Living and Reproducing on Low Gravity Worlds

1972 photograph of Apollo astronaut, Eugene Cernan, walking towards the LRV on the lunar surface (Credit: NASA)

by Marcel F. Williams

Establishing a permanent human presence beyond our planet of evolutionary origin is one of the long term goals of human space travel. The expansion of human settlements throughout the solar system has the potential to dramatically increase the economic wealth of human civilization while also greatly enhancing the survival of our species.

Some space advocates believe that the long term colonization of the solar system will require the manufacture of titanic artificial worlds that rotate to  produce simulated Earth-like gravities within their interior surfaces.  But there are still others  who believe that low gravity worlds such as  the Moon and Mars could be utilized as near term destinations for human colonization.

But can Homo sapiens really live and reproduce on hypogravity worlds?

In his classic 1972 song 'Rocket Man', Elton John says: "Mars ain't the kind of place to raise the kids..."

Well, maybe!


Planets and Moons within the solar system that are potentially suitable for human colonization:


Moon

surface gravity relative to the Earth: 0.17g 

diameter relative to the Earth: 27.3%

surface area relative to the Earth: 7.4%


Mars

surface gravity relative to the Earth: 0.38g 

diameter relative to the Earth: 53.1%

surface area relative to the Earth: 28.4%


Mercury

surface gravity relative to the Earth: 0.38g 

diameter relative to the Earth:  38.3%

surface area relative to the Earth:  14.7%


Callisto 

surface gravity relative to the Earth: 0.13g 

diameter relative to the Earth:  37.8%

surface area relative to the Earth:  14.3%

Note: Land area comprises ~ 29% of the Earth's surface with ~71% covered by water

Continuous exposure to microgravity conditions over weeks and months is inherently deleterious to human health. And there is growing evidence that long term microgravity exposure can also significantly  lower  fertility in humans and other mammals, possibly leading  to sterility. This suggest that crewed interplanetary missions requiring several months of space travel may require interplanetary vehicles capable of producing artificial gravity during the journey. Obviously, humans can't colonize Mars by sterilizing their passengers before they get there!

But it is currently unknown how much gravity is required to mitigate or eliminate significant infertility in humans. However,  if the lower gravity of the Moon or Mars turns out to seriously effect the long term fertility of humans then daily exposure to-- hypergravity-- through short armed centrifuges may be a possible solution.

Short radius hypergravity centrifuge could help to mitigate the possibility of infertility on lower gravity worlds such as the Moon and Mars.   (Credit NASA)

But the lower gravity on extraterrestrial worlds could have another deleterious effect that may effect human reproduction and even the ability of people to return to the normal gravity of the Earth's surface. Bone mineral loss under microgravity conditions is already known to occur in astronauts living in space for several weeks. And significant bone loss could distort the shape of the female pelvis to a degree that endangers her and a potential infant during attempted childbirth. Unfortunately, while hypergravity centrifuges may mitigate muscle loss in low gravity environments, they appear to have no effect on bone mineral loss.  Rigorous exercise in microgravity, however,  does seem to lower the rate of bone mineral loss-- but does not stop it.


Predicted time limits beyond the Earth  for significant  bone loss in humans that could  risk  skeletal fractures once astronauts return to Earth

Space (microgravity) - 36 weeks (60 weeks with exercise)

Moon (1/6 gravity) - 96 weeks

Mars (2/5 gravity) - 159 weeks


The predicted level of tolerable bone loss for humans in space is about 36 weeks. However, if astronauts exercise rigorously for a few hours every day then their stay in space can be extended to 60 weeks (more than a year). So it seems logical that rigorous exercise should enable humans to mitigate or even eliminate significant bone mineral loss under the hypogravity conditions of the Moon and Mars.

Having some gravity could make it possible for people to use heavily weighted vest or backpacks in order to avoid bone mineral loss while maintaining their Earthling strength-- even without daily strenuous exercise.  While lifting weights can strengthen the arms, weighted vest or backpacks producing an Earth-like weight to be carried by their hindlimbs would strengthen the legs which are normally physiologically weakened under microgravity and low gravity environments.,Within pressurized habitats on the Moon an Mars, heavily weighted vest could be worn throughout the day, providing exercise for the leg muscles when standing, walking, running and jumping.

However, children and infants who are born on the Moon and Mars  may also have to wear weighted vest on a regular basis soon after they are born if their bodies are to grow and develop properly on such low gravity worlds. But  it would appear that humans should be able to live and reproduce on low gravity planets and moons such as the Moon and Mars if they wear the appropriate clothing (weight vest or weight packs) while periodically experiencing hypergravity on a short armed centrifuge.

Of course, Homo sapiens is a species that it use to modifying  its clothing and its habitats in order to survive in more hostile environmental. That's why human ancestors were able to radiate from the tropical regions of Africa into the wintery weather of  Europe, Northern Asia, and eventually North America-- especially during the Earth's glacial periods.



Links and References

Bone Loss and Human Adaptation to Lunar Gravity

 Effects of artificial gravity during bed rest on bone metabolism in humans

 How Much Gravity Is Needed to Establish the Perceptual Upright?

Impacts of Altered Gravity on Male and Female Reproductive Health

 Detrimental Effects of Microgravity on Mouse Preimplantation Development In Vitro

 Morphological and Morphometric Study on the Effect of Simulated Microgravity on Rat Testis

Pioneering and Commercial Advantages of Permanent Outpost on the Moon and Mars

SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots