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My question is how much gravity do Human Beings need to be reasonably healthy long-term?

I am aware that the current answer to this question is "We don't know", but I wish to know if we have any educated guesses for how much Humans require based on our existing knowledge.

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    $\begingroup$ Highly related, possible duplicate space.stackexchange.com/q/16571/6944 Also space.stackexchange.com/q/55076/6944 Consider searching other questions under the gravity tag. $\endgroup$
    – Organic Marble
    Aug 24, 2022 at 21:31
  • $\begingroup$ "We don't know" is the best that can be said. That zero g is not good is known as those experiments have been performed. $\endgroup$
    – David Hammen
    Aug 25, 2022 at 5:17
  • $\begingroup$ How long is long term? $\endgroup$
    – user3528438
    Aug 25, 2022 at 12:45
  • $\begingroup$ And do these humans ever need to return to a higher gravity place? $\endgroup$
    – Jon Custer
    Aug 25, 2022 at 12:50
  • $\begingroup$ This might need to be broken into 2 categories. “How much gravity do adults need to live out an unrestricted life?” and “How much gravity does a developing infant or child need?”. Plenty of data on adults, practically nothing but animal studies on fetus or infants. $\endgroup$
    – Anthony Stevens
    Mar 27 at 20:58

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This is an excellent (if unanswerable) question.

There is insufficient evidence to provide an answer which meets SE standards. But you asked for an educated guess. I’m educated. Here’s my guess:

First, the question requires some qualifiers to reduce it to a bite-size chunk.

You need to specify if you are including re-adaptation to Earth gravity. Some health effects of low gravity (muscle atrophy, decreased bone density) could be considered adaptive in that they are only maladaptive on return to higher gravities. Maybe a bunny-hopping Lunar Citizen doesn't plan to return to Earth and is not concerned about muscle atrophy. And if the subject must re-adapt to Earth gravity, what do you consider a reasonable period for re-adaptation? If it takes 10 years to regain bone mass (during which the subject is at risk of fractures), is this “reasonable”?

Does your definition of “healthy” include treatable health effects of low-g? For instance, calcium loss from bones will increase the risk of urolithiasis (kidney stones). The risk can be reduced by diet. Both medical and surgical treatment is available. Ask someone with renal colic if they feel “reasonably healthy”.

You ask about long-term health. How long? In the long term, we all die. Do you mean a small decrease in life expectancy? Say, maximum 5 years of life lost? (By comparison, smoking shortens life expectancy by 10 years). Let’s say the heart muscle atrophy produced in low gravity results in a 1:10 chance of early cardiac death and a 2 year reduction in life expectancy. Do you consider this reasonable?

An apparently healthy person with a compromised immune system https://www.ucsf.edu/news/2021/06/420756/space-travel-weakens-our-immune-systems-now-scientists-may-know-why and living with higher mutation rates in microorganisms https://pubmed.ncbi.nlm.nih.gov/11027966 may appear “reasonably healthy” but have increased risk of infectious disease.

One of the worrisome effects of microgravity is Spaceflight-Associated Neuro-Ocular Syndrome (SANS) https://en.wikipedia.org/wiki/Visual_impairment_due_to_intracranial_pressure#Non-invasive_ICP_measurement . This condition, found in 15 long duration astronauts, shares features with Idiopathic Intracranial Hypertension. The cause of these conditions is unknown, but anything amiss inside the brain is worrisome for astronauts (pre-retirement, anyway).

So, here’s my educated guess:

For no Earth return, allowing for an acceptable 5 year loss in life expectancy and allowing loss of cognitive functioning after age 65, I’d guess 0.5 +/- 0.4g.

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The obvious answer is near enough exactly 1g. That is what we have evolved for so that is what we cope best with.

"Reasonably healthy" just leads us into deciding on what we could we want "reasonably" to mean - and we could define that any way we want. Is it no less than 5% bone mass lost in a year? Is it surviving more than 10 minutes?

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    $\begingroup$ The "obvious answer" of 1.0g may not be the "best". Exercise is good for us. Maybe the added exercise of 1.2g is more good. But then, stress is bad. Maybe the reduced physiologic stress of 0.8g is less bad. $\endgroup$
    – Woody
    Aug 25, 2022 at 18:55
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    $\begingroup$ It depends to the health state of the person. There are some the added exercise of 1.2g would be better. But others would be better off with less than 1 g. $\endgroup$
    – Uwe
    Mar 26 at 18:25
  • $\begingroup$ Many overweight people are at least 1.2 g of what the doctors say they should. The human body is amazingly adaptive. $\endgroup$
    – Robert DiGiovanni
    Mar 29 at 23:03
  • $\begingroup$ @RobertDiGiovanni - no. Almost all people live on Earth, so overweight people still live in 1g. $\endgroup$
    – Rory Alsop ♦
    Mar 31 at 9:15
  • $\begingroup$ @RoryAlsop OK, let's do Newtons. F = ma. You're right. I'll just head for the health club and lose some mass. $\endgroup$
    – Robert DiGiovanni
    Mar 31 at 10:00

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