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Near Term 1 G Habitat on the Lunar Surface

Summary

The detrimental effects of zero gravity on the human body during long durations in space are well known. It is also well known from experiments in zero gravity in space that there are serious issues with reproduction and development of young animals and at least some kinds of plants, which may compound over the generations. Besides zero gravity, low gravity environments probably also have detrimental effects. It seems abundantly obvious that humans (and other mammals) simply cannot have healthy offspring in zero or low gravity, and that it would be much lower risk to reproduce, raise children, and live in a gravity environment similar to the one we are adapted to on Earth, "one G".

Further, many plants have had issues in zero gravity, so that many agricultural crops may have serious issues as regards productivity.

This is not a time for unbased optimism or political arguments. Anybody with a sense of urgency should be planning for small 1 G habitats in the near term. There are still a lot of unknowns, but we do know that the surest way to successfully settle space and have healthy growth in population independent of Earth would be to live in habitats with artificial gravity the same as on Earth, one G. To try otherwise would be an unnecessary risk.

This is a vital issue of space settlement and the survival of our species in space -- a human existential matter.

We will need artificial gravity by rotating habitats, most readily done in orbital space, but we can also try to make them on the surfaces of the Moon and Mars.

Lunar gravity is 16.6% that of Earth gravity, and Mars' gravity is about 38%. Therefore, we have a lot of gravity to make up for.

In the near term, just a couple of large cylindrical tanks on two ends of a cable or beam, with the entire system rotating for artificial gravity, are enough for first steps, for orbital based settlements. They can grow as more tanks are added. (Of course, in the long term, we can create wheel settlements such as those designed in great detail and illustrated artistically in the 1970s under the leadership of Dr. Gerard K. O'Neill, which were in turn based on prior concepts. However, those are very long term concepts and not worth working on now.)

On the surface of the Moon and Mars, to create 1 G, we would need some kind of rotating structure of much greater complexity than what would be sufficient in orbital space, due to the large and heavy habitat rotating on the surface under gravity, whereby there must be a mechanical interface to hold it up, minimize vibrations and noise, tolerate movement within, and be designed for reliability as regards maintenance of the means for holding it up and keeping it rotating smoothly and safely.

The ring could be held up by wheels, rails for magnetic levitation, and/or a central axis to hang it on.

These concepts cannot have too small a radius for humans, due to "motion sickness" when people are in small enclosures spinning fast. Large enclosures spinning slowly are comfortable with no effects, but not as economical in the earliest years of space settlement. Research has indicated where a "comfort zone" starts, as regards rotation rate and radius, based on research of humans in rotating enclosures over long periods of time.

The effects of a rotating habitat become tolerable to imperceptible as the radius of the rotating enclosure increases, which allows lower revolutions per minute for the same artifical gravity. People vary in their susceptibility to motion sickness. On rotational rates in between too high and very low, there are also speeds in which many people may feel a little uncomfortable in during the first day but adapt quickly. In small rotating enclosures, there are levels of human disorientation when going from a sitting to a standing position and moving around.

A reasonably safe design for comfort seems to be a habitat of radius 55 meters with a rotation rate of 4 revolutions per minute (RPM). This fits with calculations of what would be required on the Moon after taking into account the Moon's gravity in addition to the centripetal/centrifugal acceleration of the rotating habitat to create a total of 1 G.

Due to the low lunar gravity, a 1 G rotating habitat would have a floor which is almost perpendicular to the ground, at 80 degrees from horizontal.

A first generation lunar 1 G rotating habitat based on two SpaceX Starship-based Human Landing System (HLS) tanks could look something like the illustration below, roughly, based on the HLS tanks being 50 meters tall and 9 meters wide:

Illustration by Mark Prado.

Additional tanks could be added in pairs, until the entire circumference is full.

Of course, walking around in the rotating habitat, people wouldn't feel like their bodies are nearly parallel to the ground outside, unless there were windows to look outside from. It would be better to just have electronic displays on the walls from stationary cameras. Besides, transparent windows are expensive and can add some risk.

(Actually, the floors may be curved slightly, since only one place on a flat floor is at exactly 55 meters radius, so that the G level would vary a tiny bit within the tank. Either that, or the simplicity of a flat floor could be kept and occupants could make any adjustments to any significant slight slope effects. Also, additional tanks could be added at a shorter radius for lower gravity areas, of course.)

One concept is to have a maglev rail mounted on the surface of the Moon as a stationary structure on top of trusses, and things similar to a maglev train car on Earth riding on a maglev rail, except it is tanks instead of train cars.

A second generation, rotating habitat manufactured on the surface of the Moon could have a continuous internal space with one floor all around the circumference, and look something like the illustration below, showing a cross section:

Illustration by Mark Prado.

Additional floors could be added, either stacked above, or stacked within for inner rings. However, only the ring floor along the perimeter would have full Earth 1 G, and any inner ring floors would have less and less gravity as you go in.

We could grow crops in the inner floors, using plants which perform well in gravity ranges between 1 G and lunar gravity. Of course, we could also use inner floors for storage.

