Tapered Rotating Habitats for Settling Low-Gravity Worlds

This has been an idea that has been bouncing around in my head for a while now. My original idea was to turn it into a paper, and I still might one day if I flesh it out more.

The long-term effects of living in a microgravity environment have long been known to be severely detrimental to the human body. One simple solution to this problem in free space is to build space stations which rotate at several rotations per minute (RPM) in order to provide a kind of artificial gravity through centripetal force. In order to reduce the negative effects brought about by Coriolis forces, such as nausea and disorientation, such space stations are typically quite large, often reaching several kilometers in diameter. Utilizing the constant and unobstructed sunlight available in free space for effectively unlimited power, such habitats are typically estimated to be able to house populations of several thousand to several million people, depending on size. However, when it comes to settling on a planetary surface, one is stuck with whatever natural gravity the object has, which for every solid world in the Solar System, is typically much less than Earth’s. Currently, it is not known how the human body adapts to living in fractional gravities, and the only data available is in the case of “zero gravity” and Earth gravity; opposite ends of the spectrum (the time Spent on the Moon, a few days total for each Apollo mission, was not enough time to establish the long-term effects of Lunar gravity on the human body). Assuming that physiological responses to gravity scales linearly with gravitational force, it can be surmised that the kinds of long-term degradation observed in extended periods of freefall would likely happen in reduced gravity, albeit to a degree relative to the gravity experienced. If this is the case, safe and long-term habitation of low-gravity worlds can still be made possible by constructing large rotating habitats buried in the surfaces of these worlds, with walls sloped at an angle so that the local force of gravity and the artificial centripetal gravity balance out to create a net resultant force.

When it comes to settling the Solar System, most people envision sprawling arrangements of habitats and other buildings scattered across a planetary surface, typically on the Moon or Mars. In science fiction, these are often seen as the precursors to urban vistas similar to what we see on Earth. However, this vision ignores two critical factors about living on most planetary surfaces; the lack of protection from radiation (in the form of cosmic radiation and solar flares), and the strength of gravity. In most science fiction settings, both of these are typically handwaved away through some kind of unexplained technology, typically some kind of artificial gravity generator in the case of gravity (if it’s mentioned at all), and radiation is typically ignored entirely. In reality, the reduced-gravity environment of many worlds throughout the Solar System may present a challenge when it comes to human settlement. While radiation can be mitigated by simply surrounding habitats with regolith-filled sandbags or burying them in trenches, the natural gravity of a planet or moon cannot be changed.

The only known method for generating artificial gravity is through rotation. This concept has been included in various designs for fictional spacecraft, such as the Discovery from 2001: A Space Odyssey, ISV Venture Star from Avatar, and the Endurance from Interstellar, as well as a few designs for future spacecraft such as NASA’s Discovery II and Nautilus X spacecraft. It has also been used in the design of massive, permanent orbital settlements meant to house many thousands or hundreds of thousands of humans in free space, popularized by physicist Gerald O’Neill in 1977 in his book The High Frontier, and expanded upon by many authors and researchers since. Indeed, I plan on writing about this topic extensively in the future. The idea of such a rotating space habitat can also be applied to the surface of a low-gravity world, after carefully considering some subtle alterations to account for the host world’s weak but non-zero natural gravity.

The ideal design for this, I think, would be a tapered cylinder. The diagram below shows what I mean:

diagram2

Diagram of a tapered cylindrical habitat

The habitat consists of an outer shell to house the rotating section, similar to the frame of a clothes drier in which the spinning drum is housed. A modest layer of regolith on top of the shell provides radiation shielding from cosmic rays and solar particles. A central, non-rotating axial shaft serves as both an axis of rotation and also an access elevator. Driven into the deeper bedrock are the radiators for the habitat’s life support and power systems, which conduct waste heat into the surrounding rock.

The habitat itself is shaped a bit like a bowl with straight sides. The side walls are angled at a slope which, when the habitat is under spin, creates a sensation of normal, “flat” gravity near the equator. One glaring issue with this design is that as the diameter becomes smaller towards the bottom, the centripetal force becomes stronger and stronger while the natural force due to gravity remains constant, resulting in a sensation of walking uphill.

diagram1

Force diagram of the rotating tapered habitat.

The following tables list values of θ required to generate a given amount of artificial gravity for selected Solar System objects with low but non-negligable gravity:

Lunar Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

9.5º

0.75 G (7.35 m/s2) 12.7º
0.5 G (4.90 m/s2) 19.7º
0.38 G (3.72 m/s2) 25.8º

Ceres Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

1.6º

0.75 G (7.35 m/s2) 2.2º
0.5 G (4.90 m/s2) 3.3º
0.38 G (3.72 m/s2) 4.3º

Europa Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

7.7º

0.75 G (7.35 m/s2) 10.3º
0.5 G (4.90 m/s2) 15.5º
0.38 G (3.72 m/s2) 20.7º

Ganymede Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

8.4º

0.75 G (7.35 m/s2) 11.2º
0.5 G (4.90 m/s2) 17º
0.38 G (3.72 m/s2) 22.6º

Callisto Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

7.2º

0.75 G (7.35 m/s2) 9.6º
0.5 G (4.90 m/s2) 15.5º
0.38 G (3.72 m/s2) 20.7º

This is one possible solution for how to deal with living on low-gravity worlds, but it probably isn’t the only one. Personally, I think that if we’re going to try to settle a broad array of planet types, we should adapt ourselves to their environments rather than attempting to adapt the environment to us, through a combination of cybernetic augmentation and genetic engineering. Of course, these are not strict binary paths. A mixture of both is a likely and desirable outcome.

Until next time, keep looking up.

References and Additional Reading

O’Neill, Gerard K. “The Colonization of Space.”Phys. Today Physics Today 27.9 (1974): 32. Web. http://www.nss.org/settlement/physicstoday.htm

 Miller, J., L. Taylor, C. Zeitlin, L. Heilbronn, S. Guetersloh, M. Digiuseppe, Y. Iwata, and T. Murakami. “Lunar Soil as Shielding against Space Radiation.”Radiation Measurements 44.2 (2009): 163-67. Web. http://www.lpi.usra.edu/meetings/nlsc2008/pdf/2028.pdf

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