Expansive Zero Gravity Atmosphere Concepts Gaining Social Momentum

The Virga comic series has recently hit the (virtual) shelves, and it offers quite a vision of a world that continues seemingly infinitely in all directions, and yet is filled with air that a human can breathe.  This is an installation onto the Virga series by Karl Schroeder, a Canadian science fiction author.  At the time of writing this, Virga comics 1 through 3 are out, and I'm rather excited to see these vivid illustrations of such a world.  Naturally, I want to make a post that covers where it fits into my own views on such worlds.

I will continue to pick at the parts that I find to be particularly bad abuses of physics, but I should note that I find the comic and series extremely cool overall.  The comic, in fact, touched on a few elements of a mixed gravity (my own term) world.  This is a very new kind of environment that is being described, and the author has done a great service by working to popularize it.

Birds and Fishes

The Virga comic portrays fishes with wings.  Even Gerard O'Neill, in his book The High Frontier talked a good deal about fish, which I did discuss on Space Exploration Stack Exchange.  Other people were apparently interested in zero gravity dolphins, which I have to admit, was a little shocking.

(image copyright Virga Comics, used as fair use here
this is from Comic 3 which can be purchased from Comixology for $1.99)

And I feel like the odd one out, thinking that birds would be considerably more interesting.  Of course, my visions hinge around the "friction buffers".  In my own vision of the gravity balloon, a flying fish that gets caught up in the rotation of a cylinder will not fare well at all.  I have no idea how birds will manage to cope with zero gravity.  Still, I'm satisfied thinking "life will find a way", and imagine that birds will at least figure out how to propel themselves long enough to get to some food.  If they flew into an artificial gravity cylinder, then they would revert to their familiar flying mechanics.  I imagine birds would be quite sensitive to Coriolis forces as well, and learn to use them - getting extra "lift" by flying in the direction opposite of the rotation.  Birds are quite good at minimizing effort on Earth.  I doubt that zero gravity worlds would be comfortable to them, but they're also some of the most adaptable species on Earth.  But that's just my 2 cents.

In addition to the bird-fish in Virga, there are also some illustrations of humans with wings or just plain flippers on their feet.  That's very entertaining to see.  It really brings to life the world that we once thought would be the promise of air travel in the early 20th century.

The author also seems to have done a great deal to realize a vision he talked about in an article written about Virga when the idea was much younger.

"For years I’ve had flashes in my mind’s eye of an endless sky, and of towers and buildings floating in that sky. A particularly persistent image was of a woman standing at a tower window looking out over an ocean of cloud, with no ground beneath the tower."

This vision was basically realized.  I believe this is the exact vision the author had in mind when the comic's sort of castle-scape was drawn.

(image copyright Virga Comics, used as fair use here
this is from Comic 1 which can be purchased from Comixology for $0.99)

Novel Environment Physics

Several phenomenon are referenced in the Virga in-world that are quite unique to a mixed gravity world.  Things that stood out in particular:

• spin downs, where the ring looses gravity
• dropping out the bottom of the ring on a air-bike

While I haven't read much of the book series, they very quickly got the plot twist of a spin down in the first book.  This is an event where they stop spinning the ring, and everyone on it loses gravity.  This was reference in the comic, although not specifically illustrated to my knowledge.  This sort of thing is only really thinkable in a mixed gravity world, and so is the dispatch of the air bikes (there may be an official term for these, but I do not know it).  The idea there, is that people launch themselves out the bottom of the ring structure to provide quick access to the larger world.  That makes sense certainly, but it would definitely be a rush as you fall, fall, fall, and then wind up without any ground left to catch you.

(image copyright Virga Comics, used as fair use here
this is from Comic 1 which can be purchased from Comixology for $0.99)

I would have liked to see some mention of false forces in artificial gravity rings, but there's still plenty of material in the series I haven't got to.  I think the "preferred" direction in artificial gravity will be a very huge concept for people who ever live in such a place.  I think it will be almost incomprehensible to them that on Earth people can't distinguish easily between north and south.  But then again, in the free floating space between habitats, none of the 3 dimensions we live in can be distinguished.

There are some other details that are generic to large zero gravity environments too.  For instance, the way people move around sticks out.  The way people structure the beds sticks out.  Also, the ports are often in apparently zero gravity environments.  I think it would be difficult to manage in such an area, but I guess that's the idea, and that the people who live there are quite used to the concept.

These zero gravity artificts strikes me as similar to a particular vision of the future (actually the year 2000) by french artists Jean-Marc Côté and other artists, made for the 1900 World Exhibition in Paris.  These visions show people prancing around the skies in only minimalist wingsuits.   It's well past the year 2000 now, and all we have are wingsuit gliders which can't sustain horizontal travel for more than a few seconds.

 (Public domain images, source)

I find the similarities between these and the Virga comic rather stunning actually.  Sure, the physics and technology itself is different.  But in both we see impossible-seeming air vehicles, and people propelling themselves around with simple flaps.


Intermediate Scale Environments

I was very happy to see a number of creative environments that exist often on the scale beyond a single artificial gravity ring.  In Schroeder's vision, the Virga world has many small suns (really fusion reactors) which define a nation, which then consists of many gravity habitats.  If I am to take the expanse of such an area to be defined by the path length of photons in the atmosphere, this is quite large.  Actually, the heat would be intolerable near the sun itself because it would entail huge intensities, which would heat the air, which would cause other problems.  But I'm not determined to ruin everything.

Aside from the "suns" there are lots of other structures depicted.  In particular, green things that look like they have bushes growing on them, which I can only assume are some type of growing platform.  In the Virga comic, they portray many artifical gravity rings hanging from these structures.  This is very cool, as it represents a sub-scale to the "suns" or "nation" scale.  Larger than the suns/nations, there's still the air circulation patern in the Virga world.  I completely agree that this is sci-fi gold, because there's so much potential expanse to this world.  The scales that are or could be at work in a science fiction world are:

• The individual rotating ring / cylinder
• A cluster of rings, possibly tied together
• Suns/nation level which spans the distance light can travel
• Circulation patters, which are the largest grouping in super-large habitats
• The gravity balloon itself
• Possibly multiple gravity balloons within a solar system

The author did mention the possibility of the final scale somewhere in his online writings, but it's not central to the Virga story at all.

