Eleven years ago on this blog, I posted the idea of the gravitat. The concept was first envisioned by Herman Potočnik, a.k.a. Hermann Noordung (1928), and popularized by Werner von Braun in 1952. Here I discuss some refinements and wider uses of the gravitat. Using this system, humans could live under Earth-normal gravity conditions on the surface of the other solid planets, moons, and asteroids in our solar system (see Figure 1).
Figure 1. A gravitat on Asteroide B-612 (thank you, St-Exupery). Any decent-sized rock would shield the habitat from at least half the meteors and much of the radiation of deep space.
Part of the reason I bring up this issue again is that I have been reading James S.A. Corey’s book series called The Expanse. I really like the books and TV show as opera; and the authors also take technology very seriously. But they do not seem aware of the idea of inducing gravity while living on the surface of other planets, and I am not sure if anyone else has proposed it!
Rationale for the gravitat
Since the 1960s, NASA has studied human physiology under conditions of microgravity, and the results are not encouraging. Not only do muscles atrophy, but astronauts also lose about 1% bone density per month in microgravity. So: microgravity may remain useful for scientific experiments and industrial production, but humans shouldn’t spend too much time living in micro-g. On the other hand, it would be very helpful to have humans making the complex decisions in places that are light-seconds and light-minutes from Earth. Even the signal-distance to Luna is enough to cause potential problems with remotely-operated machinery there.
Until we know otherwise, I propose that a standard gravitat should produce centripetal acceleration equivalent of 1 Earth-gravity (1.0 g, or 9.81m/s/s). Why? Because we have evolved under 1g, and numerous biological systems from our musculature to our bones to our lymphatic system are adapted to the 1g environment. There are other biological stressors in space (radiation, confined living conditions) so it would be wise to eliminate at least this one stress-factor.
Unfortunately the result is a structure with a lot of mass. Gilruth (1969) argued for 2 RPM as the maximum revolutions/minute of a centrifugal habitat to avoid causing vertigo. However Globus and Hall (2015) recently reviewed the literature, and 4 RPM seems generally feasible, although newly-arriving people might need a few hours to adapt. Theodore Hall (2000) created a “spin calc” program which yields the following results:
For a centripetal acceleration of 9.81m/s/s, with an angular velocity of 4 RPM,
The radius is 56 meters; diameter = 112m. That means a relatively big structure.
Right away, this suggests some empirical research that needs to be done adjacent to the International Space Station:
(1) What is the highest angular velocity that humans can comfortably tolerate? If a structure can spin faster, the radius (and thus the overall mass) of the structure can be reduced proportionally while still producing 1g of force. For a ring, the savings in mass vary geometrically with radius, so the gains would be significant.
(2) What is the lowest centripetal acceleration (spin-gravity) that can maintain long-term health? I suspect that permanent gravitats will always need to produce something close to 1.0 g, but for a multi-month interplanetary voyage, much lower forces could still maintain far better health than traveling in microgravity.
Two basic configurations: Mast-and-Yardarm, Mast-and-Ring
The lowest-mass gravitat would be a two-capsule Mast-and-Yardarm (MnY) design (see Figure 2). I chose these terms carefully to describe the multiple roles and historical corollaries for each component. Normally the central mast would not spin. Solar arrays, engines, observation equipment, communication equipment, and docking equipment would all be attached to this central mast. So the mast needs to be relatively strong, and it needs to be able to maintain a fixed orientation.
The yardarm is designed to rotate around the mast. A low-mass yardarm structure can hold the capsules in position mostly through tension while it is spinning; but that would also require fore-and-aft stays to prevent the yardarm from resonating or oscillating. Similar rigging was used on square-sail ships, for the same reason of structural efficiency. Therefore I use the same terms: stays and rigging. Figure 2 is a modified image of an existing ship’s communication mast; it gives a rough impression of the MnY configuration.
Figure 2. A mast-and-yardarm gravitat configuration.
Permanent stations on other planets, moons, and asteroids, or in orbit or at Lagrange points, could all be the much more massive Mast-and-Wheel (MnW) structure. When positioned on a planet, the mast would become the “maypole” around which the wheel would turn. All the other (known) solid bodies in the solar system have a lower surface-gravity than Earth; so the purpose of a surface-mounted gravitat would be to supplement natural gravity with centripetal force, to maintain 1g for long-term health. Surface-mounted gravitats would spin more slowly in proportion to the amount of supplementary centripetal force required to get 1g. Also, the direction of force would no longer be strictly radial, so the decks within pressurized capsules would need to gimbal so that they remain force-level within the combination of planetary and centripetal force. However, I still suggest the same standard design so that standard components can be built and debugged most efficiently.
The wheel-structure of a gravitat would look most like the London Eye or Singapore Flyer, which are both designed with tension-cables rather than rigid Ferris-Wheel spokes. Figure 3 is a mashup of an image of the Singapore Flyer with the solar arrays of the ISS, to give an impression of what it would look like. Gravitats are going to need very large solar arrays, not just because of the power-demand of such a large structure, but also because most of them will be positioned farther away from the sun than the Earth itself.
Figure 3. A full gravitat.
Capsules as triple-bagged trusses
Hard-shelled pressure vessels are not an efficient combination of function and mass for space exploration and occupation. NASA and Bigelow Aerospace have the right idea experimenting with inflatable structures. From my life-experience, I make a further recommendation: triple-bag the human-occupied areas (see Figure 4). I think a triple-bag structure will yield the best compromise of low mass, radiation-shielding, and minimal risk of catastrophic decompression due to meteorite-impact.
My guess is that each layer could be pressurized with the following gases:
(A) Innermost inhabited space: N2/O2 atmosphere at 0.6 bar pressure;
(B) Intermediate pressurized layer: N2 gas at 0.2 bar pressure;
(C) Outermost pressurized layer: argon gas at perhaps 0.05 bar.
