§4The gravity problem

0.38 g. No chronic human data at that load, none. The first mammalian data arrived in March 2026, and it cuts both ways.

Mars pulls at 0.38 g. Every figure in the settlement literature about living, working, and raising children there rests on extrapolation from two endpoints: microgravity, which is well studied and bad for the body (§3), and Earth gravity, the baseline. What happens in between was, until March 2026, almost entirely unmeasured in any mammal.

Why there was no data

§4.1 The JAXA MARS centrifuge on the ISS showed in 2017 (Shiba et al., Scientific Reports) that 1 g of centrifugation fully protected mouse bone and muscle against microgravity. But that experiment compared only 0 g and 1 g. Partial gravity, the only gravity a Mars settler will ever feel, sat between the two points, unmeasured.

The first real data point

§4.2 Tsuji et al., “0.33g mitigates muscle atrophy while 0.67g preserves muscle function and myofiber type composition in mice during spaceflight” (Science Advances, March 13, 2026; DOI 10.1126/sciadv.aed2258), placed 24 male mice on the ISS centrifuge at four levels (microgravity, 0.33 g, 0.67 g, 1 g) for 27–28 days.

Soleus cross-sectional area was preserved at 0.33 g, roughly Mars. Grip strength was not: it “showed a significant decline after spaceflight in both the μG and 0.33g groups.” Only 0.67 g maintained muscle function and fiber-type composition. In plain terms: Mars gravity likely prevents muscle from wasting away, but not from weakening.

FIG 4-1 · Tsuji et al. 2026, 24 mice, 27–28 days on the ISS centrifuge. Model organism, single study, not human data. Ground-based partial-weight-bearing rodent analogs concur that Martian loading causes deconditioning intermediate between 0 g and 1 g.

Reproduction: the settlement gate

§4.3 Reproduction is the harder half of the question, and it is entirely unstudied at partial gravity. What exists is microgravity data, and it is mixed. Mouse embryos formed blastocysts on the ISS with no significant DNA or gene-expression changes, though at lower survival rates than on Earth (Wakayama et al., 2023). Male mice housed on the ISS later sired healthy offspring (Matsumura et al.). But Shuttle-era work found serious brain-development issues in mouse offspring gestated in microgravity, and a 2025 study found simulated microgravity alters sperm navigation and embryo development.

Conception, gestation, and birth in partial gravity have never been tested in any mammal. unknown For an expedition this hardly matters. For a settlement, a place where children are born, it is the whole question, and it is the Weinersmiths’ central one: can we have space kids?

What would change this chapter

§4.4 The decisive experiments are partial-gravity mammalian studies, especially of reproduction and development at 0.38 g. The report names two things to watch: JAXA’s MHU-8 partial-gravity reproduction work, and any move toward a variable-gravity research facility. Until there is multi-generation mammalian data at 0.38 g, the phrase “self-sustaining city” is biologically unvalidated. Not disproven; unvalidated. Zubrin’s position that Mars gravity is probably sufficient is a defensible prior. It is a position, not a result, and this chapter carries it as one.