"Circulation is necessary because otherwise you run the risk of having the atmosphere condense and then freeze onto the outer skin of the sphere.  (This will only occur if it's orbiting in the outer part of a star system, of course--but in my books Virga does just that.)"

So in essence, this just fills in one scale of the hierarchy of the organization of such a place.

(image copyright Virga Comics, used as fair use here
this is from Comic 1 which can be purchased from Comixology for $0.99)

The Remaining Problems

Displayed front-and-center on the cover of the first Virga comic, probably the most iconic image of the series, is a man riding an air bike counter-rotation around the surface of an artificial gravity ring.

There's only one problem with this - it would never work that way.  I know, the idea is that the rotating structure "drags" the air with it, or something like that.  The only problem is, we already have models for this type of fluid mechanics problem, and it does exactly not do that.  I've written several posts on the fluid mechanics of the issue, and I've not seen a single thing that refutes the basic estimates of power dissipation per unit area, or the consequences of that.  In fact, the worlds portrayed in Virga are not "smooth" in the thermal-hydraulic sense at all.  They consist of macroscopic imperfections, otherwise known as buildings.  These would only add to the massive stirring of the atmosphere in that environment.  Not only would such a craft be unmanageable in the situation portrayed, but so would walking.

I don't know what kind of scale these illustrations are tying themselves to.  It seems pretty indefensible to imagine that they're much less than 500 meters in diameter.  The calculation is then simple.  Acceleration is v^2/r, and that equals to one Earth gravity.  Relative to the ambient air, that results in 110 mph.  Even if we were as generous as possible to the concept, we must somehow imagine that the airflow transitions from one stream which is at rest to another stream at this velocity.


Could the Idea Catch On?

Virga represents one extreme.  As I covered in my last post, there is a fairly specific limit to how large of a habitable area you could create with this type of method, and this science fiction world isn't far from that.  The scales and the simple expanse of the world are beyond our ability to think clearly about.  There's a meaningful place for this type of thought.  Particularly since this is a fully 3D world, the transportation limitations aren't actually any greater than Earth itself, in spite of being so many orders of magnitude more expansive.

On the other extreme, there's another reason to promote the concept of expansive zero gravity space habitats.  That's because they have good near-term prospects for space development.  The sales pitch is that we can pick a nice asteroid, walk in, and then just "put up the wall paper".  In practice, it's a little bit more complicated of course, but that description is quite an honest summation.  Asteroids of the relevant size might have enough fissures to allow us to venture straight into their center, unimpeded.  Then, even if we exert a pressure on the walls we're assured stability due to the fact that gravity itself would be sufficient to hold against the pressure.  Sure, there's a great deal of exploration and then analysis which would be requisite before we ever think about doing this, but it's something that can have safety demonstrated before hand, and it's something that can scale up to significant sizes for a significant number of people, all the while being in the depths of space.

The key to all of this, both the large and the small, is the prospect of a habitable environment which is sometimes zero gravity and sometimes artificial gravity - mixed gravity.  The smallest habitat within an asteroid with the "wallpaper" model may only host a small tube of low gravity and frustrating Coriolis forces.  But you can hop off of that to do work in zero gravity.

I come at this from the perspective of space advocacy.  The sci-fi world of Virga is the fantasy part of the spectrum, but the basic concept of a mixed gravity world are the same.  It's also really really cool.

Inclusion of Air Pressure Effects for Super Large Gravity Balloons

Although impractically huge, the math of a gravity balloon changes a great deal when they get beyond a certain size, in large part due to the pressure of the air itself.  There are quite a few related discussions that stem from this.  On Earth, for instance, we need our large gravity well to hold in air, but it's not so much because the upper atmosphere extends to escape-able distances at thermal velocities - far from it.  In terms of sheer value of density, our atmosphere is basically gone when you reach 40 km.

That altitude represents a Delta V value (the speed you would accelerate to if you fell that height in a vacuum) of 0.6 km/s, whereas the escape velocity for Earth itself is around 11 km/s.  The role our gravity-well plays is, instead, to fight against the "boiling" off our our upper atmosphere by higher energy particles from the sun and the cosmos.

To put this in different terms, if you wanted to blanket the Earth in a sheet to hold in its atmosphere, that sheet wouldn't need to be very thick.  A simple mathematical treatment of this is to say that the pressure drops off exponentially.  You need a constant to use in that equation, which is simple, and given by this reference to be 7 km.  You can very easily see that at 40 km, you're left with less than 1% of the air pressure, according to [P = P0 exp(- h/h0 )].

One might correctly notice that this has implications for a gravity balloon.  This raises the possibility of a super large type of gravity balloon, which perhaps looks more like a gas giant planet than a space station.  Formalize the problem by requiring a habitable pressure in the center.  Then that pressure decreases as you move away from the center.  There are a few possible non-ideal consequences of this, which are quite interesting from a physics standpoint.  Most obvious, the outer regions might not be entirely habitable due to a low barometric pressure.  This would only happen for extremely large sizes, and represents the ultimate upper limit.  Second, you forfeit the zero-gravity environment for a large part of the volume.  While the outer regions aren't zero gravity, they can still be very low gravity.  Going from 1 atmosphere of pressure to near-zero pressure requires a certain potential difference, but unlike Earth, that difference may be spread over a large difference for a super large gravity balloon, resulting is a very low field at any point.  It's possible that any residual gravitational field could be counteracted by wind turbines which have relatively low energy requirements.

The world I am describing comes awfully close to the conception of the Virga world.  Coincidentally, they just came out with a new comic series set in the Virga world, which I hope to write more about later.  In addition to the changing pressures, this world has convection currents (which might be physically accurate).  The author also goes on to write in a great deal of politics and dynamics to this world, which is tangential to this blog.

