If we ever achieve manned missions to the stars, one of the assumptions is that we will find planets much like Earth that we might live on and colonize. But what if the assumption is flawed? There are surely many Earth analogues in the Milky Way, but we don’t know how widely they are spaced, and a near-miss isn’t necessarily helpful, as both Mars and Venus attest. People like Robert Zubrin continue to advocate terraforming as a solution for Mars, and it may well happen one day, but supposing we get to another star, would we have the moral right to terraform a world with living creatures on it, even if they didn’t meet our criteria for intelligence?
Robert Kennedy (The Ultimax Group), working with colleagues Kenneth Roy and David Fields, has been pondering these issues and went through a possible solution at the recent Tennessee Valley Interstellar Workshop in Oak Ridge. If we stop worrying about Earth analogues, a range of interesting possibilities open up, as our own Solar System illustrates. We have small planets like Mars, along with what may be a huge number of dwarf planets. We also have moons in a wide range of sizes around the gas giants. Suppose we could transform such worlds by building a spherical shell of matter around them, totally enclosing an atmosphere and living ecosystem?
Beyond the Habitable Zone
The idea seems outrageous, but Centauri Dreams readers are familiar with even more gigantic concepts like Dyson shells, engineering on levels that would require a Solar System-wide infrastructure and a Kardashev Type II-level civilization to build. If we extrapolate advancing technologies that can do gigantic things, we can consider creating an Earth-like environment (in most ways) under a shell that protects the inhabitants from radiation and provides a self-enclosed ecology. The question of a ‘habitable zone’ would disappear because artificial lighting and temperature control would be built in, and the wild card would be gravity, which would depend on which bodies were selected for enclosure. Most would offer gravity only a fraction that of Earth’s.
Kennedy and team wrote a paper for JBIS in 2009 that lays all this out. Working the math on spherical shells, they ponder the fact that if the objective is to contain a 14.7 psi Earth-normal atmosphere, such a shell would experience the same kind of pressure-induced tension found in a balloon. Assume one atmosphere of pressure at the underside of the shell and vacuum above it, and it is possible to choose a shell thickness so that the compressive stress of gravity cancels out the atmosphere-induced tensile stress in the shell. A shell made completely of steel, for example, built to enclose a world 20 kilometers above its surface, would need to be 1.31 meters thick if enclosing the Earth, and 8.05 meters thick if enclosing the Moon.
Moreover, the shell mass used is there simply to create compressive force — opposing the pressure of the atmosphere within the shell — and can be no more than dead weight. The authors figure that enclosing the Earth’s Moon could be done with no more than a 1-meter thick layer of steel if it incorporated 62 meters of regolith on top of it, with open-ended combinations of steel, ice, dirt and rock possible for the job:
It is not actually necessary to use a metal such as iron or steel. Stony materials such as concrete can handle a lot of compression. A strong fabric material that is airtight and in slight tension could be used to support the mass of the shell, which could be mainly rocks and dirt.
The authors contend that a shell with mass equally distributed across the surface of the shell will be stable with respect to the more massive body at the center of the shell:
If the central mass is displaced a given distance inside the shell, gravity will act to restore the shell’s original position with respect to that body. Such is not true for a ring. If there were no way to damp the movement, the shell would oscillate back and forth. A viscous atmosphere will tend to dampen oscillations until the mass center is once again congruent with the center of the shell.
The Riches of Ceres
Now consider the asteroid Ceres. Here the shell, depending on which mass estimate for Ceres we choose, would have to range (if made of steel) from 45.2 to 90.4 meters in thickness — this is the amount of mass that would be necessary to hold an Earth-normal atmosphere. This is one thick covering, providing enough shielding to survive a nearby supernova. Assume you have a terraformed Ceres that is half ocean and you wind up with enough dry land area to approximate the area of Indonesia, on a world where gravity is 1.5 percent that of Earth. Could a human colony survive in conditions of micro-gravity? At this point we simply don’t know the answer.
But think about the scenario for a moment. In an enclosed Ceres, climate is a design variable and lighting can be adjusted to approximate whatever day/night cycle the occupants desire. Imagine the underside of the shell as the urban area, a place where residents live in housing that overlooks the spectacular vista of the interior, which could be maintained as farmland or a nature preserve filled with whatever species the designers choose to introduce. With normal atmospheric pressures and light gravity, human-powered flight would always be an option. The outside of the shell would be devoted to heavy industry for manufacturing and power plants.
Taken to an extreme, we get this:
… the subterranean zones of small celestial bodies would offer vast – virtually unlimited – cubic for support functions and resource extraction. Consider that the interior of Ceres – half a billion cubic kilometers – could contain almost exactly the same working volume as a world-spanning city which packed the entire surface of Earth, oceans included, with billions of 1 km high skyscrapers, each the rival of Burj Dubai. In the light gravity of Ceres, every bit of that volume would be easily reachable and cheaply exploitable, unlike the deep wells and mines of Earth. A shell world might well be the richest planet in its solar system, once the huge cost of englobement was paid off.
Building for Safety (and Aesthetics)
Numerous dangers could beset a shell world, including many that already threaten our planet, such as the impact of large asteroids, but we could avoid some problems — volcanoes and earthquakes spring to mind — if we choose or build worlds without plate tectonics, and issues like solar flares would have little effect given the shielding the shell world’s inhabitants could rely on. A rupture in the shell would be a hazard, but a small shell world like Ceres would have a shallower gravity well than Earth and be less likely to draw in an asteroid. Moreover, any shell world would include the kind of planetary defense systems that a civilization capable of building the shell in the first place would be able to deploy. Shell maintenance, safety and improvement would doubtless be an ongoing project.
The paper works through one possible construction scenario involving the Moon and considers the massive amounts of energy required to move the needed terraforming materials (roughly one quadrillion tons), obviously requiring huge advances in energy production and space transportation. But it’s a fascinating vista, one that sees the creation of hanging cities on the underside of a shell that represents an area equal to four times the area of the United States. The surface of the re-made Moon can be tuned up to be as Earth-like as we choose to make it, the entire project taking hundreds and more likely thousands of years to see to completion.
The presence of hanging cities will diminish the required surface loading by inert material. Lighting would be artificial, with solar energy (assuming the shell world is near a star) powering up the lights, or power plants on the surface of the shell doing the job if the world were built in deep space. The paper argues that shell worlds all the way out to the Kuiper Belt could have an Earth-like insolation, ecology and diurnal cycle. And imagine this:
Existing electro-luminescent displays (ELD) only provide about 1-2 W/m2 of radiant energy in various colors from red to blue-green, but their state of the art (brightness, efficiency, cost) is rapidly advancing… Since ELD materials are presently available in all three primary colors and can be subdivided into addressable segments, we can imagine a pixelated ceiling of video wallpaper simulating the natural sky of Earth (clouds, sunsets, stars, etc.) or generating any arbitrary scene. The postindustrial motto “everything is media” means art can reach its fullest expression in the canvas of a shell world.
If we do become the Type II civilization capable of building such structures, we’ve not only opened up numerous worlds within our own system for colonization, but have also gained the experience needed for constructing stable generation ships for long-duration interstellar flight. And because shell worlds could be located anywhere a suitable moon or planet is found, we should consider the possibility that alien civilizations may already have constructed such worlds around red dwarf stars or even brown dwarfs, which may outnumber all other kinds of stars. The traditional concept of a habitable zone may not be the marker we’ve always assumed it to be, with prospects for SETI extending to worlds that would not before now have gained our attention.
The paper is Roy, Kennedy and Fields, “Shell Worlds: An Approach to Terraforming Moons, Small Planets and Plutoids,” JBIS Vol. 62 (2009), pp. 32-38. If you’re a science fiction writer in search of a setting, you must read this paper. I’ll also let everyone know when the Oak Ridge presentations become available online so you can see Robert Kennedy’s talk and slides.
I would think the main problem for these worlds is power. Ceres is pretty cold, and any solar power solution will be less efficient than just using the sun directly to heat the surface. Any body at this distance or further from the sun that has no internal heat source would require a massive power source.
Shell worlds might actually make more sense closer to a star, where their shading could make worlds habitable that would otherwise be too hot. The high insolation on the shell could be captured to provide a huge amount of energy to the habitation inside. (Granted, bodies close to stars typically don’t have much in the way of the volatiles necessary for human life, but we’re already talking massive engineering, so dropping a few comets into place to make up for that shouldn’t be too hard.)
This is old stuff. I bring your attention to this 1991 paper on “Supramundane Planets”:
http://www.paulbirch.net/SupramundanePlanets.pdf
Which talks about building a dynamically-supported solid shell around a Jovian planet at the 1-gravity level.
This is actually just one of a (very interesting) series of papers on astroengineering by Paul Birch for the JBIS from 1982 to 1993:
“Orbital Ring Systems and Jacob’s Ladders” (1982)
http://www.paulbirch.net/OrbitalRings-I.pdf
http://www.paulbirch.net/OrbitalRings-II.pdf
http://www.paulbirch.net/OrbitalRings-III.pdf
“Dynamic Compression Members” (1989)
http://www.paulbirch.net/DynamicCompressionMembers.pdf
“Terraforming Venus Quickly” (1991)
http://www.paulbirch.net/TerraformingVenusQuickly.pdf
“Terraforming Mars Quickly” (1992)
http://www.paulbirch.net/TerraformingMarsQuickly.pdf
“How To Spin A Planet” (1993)
http://www.paulbirch.net/SpinAPlanet.pdf
“How To Move A Planet” (1993)
http://www.paulbirch.net/MoveAPlanet.pdf
This entire concept seems very odd to me. The value of the shell seems to be quite low yet the expense is high. It would seem easier and better to go with the subterranean construction for habitation and resource extraction since you only need to “finish” those small spaces rather than the entire body. If you want sunlight or pseudo-exterior recreation areas, build a small number of surface domes that can be made largely transparent (no need for wallpaper!). The rest of the surface can be left in vacuum and used for industrial plants that may benefit from the surface placement and for solar energy collection.
Doing away with the shell allows a more scalable solution: the entire shell must be built and pressurized with a breathable atmosphere before the body can be put to real use. With my alternative approach the body is habitable almost immediately at low cost, and you only extend the settlement and incur expense/resources as needed. So why the shell?
Wow. Mind-bending!
One might as well dump a small atmosphere on top of the shell, shielding it from small impacts. A 5mb atmosphere on Ceres should do the trick, making it much more resilient.
Not that that matters much, since anyone building a shell world doesn’t care much about resiliency. Shell worlds miss the point of terraforming. It makes much more sense to build O’neill colonies than this; at least if the air escapes you won’t be trapped under a mass of rock.
I fail to see how this is in anyway better than a worldhouse…? I can see the slight appeal of using them in the Kuiper Belt and Oort cloud, but if you can use natural light and “conventional” terraforming, it’s best to do it.
