# Safe and non noticeable changes in gravity

I was wondering what would be the most percentage change in gravity that a human would not notice and would be safe for the body functions?

I am designing a rotating space habitat and I want to know what the range of height I can have for living spaces or just areas that humans can access without feeling the change in gravity and having to adapt to it.

I used an artificial gravity calculator and for a 10km radius habitat, raising or reducing by 1km gives a 10 percent change in gravity (if I calculated it correctly) So a total depth and height of 2km is pretty good but 10 percent change in gravity may be too much.

What would be an acceptable percentage change in gravity to leave the humans feeling no different and their body unaffected when they move higher or lower in the habitat?

• Hmm... If perception of sustained weight works the same way as warmth feeling, this could extend a bit your gravity range. How fast people move from high to low gravity habitat? Commented Jan 3, 2022 at 1:37
• Well, the gravitational acceleration on the surface of the Earth varies by about 0.5% (from 9.775 m/s² in Mexico City to 9.826 m/s² in Anchorage, Alaska), so we can be absolutely sure that a variation of 0.5% is completely insignificant. We also know that zero gravitational acceleration induces a host of health problems. Unfortunately, we have no data for any value in between. Commented Jan 3, 2022 at 1:53
• Why does it matter if people are aware of the gravity change? All that matters is if the gravity gets too strong for the occupants, or if it's too low to maintain health. (And note that low-g work spaces might be fine if they live at normal gravity.) Commented Jan 3, 2022 at 3:33
• This is a really interesting question. A 10% +/- change would be significant. Newtons Laws. Picking up that 10 kg. bag of sugar, and it is only 9 kg? The inertia would be the same, but if you picked it up with the same acceleration as when it was 11 kg., you would send it to the ceiling. Not quite, but it WOULD rise significantly higher, and it would land with the same splat. Moving to lower sections and lower gravity one would have to automatically do everything slower, make slower movements, for the same effect. You would have to adjust your means of locomotion as you descended and ascended. Commented Jan 3, 2022 at 4:22
• This might not be helpful at all, but you could have concentric rings of rotating habitat, where each ring is rotating at a different rate so that it approximates the correct gravity at all levels. You'd have to designs some pretty funky elevators to switch between levels though.
– TKoL
Commented Jan 3, 2022 at 13:09

There is a massive difference between 'human would not notice' and 'safe for body functions' with the intermediate stage of 'can adapt to without much trouble'.

From real life experience in elevators and trains you can see that a temporary change in acceleration/ gravity of only a few percent is easily noticeable. You can also adapt to it without actively thinking about it the same way you can lift objects of different weight.

For limits on safety for health one can look at experiments and design of figher planes. Exposing the pilot to say 3 or 5 g for a short amount of time is often unpleasant but causes no permament damage. Something like 10g is survivable with training and suitable equipment.

So if you want to design a space habitat I would say a gravity range from mabye 0.5g to 1.5g is perfectly fine for your inhabitants. They will certainly notice the differences and may prefer higher or lower gravity for some activities but they will manage just fine.

• 1.5 G would be pretty savage. Given that the advice I've seen for the weight of a backpack a child should be hiking with is 15% of body weight, and even adults should try to stay under 30%, I'd peg the upper limit for sustained activity at 1.15 G, with maybe service / storage areas that won't be occupied for long at up to 1.3 G. Commented Jan 3, 2022 at 14:22
• @Quarague Stephen H. Dole, in Habitable Planets for Man, 1964, considers the evidence and on page 12 says: "On the basis of the available data, one might conclude that few people would choose to live on a planet where the surface gravity was greater than 1.25 or 1.50 g." rand.org/content/dam/rand/pubs/commercial_books/2007/… Commented Jan 3, 2022 at 19:52

The question is a little uncertain whether the design is:

one) a rotating cylinder with a outer radius of 11 kilometers, and inhabited levels between radii of 9 and 11 kilometers, and exactly 1 g at eactly 10 kilometers radius.

or;

two) a rotating cylinder with a outer radius of 10 kilometers, and inhabited levels between radii of 8 and 10 kilometers, and exactly 1 g at eactly 9 kilometers radius.

They need to decide which design they prefer, and also calculate exactly what the similated gravity range will be for that design.

I found an online spin calculator.

https://www.artificial-gravity.com/sw/SpinCalc/

Going with design one, for 1 g at a radius of 10 kilometers, the rotation rate will be 0.2990416771972627 rotations per minute. At a radius of 9 kilometers and the same rotation rate, the similated gravity will be about 0.9 g, and at a radius of 11 kilometers the similated gravity will be about 11 *g.

With design two, for 1 g at a radius of 9 kilometers, the rotation rate be rotations per minute. At a radius of 8 kilometers and the same rotation rate, the similated gravity will be about 0.8888 g, and at a radius of 10 kilometers the similated gravity will be about 1.1111 *g.

Stephen H. Dole, in Habitable Planets for Man, 1964, considers the evidence and on page 12 says:

On the basis of the available data, one might conclude that few people would choose to live on a planet where the surface gravity was greater than 1.25 or 1.50 g.

[https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf]

I would definately limit the maximum gravity in the outermost habitable levels to under 1.25 g, and possibly to a lot under 1.25 g.

If your habitable is a cylinder with a radius of 10 kilometers, the surface area will be about 62.8318 square kilometers for each kilometer of length of the cylinder.

And suppose that there are equal numbers of levels above or below the exact 10 kilometer radius, as in design one above, and the radius range is not very large, the total surface of all the levels should be approximately equal to 62.8318 square kilometers per kilometer of cylinder length mutliplied by the number of levels.

If your habitat is designed like a gigantic building with levels separated by the same separation as floors in a building aboout 14 feet or 4.3 meters, 2,000 meters of height should have room for about 465.11627 levels.

So if your habitat is designed like a building, and has levels separated by as much as the average level in a building, it could have about 465 levels in about 2,000 meters of height, and thus it would have about 29,216.787 square kilometers of floor for each kilometer of length.

Earth has a total surface area of 510,072,000 square kilometers, so if your habitat was 17,458.18252 kiometers long it would have as much total surface area as Earth.

Of course the habitat might be designed with much greater spacing between levels, and thus much less total floor space.