Partial-gravity unknowns: how much g is enough?

The central scientific question behind the whole thread: what fraction of 1 g keeps humans (and their offspring) healthy?

Key results

Data available by 2026

  • JAXA 2026 mouse study. One month on an ISS centrifuge at 0, 1/3, 2/3 and 1 g Paul451, 2026-03-14.
  • Significant negative effects at 0 and 1/3 g; 2/3 g close to 1 g.
  • Half the 1/3 g mice struggled to walk afterwards Greg Hullender, 2026-03-14.
  • 1/3 g was still clearly better than microgravity Vultur, 2026-03-14.
  • Paper: Science Advances, doi:10.1126/sciadv.aed2258.
  • Caveats (Paul451):
  • The levels are coarse, so the threshold could be just above 1/3 g, around 1/2, or near 2/3. Whether it is a sharp cutoff or a gradual curve is unknown Paul451, 2026-07-07.
  • Runs lasted 1 month, not 6–12.
  • The 2/3 g and 1 g centrifuge groups had the lowest body weight, which may reflect higher-rpm noise or stress.
  • Mice self-exercise ("racetracking"), so exercise is already baked into the results.
  • Earlier JAXA results.
  • Hayashi et al. 2023: lunar gravity prevents skeletal muscle atrophy but not myofibre type shift.
  • A 2023 poster suggested Mars g preserved health LMT, 2024-02-09; Greg Hullender, 2024-03-22.
  • The poster had small samples, and 1/3 g sometimes beat 2/3 g Greg Hullender, 2024-03-23. Some graphs suggest 1/3 g ≈ 1 g for some effects mikelepage, 2024-04-02.
  • Cellular thresholds. Rat macrophages on an ISS centrifuge stepped 0 → 1 → 0 g showed sensitivity at 0.3–0.5 g mikelepage, 2019-03-28.
  • Reproduction.
  • JAXA hyper-g mice showed reduced fertility and sperm motility; micro-g sperm motility was higher; embryo development is the main concern Paul451, 2018-07-17; Paul451, 2018-07-20.
  • mikelepage expects pregnancy and some childhood development to need about 1 g ("salmon returning upstream to spawn"), so habitats would host many pregnant women and children mikelepage, 2023-03-06.
  • Human anecdote. Apollo crews found lunar g the most comfortable environment they experienced Paul451, 2024-04-03. Pete Conrad ran around Skylab's ring at about 8 rpm (~0.2 g at his feet) without apparent trouble Paul451, 2015-02-14.

Physics and priors

  • Equivalence. Spin gravity and "mass" gravity are equivalent except for Coriolis and gradient effects; embryos are small, so Coriolis is negligible for them Vultur, 2026-03-16; Robotbeat, 2026-03-16. JohnFornaro held that spin and mass gravity must differ biologically JohnFornaro, 2026-03-16.
  • lamontagne proposed a 1 rpm merry-go-round on Earth to test Coriolis on foetuses lamontagne, 2026-03-16.
  • Complex-life pressure analogy. Complex life tolerates 0.5–2 atm, so mikelepage guessed the partial-g threshold is ≥0.5 g mikelepage, 2015-03-16.
  • Head-to-foot gradient. The "1% differential" rule (2 m person ÷ 1% = 200 m radius) was admitted to be a guess. Differentials of 1/2/5/10/20% have never been tested below 1 g A_M_Swallow, 2015-09-02; A_M_Swallow, 2015-09-02.

Intermittent gravity

  • Sleeping in gravity gives virtually no benefit, because bed rest is the ground analog for 0 g. High-impact exercise is the best time to have g MGoDuPage, 2022-09-20; Paul451, 2022-09-21.
  • A short-arm centrifuge is an effective countermeasure for bed-rest deconditioning Lampyridae, 2016-01-18. See the Reschke & Paloski 2017 bed rest + intermittent centrifugation study LMT, 2018-07-29.
  • Weight vests plus partial g: spin for fluid distribution, added weight for musculoskeletal loading, plus exercise. Radiation vests load the trunk while exercise weights belong on the limbs, so the synergy is modest Twark_Main, 2026-08-19; Vultur, 2026-08-14.

Positions on design gravity

Position Proponents
Mars g (0.38) as the standard beyond Earth, cheaper than 1 g Coastal Ron Coastal Ron, 2023-03-03; Coastal Ron, 2025-09-08
0.5 g is healthy and cheaper; good for tourists hkultala hkultala, 2015-11-10
~0.67–0.7 g after the JAXA data; transit at 0.5 g plus exercise MickQ MickQ, 2026-07-05
1 g, to allow Earth controls and settle the question mikelepage, Habitant Habitant, 2025-10-20
"What is 1 g for? We already have 1 g" Paul451 Paul451, 2018-07-14

Open questions (collected)

  1. The shape of the damage curve between 0 and 1 g: is 0.1 g enough, or is 0.9 g too little? Paul451, 2015-02-13
  2. Is the curve the same for every effect (bone, muscle, eyes, immune, reproduction)? Paul451, 2019-04-16
  3. Does degradation at 1/3 g plateau or keep progressing? Greg Hullender, 2026-03-14
  4. Can partial-g deconditioning be offset by periods of >1 g ("pre-conditioning")? Paul451, 2015-02-13
  5. Does a lifetime at 0.38 g matter if people never return to Earth? Vultur argues muscle loss is acceptable for permanent settlers; vision and pregnancy are what matter Vultur, 2026-03-13.
  6. Multi-generation effects in mammals: no data.