The rotating system could be balanced by ballast which is automatically adjustable.

Due to the much greater simplicity of orbital based rotating habitats, compared to surface based rotating habitats, for providing 1 G of artificial gravity, it may be much better to put the largest segment of the space population into orbital habitats, rather than base them on the surface of the Moon or Mars, and have pregnancies and raise children in these orbital habitats, at least until space industrialization is advanced enough to make suitable surface based habitats much more economical. We might have workers on the surface of the Moon while mothers and children stay in orbit.

(As regards DNA diversity, it may be best to not try to have conception by mating in space for at least the first generation, and instead bring a large number of fertilized and frozen IVF embryos from Earth representing diversity from the human population, which are then implanted into the wombs of surrogate mothers in space. This also reduces risks, including of an ectopic pregnancy emergency. However, that's another topic for another time. Here, the concern is mainly of a healthy 1 G for fetus development, child development, and adult health during long term space residency. Notably, in this scenario, the population would best be mainly females working as engineers, agronomists, and in other specializations, and doubling as surrogate mothers at times.)

Of course, there are also major advantages to manufacturing in orbital based factories, but the raw materials must come from the Moon, whereby we must deploy workers to the Moon, maybe in shifts.

However, if we are to have long term workers or settlers on the Moon and Mars, then we should refine designs on 1 G habitats on those surfaces to be suitable for the particular long term living concepts.

There is an unusual shortfall in the public, professional literature about 1 G habitats on the surface of the Moon or Mars for space settlement. Official artwork has people living and working in the low gravity environments. This overlooks the biological issues of long term duration in space, as well as biological reproduction and possibly having more productive crops.

As it is a relatively basic engineering and architecture matter, it could use attention from many capable people in the world. The artistic designs would also be popular on social media. So, please join us on developing designs for artificial gravity habitats on the Moon and Mars, and/or try to do something on your own.

The following will briefly cover architectural and engineering design considerations.

There are grandiose far future images which get too much attention from popular media outlets as sensational designs, but the focus here will be much smaller first generation habitats for economic and technical feasibility for the first generation of settlers. The author has chosen to ignore the long term designs and focus on the practical first generation designs necessary for survival of our species.

Details on Human Comfort

There are experiments going back to the 1960s for rotating habitats, in view of putting people into orbit in space stations, as in those early years of the space program there were many more unknowns about the effects of zero gravity on health, and also uncertainty about the plans of the manned space program. Some historical concepts before the space program had designs for rotating habitats for artificial gravity in outer space. Participants in the experiments in the 1960s were put into rotating rooms for days to over a month, and directed to move around and perform various tasks.

Going from a sitting to a standing position, and moving around, were of most interest, especially as regards head movements, and how it may cause motion sickness or significantly affect perceptions of balance. This is due to the coriolis effect of different parts of your body experiencing effects due to the varying distances of these parts of your body from the axis of rotation, including the relatively small distances inside one's head which help you keep your balance and make you aware of your orientation, and which are sensitive to motion.

People differ in their susceptibility to motion sickness while being in a rotating room, and in their adaptability over time. In experiments on Earth, at low rotation rates, even highly susceptible people were symptom free or nearly so. At somewhat higher rates, people experienced mild symptoms but did not have significant disabilities. Many people adapted to be almost symptom free in a short time, such as a day. However, above a certain threshold of RPM, even pilots who were otherwise resistant to motion sickness did not fully adapt after many days.

These results tell us what is surely comfortable at the extremely good side, such as very long radius and low rotation rates. However, it might not be economically feasible to have such large habitats on the lunar surface for first generation habitats, so we may want to make a judgment about the smallest habitat which would most probably be comfortable for chosen astronauts.

To produce 1 G, it looks like a radius of around 55 meters with a rotation rate of 4 revolutions per minute (RPM) would be comfortable. (Globus and Hall, 2017) Of course, for cost reasons, it would be economical to reduce the radius and increase the RPM, and that might be possible, but this analysis will take a more conservative and higher cost approach. It might be better to overestimate costs and have extra money for other things, than to underestimate costs and possibly incur delays later, though I'm aware that others may disagree and instead pursue foot-in-the-door techniques. It would be good to see improvements on this design, if there is truly a real basis for the improvements.

There is no need to have windows to look out of on a rotating habitat on the surface of the Moon, so we can avoid motion sickness induced visually. (There may be many video screens showing images from cameras located anywhere, but those can be showing stationary scenes.)

Below is a graph of comfort vs. rotation rate which this analysis has roughly gone by. A good conservative pick is 4 RPM, which results in a habitat of radius of almost exactly 55 meters on the Moon, and slightly more in orbital space, though we could economically increase the radius and reduce the rotation rate in orbit if we preferred. Of course, on the Moon's surface, large rotating habitats have a lot more engineering challenges. All the numbers as regards comfort are very rough, and may vary according to individual susceptibility and adaptability, which may in turn affect who is chosen from the pool of astronauts.