I have noted (as have others) that the Virga world doesn't need to have a wall of carbon nanotubes, but I haven't given very detailed consideration to how thick the walls might need to be, or even if these walls would be stable.  Wall thickness is, after all, a function of the pressure near those walls.  The technical guidance we have for Virga is that it is 5,000 miles in diameter.  I don't think we are given a pressure at any point inside this, but I will happily assume 1 Earth atmosphere in the center.

The Mathematical System

Governing equations are straightforward, but I found them deceptively subtle to get correct.  There are number of components, so I'll outline them in a list.  The goal here is to obtain a set of 2 differential equations that describe the atmosphere pressure and gravitational field within this structure as a function of the radius.  In order to get there, we have to formalize several physics equations.

Technical steps:

1. I used the ideal gas assumption and constant temperature.  This relates pressure and density in a linear relationship.  Use the specific gas constant to write this explicitly.
2. Mass is taken to be a function of radius, and includes all air mass below that radius.  This is then related to the field, making use of the shell theorem.
3. Change in mass for a differential increase in radius is the sphere surface at that radius times the air density at that radius.  This is a geometrical statement.  Additionally, density is replaced with pressure from the ideal gas equation.
4. Initial conditions are obvious.  The gravitational field is zero at the center, and this formalization specifies the pressure at the center to be the habitable goal.
5. The differential equation for mass is then changed to be in terms of gravitational field, which comes from making a substitution from the shell theorem.  Some algebra is done, using the chain rule of calculus, and then rearranged.
6. Pressure falls according to the density of air and the gravitational field, in the same way that Earth's atmosphere does, which I outlined in the introduction here.  This is written in a form usable as a differential equation.

For such a large volume, transport of heat out from the center to the outer regions becomes non-trivial.  This makes the assumption of constant temperature a little dubious.  We would probably expect some falling temperature with radius.  This is still pure presumption as to how this thing would actually be built, so I'll keep T constant for now.  The equations are complicated enough as they are.

This math is a little bit intimidating, and it turns out, the system can't be directly solved easily.  In the following equations, look at the last 3 lines.  Those fully specify the differential equation system.  With the constants filled in, you can put this directly into a mathematical software package.  Unfortunately, I have not yet found such a package that will give an algebraic answer, so I will have to satisfy myself noting that it can't be done without extremely exotic functions.

Governing Equations of the system

Last three constitute complete differential equations

The constants need definition.  Since this is a technical blog, I will list all of the physical constants employed here.  The gas constant is for air at sea level.  Generally, the values are sought to provide a normal Earth room temperature atmosphere, consistent with most of this blog and the idea of Virga.  It's possible to make an extension of this math to do primitive analysis of gas giants or stars.  Hopefully I can do that as another post some other time.

• R_{specific} = 287.058  J / (kg K)
• T = 293 K
• G is Newton's gravitational constant = 6.67384e-11 m³/(kg s)

With this, the system is fully specified mathematically.  You have the ability to input the above equations, with the above constants, into a numerical integrator and obtain a spatial picture of the pressure and field within one of these bodies.  I have made my own code to do these calculations available on Pastebin here.

When talking about a gravity balloon, we are terminating the gas by adding a wall (in literal terms).  By the shell theorem, we should be comfortable ignoring everything beyond whatever radius we're looking at, because the gravitational field contributions all cancel out.  That is why these mathematics are relevant for large gravity balloons.

I was also interested in the effect that setting different pressures would have.  I tested two cases, where the central pressure was 1 atm and 3 atm.  A surprising result came out of this - that the pressure at large radii was lower when starting at a higher central pressure.  Actually, this makes complete sense.  This is why gases consolidate into gas giants instead of always hanging out in a large volume at low density.  This is telling the story of gravitational collapse of gases.

Pressure versus Radius graph

This has interesting consequences for gravity balloons.  You would think that containing more gas in the same space would require more container material... but that's just not the case here.  This is the strange nature of self-gravitation.  The air holds itself in (to a limited extent).  Now, there's also the valid question of whether 3 atmospheres of pressure is actually usable, and it's likely not because of Oxygen toxicity.  Because of that, it's not at all clear how the usable volumes between both of these compare.  But for sake of argument, let's take the pressure range for the "habitable" volume to be 0.8 atmospheres to 1.0.  With that specifier, I can compare the habitable volume between these two cases.  We're imagining that the center area of the 3 atm case will be treated as uninhabitable, but people could live beyond that radius.  Honestly, I think this looks closer to the sketches of Virga.

Table of Radii that certain Pressures occur at and corresponding volumes with given range

Radii	for the	Pressure
(given in km)
	1 atm	3 atm
0.8 atm	0	18,384
1 atm	10,987	18,590
Habitable	volume
case	V (km3)
1 atm	5.5562E+12
3 atm	8.83007E+11

Here we see that the habitable pressure would not be increased by adding more air to the system.  That's not entirely surprising, for the same reason that Jupiter doesn't have much volume at "habitable" (again, just 0.8 to 1 atm) pressures.

Properties of the Structure

There are two mass values of interest - the mass of the wall required to hold the air in, and the mass of the air itself.  For the wall requirements, the formula I have used so far for the "large case" needs to be revised.  Going back to my original question on physics stack exchange about this question, the large case has fit the equation of [ P = 2 G pi mu² ].  That equation takes into account the self-gravitation from the wall itself, but not the gravity from the air.  So I've wrote another equation that does take it into account.  To solve this equation, it needs to be solved for the mass-thickness of the wall, and then simply multiply by the sphere surface area at that radius and that's the wall mass.


The calculation of air mass is trivial because it follows the same equation used in setting up the differential equations to begin with.  That equation is just recycled.  Now, here are the equations.  These are in terms of P(r) and g(r), which are the pressure and gravitational field throughout the air.  These are outputs of the code that I have on pastebin.

Expressions for mass of the air and wall of super large gravity balloon

(require numerical solution of previous set of equations)

We can now look at how the two independent variables (center pressure and the radius) affect the mass needed to construct the wall of this gravity balloon and fill it with air.  With my code output and these equations, I produced the following graph to illustrate this, and it gives a good picture of the general mass scales involved for different cases.

Graph of calculated masses of wall and air

given different radii of structure

Keep in mind that this graph is still using linear scales.  In terms of general observations:

• The material requirements for the wall never reaches an absolute maximum.  This was one of the primary questions that was motivating me.  The wall mass requirements grow at a rate below even the surface area of the volume, but it continues to grow.
• At super large radii, the gravitational field from the air itself dominates, which isn't very surprising.
• The cross-over point is around 20,000 km (40,000 km diameter), which has an atmospheric pressure of 0.5 to 0.7 atm.  In other words, the air gravity starts to dominate while the outer regions still remain disputably habitable.

There's now a need for better reference values.  Earth's moon has a mass of 7.3 x 10²² kg.  This is a facinating reference point, because it establishes the the maximum practical size of a gravity balloon is right around the mass of the moon.  In terms of length scales, I just want to quickly note some other bodies for comparision.

radii for comparison

• The Moon 1,738 km
• Virga 4,023 km
• Earth 6,378 km
• Saturn 60,268 km
• Jupiter 71,492 km

Compared to what's possible, Virga is somewhat small.  My expectation was that the pressure would varry significantly between different regions in it, but that expectation has proved wrong.  Virga's outer regions would only be about 4% lower pressure compared to its center.  However, I also need to volunteer the fact that if Virga was made as a gravity balloon, the walls would have more mass than all the air (and all other stuff) on the inside.

I also wanted to write a little more on the stability of such a massive construction, but I find myself at a loss on the subject.  This analysis included effects from changing pressure over the volume and gravitational effects from the air.  These will affect the stability of the walls, but I'm not entirely sure how or to what extent.  The gravity of the air is at least partially destabilizing, just how tidal forces are.  Come a little closer to center, and the gravity increases, and vice-versa.  This is characteristically unstable, but it probably isn't a game changer.  There's also the fact that the pressure increases as you go in further, and as far as I can tell, this effect will be more significant (in any case) than the tidal forces from the air.  All of this is concerning the particular deformation of one part of the wall falling in a little bit.  It seems that the dominating factor for that contingency is what it's always been - the change in the self-gravitation of the wall.  My expectation was that the wall-self gravitation would become irrelevant on large scales because the mass-thickness of the wall declines.  That happens, but probably not to the point of irrelevancy.  Even if you go so big that wall self-gravitation didn't dominate the stability discussion, the air pressure would be the dominant mechanism for these global deformations.  Of course, there's still the matter of "local" instabilities - which consist of leaks and wall Rayleigh-Taylor instabilities.

As for other observations, I want to quickly hit the escape velocity and the wall thickness.  At around 100,000 km radius, I find the wall thickness to be around 240 meters, if I assume a density of 3.5 grams per cubic centimeter.  Crowlspace was looking at about the same thing and came up with 1,345 meters at a 200,000 radius.  Of course, with larger radius the wall thickness will decrease.  This shows that Crowlspace's number is truly quite different from mine.  I believe this is because of the nature of the calculation he was trying to do, which wasn't considering any hetrogenous spatial distribution of the gas.

For the 1 atm central pressure case, I find an absolute maximum escape velocity (from the surface, including air and wall) to be 761 m/s.  This is a facinating result, because as long as you don't change the parameters like the central pressure or density, it is the maximum escape velocity that a gravity balloon can ever have.  It's also baffling because in Newtonian gravity, the gravitational potential of an infinite sheet of matter is infinite, and in large cases of this the wall starts to look a lot like an infinite sheet.  But that doesn't happen because the wall's thickness decreases with increasing radius.  Even more surprising is the magnitude of this number.  It's just not very fast, and even a bullet from a conventional gun can meet it.

Of course, the real question is where you would get all of that air from.  Wall materials for a gravity balloon can be anything, so the moon itself would literally suffice for this large limit gravity balloon's wall.  Earth's atmosphere is made out of fairly common elements, so that's not a constraint, but they would have to be processed in some sense.  I agree with the sense that such a large habitat would be in the outer edges of a solar system, but possibly they would be a complete interstellar space.  If near some clouds of gas of plentary nebula, perhaps the gases would be easier to collect.  I don't doubt that some dumb rock for the wall materials would be hard to find either.  But there's still the matter of turning whatever gas you have into molecular Nitrogen and Oxygen.  It's certainly a reasonable idea for speculation.  The scale is just so impossible for a humble human to consider.  A habitable area could literally exist accross a region 2 times the diameter of Earth.  The math tells us that is easily possible in terms of pressure alone.  But what would anyone do with all that space?

Asteroid Macroporosity and Caves

In their natural state, asteroids are not entirely solid, but reading up on the issue I've been surprised by the degree and distribution of empty spaces inside.  I posted about this on Space Stack Exchange, and obtained some really fascinating information.

For background, micro-porosity describes small voids of the type that we can measure from fallen meteoroids.  Those are to be expected (probably from some bubbling of intermixed volatiles).  Macro-porosity includes voids of large sizes.  How large?  It seems that no one knows.  In the fact of the massive deficit of knowledge, there are huge implications for our future in space.  The most shocking detail I encountered from that above question is this:

"The (scarce) theoretical works on that point predict an internal structure where most of the void are deep inside, while the shell around is pretty-much packed."

This is a fairly direct response to my own question - of how Mathilde can possibly look solid when literally half of it is empty.  No matter how long I stare at the pictures taken by the NEAR Shoemaker, I can't convince myself that there are any particular caves or anything of the like on this scale.

50% is no small fraction obviously.  A 1999 paper in Nature is a good example of what is probably meant by the scarce theoretical work in this subject.  These conglomerations would leave a large fraction of the interior as void, but it's hard to imagine it reaching half.

The mind boggles to transition from this relatively smooth surface to an interior made mostly of space, crisscrossed by boulders the size of American cities.  Would there be openings in the surface to these voids?  It starts sounding like the common sci-fi trope of a hollow planet.

Mathematics of Breaking Length

I keep returning to the importance of a number of mathematical details of structural mechanics of self-gravitating bodies.  The fearsome size (about 50 km diameter) of Mathilde is more incredible when you consider that the rock in the center is under constant stress from all that weight and is actually no worse off because of it.  The best model might be something like a gumball machine.  Each gumball is hollow and has to resist the pressures by 3 or 4 other gumballs pressed against its side.  This is the true failure mechanism.  It relies on the overall pressure of the pile, as well as its own size.

In general, the pressure that has to be resisted is the same central pressure that I've been calculating in this blog over and over again.  After all, the voids are literally a vacuum.  Actually, we don't know this for sure, but it's the best assumption we have right now.  There's no good case for how gases are held in.  If there were, that would constitute a natural gravity balloon.  While I think it's a fairly stable construction, that's after careful consideration and engineering to implement barriers that block gas movement.

This equation for central pressure is:

$$P = \frac{2}{3} G \rho^2 \pi R^2$$

to convert this into structural requirements, we need to introduce the material strength, sigma.  In a simplistic sense this can just be equal to pressure.  I have a mental image of a bar spanning the entire diameter, made of the same material as the rest of the asteroid.  Physically, this is fairly nonsensical butit gets the job done for a simple approximation.

$$\frac{\sigma}{\rho} > \frac{2}{3} G \rho \pi R^2$$

For the specific case of Mathilde's size and density, this requirement is 126 m² / s².  Compare to a specific strength for concrete of over 4,000 m² / s².  If anything, this probably indicates that the conglomeration of material inside the asteroid is, in fact, not strong.  Alternatively, it shows that a series of breaking and rearrangement has already taken place.

Consider the alternative of a building on Earth.  It also has some average bulk density that results in a pressure at the bottom.  Atmospheric pressure is disregarded because it's irrelevant to material failure.

$$P = \rho g h$$

For direct comparison we would like to put this in terms of material specific strength.  We have to decouple the "load" and "supports" to be practical, but I'm not too interested in that right now.  For

$$\sigma = g h \rho \\
\sigma / \rho > g h$$

This is the concept of breaking length.  For concrete the "breaking length" is 440 meters.  That corresponds to a specific strength value of (9.8 m/s²) (0.44 km) = 4,312 m²/s². In short, this is saying that you could stand up a concrete tower 440 meters high, but no higher without it buckling under its own weight.  Self-gravitation is very fundamentally different.  To illustrate this point, I want to compare the two cases of resisting void collapse in asteroids to breaking length on Earth.  This is simply a matter of requiring the specific strength to be the same in both cases.  In a sense, we use breaking length as a metric of specific strength itself.

$$g h = \frac{2}{3} G \rho \pi R^2 \\
R = \sqrt{ \frac{g h }{ \frac{2}{3} G \rho \pi } }$$

For reference, Mathilde has a diameter of 53 km.  So resisting collapse of the voids at this size would be roughly equal to the task of resisting breaking over a length of 12 meters of the surface of Earth.  Obviously this is because gravity is weak, and the Earth is big.  Still, it's an interesting observation that's really hard to wrap one's head around.

Implication

In this blog, and elsewhere, it's generally taken for granted that making it to the center of a large asteroid would be difficult task.  For many, yes, this is the case since the macro-porosity is effectively zero.  For the majority, however, it's either fairly major (>5%) or we just don't know yet.

It's incredible to think that drilling to the center of such an asteroid... might not require any drilling.  You might be able to just float on in a giant cave entrance.  Exactly how much will be accessible and how easily is an unanswered question.

Even candidates like Eros have a major macro-porosity value, somewhere in the neighborhood of 10%.  This is a small amount of the total, but in terms of access, it could make an unbelievable difference.  If future methods could give a good idea of the 3D distribution of matter in an asteroid, and then the composition as well, then development of their resources will be a very different matter.

There are also major changes to the pressure-volume relationships.  I've never even considered the idea of increasing the density of the rock, but this would be possible for highly porous candidates.  It would be obvious to use the existing voids to store resources or excavated rock.  Doing so doesn't quite reduce the pressure due to self-gravitation.  On the other hand, moving around large rocks while at the same time avoiding cave-ins and other material rearrangement would not be an easy task.  Still, it's something that can be analyzed and worked around.

Space Colonies Inside Irregular Asteroids

Any asteroids within the size-range suitable for a early gravity balloon colony would likely be highly irregular.  That is to say:

1. the body exhibits a non-spherical shape, like a blob
2. its rock has some amount of compressive strength

This was established with my last post, where you can see that basically any body with a natural internal pressure of 0.3 to 10 Earth atmospheres is likely to be irregular.  Those are also the internal pressures you need to use it as a colony.  Now, I'm using the term internal pressure, but so far I'm only referring to the contribution of self-gravity to internal pressure.  It could be more or less according to how the asteroid formed and what kind of internal forces it experiences as a result.  In this post I want to talk about how you would manage the most obvious kinds of internal forces for an early gravity balloon colony.


Irregular Asteroid Stresses

Call the largest dimension the polar axis.  This is chosen because it is the axis of symmetry.  In that case of planets, the axis of symmetry is typically the smallest one because they are roughly oblate spheroids, whereas 10-km scale irregular asteroids are more like a prolate spheroid.  In other words, this is the geometric model I'm using:

My terminology:

large dimension = polar axis

This doesn't look very far off for many of the asteroids we've photographed.  Eros, in particular, among others.  With this in mind, we can consider the stresses that he object's own self-gravity will cause on its interior.  For a mental model, this is somewhat similar to a building on Earth.  In order to "stick up", it has to have some strength.  The only difference between an asteroid sticking out as a prolate spheroid and a building standing up on the surface of Earth, is that the asteroid deviates from a spherical shape, while the building deviates from the flat surface of the ground.

This protrusion will only lead to compressive stress here.  After all, it doesn't have to stand up to the wind or any other dynamic forces.  If you want a rough formula for exactly how much compressive stress, then consider the gravitational head.  For a bad approximation, imagine that the gravitational field only varies with radius.  Then it's obvious that the equatorial radius is smaller than the polar radius.  That leads to an inconsistency if you imagine the object material is fluid-like.  You obtain a different pressure if you measure the elevation drop from the equator surface to the center versus if you measured from the polar surface to the center.  It is precisely the difference between these two pressure that is the non-isotropic force, or the compressive force.  It has the same units as pressure, because these are both elements within the stess tensor - the bread and butter of civil engineering.  After all, the entire proposal basically comes down to civil engineering.

For some math, you can imagine that this stress will be approximately equal to the gravity on the surface of the asteroid (which is an average figure itself) times the elevation difference at the equator and the pole.  Without going into details here, you can get a factor of 2/5 to add onto this.  This isn't perfect, but it's pretty good.

$$\sigma_z \approx \frac{2}{5} \rho g \left( R_x - R_z \right)$$

To illustrate the occurrence of this compressive force, I've used some arrows here.  Imagine that a pressure without any material stress would entail 4 arrows from all directions in this 2D approximation.  In reality pressure acts all around you.  So instead of that, we have some preferential direction where the pressure squeezes from only two sides.  I did my best to illustrate that for the prolate spheroid shape I'm talking about here.  This is briefly representative for the natural state of the asteroids I'm talking about.

You can easily extend the idea to a hole in the center.  About the same amount of net force will be present over a cross section at the equator.  If you drill a hole in the center, that means that there's less area over which to distribute this force.  Logically, that means that the stress would be intensified by the presence of this hole.

There are some finer points to this argument - mainly that in the above model the bubble would have to have some internal pressure.  This is what we're talking about for the gravity balloon.  Specifically, the pressure would have to be exactly enough such that the top and the bottom of the hole in the above image wouldn't experience any compressive stress.  You could change that with a different pressure inside the bubble.  If you increased the pressure in the bubble enough you would induce tensile stress - stress that tears the material apart, not pushes it together.  I have assumed that the pressure set-point would be carefully managed to keep all stresses compressive.

Why would you want to do this?  Because of the failure mechanisms associated with breaking.  If you compressive force doesn't hold up, then you could see some material rearrangement - just like if you had built your sandcastle too high.  That might still not result in loss of atmosphere.  Particularly if you had added some membrane to keep the atmosphere from diffusing into the rock to begin with.  There still remains a danger that whatever material rearrangement happens would destroy some part of that membrane, but it's still a smaller danger compared to failure of tensile stress holding the atmosphere in.  If you relied on the tensile stress of the asteroid, you would risk a catastrophic loss of atmosphere.  This is the same sort of event we concern ourselves with the international space station, or any similar design.  Breaks are generally fatal.

Impact of a Deformed Central Bubble

Similar thought experiments can be used for imagining that a small bubble is deformed in the shape of a prolate spheroid.  Start with the assumption that tensile forces are unacceptable.  Then imagine that we deviate from the spherical shape that I've always talked about for the inner bubble of air.  If you do this, that is effectively adding material around the equator region and subtracting it from the polar regions.

In doing this, we introduce a quadrupole moment.  This behaves as you would expect from a quadrupole field:

generic example of a quadrupole field

But in the case of a gravity balloon, we can only allow compressive stress, by adjusting the pressure of the air bubble lower.  Start out drawing the gravitational field lines from the quadrupole gravitational moment.  Then, due to the compressive stress argument, draw a compressive stress in all places where two arrows point toward each other.  This is what I've done in the image below.  You still have to use your imagination to think of this being an prolate spheroid.

There is an obvious utility to this - because the compressive stresses are in the opposite directions to the stress from the asteroid shape itself.  That means that an odd shaped bubble may be used to less the compressive forces within the interior of the asteroid.  That is exactly what you would do for management of interior forces, building a primitive gravity balloon colony.

In short, the combination of the two above stress diagrams gives a result that is overwhelmingly balanced back to isotropic forces.  That is, just pressure, no material stresses.  Like illustrated below.

gravitational balloon designed to relieve

stress within the asteroid rock

There's no reason to believe this could be done perfectly, but I haven't done the calculations.  I imagine it would be quite an involved project to do so.  Even as we imagine there will be some residual forces, it needs to be considered what the criteria is.  There seems to be every reason to believe that you could start with an asteroid like Eros and establish a colony with breathable air, several kilometers in diameter, all the while keeping the asteroid internal forces less than its natural state.

You would not want to stress the asteroid more than its natural state, because that will give you a virtual guarantee that loss of atmosphere will not happen.  That's the type of guarantee needed to have people seriously consider moving there, and that's why a gravity balloon is a competitive concept for space colonies.

Irregularity of Asteroids by Mass

The very fact that an asteroid is non-spherical proves conclusively that it has some material strength.  It would then be tempting to use its material to hold in an atmosphere, literally with no processing at all.  This would be cheap, but it would also be potentially dangerous as well as unnecessary.  The reason lies in a mass-scale argument.

Looking at what we know about asteroids, we can find that the size cutoff at which most bodies appear highly spherical is fairly close to the point where their internal pressure approaches 1 Earth atmosphere.  Because of this, it would be tempting to imagine that an asteroid's material strength may be nearly sufficient to maintain an atmosphere.  There are a few hazards with that argument, but the main takeaway is that it's not necessary.  Basically, self-gravitation is more useful than tensile strength because we have no reason to believe that an asteroid's natural tensile strength is reliable.  There are more complicated arguments involving the role of compressive strength, but I'll get into those with a later post.

Abundance of Irregular Asteroids by Pictures

Here are a few examples of asteroids at different scales that we have pictures of.  There is a combined imgage out there which preserves the length scale, although this isn't as useful as just looking at the images side-by-side, since I'm interested in their degree of irregularity.

