Asteroids as habitats

[aero]

Hollowed asteroids used as habitats are standard fare in science fiction. Often such habitats are made from a nickel-iron asteroid using heat and internal pressure to expand it into a thin spherical shell. Frequently heat is supplied by focused solar radiation or by nuclear devices.

Is it remotely feasible to make a large habitat by hollowing out an asteroid in this manner or by using any other forseeable technique?

[scienceguy]

not enough gravity

[QuantumG]

Vast amounts of literature on the subject: http://en.wikipedia.org/wiki/Colonization_of_the_asteroids

Asteroids provide two advantages over colonizing "free space" such as the Lagrange points:

 * Materials to build that don't need to be sourced from elsewhere
 * Radiation protection

Spinning the asteroid has been the traditional way to obtain artificial gravity. More recently a number of people have recognized that spinning a structure inside a hollowed out cavity of the asteroid is a lot easier and maintains the low gravity environment of the asteroid on the surface, which has manufacturing advantages.

I wrote about one of the simplest ways to generate artificial gravity - a train track inside an asteroid back in July 2010, http://quantumg.blogspot.com/2010/07/living-inside-asteroid.html

[RichardAKJ]

I wrote about one of the simplest ways to generate artificial gravity - a train track inside an asteroid back in July 2010, http://quantumg.blogspot.com/2010/07/living-inside-asteroid.html

What happens when the train breaks down?

[kch]

I wrote about one of the simplest ways to generate artificial gravity - a train track inside an asteroid back in July 2010, http://quantumg.blogspot.com/2010/07/living-inside-asteroid.html

What happens when the train breaks down?

Everybody has to be re-trained? 

[QuantumG]

What happens when the train breaks down?

You fix it?

Not sure I get the question.. the value of the train is that you can imagine getting it running within the first few months of arriving at the asteroid*. That's particularly important if you accept some finite limit to the amount of time humans can spend in zero-g before deleterious health effects kick in.

* The assumption here is that the trip out to the asteroid is brief (no more than 6 weeks) and done in zero-g, with limited radiation protection. If you have a ship with artificial gravity already, then you just need a nice cavity inside an asteroid to park and get the radiation protection. If you have both then all you get from the asteroid is resources.

[aero]

Yes - This is exactly how most sci-fy goes, jumping straight to what has been made and how it is used, totally ignoring the challange or impossibility of making it in the first place. I was hoping for this thread to focus on how the hollow asteroid became a hollow asteroid, not so much on how it was used after it became hollow.

Might it be feasible to guide two modest sized asteroids, each with a similar size crater together so that one crater capped the other and the weak gravity held them together? If not, how much super glue is needed?  Would one cratered asteroid, capped with a moderately flat asteroid be a suitable start to a habitat? How about a larger asteroid with a crater capped by a small nickel-iron asteroid that had been heated and spun into a disk?

[QuantumG]

1. pick an asteroid with an upcoming close approach to Earth
2. send a tunnel digging robot and supplies on a long duration flight
3. dig tunnels
4. flight of the humans to the asteroid
5. setup minimal artificial gravity and life support systems
6. begin excavation of the asteroid and in-situ manufacturing
7. expand the artificial gravity and life support systems to support a growing population
8. continue excavation and construction towards "worldlet" goal
9. develop propulsion systems to take control of the worldlet, so it can be directed towards other asteroids
10. use other asteroids as resources or create more worldlets

Expand humanity ever outwards, colonizing the Oort cloud and eventually to the stars.



Enjoy.

[jedsmd]

Yes - This is exactly how most sci-fy goes, jumping straight to what has been made and how it is used, totally ignoring the challange or impossibility of making it in the first place. I was hoping for this thread to focus on how the hollow asteroid became a hollow asteroid, not so much on how it was used after it became hollow.

Might it be feasible to guide two modest sized asteroids, each with a similar size crater together so that one crater capped the other and the weak gravity held them together? If not, how much super glue is needed?  Would one cratered asteroid, capped with a moderately flat asteroid be a suitable start to a habitat? How about a larger asteroid with a crater capped by a small nickel-iron asteroid that had been heated and spun into a disk?

You might want to Google "Project Plowshare - Peaceful Nuclear Explosions (PNE)".  The US set off a series of 27 atomic bombs to see if they were practical for excavation of harbors and such.  Should be applicable to your project.

[QuantumG]

Gah! What do you imagine these things are made out of?

Consider 25143 Itokawa, visited by Hayabusa. It has a mean density of 1.95 g/cm^3. This is about the same as the density of natural gravel.. which makes perfect sense when you look at it.

You don't need nuclear bombs to dig a hole in this stuff.

[jedsmd]

Gah! What do you imagine these things are made out of?

Consider 25143 Itokawa, visited by Hayabusa. It has a mean density of 1.95 g/cm^3. This is about the same as the density of natural gravel.. which makes perfect sense when you look at it.

You don't need nuclear bombs to dig a hole in this stuff.

Agreed a Tunnel Boring Machine is a far more practical approach.  But AERO wants to hollow the entire asteroid and possibly reshape it and spin it in the processes.  When you consider the amount of material you need to move to complete his vision - nuclear may be the way to go.

[scienceguy]

Gah! What do you imagine these things are made out of?

Consider 25143 Itokawa, visited by Hayabusa. It has a mean density of 1.95 g/cm^3. This is about the same as the density of natural gravel.. which makes perfect sense when you look at it.

You don't need nuclear bombs to dig a hole in this stuff.

If they're like gravel, how will the tunnels hold together after you've tunneled through them?

[aero]

Gah! What do you imagine these things are made out of?

Consider 25143 Itokawa, visited by Hayabusa. It has a mean density of 1.95 g/cm^3. This is about the same as the density of natural gravel.. which makes perfect sense when you look at it.

You don't need nuclear bombs to dig a hole in this stuff.

If they're like gravel, how will the tunnels hold together after you've tunneled through them?

And the material must be strong. Remember it has to withstand pressure - not so much force if there are only small diameter tunnels, but quite a lot of force if the cavity is large.

[QuantumG]

Agreed a Tunnel Boring Machine is a far more practical approach.  But AERO wants to hollow the entire asteroid and possibly reshape it and spin it in the processes.  When you consider the amount of material you need to move to complete his vision - nuclear may be the way to go.

Agreed, it's a nice to have, but you can imagine doing without if necessary.

If they're like gravel, how will the tunnels hold together after you've tunneled through them?

What forces do you imagine hold the asteroid together now?

It's not gravity, that's much too weak.

[jedsmd]

Gah! What do you imagine these things are made out of?

Consider 25143 Itokawa, visited by Hayabusa. It has a mean density of 1.95 g/cm^3. This is about the same as the density of natural gravel.. which makes perfect sense when you look at it.

You don't need nuclear bombs to dig a hole in this stuff.

If they're like gravel, how will the tunnels hold together after you've tunneled through them?

On earth soft bore tunnels are normally lined with concrete.  If there is ice in the asteroid a frozen ice liner might be practical.  But I think QuantumG is right, in that low of gravity a liner isn't necessary as lateral soil pressure might be next to non-existent.

[QuantumG]

If you wanted to pressurize (with air) the tunnels you'd need a liner.

[jedsmd]

If you wanted to pressurize (with air) the tunnels you'd need a liner.

Yes and steel or other tension reinforcement to resist the pressure - ugh that will probably have to come from Earth.

[QuantumG]

If you want to do it early, sure. Resupply from Earth is not so bad, you can do that with long lead times and electric propulsion (like all the asteroid missions to-date).

Do it later, when in-situ manufacturing is happening and local resources of metals can be processed, and imagination becomes the biggest limit to what future generations can achieve.

Taking the long view, there will always be something that it's better to get from Earth than it is to produce locally.. so what can asteroid colonists produce better than people on Earth? Nothing, right? I disagree. They can make stuff in space. So long as launch from Earth remains costly, and people on Earth remain interested in manufactured items in space, those colonists already in space will always have valuable goods.

Maybe that's not so clear. So here's an example: spoons. Even today, people use spoons in space. Sometime in the future there's a need for 1 kg of spoons in Earth orbit. We could launch it from Earth and pay, let's say, $500/kg in launch costs (what an amazing achievement!) or we can put it on the slow boat from the asteroid colony. It's not hard to imagine the marginal cost of transporting those spoons from an asteroid colony to Earth being a lot cheaper, because the energy requirements are so much lower.

[A_M_Swallow]

The tunnel and asteroid will also have to take the force of the train passing.

We may be able to sinter asteroid material to make tunnel liner.  A weak material simply means the liner has to be thick.  If sintering does there are other ways of melting and setting material.

[QuantumG]

The tunnel and asteroid will also have to take the force of the train passing.

Not really. "Train" is an analogy.. we're talking about a ring of steel with a carriage rolling around the inside. The track is transported in parts and put together. It would work fine on the way to the asteroid too, but I think there's more value in a fast transit in zero-g to get to the asteroid than artificial gravity in transit because you can send the track and the train on a slow electric propulsion cargo flight, reducing the mass required on the high impulse crew flight. There's a tradeoff.

[Andrew_W]

You drill down to the center of the asteroid, insert a ~1 km diameter spherical liner and inflate, then you just bring your habitation modules inside the liner, tie them together and rotate.

For a small asteroid the gas pressure inside the liner is going to be well under 1 psi.

You still need the liner, even if the expanded asteroid was self supporting, to keep dust and other debris out of the environment.

you'll probably want to use mirrors and a transparent panel to let sunlight into the habitation volume.

[A_M_Swallow]

The tunnel and asteroid will also have to take the force of the train passing.

Not really. "Train" is an analogy.. we're talking about a ring of steel with a carriage rolling around the inside. The track is transported in parts and put together. It would work fine on the way to the asteroid too, but I think there's more value in a fast transit in zero-g to get to the asteroid than artificial gravity in transit because you can send the track and the train on a slow electric propulsion cargo flight, reducing the mass required on the high impulse crew flight. There's a tradeoff.

Unless you have 2 trains, preferably 3, the entire tunnel system will move from side to side as the train passes.

[douglas100]

I would avoid rubble piles and go for monolithic asteroids. (This requires some close up survey work, of course.) I'm not convinced the gravity of even a small asteroid would not cause a slow motion cave in of the cavity dug out. I don't think we know enough about this to be sure. Nature has nasty ways of surprising us.

[QuantumG]

A "slow motion cave in" eh? Maybe you should rethink that.

[douglas100]

Please explain.

[revprez]

Taking the long view, there will always be something that it's better to get from Earth than it is to produce locally.. so what can asteroid colonists produce better than people on Earth? Nothing, right? I disagree. They can make stuff in space. So long as launch from Earth remains costly, and people on Earth remain interested in manufactured items in space, those colonists already in space will always have valuable goods.

Maybe that's not so clear. So here's an example: spoons. Even today, people use spoons in space. Sometime in the future there's a need for 1 kg of spoons in Earth orbit. We could launch it from Earth and pay, let's say, $500/kg in launch costs (what an amazing achievement!) or we can put it on the slow boat from the asteroid colony. It's not hard to imagine the marginal cost of transporting those spoons from an asteroid colony to Earth being a lot cheaper, because the energy requirements are so much lower.

Let's say you can get down to $500/kg lift to LEO.  If in-space transit costs are negligible and you can deorbit for about a thousandth of a percent of that, space-based transport competes with terrestrial (which is on the order of $0.01 per ton-km).  This is first order guesswork, but it does neatly separate the problem of competing origination costs from transport ones.

[Andrew_W]

Another approach would be to build a torus shell from blocks of asteroidal material made using solar sintering, a spin your small habitat modules inside the torus shell as it orbits the asteroid while the proper colony is constructed.

[manboy]

I always wondered if a centrifuge would unintentionally change the natural rotation of an asteroid.

[ArbitraryConstant]

Vast amounts of literature on the subject: http://en.wikipedia.org/wiki/Colonization_of_the_asteroids

Asteroids provide two advantages over colonizing "free space" such as the Lagrange points:

 * Materials to build that don't need to be sourced from elsewhere
 * Radiation protection

Spinning the asteroid has been the traditional way to obtain artificial gravity. More recently a number of people have recognized that spinning a structure inside a hollowed out cavity of the asteroid is a lot easier and maintains the low gravity environment of the asteroid on the surface, which has manufacturing advantages.

I wrote about one of the simplest ways to generate artificial gravity - a train track inside an asteroid back in July 2010, http://quantumg.blogspot.com/2010/07/living-inside-asteroid.html

I've often thought along these lines - the main challenge I've thought of is heat rejection from inside the asteroid. Reflecting sunlight in without empty space around to radiate the heat will warm up pretty quickly, won't it?

[alexterrell]

I put something in here, including a sketch
http://forum.nasaspaceflight.com/index.php?topic=18759.msg730853#msg730853

A lot of asteroids are rubble piles with zero tensile strength. You bury an inflatable habitat and then inflate. The habitat can then be primarily assembled on Earth.

As for the train idea, you can extend this into a torus with a very small inner radius (c 5m) so you still have access, and a large enough outer radius (25m) to give useful artificial gravity.

[alexterrell]

I've often thought along these lines - the main challenge I've thought of is heat rejection from inside the asteroid. Reflecting sunlight in without empty space around to radiate the heat will warm up pretty quickly, won't it?

If your using sunlight, filter out the IR first.

If you have some way of transferring liquids in and out, you can remove stale air, clean it, and reintroduce clean chilled air or even liquid air.

[neviden]

I don't see why it would be impossible to make.

First, you find nice (preferably ruble pile) asteroid that is made of iron. Use magnets to separate iron from the rest of things. Melt the iron with concentrated solar energy. Make a thick iron plate. Weld plates together to form a large tube. Rotate the tube. Fill the tube with air. If you would need protection from radiation, dump few meters of soil on the inside (preferably in some form of concrete). If you need sunlight, use mirrors and windows.

After that you can walk inside of this large space station and do whatever you would do in a large empty building on Earth that has 24/7 light. Nice house, garden, workshop, factory, shop, hospital,...

[AlanSE]

I've spent some time on related concepts, and I find a lot of strange statements in this thread.

Saying that asteroids are like "gravel" is a helpful analogy, certainly.  However, the talk about cave-ins looks pretty thoroughly incorrect up to this point.  If we assume that asteroids are made of rocks roughly the size of gravel, then yes, digging tunnels will be difficult.

But there's no reason to think this.  Asteroid macroporosity is a fascinating topic.  If you ask how large the pores are, however, we don't really know.  We still have good theoretical arguments in different directions.  As instructive example are binaries that were tidally locked, and then got close enough to make contact - what I think of as "kissing" asteroids.  This formation is probably very common, and it's not unexpected.

As we categorize more and more asteroids, we find a huge fraction are highly irregular.  These have ratios of length to width of 2-to-1 or so very frequently.  It's about as likely to find a highly irregular asteroid as it is to find a regular one, unless you look at the huge ones like Vesta or Ceres.

A collapsed binary pair is important for two reasons:

 - it gives a lower limit on the rock strength (due to resistance against its own gravity)
 - it tells us how fragments might have clumped together

In other words, the fact that we have lots of highly irregular and lots of binary asteroids gives us a picture of their evolution, where they're kind of two sides of the same coin.  That's important, because it indicates that larger bodies are likely made of parts roughly the size other asteroids.  Or at least some of its parts will be that size, and the entire rubble pile might be made of a continuous distribution of sizes.  For the technically inclined, I will note that the majority of the mass of the asteroid belt is contained in larger bodies.  There are more smaller bodies, but there is more mass from larger bodies.  So if you clumped together random bodies from the asteroid belt into a rubble pile, large fragments would dominate, QED.

What I'm trying to say is that there may be no tunnel to build.  In other words, you can float through the spaces between the rubble straight to the center of the body.  There are no concerns about cave-ins, because these caves have been there for millions or billions of years.  At some locations in it, I don't doubt we will find gravel-like rubble, but nature is much more messy than to make the entire thing out of this.

Talk about the "liner" is meaningful, however.  If there are spaces in-between the rocks that allowed you access to the center, then obviously you have to close off those spaces to hold any kind of gas inside.  You probably don't have to worry about the gas permeating large monoliths, but they could be fractured anyway.  That's what you'd cover with a liner.  I've recently been arguing that this makes sense to hold air, and there are almost certainly km-scale cavities where this would work:

http://gravitationalballoon.blogspot.com/2013/11/why-not-live-in-empty-spaces-inside.html

QuantumG made some good points, but regarding artificial gravity, I don't think it needs to be that complicated.  If you've sealed off a pressure boundary then you can put a rotating structure inside that boundary.  There's some human limit of tolerance for Coriolis forces, so you'd prefer to make them large, but you're limited by air resistance.  But that can be engineered around.

[cordwainer]

The easiest way might be to find a fast spinning asteroid and build conning towers along its axis of spin to provide artificial gravity environments. Then slowly build a ring or torus between the towers to create a larger habitat. Though I like Larry Nivens idea of inflating an iron-nickel asteroid into a hollow ball, I don't think it would be very practical or easy to engineer such a feat.

[AlanSE]

The easiest way might be to find a fast spinning asteroid and build conning towers along its axis of spin to provide artificial gravity environments.

The structural needs scale with the radius of the circle that you travel in artificial gravity rotation.  In other words, you picked about the worst possible point available.  Now, maybe you thought that starting from GEO gives some kind of benefit.  I wondered the same thing, and I assure you, it doesn't.  Having a gravitational body in the middle of a tethered rotating system doesn't help.  It makes the material needs more, not less, because the material constraints still scale with radius, because that is purely geometric, because the acceleration is a set constraint to be Earth gravity (or close to it).  You could claim that the scheme reduces Coriolis forces.  This is because it spins with a slower speed than without the asteroid in the center.  But the asteroid's gravity is small to begin with compared to Earth gravity.

Not only do the structural needs balloon, but you trashed all the radiation shielding benefits from the asteroid.  It's true that you get rotation from the spin of the asteroid itself, and these commonly have day-lengths of 4 to 8 hours.  An asteroid's rotation can certainly be used as a catapult or to impart angular momentum to other things.  But rotating a space station is not the hard part.  You can do that with a simple electric motor, a wheel, and some tensioners.  I can't identify a problem that your proposal solves.

[DavidEarlAgoura]

Long Term Living in low G environment
So here's My Idea. I've never heard of anyone else suggesting it so maybe I'm the first?

  As for long term living in a low g environment like an asteroid, space station or moon: You need 1 G gravity to stay healthy and have children. So, we don't need a train going round in circles, because, lets be honest here, that's Really impractical. Especially for a space station or any colony of significant size. It also makes a free floating wholly artificial space station much more expensive to spin the whole thing up (LOTS more reinforcement needed) than it would if only the residences and workplaces were individually spun for gravity.
  All we need to do is build peoples homes/workplaces/wherever in the shape of a toroid (donut) shape and put a spin on them so it creates a pull of 1 gravity inside. Surround the building with several concentric circles of metal or another sturdy material and float them on magnets. Put them at an ever increasing angle and speed, towards the house, and the resident can match speeds with the house or his/her workplace by simply stepping from one to the next till they get to the innermost ring that has a matching speed to their home/office/wherever. It would be like walking up or down steps.
  This way, the only time you would actually experience micro or low gravity would be going to and from your home and the office or where ever else you were going. You would live, work and sleep (1/3 of our lives) in a 1 gravity environment. It's a LOT cheaper and faster to build, and a WHOLE lot safer than spinning up a large structure like an entire space station or asteroid.
  A spinning space station has the extremely difficult problem of not becoming unbalanced and literally tearing itself apart. Think about what happens when a spinning top becomes unbalanced and starts jumping all over the table.
  This probably wont be needed on Mars, but may be needed on Earths Moon if Lunar gravity is not enough to keep humans healthy or able to have children.
  It would certainly make it easier to do something like start using the international space station as a starting point and just keep adding modules for work, manufacturing and living spaces, As opposed to the Massively, Staggeringly large amounts of money needed to build something like a Stanford torus that cant be used until it is completed. Adding/creating 1 g living/working spaces one at a time, as needed, is much cheaper, safer and more practical than a single mono-structural station that would require Huge budget approval by the ever changing, multiple administrations and congresses.
  Any one of which could kill the whole project at any time during the years and possibly decades it would take to build one.
  My way, it grows as needed and as we can get the funding for it on a case by case basis. By making it cheaper, it brings space habitats into the range of private corporations and individuals.
I hope someone who is involved in these sorts of things considers my ideas.
Cheers!

[zodiacchris]

Hi David,
Welcome to the forum with you first interesting post. Without going into too much detail, just two things to ponder regarding your concept of small, house sized individual spun habitats.
-I would recommend you read up on Coriolis force and the associated side effects in small structures. If to small, you'll have a very miserable time in your small habitat and your inner ear will not be impressed...
-We don't know if we need 1g to live and procreate, we might well be okay in 0.5 or 0.3 g, like we have on Mars.

Cheers,
Chris

[DavidEarlAgoura]

So here's my ideas on living IN an asteroid.
  The last thing we want to do is damage the structural integrity of the asteroid by melting it. and we wouldn't need a liner, or at least not a very thick or expensive one.

  If we used a large, nickel/iron or crystal iron asteroid, one that was probably part of a planet that got smashed up during the early days of the solar system when the inner solar system was something like a demolition derby and there were a LOT more planets/planetoids/dwarf planets than there are now.

Every time someone finds a nickel iron or crystal iron meteorite, their holding what used to be a part of the crystal iron core at the center of a planet that got smashed apart soon after the birth of the solar system.
 The benefits to using a solid crystal iron asteroid is that since it formed under the intense heat and pressure at the center of a planet, it's likely to be one giant iron crystal, and VERY strong and NOT porous.
We could choose a large asteroid, miles or even tens of miles (hundreds of miles?) wide and long, and then, using giant solar mirrors spun for rigidity, focus sunlight on the asteroid and ablate the surface until it is cylinder shaped with dimensions something like a can of soda.
  Then, still using the mirrors, core it out, down its horizontal center. If the asteroid was, say, 50 miles wide and 100 miles long, and the cored hollow section was 5 miles wide, that would leave 22 and 1/2 miles of solid iron crystal on all sides to protect the interior settlement from small ( and even large) asteroids and radiation.
And if we left, say, 5 miles of iron on each end as a buffer between the inside habitat and the asteroids and radiation of the outside universe, that would leave us with a living space with a radius of  15.7 miles and Ninety Miles Long. That’s an interior living space of more than FOURTEEN HUNDRED SQUARE MILES! Comparatively, the city of Los Angeles with a population of 3.8 million people,  is only 469 square miles.
That’s enough room for a LOT of people.
Once cored, the mirrors could be used to ablate/etch a varied landscape (hills, rivers, lakes, etc) on the interior. Then, a layer of carbon, easily obtainable from comets and carbonaceous chondrite asteroids, refined using focused sunlight from mirrors, could be put down on the inner surface to insulate the habitat against the cold of the interior of the asteroid.
  Once cored, landscaped and insulated, and with one end left open, large chunks of stony asteroids already carved/landscaped themselves (by the mirrors again) could be maneuvered in and attached to the interior,  with some ground up to make soil for plants to grow in (plants LOVE ground up stony meteorites).  Comets, which are rich in organic molecules and would help build living soil, could be brought in for air and water.
  Once everything is in place, the end chunk of the section that was removed from the core could be cut off and put back as a plug and (using mirrors again) welded back into place to seal the interior.
  Then, the asteroid could be spun up so the interior surface of the habitat would have 1 G. Various methods could be used to spin it up.  Perhaps using mirrors again, to drill holes at an angle into the asteroid. As the iron is vaporized it would expand outward and act like a rockets exhaust and spin up the asteroid.
Fixed onto the outside of the asteroid,  mirrors with lenses that connect to fiber-optic lines routed into the interior, could be used to gather sunlight and transfer it to the habitat providing light and heat. And a look at the outside world.
The same process could be used with the core that was removed to make a smaller version, and perhaps even one more from that ones core to make an even smaller version of it.
  I sincerely hope I live long enough to witness something like this happen.

Best!

[momerathe]

Depending how far in the future we're talking, it may be easier for us to adjust ourselves the the habitat, than the habitat to ourselves; to genetically engineer traits for radiation protection and zero-g living.

[Nilof]

I'm not convinced that we will be digging ourselves into asteroids. It gives some extra shielding potential, but it really makes artificial gravity a PITA, and frequent activity around an asteroid that you can dig down into will create a very dirty environment with asteroid dirt flying around everywhere.

My general opinion is that one is better off avoiding schemes like this and just use the asteroid for resources. For early habitats, bring inflatable habitats from Earth, surround them with properly sealed bags of gravel from the asteroid for shielding, and use a tether and counterweight system for artificial gravity in free space near the asteroid. The marginal benefit of digging yourself down early just does not outweigh the benefits of a pristine vacuum, easy access to sunlight, and the ability to plan ahead easily.

By digging yourself into an asteroid, you can run into unexpected problems, such as metallic dust from the asteroid cold welding itself into critical parts, sharp-grained dust getting on your solar panels and inside your habitats, unpredictable thermal changes, ect ect. I expect free space habitats with asteroid material as shielding is likely to become the superior option much like buildings above ground are generally both cheaper and more practical than buildings below ground on Earth.

[DavidEarlAgoura]

Hi Chris! Thanks for the input. I did some research on Coriolis Force (see "Principles of Clinical Medicine for Space Flight").  Most research on Coriolis Force complications and negative effects done was by the Russians in very small capsules about 10 meters in diameter. Even then they noticed that the Cosmonauts seemed to be adapting to the disorientation after a while.
 
What I envisioned was something about the width/diameter of 2 land sea containers side by side (about the size of a double wide mobile home here in the U.S.) about 6-8 meters wide and 3 high,  curved around in a cylinder (looking like a donut) and probably broken up into several separate living quarters but with one entrance for the free floating space hab. The structure would probably have an exterior diameter of 50 meters or so. At that diameter, based on what I've read, Coriolis Force shouldn't be an issue as a 6 foot (2 meter) tall man standing would have his head be about 23 meters from the central axis of the wheel/hab.
 For moons and asteroids square footage is probably not an issue as if we go there to stay we will have learned to use local materials to manufacture habs. They will almost certainly be underground so radiation and asteroid strikes probably won’t be an issue. Which means all they would need is something to seal in the pressure and heat (probably spayed on the exterior walls) and then just something to enclose their residence. I imagine it would be like living in a REALLY long, thin house, albeit, a curved one.
It's looking more and more like any large structures put in space in the near future will probably be pneumatically reinforced, inflatables as its easier and cheaper to ship them.  Probably filled with water (as it also shields radiation) possibly with an additive to automatically "clot" any small punctures. When frozen at 250 F below zero, (shielded from sunlight) water ice is harder than steel. If it was even 25 centimeters thick it would shield against almost all micro asteroid impacts.  The designs I have heard of proposed an exterior of frozen hard ice at -250F with another layer underneath that of liquid water (possibly with the “clotting” additive) so that if something does manage to puncture thru the hard ice layer,  the liquid layer below would be forced up by the vacuum and freeze/foam/clot  and seal the damage.
Larger inbound objects will probably be targeted and ablated with lasers before they can be a problem. NASA and DARPA are working on that sort of system as we speak to deal with all the space debris floating around in space right now.
  Blame the Chinese for a lot of that as they blew up a perfectly good satellite just for “target practice”. But mostly, to let everyone else know they can target and destroy any satellite they want, whenever they want. The problem with that is it risks the “Cascading Catastrophe” scenario where the debris from the destroyed satellite hits and destroys other satellites, their debris hits still more and so on and so on until all of near earth space is completely off limits to us and no satellites remain intact and no more can be launched.  The results would be catastrophic not just for a completely cancelled space flight of any kind but with the GPS satellites destroyed, airplane travel would effectively end for the foreseeable future.
I’m in California and got to get some sleep.
Cheers!
P.S. you may get a kick out of this, see my website @ donearlslates.com. I’m in the motion picture industry and live near Hollywood. My dad designed the modern slate/clapperboard.
Night!

[llanitedave]

I'm not convinced that we will be digging ourselves into asteroids. It gives some extra shielding potential, but it really makes artificial gravity a PITA, and frequent activity around an asteroid that you can dig down into will create a very dirty environment with asteroid dirt flying around everywhere.

My general opinion is that one is better off avoiding schemes like this and just use the asteroid for resources. For early habitats, bring inflatable habitats from Earth, surround them with properly sealed bags of gravel from the asteroid for shielding, and use a tether and counterweight system for artificial gravity in free space near the asteroid. The marginal benefit of digging yourself down early just does not outweigh the benefits of a pristine vacuum, easy access to sunlight, and the ability to plan ahead easily.

By digging yourself into an asteroid, you can run into unexpected problems, such as metallic dust from the asteroid cold welding itself into critical parts, sharp-grained dust getting on your solar panels and inside your habitats, unpredictable thermal changes, ect ect. I expect free space habitats with asteroid material as shielding is likely to become the superior option much like buildings above ground are generally both cheaper and more practical than buildings below ground on Earth.

I agree.  asteroids are resource bodies, living in one would be like living in a mineshaft that you're continually excavating around yourself.  Far better would be a free-floating habitat made of standardized parts, some of which might well be constructed using resources from the asteroid at hand.  The other advantage of the habitat is its mobility.  You can reposition the habitat to have the asteroid's bulk block it from the Sun in the case of a large mass ejection event.  Otherwise, that bulk will block a reasonable number of cosmic rays from all-sky sources.  And when the asteroid is played out, as they may happen after some period of mining (some asteroids may be more resource-rich than others), the entire habitat can be moved to another asteroid in the belt with a minimum of delta-V requirement.  This will allow us to exploit a huge number of asteroids eventually, rather than just having to choose one and stick with it.

[Hanelyp]

If I were designing a structure to be built from raw asteroidal material, even from an iron rich asteroid, I'd assume the material had negligible tensile strength until examined and demonstrated otherwise.  Just carving a hollow in an asteroid and expecting it to hold together when you spin it up for artificial gravity or fill with air is unlikely to work out.

[QuantumG]

Well, you need at least 10 tons of shielding per m² to protect your population from radiation. So, if your goal is a multi-generational colony, you will be tempted to at least start building inside the asteroid. The first problem you'll encounter is the availability of natural sunlight for growing crops. I think the solution to that is simply to genetically engineer crops for zero gravity and high radiation tolerance. This leads to remotely tended farming, which I think makes more sense anyway.

If you go with a cylindrical design with rotational stability, ala KalpanaOne, the surface area is 2πrh (ignoring the end caps) and h = 1.3r, so the total liveable area is 8.17r² m². If you consider 200 m² to be the average living area of a family unit (which is quite luxurious), then r = √24.48n. Where n = 1200 say, r = ~172m. The removal of farming area from the colony reduces the required radius by almost a half.

[Robotbeat]

Or just chemosynthesis. Methane and ammonia and oxygen (with minor amounts of other minerals) into edible food grown in big vats. Here: http://www.unibio.dk/

That could make up bulk of calories. Then, just grow the rest via LED growlights.


...Shielding is fairly easy. Grind up regolith/rocks to gravel and put it into sacks. (The sacks, if you prefer, can be made of rockwool produced from the regolith.) But rock makes poor shielding due to the low hydrogen content. It produces lots of secondaries, so you have to make sure you use LOTS of shielding to also stop the secondaries. But sacks of regolith scales pretty well if you're not going anywhere.

[QuantumG]

I think what it comes down to is the size of the cavity inside the asteroid. If you're tempted to rotate your habitat mere meters away then it will have the same failure mode as Lewis One - crashing into the shielding - but I think that temptation is driven by a lack of shielding material. If you have an abundance of material, which obviously is the goal of co-locating your habitat with an asteroid, then a static shield with some stand-off distance is more desirable than packing the shielding into the hull as your habitat doesn't have to carry all that weight, you don't have to expend power spinning it up, etc.

[AlanSE]

How much do you reasonably think that removing the shielding requirement will decrease the mass per unit area metric? Various optimized shielding options in ideal orbits around Earth can be as low as 1 to 2 ton/m^2, according to papers that even you have recently shared on this forum. 5 ton/m^2 would be more probable, particularly if you didn't have a choice of the materials. Going all the way to 10 ton/m^2 seems pedantic because nothing about our biology says that we need sea-level shielding, and plenty of people live with less attenuation from the atmosphere anyway.

So let's assume we get rid of that. The floor still needs to support the artifacts that people like to live with. It also has to support the people themselves. If you take the shoe contact area of a normal person versus their weight, that itself is around 2 ton/m^2. Since the floor is rigid you don't need this specifically (local members distributes the weight), but you still need enough mass so that walking and running about won't deform the circular shape. If you wanted a ridiculous lower limit, it needs to be at least (70 kg)/(65 m^2) = 1 kg/m^2 to support the humans themselves.

Provided that you have a pressurized colony, the hoop stress will be the primary thing keeping the shape. You still need to resist the stress from the air itself, and this will probably dominate the loading. Sea-level pressure still "feels" like 10 ton/m^2 even if the wall materials weigh far less than this under the artificial gravity.

All we need to do is build peoples homes/workplaces/wherever in the shape of a toroid (donut) shape and put a spin on them so it creates a pull of 1 gravity inside. Surround the building with several concentric circles of metal or another sturdy material and float them on magnets. Put them at an ever increasing angle and speed, towards the house, and the resident can match speeds with the house or his/her workplace by simply stepping from one to the next till they get to the innermost ring that has a matching speed to their home/office/wherever. It would be like walking up or down steps.

Magnetic bearings are:

1) not naturally stable
2) best for high speeds

Concentric walls which have a staged relative velocity difference are about the last thing you would use in conjunction with magnetic bearings. This why O'Neil proposed one single track with magnetic bearings for super-massive rotating colonies that couldn't be supported due to limitations in material specific strength values (he was too early to be on the nanotube bandwagon). But he only proposed 1 stationary wall and 1 moving wall. Then you produced 1 g at a large curvature with magnetic bearings because that's what it works for. This scheme could produce huge areas along with virtually insignificant Coriolis forces - all while having no global material strength constraints.

The only problem is that for every square km of living space, you have to manufacture a crazy array of magnets which are all on active control of one form or another and probably use electromagnets. There is no magic bullet passive scheme to obtain magnetic levitation. You can use inductive windings, but you'll add more power dissipation and these will all only be conditionally stable anyway. For super large structures, I'm personally convinced that you'll also require active control of positioning of all the magnetic "rails" because the structure is so large it can't be thought of as a solid structure either.

If you take this scheme and just multiply it by multiple repeated layers you don't buy yourself anything. The appeal of magnetic bearings in vacuum is that they're mostly insensitive to speed. If the fields are mostly symmetric in the direction of motion, then the electrons can't tell the difference between one velocity and another.

[nadreck]

Now I certainly have a biased interest in icy asteroids because I believe that in the next 50 years they will be the most valuable resource as source of water that can become fuel, breathable air, water which will make up a large part of grown biomass for food and fibers in extra terrestrial settlements. This, by mass, will be more in demand early on in our exploitation of inner system space than other materials, which will existing in some quantities on an icy asteroid anyway.

While I can imagine some romantic and colourful space created inside a hollowed out space iceberg, with atmosphere etc. an ice asteroid would not be strong enough to rotate for a significant centrifugal force, but, if one hollowed out a large space in an icy asteroid that one wanted to move, attached a very solid anchoring mass at the axis of a torodial spinning space station with a stationary central axis, you could use the excellent shielding properties of all that hydrogen in ice.  The spinning station could be engineered to be part of the orientation control of an asteroid (and whether you intended it as such you have to account for it in planing the effect of the spinning mass inside on the orientation/spin of the asteroid in question) and I only see such an asteroid being of real value placed (eventually) in orbit of Earth, Mars, Venus, one of the Jovian moons, or, in the very long run, somewhere useful in the asteroid belt as a station that provides fuel and other products to transiting ships.

I would envisage the hollowed out space to have an opening at one end large enough to bring the largest of ships in and plenty of space and non spinning (anchored to the central axis) working space to dock and support transiting craft while keeping them sheilded

Obviously to keep these asteroids from subliming away when they are brought to the inner system they will need some sort of reflector to keep the sunlight off the surface, to save mass that needs to be brought along it only makes sense to me that you constantly have active control of the asteroids orientation (besides which, however you moved it in the first place required orientation control along with what ever propulsion system was involved).

[Nilof]

I think what it comes down to is the size of the cavity inside the asteroid. If you're tempted to rotate your habitat mere meters away then it will have the same failure mode as Lewis One - crashing into the shielding - but I think that temptation is driven by a lack of shielding material. If you have an abundance of material, which obviously is the goal of co-locating your habitat with an asteroid, then a static shield with some stand-off distance is more desirable than packing the shielding into the hull as your habitat doesn't have to carry all that weight, you don't have to expend power spinning it up, etc.

One interesting special case to consider is a Phobos colony. Because Mars takes up such a huge part of the sky, the Mars-facing side if Phobos is very well protected against radiation. You could have a tidal tether with a counterweight hanging down towards Mars to allow the habitat to be suspended over the surface with a few hundred meters of clearance. It could also provide torque for gyroscopic precession so that the direction of rotation of the habitat is constant with respect to the surface.

Mars and Phobos provide the shielding in this case, and you only need a thin annulus if you want to shield against the rest of the GCRs, which would admittedly not be too dangerous. This also means that a Kalpana One-type habitat can be made wider without being rotationally unstable, since you don't need heavily shielded endcaps.

The most efficient solution for power generation is likely to put solar panels on the tidal counterweight. The orbit of Phobos is tilted by 26 degrees to the ecliptic, so the sunlight there should be the same as for any low Mars orbit if it hangs down low enough. Better than at Ceres distance or Mars surface, but worse than LEO or high Mars orbit.

[kato]

...Shielding is fairly easy. Grind up regolith/rocks to gravel and put it into sacks.

That bears the question whether it wouldn't be easier to first build your colony separately and grind up the asteroid to build a concrete-like shield around it. Way simpler than hollowing it out or anything like that anyway, because a tunnel-boring machine for a 1-km wide hole would probably have a mass of 10^6 tons by order of magnitude. Minimum.

We basically just need an asteroid with water, calcium, some metals and a whole lot of regolith - should be doable. Oh, and a couple dozen fusion reactors to power this whole thing, which can later be reused for the colony.

Just build a mining base on the asteroid from which you grow the basic colony structure into the open space "above" it, converting in-situ material as you go - sort of like building a skyscraper (in reverse, shifting it out) and heaping a concrete tower up to a couple hundred meters thick around it as a shield.

[QuantumG]

This seems helpful:

Apart from a few asteroids whose densities have been investigated, one has to resort to enlightened guesswork. See Carry for a summary.

For many asteroids a value of ρ~2 g/cm³ has been assumed.

However, density depends on the asteroid's spectral type. Krasinsky et al. gives calculations for the mean densities of C, S, and M class asteroids as 1.38, 2.71, and 5.32 g/cm³. (Here "C" included Tholen classes C, D, P, T, B, G, and F, while "S" included Tholen classes S, K, Q, V, R, A, and E). Assuming these values (rather than the present ~2 g/cm³) is a better guess. - source

[mikelepage]

I saw this video by the Australian science program Catalyst back when it aired and it came to mind just now - it mainly focusses on the use of Lunar regolith, but having had an email conversation with the Sydney-based lead scientist on this technique he informed me that they also think it would work with asteroid regolith.

It's relevant to any construction techniques being talked about here I think - the system is quite efficient with its use of the pneumatic gas to harvest regolith.

http://www.abc.net.au/catalyst/stories/4052664.htm

[JasonAW3]

Actually, an O'neil tube or Stanford Torus built inside of an asteroid makes a considerable amount of senseso long as you line the cavity with a structure that can handle shifting masses.  If you physically attach the Torus or O'Neil Tube to the inside of the asteroid, regardless of how effecient the coupling joint is, some precession will occure with the asteroid itself.

If we're talking a rubble pile asteroid, unless the precession rate creates a centripedal force greater tyhan the Asteroid's gravity, then it should gradually reshape that part of itself that the rotating structure is in, to conform to the protective structure around the rotational mass.  The same is true of any mass beyond the location of the rotating mass.  mass will be lost by a spin higher than the forces holding the asteroid together, or will migrate to the centerline of the spin.

Iceball asteroids are much the same, but will change over a MUCH longer period of time.

Carbon Asteroids?  depends on if it is solid, tightly packed or just a dustball.

Nickel Iron?  Just mine them.

[AlanSE]

Actually, an O'neil tube or Stanford Torus built inside of an asteroid makes a considerable amount of senseso long as you line the cavity with a structure that can handle shifting masses.  If you physically attach the Torus or O'Neil Tube to the inside of the asteroid, regardless of how effecient the coupling joint is, some precession will occure with the asteroid itself.

If we're talking a rubble pile asteroid, unless the precession rate creates a centripedal force greater tyhan the Asteroid's gravity, then it should gradually reshape that part of itself that the rotating structure is in, to conform to the protective structure around the rotational mass.  The same is true of any mass beyond the location of the rotating mass.  mass will be lost by a spin higher than the forces holding the asteroid together, or will migrate to the centerline of the spin.

Precession will not occur if the axis of rotation of the colony is aligned with the axis of rotation of the asteroid. So wouldn't you obviously design for this? Sure you would have some precession if you didn't have accurate survey equipment or use cruddy mechanical stabilization, which seem like ridiculous assumptions to make. You'll locate the anchoring points to form a line parallel to the asteroid's axis of rotation. You frankly don't need anything from the coupling joint aside from some accommodations for shifting masses, power cables, airlocks, etc. It's not a difficult problem to solve either. Before building the colony, rotate a test mass connected to a long pole which spans between the two contact points. Then, just like Earth pendulums, it will sweep a circle that tells you the relative rotation of the asteroid.

You will need some mechanical force between the colony and the asteroid to counteract its microgravity. Even a tiny acceleration will have a force like the weight of a dump truck when applied to the millions of tons of the colony. These forces also need to act through the colony's center of mass, so you need bearings on both ends which act symmetrically. But there doesn't need to be any change in the artificial gravity rotation vector. I think that would be a bad design, and almost always avoided.

[llanitedave]

The asteroid itself is likely to be precessing, and the process of construction will inevitably affect it in complex ways.

[AlanSE]

On the Axial Precession of Asteroids

http://link.springer.com/article/10.1007/BF00653617

The observed fact that light changes of the asteroids exhibit no beat periods is interpreted as an indication that they do not wobble in space like spinning tops, but spin about only one axis (possibly — but not necessarily — inclined but little to the plane of their orbits). Since, moreover, the damping of three-dimensional rotation by jovi-solar attraction would require a time which is long in comparison with the age of the solar system, it is concluded that the present uni-axial rotation must represent a property preserved from the time when the asteroids were formed.

Why? Because here would be the equation for the precession in relative terms. By that, I mean the answer has units of 1/s, which is a metric of the change in relative angular momentum over time. This is what you need to ballpark the magnitude of angular change over some millions of years. In other words, this answers the question at hand without units specific to the body's overall size. However, the angular rate of precession does depend on the body's size because this is about tidal interactions with the sun.

http://en.wikipedia.org/wiki/Axial_precession

Combining dphi/dt with Tx:



C and A are equatorial and axial moments, and the first term (C-A)/C is a measure of the equatorial bulge's moment relative to the total body. In other words, this is a geometric ratio metric. For our purposes, variable "a" is only different by maybe 1 AU versus 4 AU or so. In the big picture, this is practically the same. Spin rate is also the same order of magnitude, so it just about everything else.

I could easily agree that an asteroid would precess faster than what Earth does. But not by a factor of 1000s, and Earth precesses really slowly.

And even if we assume a huge colony on the order of 1 million tons in the middle of Phobos, that's less than 10 million times less than the mass of the entire body. There's no potential for impact to global mechanics unless we're talking about a tiny NEA, and those are unlikely to be rubble piles.

[veedriver22]

 I have a question regarding using rotation for gravity.   Would it not only work for things in contact with the floor?
I wonder if you jumped would you come down or would you just sail up to the ceiling.   Or if you were to throw something up what would happen.  If you were in a hollowed out asteroid and tossed up a baseball I am thinking the ball would end up on the other side of the asteroid.   

[meekGee]

I have a question regarding using rotation for gravity.   Would it not only work for things in contact with the floor?
I wonder if you jumped would you come down or would you just sail up to the ceiling.   Or if you were to throw something up what would happen.  If you were in a hollowed out asteroid and tossed up a baseball I am thinking the ball would end up on the other side of the asteroid.

Dynamics in a rotating reference frame is really screwy.  Specifically, your baseball will meet Mr. Coriolis on the way and end up somewhere else than you'd think.  It might even hit you in the head, but the motion path you'd perceive for the ball would not be a parabola.

There are some youTube vids that show similar behavior in centrifuges (or carousels)

[Nilof]

I have a question regarding using rotation for gravity.   Would it not only work for things in contact with the floor?
I wonder if you jumped would you come down or would you just sail up to the ceiling.   Or if you were to throw something up what would happen.  If you were in a hollowed out asteroid and tossed up a baseball I am thinking the ball would end up on the other side of the asteroid.

Basically, inertia means objects want to move in a straight line in the nonrotating frame of reference. But draw any line from the inside of a circle and it will intersect the circle at some point. So objects do indeed tend to fall down. Those that don't or do so very slowly need high velocities relative to the floor, so that situation is more or less analogous to being in Orbit.

[Paul451]

I have a question regarding using rotation for gravity.   Would it not only work for things in contact with the floor? I wonder if you jumped would you come down or would you just sail up to the ceiling. Or if you were to throw something up what would happen.  If you were in a hollowed out asteroid and tossed up a baseball I am thinking the ball would end up on the other side of the asteroid.

While in contact with the surface, you have the same tangential velocity as the ring. Basically, you are moving sideways. The floor is attached to the rest of the station, so it is pulled in a circle. From your position, the floor is being pulled upwards/inwards as you move sideways. As a result, the floor pushes you "up", and your inertia makes you feel like you are being pulled "down". That constant push "up", simulated pull "down", is what gives you artificial gravity.

So you are holding a ball, you let it go. It is in freefall, yes, but it is not stationary relative to the hub so it doesn't float, it already is moving "sideways" (just like you) with that initial tangential velocity. Because it is no longer being pushed up by the floor (or by you), instead it is free to move sideways in a straight line, but you and the floor move sideways and "up". So when the ball hits the floor, the you and the floor have rotated around to the path of the ball. So the apparent net motion is "down" to the floor.

Hang on, I'll throw in an image, probably easier to see.



Stick-guy in the middle is standing on the floor, but because the ring is rotating, he's actually moving sideways tangentially to the ring. If the floor wasn't there, he'd move in a straight line to the left. But because the floor is pushing him up and inwards, he instead moves in a circle.

If he drops his little red ball, it is free to continue in a straight trajectory as if the ring wasn't there, until it hits the floor. During that time, the floor will carry stick-guy around further, and so the ball will hit the floor just as his feet arrive. From his point of view, the ball fell straight down.

(In reality, because the ball is closer to the centre, it will be moving slower than the floor. So stick-guy's feet will move around slightly further by the time the ball arrives (shown by the faded image) hence the ball will hit the floor slightly to the right of stick-guy's feet. To stick-guy, it's as if the ball has drifted to the right instead of falling straight down.)

No idea if that actually makes things clearer.

[Paul451]

No idea if this thread is still being watched by the original participants. But...

Re: Dust/difficulty.

It's interesting that when people criticised the idea of building a base inside the asteroid, their alternative suggestions always contradicted the very reasoning they use to criticise the asteroid-base.

For example, paraphrasing, "tunnelling will be difficult/expensive, and tunnelling will produce dust", therefore... "you should build a stand-alone rotating space-station with shielding made from the asteroid material..."

... which you have to dig off the asteroid, separate out the preferred ore (high hydrogen, low metal), process (shape/sinter) the chosen ore into suitable blocks/panels or bag and seal, and transporting it to the ring-station construction site, presumably using fuel also made from asteroid material (which you have to dig off the asteroid, separate out...) And somehow all that is not only going to be easier and cheaper, but create less dust around the asteroid, than digging a hole - once - and then just living in the hole?

Pretty much by definition, if you can make enough shielding for a rotating station by mining an asteroid, then you've figured out how to dig material off the surface of an asteroid without ruining your own equipment with dust. If so then wouldn't it be even easier to dig one smallish hole, line it, add a dock, and move all further mining/etc, underground. No more dust problem except at the "coalface". Solves your radiation problem instantly, long before you've mined enough material to shield a large space station, long before you've even built the large space station. Plus fewer problems attaching equipment to the "surface" (inside), and fewer problems even attaching equipment to the actual surface (outside) since you have a rapidly deepening "anchor" to attach to.

because a tunnel-boring machine for a 1-km wide hole would probably have a mass of 10^6 tons by order of magnitude.

I think you misread the idea. The tunnels for the ring-hab were not 1km wide. Someone mentioned inflating a 1km bubble, but that's a different concept and doesn't require a TBM (except perhaps to deliver the inflatable.)

(However, since you brought it up, a TBM able to bore a 1km wide tunnel doesn't have to be 1km wide. You can use a regular TBM running in a helical pattern with a 500m radius, separating out an "apple core" 1km wide, even though the actual TBM, and hence the width of the cut, might only be a few tens of metres wide.)

[Paul451]

(Last one, promise. But it goes on for awhile...)

I wrote about one of the simplest ways to generate artificial gravity - a train track inside an asteroid

Is there a reason you went with a train on tracks? Why not a torus-station spinning in free-fall inside the ring-tunnel? Then your wall-track is merely to keep it centred, which should create much less pressure and vibration grinding at the walls of the tunnel compared to having a multi-tonne train, off-centre, unbalanced.

Your track has to be strong enough to support the entire mass of your train anyway. So why not separate it from the walls and have it spinning with the modules attached. Then the modules don't need heavy rail-bogies able to support the whole weight of the module (in 1-artificial-g) on the track. You'll just need a much lighter guide system to keep the ring centred.

You also don't need the habs to immediately fill the entire ring, you just need any modules balanced by a counterweight. (Nods towards the trillion tonne asteroid to hint where you can find free counterweight mass even at the earliest stages of development.) That way, the big stuff in your very first construction (ring-spine and guide-track) is dumb bulk material, while the complicated stuff (habs) don't have to be the equivalent of heavy rail cars. (The first ring-spine might be a clever/expensive composite truss, to make it as light and low volume for transport from Earth. But it's still easier to engineer than the habs, so off-loading the loading to that bulk-structure makes more sense than adding that complexity to the habs.) You still get all the benefits of radiation and solar shielding, thermal uniformity, etc. And without the bogies, the modules can initially be simple inflatables attached to the spinning ring, until your IRSU is capable of building new habs.

Speaking of thermal issues. Have you considered how you are going to cool the damn thing? I mean it benefits from not being in direct sunlight, and the asteroid has a huge thermal mass so the tunnel walls will radiate a uniform (IR) temperature. But eventually the habs will warm the tunnel walls and will be reabsorbing their own reflected heat. You'll need to connect it to radiators somehow.

(Obvious solution, pressurise the tunnel not the modules, circulate the gas in the tunnels through external radiators. But, even though you're only moving at 60kmh relative velocity, that leads to issues with drag, hence energy costs, plus a more rapid altering of the asteroid spin. And the solution to the latter (more rings, counter-rotating), increases the energy cost further. Still, unpressurised modules is a nice bonus.)

Speaking of energy. Another useful thing about asteroid habs, you can shove a nuclear reactor a few tens of metres underground, but well away from the base, and not only is the hab/work-area shielded by the asteroid bulk, but even the reactor's external radiators are shielded simply from having the reactor itself 20-30m beneath the ground under them. Not only reduces degradation of the radiators, but allows astronauts/vehicles to work around them freely without worrying about their own position relative to a reactor-shield shadow.

--

Additional observation: QuantumG's proposal is a 894m radius, that's simply the minimum needed to produce 1g at 1RPM. But since radius is proportional to the square of the rotational period, increasing the RPM gives an exponential reduction in radius. Experiments suggest people can probably handle 2-3 RPM without difficulty. And we can expect the earliest workers to be trained astronauts, selected against motion sickness and experienced in the weirdness of micro-g; many of them should be able to handle 4-6 RPM.

At 3 RPM, your radius is reduced to just 100 metres for 1g. At 6 RPM, it's down to just 25 metres. At 4RPM and 0.2g, your radius is just 11m. At 6RPM and 0.1g, radius is just 2.5m  If we ever do this, it's worth exploring higher RPMs, the potential savings are enormous.

(At 6PRM and 0.1g, your 5m diameter fits entirely within a BA-330 inflatable module. At 4g, 0.2g, rotational velocity is just 16kmh and the 22m diameter is small enough to carve out the entire disk. Line the cavern and pressurise it, and have an unpressurised module and counterweight on a pair of simple rotating arms, with the hub simply bolted to the walls. BTW, I'm not suggesting these are optimal sizes, just using them to illustrate the size of the effect.)