Original source: Robert R. Gilruth, "Manned Space Stations - Gateway to our Future in Space", Manned Laboratories in Space, edited by S. Fred Singer. Springer-Verlag, 1969. Cited by Theodore Wayne Hall, The Architecture of Artificial-Gravity Environments for Long-Duration Space Habitation, 1994. (The latter is a 304 page Ph.D. dissertation for a Doctor of Architecture submitted to the University of Michigan, which he has put online in PDF form on his current website, artificial-gravity.com This is an excellent reference, and Dr. Hall is still very active in this field. Here is a direct link to the PDF of Dr. Hall's dissertation within that website.)

A 110 meter diameter (55 meter radius) habitat rotating on the surface of the Moon is fairly massive, but of course at 16.6% Earth's gravity would "weigh" much less than a similar structure on Earth's surface. For a continuous ring floor around the perimeter, the walkable surface space would be proportional to the height of the rim. Each meter of floor width (i.e., height above the lunar surface) would give 173 square meters of floor space for a 55 meter radius habitat. Therefore, if we made it 5 meters tall, for a 5 meter wide corridor, we would have roughly 865 square meters (9311 square feet). This would be for the "ground floor". If we make the ceilings 3 meters tall, then it would have a volume of approximately 2500 cubic meters on the ground floor.

For comparison, the International Space Station has a size of slightly under 400 cubic meters.

Unless, of course, we put the main human population into orbital space, as I would recommend for many reasons, instead of trying to have children on a planetary surface, but this presentation will go ahead and discuss a design for the latter.

Actually, we could have more than a "ground floor" on our lunar habitat. There can be multiple levels between the ground floor and the hub, which provide lower levels of gravity, and which can be used for growing food from plants which are still healthy at lower gravity levels, and storage of various things. Some people may even live and work in the lower gravity levels. The 1 G level can be mainly for mothers and non-adults to live in. However, we might alternatively choose to not have many levels, in order to minimize the weight of the entire structure and thereby the engineering design to hold it up.

There are a variety of concepts for space settlement, so this presentation is not intended to endorse any particular design. It is mainly to cover some of the basics of first and second generation habitats for space settlement.

Engineering Design Considerations

To hold up the entire rotating structure on the surface of the Moon, the options are any or a combination of the following:

• A rail mounted on the surface of the Moon on trusses, with wheels on the tanks. The downsides include mechanical wear and maintenance, and vibrational noise. (Of course, some sort of dampening mechanism would be emplaced to try to minimize noise.)

• Magnetic levitation (maglev) rails would greatly reduce drag, require much less maintenance due to mechanical wear and tear otherwise, and make for a relatively quiet structure, but it should be designed for exceptional reliability such as any cryogenic systems. Wheels would still be required to get the tanks or structure moving up to the speed where the maglev can take over, and these wheels would be a backup system which could be quickly deployed if the maglev system failed or needed major maintenance. (It's also possible to have active maglev at zero velocity, as there are different kinds of maglev systems with tradeoffs, but wheels may be a better option for getting the maglev started and to serve as a good backup in case there are problems in the maglev system.)

• In the center, a central post or a more central ring could help hold up the entire system, but the mechanical stresses of central bearings would be higher, and the spatial stability of the hanging system with motion within would complicate the design. While rotating habitats in orbit use a central axis, it may be better to lift rotating surface habitats along the perimeter.

The perimeter speed of a 55 meter radius structure rotating at 4 RPM would be 23 m/s, which is around 83 km/hour (52 mph). Maglev trains using electrodynamic suspension (EDS), which have passive suspension so that they are simpler and reliable, have a minimum speed of around 30 km/hour (19 mph), under which they deploy wheels. However, EDS systems require the superconductive magnets to be kept cold. Another kind of maglev system, Inductrack III, is designed for heavy loads at low speeds using permanent magnets, and can support up to around 50 times the magnet weight, though the weight ratio depends upon the materials of the magnets.

On the lunar surface, unlike on Earth, there are abundant nickel-iron metal granules which can be extracted from lunar regolith using simple magnets and grinders. These granules also have a little bit of cobalt.

The entire system must be isolated from abrasive dust in the environment. This could be provided by an outer stationary wall and flooring which are built in advance of construction of the rotating systems. The whole system would best be enclosed and sealed.

Since we need radiation protection in space, it would be best to dig a deep hole to put it in, and erect a thick roof above it, which may in turn be covered with regolith. Water storage could also be placed in that area, which provides additional radiation protection.

Locating the habitat underground would also provide thermal stability between day and night.

We don't need natural sunshine, since suitable light for plant growth and human health can be created from electricity, providing only those parts of the spectrum useful for plant growth and visible by humans. This offers design and operational simplicity.

Final Words At The Moment

This article was written in the hope of stirring up interest in working on realistic 1 G habitats in space for near term space settlement.

Any comments would be appreciated.

If there is previous work not cited here which you think may be of interest, please send me the information.

If you would like to work on this problem, either in collaboration or on your own, please let me know.

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