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Most discussion I have read about using tethers and rotation in order to simulate gravity on spacecraft, talk about simulating Earth's gravity - 1g or 9.8m/s^2.

Baked into the 1g figure is the assumption that humans evolved on Earth where gravity is 1g so it's probably healthiest for us. But is that really necessary? Have there been any studies or research into how much gravity is actually needed in order to minimize the long-term health effects?

  1. A spaceship that rotates to generate 1g of gravity would either require a (debatably) impractically long tether, or have to spin so fast that it would cause a disorienting Coriolis effect. Wikipedia says that the human factors of Coriolis effect would be mostly negligible at 2rpm. By my calculations, At Earth's gravity, that yields a radius of 224 meters, whereas at Mars gravity, that is reduced to 84 meters. One could potentially imagine a spacecraft with two manned modules connected by tethers and a 168 meter long inflatable tube to allow crew and supplies to pass between them; however, bring that up to 448 meters for 1g, that's over a quarter of a mile - and you can see that it starts to become impractical.

  2. A spaceship that is enduring 1g of centripetal acceleration would have to be built with the same rigidity and structural properties of a similarly sized structure on earth, meaning that it would require more materials and therefore have more mass. If we assume the entire craft is assembled on Earth and launched as a unit, Perhaps this is not such a problem - since the spacecraft would have to be built under Earth conditions, and in fact endure acceleration significantly greater than 1g during launch. Even so, the ship would be in a different configuration during launch that could be designed to be more rigid, or perhaps an aerodynamic fairing could be used to provide extra rigidity during launch, or perhaps the acceleration during launch would be on a different axis than the planned rotation. However, assuming some amount of orbital assembly, which is in fact rather likely, there would have to be at least some additional mass overhead to design for 1g of simulated gravity.

  3. If we're going to send astronauts on multi-month or multi-year missions to Mars, or even start a colony there, they're going to be exposed to Martian gravity which is (VERY approximately) one third that of earth. Similarly, gravity on the moon is 1/6 that of Earth. If we think that's OK then why bother with 1g on board the spacecraft to take them there?

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    $\begingroup$ The actual answer is "we don't know." The problem is too complex to model; it must be answered experimentally - and the experiment is a little too expensive for current political climate. $\endgroup$ – SF. Jul 1 '16 at 17:18
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    $\begingroup$ Can any insights be gained from those who have spent long periods of time confined to bed (in 1g)? If we see effects on astronauts linked to bone and cardiovascular health due to the absence of forces against which the long bones and the heart have to work, isn't being in a prone position comparable? Haven't there been studies based on that exact premise? And if so, wouldn't that create the opportunity to investigate thresholds at which effects begin to appear? $\endgroup$ – Anthony X Jul 1 '16 at 18:48
  • $\begingroup$ @AnthonyX: Yes, bed rest has many effects very similar to zero gee. For bone density, there is evidence for a very low threshold: nytimes.com/2016/04/02/health/… $\endgroup$ – Ben Crowell Jul 2 '16 at 1:42
  • $\begingroup$ At least we have two practically relevant G-levels to consider. That of the Moon and that of Mars. If neither is enough for human health, then our settlement of space will be very different than if that medium gravity turns out to be healthy for us. I speculate that even slight gravity is enough to solve many problems, and gymnastics in low gravity environment could take care of the problem during a couple of years at least. Just rotating the Mars transfer spacecraft slightly would get rid of alot of microbe microgravity problems as well as gym equipment problems. $\endgroup$ – LocalFluff Jul 2 '16 at 11:43
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We do not know yet. The main issue is a lack of empirical data.

There are only four specially trained volunteers with more than one year exposure to microgravity. We'd need hundreds of volunteers under different gravities to measure the difference (it is however plausible to assume that the effects are gradually dependent on the level of gravity).

Conclusion: For any long-term mission we should provide as much (up to 1g of course) gravity as is reasonably possible.

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  • $\begingroup$ Certainly it is not possible to reasonably provide 1g while on Mars (who wants to live in a giant centrifuge?) So of course this applies to transit. Assuming an approx. 2 year cycle for Mars and Earth to be in alignment for transportation between the two worlds, and given a reasonable transit time (three months?) is the benefit of a full 1g worth the extra cost? Would six extra months at Mars gravity of 1/3g make much difference? Would some intermediate value (1/2g or 2/3g) make sense as a compromise and serve to help astronauts "transition" more gradually between the two environments? $\endgroup$ – orulz Jul 1 '16 at 18:18
  • $\begingroup$ As I said: We just do not know. All we know is that the risk of exposing a well-trained person under a strict regimen for roughly a year seems to be tolerable in some cases. In case of a Mars mission that might still have fatal consequences to at least some of the crew. $\endgroup$ – choeger Jul 1 '16 at 18:39
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    $\begingroup$ I find it frustrating that more hasn't been done to find this out. A year at zero G might be acceptable, for some definitions of acceptable. But you don't want to get someone to Mars only to find out they can't walk when they get there. And you don't want to build a colony on Mars only to discover that we degenerate and die under Martian gravity. Somewhere between zero and 1 gee is an amount we can live with long term, and that number has a huge impact on a Mars mission. Yet here we are, blithely designing Mars missions without knowing that. $\endgroup$ – Eric Shafto Jul 2 '16 at 3:04
  • $\begingroup$ @EricShafto I completely agree. The gravity question is the only one that matters long-term because it's the one problem that can't be solved by engineering in the foreseeable future. I certainly wouldn't want to live in 38% gravity. One would weigh 2.63 times as much on Earth. We should build large rotating space stations and experiment. Maybe slightly more than 1G increases life span, who knows? We need to find out. $\endgroup$ – nmit026 Jul 19 '17 at 22:30
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We don't know.

We currently only have good data for how humans are doing in 9.81 m/s² or in 0 m/s² acceleration.

The only case where humans were ever exposed to anything between 1g and 0g for longer than a few minutes was during the Apollo moon landings (1.62 m/s²). The longest was Apollo 17 with 75 hours. Still not nearly long enough to notice any long-term health effects.

To find out how exactly different health problems scale with decreasing gravity we would need to put some humans in different gravity environments for several months. Options could be a permanent moon base or a centrifugal space station in Earth orbit. As far as I know, neither is currently in the budget of any space agency.

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