How about a small world in a plastic bag? iItead of depending on gravity , an icy body say 10 km in diabeter could be practically enclosed in a bag to contain gas at say 0.5 atmosphere. This would be enough pressure containment to support an oxygen atmosphere, water vapor and some buffer gas like nitrogen. Back of the envelope calculations makes it look like a double shell with cable reinforcement might only need to have a combined thickness of a couple of centimeters think ( poly carbonate) or a bit thicker for poly ethylene. Cableing to the surface could also help hold the thing together. Such an object would have have about 300 square kilometers of planet surface area ( 4 pi X25) and because of low gravity , presumably the INNER surface of the “bag ” would also be habitable, and the outer surface a work and storage zone. There would be enough gravity to keep liquid water on the surface, and “soil ” in place for plant growth. The energy requirements would be modest if the bag was reasonably insulated and could be provided by fission or fusion power.
Hey I am excited by this idea! .
How about we find a moon with an circum-moon ocean, pump most of the water to the surface to refreeze and have a shell around a core with the space between available for inhabitation.
Say a habitation layer on the moon would be surface, shell, atmospheric layer, buildings – an effective layer of a few hundred kilometers. What’s then the objection to creating an ‘onion world’?
Just this morning, NASA confirmed that Kepler 22b is in the habitable zone. It has a radius of 2.4 Earth’s, but we don’t know what the density of the planet is to infer its gravity and if it would be colony material.
Still, no data on the albedo characteristics, what’s the make up of the planet, if it’s even got a rocky surface. Lot’s of unknowns. Still, very exciting.
I predict once we find that one Goldilocks planet, that matches the Earth’s characteristics like mass, similar albedo, temperatures, same type star, etc. there will be a mad dash to get there. Space tech will explode (not literally) and the huge world governments will race to build a ship to get first dibs on it. Best case scenario is a world-wide effort, of course.
This also gives the possibility of building a world that, externally and visually, looks like any other asteroid or planet.
The original paper reads
“While it will not be demonstrated in this paper, it is possible to prove that a shell having mass equally distributed across the surface of the shell will be stable with respect to a much more massive object located at the center of the shell. If the central mass is displaced a given distance inside the shell, gravity will act to restore the shell’s original position with respect to that
body.”
I really wish they would have demonstrated that because I thought for a symmetric and uniform shell around a spherical planet there would be no net gravitational force for the gravitational potential inside the shell is constant. Any arbitrary body inside the shell should feel no force. I can see how if they assumed the shell acted as a point mass they could think there would be a restoring force but it seems to me that the assumption is wrong for the region inside the shell. I wonder what they assumed with regard to the relative masses? I think mass asymmetry would not change the Shell Theorem.
The Wikipedia entry on a related concept of the Dyson Sphere reads;
“There are several serious theoretical difficulties with the solid shell variant of the Dyson sphere: Such a shell would have no net gravitational interaction with its englobed sun (see Shell theorem), and could drift in relation to the central star. “
If I lived in such a world I could not resist the temptation to circumvent all safety measures and fly under my own power. Depending on atmospheric density terminal velocity should be in the 6 – 10 m/s range so it looks as is these safety measure would not need to be very high even in a world that places a much higher value on human life.
I just wonder why build the shell? Why not just hollow out a dwarf planet or a large asteroid and let people live under ground.
Paul, when you mentioned ‘shell maintenance and improvements’, I had a thought: We might add layers on the outside of the first shell, as improvements. Each successive layer would more than double the surface area, and we could even add farms and parks. The living area would be nearly limitless, as long as we could supply enough power. Might as well dream big.
The end result doesn’t seem much different than pressurized subterranean buildings. It seems much more difficult to get started with a shell though. Nice thing is we can live fairly well under lower pressure if needed.
“lepton wrote “I just wonder why build the shell? Why not just hollow out a dwarf planet or a large asteroid and let people live under ground.”
Exactly. Networks of tunnels and large caverns would serve the same purpose and be a lot easier to build. One could also or alternatively make a large series of relatively cheap interconnected domes on the surfaces to both allow light and warmth in while filtering out harmful radiation. Imagine a sort of bubble wrap covering large parts of the Moon’s surface or of Mars.
The whole concept of the shell world seems like a reworking of the 1970’s O’Neill Space Colony concept. It was proposed then the such Space Colonies could extend throughout the solar system and be built with the resources found off Earth.
Bob, I thought about that gravitational restoring force also, and concluded that it must have been mediated through gravity induced atmospheric pressure differences with altitude. This would explain why the enclosed atmospheric height is so extensive.
I read the Roy, Kennedy and Fields paper, and was less than impressed. As Tobias Holbrook says above, the worldhouse concept (also called paraterraforming) is superior. The shell world idea seems to require inflating an entire global shell in a single operation. The worldhouse concept, which is similar, can be modularised, so a minor or dwarf world can be occupied incrementally, as investment conditions permit. The shell world idea therefore dispenses with one of the major advantages of the worldhouse for no apparent gain.
The worldhouse refs are: Richard L. S. Taylor, “Paraterraforming: The Worldhouse Concept”, JBIS, 45 (1992), pp. 341-352; Richard L. S. Taylor, “The Mars Atmosphere Problem: Paraterraforming — The Worldhouse Solution”, JBIS, 54 (2001), pp. 236-249.
For what it’s worth, I’d take Ceres completely apart and build rotating space colonies which have control over their gravity. If human embryo development proves difficult under reduced gravity, as some have suggested, the space colony solution could be the only way to use these small worldlets.
Thank you for recognising that the term “habitable zone” applies only to habitation by one of the several possible forms of life! I would suggest it be replaced with “surface water zone” or “Earth-analogue zone”, in the interests of scientific accuracy!
Stephen
Oxford, UK
I’ll respond to five or six comments at once.
Terry Koberstein December 5, 2011 at 14:46 wrote:
> How about we find a moon with an circum-moon ocean, pump most of the
> water to the surface to refreeze and have a shell around a core with the
> space between available for inhabitation.
Yep, we thought of this construction technique while figuring out a plausible plot for an SF short story to be set in this environment. It’s an approach that ice-worlders, who evolve in an ocean, might take. For an ice-worlder, this might well be the normal method of “terra”forming!
***
Michael the Civilized December 5, 2011 at 15:24
> This also gives the possibility of building a world that, externally and
> visually, looks like any other asteroid or planet.
This is precisely what we mean when we say this sort of living arrangment might appeal to the very shy, or very paranoid. Imagine taking a detailed photosurvey before construction commences. Once the shell is complete, the regolith deadweight on the outside can be re-sculpted to mimic the original contours. (Within some limits on the amount of overburden, b/c one can’t have too much of a mascon on the surface, else the shell will deform.) In addition, the paranoid race can build their camouflaged shell world around a very common very long-lived red star that nobody else would think to look at twice.
This camouflage wouldn’t fool everybody of course, or the very persistent (which is the basis of another good story). But it might limit the external signatures enough to prevent observation by a flyby mission, which would mollify the paranoid dwellers within.
***
lepton December 5, 2011 at 17:29
> I just wonder why build the shell? Why not just hollow out a dwarf planet
> or a large asteroid and let people live under ground.
Because in a shell world, the occupants could live and walk around in a shirtsleeve environment on the *exterior* surface of the central body but underneath the shell, to maximize what gravity there was. Or they could live in cities hanging off the ceiling, enjoying nearly the same gravity. Living on the *interior* of a thick hollow shell, on the other hand, there’d be almost nothing to hold them to the “ground” whilst standing “upside down” on it.
It is interesting to ponder whether the first shell world would be purpose-built, or merely become one via the termite approach in the outer layers of a solid world. Imagine the “pillar and room” method of coal mining, or the Great Hall of Durin in Tolkien’s Middle Earth, extended everywhere with the pillars eventually disappearing. This would take some time.
Note that our shell world is not intended to provide artificial gravity on its inhabited surfaces by spinning! It is a true 3-D shell.
***
Bob December 5, 2011 at 16:05
> I really wish they would have demonstrated that [Shell Stability] because
> I thought for a symmetric and uniform shell around a spherical planet
> there would be no net gravitational force for the gravitational potential
> inside the shell is constant. Any arbitrary body inside the shell should feel
> no force. [snip]
We considered, analyzed, and cited the Shell Theorem in a later version of this paper presented to the IAA at Aosta, Italy this summer. Realize that the shell stands off the central body by just a few percent of the central body’s radius at most. In addition, the shell’s mass is at most 1% that of the central body. The Shell Theorem breaks down under these extreme conditions. We did a finite element analysis (sliced into thin rings) of the resultant force on the shell when the central body is displaced by the thickness of the atmosphere from the center of symmetry. We found a restoring force on the order of a few percent along the axis of displacement working to restore symmetry. The end caps dominate the contribution compared to the ring elements. The result is simple harmonic motion which eventually damps out viscously in the presnece of a contained atmosphere. In addition to this force, there is another physically-independent restoring force due to the nonuniform pressure across the contained atmosphere’s scale height, which will also act to counteract central-body-displacement relative to the shell. Granted, the scale height effect of a small world like Ceres is much smaller than on a big dense one like Earth, but it is nonzero.
***
Rob Henry December 5, 2011 at 16:14
> If I lived in such a world I could not resist the temptation to circumvent
> all safety measures and fly under my own power.
On an enshelled Ceres, one could probably do this — step off one of the platforms hanging from the ceiling and glide/fly to the surface. Depending on absolute pressure, and partial pressure of O2. That’s the idea behind our comment “new modes of existence”. Flying the other way might not be possible without artificial assistance, as I recall. Even in a shallow gravity well, 10 km is a long way to lift a body on internal energy alone.
***
Daniel Suggs December 5, 2011 at 17:40
> We might add layers on the outside of the first shell, as improvements.
> Each successive layer would more than double the surface area,
We thought of this way back when, but if that additional layer were pressurized also, then its pressure would negate the need for deadweight on the first shell. So then, each time the builders added a layer, they would have to shift the regolith/overburden onto the new layer, then inflate the new shell. Tricky to do while the shell zone underneath is occupied. Each additional shell going out would have to be maintained at slight lower pressure than its inner neighbor, else the neighbor’s ceiling would crush inward. Also, with multiple concentric shells, one would have to exercise *very* careful temperature/pressure control on *each* nested environment in order to prevent unwelcome effects (due to expansion/contraction) on the abutting shells.
***
thank you all for your thoughtful comments,
Robert Kennedy aka “Sputnik Wrangler”
Paraterraforming (or the Worldhouse concept) has been around a while. I believe it has structural problems. Take a transparent roof, each section one square kilometer with supports at each corner. If you provide pressure under it and vacuum above it, then 14.7 psi x 1 square kilometer yields 2.27E10 pounds of upward force. Each support then must pull on the roof to keep it from flying away (tension). I calculate that each support column will need to have a cross section of about 2500 feet square of solid steel (no safety factor). Please check my math. You can reduce that by putting atmosphere on top of the roof to reducing the delta P but then you’ve lost the advantage of a contained atmosphere, plus atmosphere above the roof will slowly leak out into space and will need to be replaced. A properly designed shell will have very little stress and very little leakage.
Bob hits on an important point about the stability of a shell around a central body, one the authors of the paper have struggled with, with limited success. The Shell Theorem states, “If the body is a spherically symmetric shell (i.e. a hollow ball), no gravitational force is exerted by the shell on any object inside, regardless of the object’s location within the shell.” But it doesn’t state that the central body exerts no net force on the shell. Clearly all forces balance out when the central body is at the center of the shell, but if offset from the center of the shell, forces within the shell seem to force the shell to move so that it is centered around the central body. Odd, I know and any help on this subject is welcome.
But even assuming that there is no gravitational restoring force, the atmospheric scale height results in a pressure force that tends to force the shell so that the central body is centered. Failing that, one might have to resort to cables or active controls to maintain the proper orientations, but it is doable.
The shell could be made of glass. More common elements, and also a better view.
I think Bob is right and the authors wrong: A shell around a central body would not be stable, metastable at best. Some rigging would be needed to keep it centered.
Hanging cities strike me as dangerous, it would be easy for people or things to fall off and hit the ground below. I like the human powered flight idea, though.
Finally, Daniel has a good point. A shell is really like a big building. Why bother putting it up in the air, when we can more easily build it on (or under) the ground? As many stories as we want to, one huge contiguous building around the entire planet/moon. Sure, the empty space would be nice, but probably not worth the price. Many of us hardly go outside anymore, anyway….
I don’t understand the logic of this at all. Remove the resource moon, spin the shell and you have an… O’Neill space colony. This has far more benefits than the shell world :
1. scalable
2. desired gravity level
3. located where desired.
If you want habitable surfaces, go with domed craters, which at least can be built on a reasonable scale and then expanded, rather than all-or-nothing sizes.
Michael the Civilized said on December 5, 2011 at 15:24:
“This also gives the possibility of building a world that, externally and visually, looks like any other asteroid or planet.”
So we could have residents in the Planetoid Belt or under the surface of Phobos and be none the wiser. So much for detecting artificial lights on them.
Europa is a ready-made shell world with its ice crust and liquid water interior. And all the sea food they can eat!
Of course this assumes that the residents won’t mind not seeing the sky on a regular basis. Arthur C. Clarke did not find it a pleasant prospect in 1963:
http://thespacereview.com/article/1981/1
We need to be very careful about terraforming or colonizing any worlds that may or may not have life. My view is that we should not terraform or colonize any worlds with any type of life, even microbial life, so that they can have the right to evolve on their own. Life must be respected in all its forms. How would we like if a type III, IV, V, or VI civilization decide that they need our world for some unknown purpose known only to them? To one of these type of civilization we would be more like a virus. Life is precious!
James: why would you grant microbial life the right to evolve on its own, but deny colonising space-faring civilisations that same right? Remembering that competing with other species for food and territory is intrinsic to evolution.
Stephen
Robert Kennedy said:
“In addition, the paranoid race can build their camouflaged shell world around a very common very long-lived red star that nobody else would think to look at twice.”
We are just now starting to take red dwarfs seriously as a system with planets that may harbor life, either native to those worlds or having come from somewhere else. I have the feeling that ETI with much more knowledge and experience with interstellar matters than we will have figured out long ago that red dwarf systems are no barrier to life in one form or the other.
My next question: Can shell dwellers keep their waste material and heat inside their world or do they have to dump it out onto the surface and into space to avoid polluting themselves? If so, there goes the camoflague idea.
Ken Roy writes regarding the Shell Theorem;
“But it doesn’t state that the central body exerts no net force on the shell. Clearly all forces balance out when the central body is at the center of the shell, but if offset from the center of the shell, forces within the shell seem to force the shell to move so that it is centered around the central body. Odd, I know and any help on this subject is welcome. ”
I think it does. There are always mutual gravitational interactions between any two bodies regardless of shape or orientation however if the shell is truly uniform then the resultant force between shell and central mass will always exactly cancel for any location of the central mass inside the shell. If the shell has no net force on the central mass then the central mass has no net force on the shell and any purely internal forces in the shell cannot move it.
However, I would be interested in seeing the details of the calculation and the assumptions that went into it.
As an aside, if you add some dynamics such as pumping liquid around the shell to artificially create a non-uniform gravitational potential thus violating the conditions of the Shell Theorem, you might be able to create a restoring force to adjust for slight drifts so you might be able to actively keep the situation stable.
It’s an interesting idea, and no doubt might have application in some circumstances (see below), but I remain far more comfortable with cylindrical or open ended colonies which are at least moveable and escapable and presumably a lot easier to engineer. It may sound weird but I think many would find the shell world to be unpleasant. Maybe other claustrophobic types feel the same. I honestly doubt I could psychologically handle it, but that may be just a lack of imagination on my part. People born and raised in such an environment might be quite comfortable with it. Good for them. Of course, I try to avoid people-centric arguments, but this seems like a giant step backwards. Even if it does work, why go into space to live like that?
I note that a number of commentors question the stability of the arrangment. So do I. Whether from an engineering or a human factors arrangment, the shell world seems unworkable. Maybe as a prison planet or a psychological testing laboratory or a place to spy on unwary or unfriendly solar systems (just kidding).
>Life is precious!
Yes, unless I am hungry for a burger or need to be cured of a disease. But if you mean intelligent life, I mostly feel the same — unless the life forms are tall (too tall in my opinion), blue aliens running around in their skivvies. Then I really have no use for them.
James said on December 6, 2011 at 1:06:
“We need to be very careful about terraforming or colonizing any worlds that may or may not have life. My view is that we should not terraform or colonize any worlds with any type of life, even microbial life, so that they can have the right to evolve on their own. Life must be respected in all its forms. How would we like if a type III, IV, V, or VI civilization decide that they need our world for some unknown purpose known only to them? To one of these type of civilization we would be more like a virus. Life is precious!”
It is a nice, humane, and civilized idea, James. In an ideal universe, I suppose every life form that appears on the scene has the right to live and evolve.
However, this reality is far from ideal both on Earth and elsewhere. As we have seen in nature on this planet, even long before humans arrived on the scene, countless organisms of all stripes have been rendered extinct by internal and external natural forces. As just one but large example, the Permian-Triassic Extinction event, which took place 252 million years ago, wiped out almost all life on Earth. Life did recover and thrived again, surviving more mass extinction events to the present day.
Citing more recent deliberate examples, humans have historically not only ignored any wills of the microbes on this planet, but they have often rendered extinct any number of their fellow humans and seriously depleted other mammals no matter how smart and/or harmless they were, such as the whales.
Will we be wiser/kinder when we start colonizing space should we come across an alien life form, be it a microbe or macrobe? Will necessity make that pointless? Is an intelligent “assault” on another species any worse than, say, a comet impact or supernova? Maybe we can avoid harming other life forms in the galaxy as we spread out, but will other intelligences with similar plans think and behave in the same manner?
My feeling is that while perhaps life and existence is not exactly the harsh Darwinian survival of the fittest mode as portrayed in the past, nature does not always wait for a species to get smart and capable enough to survive any problems that arise. Humanity may also find itself one day in a position that it cannot afford the luxury of leaving alone a world like Mars so that some simple bacteria may live under its surface, as an example.
Would it be possible to put a spin on a shell surrounding a moon? Then you could have gravity as well as the boon of resources.
I hadn’t thought of this before, it’s a fascinating concept. But the huge upfront cost is a real consideration.
Ken, you are correct that Richard Taylor describes a worldhouse roof under tension. However, the worldhouse, just like the shell, is equally suitable for the method of balancing the internal atmospheric pressure with a thick and massive roof. The important practical point which seems to be lacking from the shell world concept is modularity.
Perhaps the two concepts could be merged, combining the piecemeal approach of paraterraforming with the massive roof of the shell world?
Stephen, Oxford, UK
Ken Roy, I have not done the maths, but if it really works out that a central mass can exert a restoring force on a central body, and given that it is school level calculus from Newtons principles that a uniform spherical shell exerts absolutely no force within it you have invented the first space drive. Either that or your calculations are wrong.
Bob is definitely right with what he says about the instability of the position of the shell relative to the central body — “mutual gravitational interactions between any two bodies” etc. The shell and the central body would collide.
I remember doing the pertinent calculations many years ago already (and, no, I won’t repeat it here; and, no, you don’t need a finite element analysis, the problem can be solved analytically). By the way, its not even possible walking on the inner surface of the shell without drifting away.
For the readers who like science fiction literature — i.e. for nearly all readers — I should mention, that there has been a similar discussion about the stability of Niven’s ringworld, which induced him to present a solution in another story.
When it comes to paraterraforming, one might choose to use water as the deadmass. If you’re not too far from the star that it would attentuate light beyond use, a 10-20m layer of water on top of your world would seem to be an excellent means of pressurising your atmosphere and shielding from radiation, as well as giving you another habitat.
Taking the case of Ceres, the first thing I would do in terraforming is give it an atmosphere. Not too much, just a small 40-50mb one of CO2. Then, I would construct a global transparent roof over it, duel layered with say 50m gap for water to be pumped in. I would then begin to pressurise under this roof, filling the layer with water – it would provide enough counter pressure from it’s weight alone for a 125mb atmosphere. Add in the 50mb from the outer atmospheric layer, which provides meteorite protection, and put the thing under tension, and you can get a 250mb atmosphere, which is enough for humans to survive quite easily.
I would say that there is a good case for doing as James suggest. If after having developed the skill to travel between the stars it turns out that we are the only one (or one of very few) that are able to do so then we would have plenty of available planets to settle. If on the other hand space travelling civilizations are common then our existence tells us that none of them have decided to exterminate us in order to settle here.
I should try some qualitative explanation regarding the shell stability problem. (Now that I had a meal ;-)
A homogeneous, symmetric, spherical (did I forget anything? you know what I mean) shell does not induce any non-zero gravity field inside itself — nowhere (yes, as Rob Henry said: school level calculus from Newton’s principles). This implies that the shell does not exert any force on any body inside in any position, central or not, and — the other way round — any body inside in any position, central or not, does not exert any force on the shell.
Robert Kennedy (a.k.a. Sputnik Wrangler) said in his comment above “… the shell stands off the central body by just a few percent of the central body’s radius at most. In addition, the shell’s mass is at most 1% that of the central body. The Shell Theorem breaks down under these extreme conditions.”
These conditions do not change anything. Robert Kennedy’s statement is not true — simply.
He continues “We did a finite element analysis (sliced into thin rings) of the resultant force on the shell when the central body is displaced by the thickness of the atmosphere from the center of symmetry.”
The resultant force is always equal to zero. So, any analysis — including a finite element analysis — will give a restoring force equal to zero. Numerical errors?
Above that a finite element analysis, which is an approximation of the analytical model, is not necessary, because the analytical model already shows what’s going on. Useless effort.
Alex Tolley on December 6, 2011 said
I don’t understand the logic of this at all. Remove the resource moon, spin the shell and you have an… O’Neill space colony. This has far more benefits than the shell world :
1. scalable
2. desired gravity level
3. located where desired.
Except that you don’t. Remove the central body from a shell world and it will explode due to atmospheric pressure. The thing to keep in mind is that small pressures applied over large areas result in huge forces. Paraterraforming and the Supramundane Planet idea require materials that are orders of magnitude stronger than anything we have today. O’Neill colonies can be built using standard materials we have today but they will be highly stressed. Such structures will have a limited life expectancy due to metal fatigue. I’d like to see a study on that but I’d expect that you might get a few hundred years out of an O’Neill colony. Radiation shielding is a big problem for O’Neill structures and their occupants.
The shell world idea results in very low stress levels within the shell itself and massive shielding for anything underneath. A shell world at Ceres could withstand a hit from a nearby supernova due to shielding where Earth and O’Neill colonies would be cooked.
Make no mistake, shell worlds are long term projects, expensive, risky, prone to repeated failures before the builders get it right, requiring massive quantities of energy and materials. But once we build one we’ll know how to build many, and they can be self contained worlds lasting thousands of years, a home not only for humans but for all other Earth life we choose to bring with us.
Again, the Shell theorem, which we cited in our Aosta paper, is predicated on a small test mass inside a large shell. Indeed, in that case, there is no net force to restore a displaced test mass to its original position. The internal g-field is flat across the shell. However, the Shell theorem breaks down at geometric extremes:
– when the “test mass” is an entire planet, and,
– when the shell is a small fraction of that “test mass” (no more than a percent, say), and,
– also when the shell is in close proximity to the “test mass” (standing off by a percent).
In such a case which we have proposed, if the shell is displaced with respect to the center of mass of the central body, then the gradient of the g-field would not be flat everywhere across the shell — while it would remain uniform orthogonal to the axis of the displacement, it would become *nonuniform* along the axis. This effect of this nonuniform field on differential mass elements, which we modeled as rings perpendicular to the displacement axis, would be a force tugging the shell back on center. Furthermore, the force is linear with displacement.
Consider this thought experiment: Shells around dense heavy worlds would be much thinner than shells around light worlds. (See the table in the paper. However, it’s pointless to build a shell around a dense heavy world because it already has a thick atmosphere on the surface.) Earth has a strongly curved geodesic, due to its density (densest world in the inner solar system). Therefore, on Earth one experiences a significant difference in g-force, even over a small distance, as one moves in or out along radial lines. Now imagine suddenly wrapping a balloon of tin foil around the entire Earth, if one cares to, like the Echo satellite but vastly larger. If one chooses an altitude high enough with pressure low enough (near vacuum), this thin shell will float, tugged in by Earth’s gravity but held up by air pressure. Would this insignificant envelope suddenly make the strongly curved geodesic of its contents (the entire planet below) flatten out? No. At any given point in the Earth-shell system, the fields of the two masses would superpose.
No, this is not a reactionless drive, or emphatically a drive of any kind. Move the shell by internal agency, and the shell returns. Moving the shell by external agency is an interesting idea, but more than a little wouldn’t be safe and we don’t recommend it…
Regarding spinning the shell: the shell is free to rotate as it will. After thought, we decided rigging would complicate matters. No, we don’t recommend deliberately spinning the shell — for one thing (aside from the obvious mechanical problem of spin-induced hoop stress) a spin fast enough to generate g-force *on the ceiling* would cause all the dirt overburden to fly off the plane of spin. Without the overburden of regolith to counteract the internal pressure on that newly-bared belt, the shell would burst there.
Long term, the Yarkovsky Effect is something the maintenance crew would have to deal with.
As for being claustrophobic, our example ceiling would be 20 km over one’s head, which is far more headroom than one would get in an O’Neill colony. And, as we point out, the ceiling (in fact all surfaces, have you seen the nighttime light shows on modern skyscrapers lately?) could be arbitrarily pixellated with light-emitting stuff, such that it could be indistinguishable from a real sky if the designers so chose. The Rayleigh effect assures this for almost every sight line except straight overhead. (Recall the fake skydome in the movie “The Truman Show”.) And, as we show, one doesn’t need to generate anything like the solar constant (~1400 W/m^2) for illumination or growing plants. Less than 100 W/m^2 average over the day/year would suffice.
Regarding waste heat spoiling the camouflage: the shy inhabitants could:
– utilize the frozen interior of their world as a heat sink for their post-construction industry for a very long time, or,
– exercise heat / vapor discipline, or,
– select a candidate for enshelling close enough to the star that the anomalous heat emission from the hidden dwellers would be lost in the noise. A number of worlds in our own system are warmer than they should be.
Understand that we come at this with humility, which is why the SETI people should maybe consider other possibilities.
Regarding which surfaces are inhabited: we proposed hanging cities from the ceiling for the interesting view (glass floor under one’s feet). But normal acrophobic denizens of the shell world would mostly walk around on the open surface of the central body below, with their feet in real dirt, not upside-down on the ceiling like ants.
Regarding high cost: in the introduction to /The Next 200 Years/ (1976), the late great Herman Kahn wrote, “Today we are few, poor, and weak; tomorrow we will be many, rich, and powerful.” I’m sure the Extropians lurking here have an opinion on ultimate human capabilities…
As for life itself, we predicated the whole idea on the moral hazard of introducing aliens (i.e., us) into somebody else’s ecosystem, and on the high probability that a functioning ecosystem elsewhere (even if lacking sentient lifeforms) would be toxic or at least biochemically incompatible with us, due to the vast number of random paths evolution can take.
Robert Kennedy
Astronist: Maybe it is or maybe it isn’t Darwinism, or as you say, intrinsic of evolution to complete for food and territory. We don’t know until we actually observe life on another planet. What gives a space faring civilization the right to take over a world that has life on it? Would you advocate taking over a world that has intelligent life; even if we destroy life that is indigenous to that planet. Should the day come we are on a world with unintelligent life I hope we protect it better than we do the life on our own planet.
I especially hope we don’t colonize a world with intelligent life on it unless they want us there.
I suspect that the overwhelming majority of the worlds out there with life will not have progressed beyond the Proterozoic. I think humanity would be morally justified in supplanting such simple lifeforms. Phanerozoic worlds, OTOH, should be considered completely off limits as far as major alteration goes. Exploration only.
If there are Kardashev Type 3 civilizations out there, ones which utilize the energy of their entire galaxy, do you think they are worried or worry about lower life forms as they transform the galaxy to their needs?
Sputnik_wranglers light level for their crops is interesting, but they did not state what their productivity was expected to be compared to, say, hydroponically grown crops here on Earth. The average surface insolation hitting the top of the atmosphere across a horizontal surface area element for Earth is 350W, so it seems reasonable that the average level of visible light after cloud cover considerations is 100W/m^2. If photosynthetic conversion is equivalent to that of our most efficient food crop, sugarcane, this would give 8W/m^2 for human activity, So we can support a population 100,000 famished people per sq km, or 10,000 highly active ones.
I suspect that these city densities would actually be needed to justify the cost.
James said
“My view is that we should not terraform or colonize any worlds with any type of life, even microbial life, so that they can have the right to evolve on their own.”
I have heard many similar comments and can make no sense of them, save that they are part of a new religious movement. To illustrate how ill formed such blanket sentiment is, take the example of the domestic dog, the average member lives with far less stress than its ancestors. Also note that they induce a measurable reduction in stress related illness on their owners, the most famous example being the dramatic reduction in cardiac disease that their companionship brings.
So, first I ask you to contemplate what a tragedy it would have been for both parties if our ancestors had also thought this way. Next I point out how difficult it would be to sterilise an existing biosphere, and how much more likely planners are to work by including as many elements of the old biosphere in their new one as they can. To me it seems that we could also ask if was moral to deprive pre-existing life-forms on such a planet of the new opportunities that terraforming might bring.
Robert Kennedy (a.k.a. Sputnik_wrangler), thank you for your comment.
My point is *not* a “Shell theorem, which we cited in our Aosta paper,” which “is predicated on a small test mass inside a large shell”, but the fact that a “shell does not induce any non-zero gravity field inside itself” (as I said above). This remains true regardless of the geometry of the body (or the bodies) inside, including the “geometric extremes” you mentioned above. No gravity field — no interaction — especially no interaction of the kind you describe. You can’t escape here.
Talking about the shell theorem — it is *not*, as you say, predicated on a small test mass inside a large shell. Instead, its derivation uses a *point* mass — a special physics concept — to show that the net gravitational forces acting on it from the mass elements of the shell sum up to zero. This is true for each and every point of a body inside. This implies, that the shell does not exert any force on a body inside as a whole. This derivation does not depend on the absolute or relative size or on the nearness of the participating bodies.
Your and Kenneth Roy’s and David Fields’ work starts with something which is not true, something which is counterfactual, and your work is full of misconceptions from the outset. You should know what you get from this regarding your conclusions. You should revise your position, and above that you should withdraw your work from the Journal of the British Interplanetary Society — I’m serious. And I’m out.
Ducan Ivry, I am also a sceptic of this model but it is still possible that the problem might reside with the way they presented it here. Perhaps they calculated with a shell that was not as uniform as they are leading us to believe in the above précis. Alternatively, perhaps the effect is mediated by induced distortions of the shell by atmospheric pressure differences with height if it is moved off-centre, and if so, this would also explain the symmetry along the axis of dislocation that they use.
Duncan is right, in my opinion.
The other thing to keep in mind is that larger forces can be sustained by thicker walls. Pressure containment is scalable, i.e. if you scale up a structure, including wall thickness, it will be able to hold the same pressure using the same material strength. As a bonus, you get good radiation shielding if the structure is large enough. The size of O’Neill colonies is therefore not limited by the need to contain air pressure, and in fact bigger is better because the structural material can double as radiation shield.
Size is instead limited by the desire to generate Earth like gravity. Current maximum strength materials do not allow structures much larger than 1000km to sustain an artificial gravity of 1 g by rotation.
The general Idea here seems to be that humans would have no moral right to “terraform ” another planet , if this might endanger its original life forms , EVEN if these are non intelligent . And so we should instead concentrate on learning to build “shellworlds” ,here in our own solar system .
For me it is the first time I have ever encounterred these to basic strategies as ALTERNATIVES . Until now , or very recently , I believe most people would have seen them as two complementary aspects of “reaching for space” .
Perhabs the next step in this moralization process will be to start doubting if we have any moral right AT ALL to invest more resources in space , when (as will probably happen relatively soon ) , milions of people wil start dieing from hunger because of overpopulation ?
If American and European thinking increasingly , and with no end in sight, are becomming dominated by this kind of moralistic paralyssis , the end result will be that other cultures will take the lead . The Rusians, Chinese , Japanese and Indians does not suffer from any comparable self-doubts . They may be somewhat behind in teknology , but thats only a matter of time .