from 1 g on Earth or from microgravity aboard the ISS, where bone density in load-bearing bone falls 1 to 2 percent per month. Whether 0.38 g is sufficient to trigger normal loading responses has never been measured.
It is entirely untested. No mammal has gestated and developed in reduced gravity, and skeletal, vestibular and cardiovascular development are all load-dependent. A person raised at 0.38 g might never be able to visit Earth comfortably.