Picture	Name	M (kg)	Center Pressure due to self-gravity (in Earth atmospheres)	Type
	1 Ceres	9.4 x 1020	221	spherical
	4 Vesta	2.6 x 1020	185	borderline
	21 Lutetia	1.7 x 1018	6.3	irregular
	253 Mathilde	1 x 1017	0.27	somewhat spherical
	243 Ida	4.0 x 1016	0.36	irregular
	951 Gaspra	2.0 x 1016	0.24	irregular
	433 Eros	7.0 x 1015	0.12	irregular
	2867 Steins	1.0 x 1014	0.0032	irregular
	4179 Toutatis	5.0 x 1013	0.0027	somewhat spherical
	25143 Itokawa	3.0 x 1010	2.0 x 10-5	irregular

There is somewhat a deficit of information beyond this.  If you want to see more pictures of asteroids (like I do), you might be out of luck, because almost every one of the pictures above represents a major space exploration mission.  There's also a pronounced deficit of information for Earth-like center pressures, since the above tables skips over an order of magnitude between Mathilde and Lutetia.

Note that pressure doesn't follow directly from mass.  This is because the objects have different densities, and I used those when calculating the center pressure.  A lower density will result in a lower central pressure, even for the same total mass.  That is simply a quirk of self-gravitation.

Once we get to the extremely small bodies, they seem to commonly take on the shape of an prolate spheroid.  Beyond one Earth atmosphere, the bodies all seem to conform to a roughly spherical envelope, even though there are a lot of irregularities on the surface.

For those small bodies, it's interesting to note that the speed of rotation also puts a limit on the amount of material stress we could expect from them.  A list of fastest rotating objects shows that some have a rotation on the order of a minute, but these are all several meters in diameter.

Largest Bodies Identified Irregular

Another reference for this is the Wikipedia list of solar system objects by size.  I went through that table and grabbed the largest objects that were identified as irregular.  This is a good approach because it gives a mass figure below which irregular objects start to appear is large number.  However, its major shortcoming is that we have no idea how many corresponding regular bodies exist around their mass scale.

Object	M (kg)	Shape
Proteus (moon)	5.0 x 1019	irregular
Nereid (moon)	3.1 x 1019	irregular
52 Europa	1.7 x 1019	irregular
Davida	4.4 x 1019	irregular
Sylvia	1.5 x 1019	irregular
Cybele	1.8 x 1019	irregular

Reasonable accounting would put the limit for irregularity at somewhere between 10¹⁷ and 10¹⁹ kg.  However, the limit I've found for a habitable inner pressure is around 10¹⁶, and can even be lower than that.  Engineering limits are still more complicated.  Even though bodies in this size range are irregular, they're still roughly spherical in many cases, indicating that material strength doesn't hold up on the scale of self-gravitational forces.  Within the size range we're interested in, the bodies are showing that they withstand some deformation against the equipotential criteria, but exactly how much is unclear.

This evidence does give a clear message - that using an asteroid to hold breathable air would involve both self-gravitation forces, as well as some material pushback.

Centrifugal Field Divergence for a Rotating Planet

Centrifugal Potential is a false potential within the context of a rotating reference frame, which can be thought of as coming from the false Centrifugal Acceleration.  I was reworking through some of the details of this and was going to post a question to Physics Stack Exchange, but it turned out that my knowledge was complete enough to not really merit a question, because I had nothing to ask, so I'll post the work here.

This has application to a rotating planet.  The mathematics for Centrifugal Potential is general to any rotating field.  I'll call it u_r.  Call the angular speed omega, and distance from the axis of rotation rho. The potential can be arrived at several ways, and they all give the following.

I would like to find the Laplacian of this.  This would be the same thing as the divergence of the Centrifugal Acceleration.  With the potential in the above form, if I understand correctly, we have to use the cylindrical version of the Laplacian.  The cords (rho,phi,z) are (distance from axis, azimuth angle, and vertical position).

The last 2 derivatives are zero, and we can expand the first.

I questioned the factor of two, but now I think that's probably correct.  This would be useful to compare this to the divergence of Newtonian gravity which can be found from Gauss' law, which I'll call u_g.

If we use Gauss' law again, then would we be able to make a 100% true statement about the integral of gravity over the surface of a rotating planet?  We would need to take dot product of gravity with the surface, but if it fits the hydrostatic condition then all gravity is normal anyway.

The two volumetric terms only depend on density.

Is it then permissible to write the following?

With this, we have two extremely interesting terms.  I think the most interesting thing is the ratio of them.  For instance, with Saturn, I calculate the rotational term is about 10% that of the gravitational term.  This means that the rotation subtracts 10% of the surface gravitational flux that its gravity creates.

It's hard to extend that statement very much further.  We would be tempted to say that someone standing on the surface of Saturn (ignoring the actual complications in doing this) would weigh 10% less due to the planet's rotation, but that's not completely correct because gravity isn't the same everywhere to begin with.

This does have some application to gravity balloons.  We can apply the concept of Gauss' Law balance to different volumes.  For instance, if we assume that the inner pressurized volume has effectively zero mass, then that implies that a rotating gravitational balloon would have a net outward field on the walls.  That means that over time things would avoid the center and move toward the walls.  That's an interesting effect and it would have a positive impact on stability, causing the Raleigh-Taylor effect to no longer want to mix the walls up.

If you wanted to take this to the absolute extreme case, you could imagine creating artificial gravity on the inside walls by rotating the structure fast enough, while still holding it together through self-gravitation.  In principle, this could possibly work, but to make Earth-like gravity, the mass required would be much greater than Earth itself.