Each layer would have some insulation on the outer face, and whatever is the lightest way to block the most harmful radiation.
Within the innermost bag, the inhabited deck would be supported by an open truss.
Thus, there would be no “hard pressure vessel” in this design, except perhaps the junction-airlocks that link each capsule to the next capsule along the ring.
Also, this means inhabited capsules would have no windows. Maybe in the junction-airlocks. But then the junction-airlocks would admit more radiation; at some point the residents would need to decide their own trade-off of radiation-exposure for a direct view outside.
Figure 4. Schematic section of one of the pressurized capsules of a gravitat. Note: this sketch represents one segment in a much larger (225m) structure, based on my prior impression that the maximum comfortable rotation rate would be 2 RPM.
Remotes for most EVA maintenance work
Since there is a fair amount of hard radiation in space (and on the surface of most bodies in our solar system), extravehicular activities (EVAs) should be minimized. Remote robots should be used as much as possible for maintenance, repair, scientific research, and commercial production.
Two other reasons to minimize EVAs on planets, moons, and other bodies: to avoid biologically contaminating them, and to avoid having their dust contaminate us. For the next few decades at least, we should try to avoid spreading bacteria to our neighboring planets and moons until we can verify ‘beyond a reasonable doubt’ that there are no indigenous microbes on those planets and moons. It is pretty likely that some of the “extremophile” microbes on Earth could survive on Mars. If we allow that contamination, we may never know if life emerged there independently.
Conversely, dust is a problem for us. On Earth, our rich complex of biota locks down most forms of dust in soil and water. But the other planets, moons, and asteroids in our solar system have superfine dust that would cause serious long-term lung damage for humans. When the Apollo astronauts climbed back into the LEM with their suits on, they tracked in a lot of hazardous dust. This will be a problem for EVAs on the surface of every other solid body in our solar system.
Super-fine dust is also a problem for the moving parts within machinery, and I assume NASA has been spending time designing for that.
Re-visiting the rationale for human space exploration
Should humans be in space at all? I think that question can only be answered in a moral framework. As an urban planner, one of my core concerns is social justice; and the argument against the Apollo program was that we should not be spending tax dollars sending people to the moon when people in our own country are suffering from poverty. In the 1960s and 1970s, space-exploration looked profligate; and I think it only really gained Congressional funding because it was a thinly-veiled weapons demonstration against the USSR Today, we use satellite imagery for weather-predictions and disaster-coordination, so the argument against satellite-launches has abated, since rich and poor benefit from this technology. And there are other U.S. budget-items which are indeed more profligate—like the unnecessary invasion and occupation of Iraq for $2 trillion. To put the human spaceflight program in perspective, Elon Musk recently said, ‘I think we should spend at least as much on space exploration as Americans spend on lipstick every year.’
There are two reasons why I think we should push much more human space exploration. First of all, I think we will make discoveries that will grow our economy. Not merely technologies like velcro and mylar, but the sort of widespread kick to a whole economy when a population feels inspired. When we use a really big space-based telescope to get high-resolution images of the planets orbiting nearby stars, I think the effect on our culture and economy will be very positive.
The second reason I advocate space-exploration is the lessons in environmental humility we will encounter. A long-term life-support system is hard to design, hard to maintain. I believe NASA-AMES should promote the difficulty of long-term, closed-loop life-support systems. The Initiatives-List of Ames shows that NASA has already linked the idea of offworld life-support systems with Earth-based sustainability research. Their Sustainability Base building is an application of NASA tech to what Architects call ‘green building systems’. During the next high-profile mission, such as to Mars, NASA could tie the problem of long-term life-support explicitly to thinking about the emergent complexity of natural ecosystems. In this way, long-journey human spaceflight could shift our collective attention much more to the gritty details of soil micro-biomes, algae growth, and managed nutrient-cycles.
Here is where planning for deep space exploration links most profoundly to city planning. Cities are complex emergent systems. One dimension of urbanism is that cities are integrated into multiple ecosystems; they are part of the environment and we need to plan accordingly. For example, Singapore has dammed off all the coves and bays of their island-city, so that all rainwater runoff is literally captured before it reaches the sea. The political tensions with the Malaysian national government have pushed Singaporean leaders to think about how political isolation of their island affects access to water. Since it is easier to remove bacteria than dissolved salts, Singaporean planners are willing to recycle everything from stormwater runoff to sewage into potable water before engaging in large-scale desalination. That is an entire city thinking about their life-support systems along the lines of spacecraft planning. Thus, political tensions have caused Singaporean planners to think about sustainable urbanism well beyond what most planners have considered–yet. It will help if high-profile missions in deep space draw attention to humbler things: the beauty of composting, window-box gardening, and water-cycles.
Cramer (1985). “Physiological considerations of artificial gravity.” in A.C. Cron, ed., Applications of tethers in space. NASA CP-2364, v.1, pp.3.95-3.107.
Connors et al. (1985). “Living aloft: Human requirements for extended spaceflight.” NASA SP-483, pp.35-51.
Gilruth, Robert R. (1969). “Manned Space Stations–Gateway to Our Future in Space.” Manned laboratories in space, p. 1-10. New York, New York, USA: Springer-Verlag.
Globus, Al and Theodore Hall (2015). “Space settlement population rotation tolerance.” Preprint PDF.
Hall, Theodore. (1994). The Architecture of Artificial-Gravity Environments for Long-Duration Space Habitation. Ann Arbor, Michigan: PhD Dissertation.
Noordung, Hermann [Herman Potočnik] (1929). Problem der Befahrung des Weltraums: Der Raketenmotor. Berlin: Schmidt & Co.