Sources

Forum posts

  • Paul451, 2015-02-13 (reply #272): Paul451 unknowns list 2015. Post.
  • Paul451, 2015-02-14 (reply #276): All Earth centrifuge data >1 g; Conrad ran in Skylab. Post.
  • mikelepage, 2015-03-16 (reply #534, 1 like): Pressure analogy → guess ≥0.5 g. Post.
  • A_M_Swallow, 2015-09-02 (reply #587): 1% rule was a guess. Post.
  • A_M_Swallow, 2015-09-02 (reply #588, 1 like): Gradients never tested <1 g. Post.
  • hkultala, 2015-11-10 (reply #745, 1 like): Why 1 g? 0.5 g healthy & cheaper. Post.
  • Lampyridae, 2016-01-18 (reply #831, 1 like): Short-arm centrifuge effective in bed-rest studies. Post.
  • Paul451, 2018-07-14 (reply #926): What is 1 g for? need different levels. Post.
  • Paul451, 2018-07-17 (reply #968): Hyper-g fertility drops; micro-g sperm motility higher; embryo issues. Post.
  • Paul451, 2018-07-20 (reply #1003, 1 like): Mice may be more fertile in low g; embryo development main issue. Post.
  • LMT, 2018-07-29 (reply #1050): Reschke & Paloski 2017. Post.
  • mikelepage, 2019-03-28 (reply #1415, 2 likes): Macrophages sensitive at 0.3–0.5 g. Post.
  • Paul451, 2019-04-16 (reply #1554, 1 like): Same curve for all effects?. Post.
  • MGoDuPage, 2022-09-20 (reply #1787): Gravity during sleep little benefit. Post.
  • Paul451, 2022-09-21 (reply #1788): Exercise is best time for g. Post.
  • Coastal Ron, 2023-03-03 (reply #3459): Mars g will be most prevalent. Post.
  • mikelepage, 2023-03-06 (reply #3494): Pregnancy/child development may need ~1 g. Post.
  • LMT, 2024-02-09 (reply #4463, 1 like): Hayashi 2023 lunar-g paper; encouraging Mars-g poster; MARS system. Post. Attachments: Shiba et al. 2017 Fig1.png, Wadhwa et al. 2023 poster session detail.png.
  • Greg Hullender, 2024-03-22 (reply #4604): Mars g seemed fine for preserving health (15 cm radius). Post.
  • Greg Hullender, 2024-03-23 (reply #4608, 1 like): Poster not paper; small samples. Post.
  • mikelepage, 2024-04-02 (reply #4620): 1/3 g as good as 1 g for some effects. Post. Attachments: Screen Shot 2024-04-02 at 1.17.07 pm.png.
  • Paul451, 2024-04-03 (reply #4633, 1 like): Apollo: lunar g most comfortable. Post.
  • Coastal Ron, 2025-09-08 (reply #4941): Mars g as a standard; cost. Post.
  • Habitant, 2025-10-20 (reply #5022, 1 like): 1 g for productivity and minimal risk (Habitant). Post.
  • Vultur, 2026-03-13 (reply #5043): Muscle loss not issue for permanent settlers; vision/pregnancy matter. Post.
  • Paul451, 2026-03-14 (reply #5045, 6 likes): JAXA study: coarse levels, threshold between 1/3 and 2/3 g, confounders (6 likes). Post.
  • Vultur, 2026-03-14 (reply #5046, 2 likes): 0.33 g clearly better than µG. Post.
  • Greg Hullender, 2026-03-14 (reply #5047, 1 like): Half the 1/3-g mice struggled to walk; eyes; plateau question. Post.
  • JohnFornaro, 2026-03-16 (reply #5051): Spin vs mass gravity must differ (JohnFornaro). Post.
  • Vultur, 2026-03-16 (reply #5052, 3 likes): Equivalence principle; only Coriolis differs (3 likes). Post.
  • Robotbeat, 2026-03-16 (reply #5053, 2 likes): Earth also has Coriolis. Post.
  • lamontagne, 2026-03-16 (reply #5054, 1 like): Test Coriolis with Earth merry-go-round. Post.
  • MickQ, 2026-07-05 (reply #5065): 0.67–0.7 g sufficient; transit 0.5 g + exercise. Post.
  • Paul451, 2026-07-07 (reply #5067): Coarse levels; sharp or gradual unknown; mice self-exercise. Post.
  • Vultur, 2026-08-14 (reply #5075): Radiation garments + distributed weight?. Post.
  • Twark_Main, 2026-08-19 (reply #5079): Spin + weights + exercise; modest synergy with radiation vests. Post.

External references

  • JAXA partial-gravity mouse study, Science Advances (2026), doi:10.1126/sciadv.aed2258.
  • Hayashi, T. et al. (2023). Lunar gravity prevents skeletal muscle atrophy but not myofiber type shift in mice. Communications Biology 6:424.
  • Shiba, D. et al. (2017). Development of new experimental platform "MARS" (Multiple Artificial-gravity Research System). Scientific Reports 7:10837.
  • Reschke, M. & Paloski, W. (2017). Bed rest + intermittent centrifugation study.
  • Gizmodo (2026): "Mars doesn't have enough gravity to keep humans healthy, study suggests"; ScienceAlert coverage of the same study.