Clover Orbits and Heat Removal

Without engineered solutions, any gravity balloon is thermally isolated.  The 10 km of rock would prevent any significant heat loss by conduction, and there is no other mechanism to get the heat out.  An engineered solution would have to entail some kind of matter exchange.  For small conceptions of the idea, this could probably be done in batch processes, which would also make more sense in the context of access tunnels and the airlock structure.  Once it gets much larger, however, there's a bit of a different challenge.  As the size grows, the habitable volume grows with the cube of the linear dimension.  In light of that, I want to talk about solutions for cooling that scale very well with large gravity balloons.

Pipelines in Zero Gravity

The idea of machines (and industrial society in general) spanning large swaths of volume in zero gravity is an exotic idea.  You can reliably have a process that involves huge exchanges of mass and even huge velocities while at the same time putting very little energy into it, and actually having very little rigidity to it.  The engineered systems in this environment could be large, complex, but fragile.  Imagine a normal train and the sheer impossibility of stopping such a thing, and multiply that concept by a thousand.  People won't even be worried about such huge huge things moving about because it would be so easy to move around it.  This is a true embodiment of the relative-ness of motion.  For a simple example, image a large pipeline of things moving in a giant circle.  This pipeline could be a simple ring, but with a thickness much smaller than its radius.

Moving from the inner surface to the outside surface requires doing work because there is a gravitational field.  If you're moving individual objects it also requires the energy to move in and out of the airlock, but we can imagine a smooth pipe that doesn't have this burden.  You don't necessarily have to put in the work against the field every time.  If the stuff you're moving is already moving sufficiently fast then it can make it out without much problem.  In a simplistic sense, we can imagine a set of tunnels in the rock, some going out and some going in.  After the cooling mass goes out a hole, then it makes its way to the "in" hole moving along the simple geodesic.  In this case, by geodesic, I mean a path traveled in free-fall.

Pragmatically, making a large pipeline of material going through an access tunnel would be quite an engineering feet, but mostly due to the challenge of keeping the air in.  In fact, the challenging of maintaining a pressure boundary over a moving surface is quite common in engineering and there is certainly no simple solution.  However, in the case of the shell of a gravity balloon, there would be abundant space over which to implement this solution.  Ultimately, this could provide an easier way for even people to get from the inside to the outside than by constantly messing with batch-process airlocks.  Another complication is that the shape would have to deform as the speed changes due to gravity.

Orbits Moving In and Out

Interested in how you would actually have to configure these holes, I asked about the problem in a limit case on Physics Stack Exchange.  To make the answer simple enough, we imagine that the rock is thin relative to the overall dimensions of the thing.  That now fits the same problem of a charged particle orbiting in and out of a spherical wire mesh at some voltage.  There was a very intriguing answer to that question.  Basically, there are huge number of ways to configure these odd orbits.  Not only can you have any given number of "dips" outside the shell, but for a given periodic number, you can have any degree of roundness you like.

The most practical case for industry in a gravitational balloon is obvious to me as the 2-exit orbit.  I've dubbed this a "2-gon" in the question on Stack Exchange because it exists within a family of chopped polygons.  This would be an ideal orbit for a pipeline into and out of a gravity balloon because it hits the walls at a high angle, limiting the distance of tunnels needed, and it can have a long path length within the center.  You can see in the question that for a larger number of dips it skirts the edge more, as opposed to making deep secant lines.

The 2-gon orbit image by Physics SE user JJ Fleck  

For gravity balloons of more moderate sizes, a large fraction of the orbit would be spent within the rock itself.  I tried to give some thought to the orbital shape within that rock.  I asked a question about a similar case of something orbiting within a field that was proportional in strength to the radius.  However, I realized that this doesn't fit the shell volume that I'm speaking of here.  The field is linear with radius but it is not directly proportional to radius.  That makes the solution for the orbit rather complicated.  Ultimately, it's sure to be a relatively small revision on what kind of shapes you can expect, but it needed to be mentioned.

The question remains of how practical this would be.  Particularly, how fast would such a cooling pipeline move?  We could use orbital dynamics, but there's another way to cheat and get an answer simply.  In the large case we know exactly what the gravitational field on the surface will be, provided we have the required internal pressure.  In this case the rock density doesn't even matter due to interesting mathematical conclusions.  The field is just:

$$g = 2 \sqrt{ \left( 2 \pi G P \right) } = 0.013 \frac{m}{s^2}$$

With this, I can generally answer the question of the orbit time for a coolant loop.  Again, for the large limit, there is a helpful simplification.  Just imagine the field on the surface to be constant.  Then we can get a time for the turn-around as well as a velocity.  The velocity should probably be kept well below the sonic velocity of the atmosphere, and ideally we would like to see round trip time less than a day or so.  To make this workable I assumed a generic velocity of 50 miles per hour, or 22.3 meters per second.  With that, the height a pipeline will protruce above the surface before naturally falling back down is calculated.

$$1/2 a t^2 = h \\ v = a t = sqrt( 2 h a ) $$

Numerically, plugging in the 50 mph figure:

$$ h = 1/2 v^2 / a \approx 20 km \\ h \approx \frac{1}{2} \frac{\left( 22.3 \frac{m}{s} \right)^2 }{ 0.013 \frac{m}{s^2} } \approx  20 km $$

This is a nice answer because it is on the order of the shell thickness in the first place.  That means that even if the case we're looking at isn't all that big, if the center bubble is just a few times greater than the shell thickness, the idea will be perfectly workable.  Now, with this speed we can imagine the travel times for certain things.  Even for a giant diameter of 250 km, it would still only take 3.1 hours to traverse the diameter at the speed.  You can then imagine multiplying this by 2 for the reverse trip.  Then, for the times spent outside the shell we'd be looking at about half the average velocity.  So credit about 10 km shell thickness, 20 km turn around distance, then a full orbit takes about 30 km of that distance.  This adds about .3 hours.  So for a full pipeline orbit through a 250 km gravity balloon, we'd be looking at a time on the order of about 7 hours.

There are several variables I haven't delved into, like the overall cross sectional size of such a space pipeline, but even with a normal "train" size, such a thing could be used to transport massive amounts of heat.  There are a lot of remaining complications, but it would scale very well.