Supposing one wanted to place a ring in equatorial orbit of the Moon. What do the general details of that look like?
Its purpose is manufacturing and as a launch site for Robotic, and eventually Crewed Interstellar transport category ships.
The assumptions I’m using are that there would be a series of captured asteroids to hang/suspend the general structure from.
I know they will not be lunar-synchronous, given that would intersect the Earth’s own solar orbit at some point.
Is there a lot of circumferential flexibility? I feel like there would be so long as we’re not precious about orbital periods.
Practical limitations WRT Ring Cross-sections, torsional loads from tidal forces, etc?
Strangely enough, the internet really doesn’t seem to have much about the physics of this general arrangement.
My understanding of Orbital Mechanics is about what you’d expect for a 'Doper. A lot of people know less than I do and a bunch know quite a bit more. So I’m appealing to someone to not only know this, but to be able to relate it.
This is for a fictional book I’m writing (totally willing to credit valid research in case that was not obvious), so while I’d like it to work, I can McGuffin up enough things to keep the story moving.
You can’t put a solid ring in orbit. Or rather, you can, but it’s way more trouble than it’s worth, because its orbit will be unstable: If anything at all disturbs it, then you need to disturb it right back immediately (presumably, using expensive rocket engines), or it will fall. If you want something like a ring, then you make it out of unconnected satellites which are able to drift a bit relative to each other.
Nor is there really any benefit to having a complete ring. You want to put a space station in orbit around the Moon? Fine, put a space station in orbit around the Moon. What’s the rest of the ring for?
You can’t have a solid ring because besides the structural loads being ridiculous there’s no stable orbit because it’s not really in orbit. It’s centre of mass is co-located with the centre of mass of the moon. Any perturbations to its position will result in those perturbations increasing - if you nudge it off-centre, the closer part is now more strongly attracted to the moon than the further part so it will be pulled ever closer and closer. For more information, google “Ringworld isn’t stable.” All the details about how Larry Niven’s Ringworld isn’t stable would also apply to your lunar ring.
You could try a Saturn-style ring, but it won’t last very long as there aren’t any really long-term stable lunar orbits. Earth fucks things up by exerting too much gravitational influence. So over time the bits of the ring would bash into each other, coalesce into bigger bits, be flung out of lunar orbit, or crash into the moon.
Here are some calculations regarding a selenostationary orbit. Basically, it says what people herre have already said – too much influence from the Earth and sun
I think that the OP’s thinking was that you might be able to essentially make up a ring by linking a series of “Beanstalk”-style space towers. People have depicted such a ring around the Earth. Of cours, you need ludicrously strong materials to build such a thing. But with the moon’s lower gravity well, maybe you could build selenostationary towers without ridiculously strong materials. But it seems that, unless uou get the moomn away from the Earth and the sun, that ain’t gonna happen.
If you could, though, you’re in a different position than Ringworld Instability – the ring would be “tethered”. But i still think you’d need ludicrously strong materials.
Whats the in-story rationale for this to be ring-shaped and located around the Moon?
You can’t have a perfectly circular orbit around the Moon because of irregularities in the Moon’s density as well as gravity from Earth. So the ring needs to be rigid and very strong, to force every part into circular motion around the Moon. And as already mentioned, it’s in an unstable equilibrium, requiring active control to keep it in place. So there has to be a strong benefit to justify all this engineering and risk, and I’m just not seeing it. If you need a ring-shaped space station a few thousand miles in diameter, it’s much easier to put it in a Lagrange point (either Earth-Moon or Sun-Earth) than around the Moon.
An orbital ring would be enormously useful around Earth, because it would give us a way to manufacture things up in space, and it would also allow us to get out of Earth’s gravity well and atmosphere using things like elevators that don’t require the use of enormous quantities of fuel due to the tyranny of the rocket equation. (Not that this is practical – but if we could do it, it would be useful).
On the moon, you don’t NEED a ring. If you’re building a manufacturing operation on the moon, you’re using local resources-- that’s the only reason to build on the moon rather than in low Earth orbit. But getting materials off the moon and into orbit doeesnt require an orbital ring, because there’s no atmosphere. You can accelerate your newly assembled spaceships into orbit using a simple magnetic railway only a few miles long (and you can go even shorter for raw materials or sturdy manufactured goods; you only need that length if you’re going for g forces that a human can sustain.
Since the moon’s gravity is so low, and since there’s no atmosphere, you can just launch things off of these magnetic ramps and toss them straight into orbit. All you’re using to do this is electricity, which one way or another will be abundant on a lunar base. Estimates for the cost of a lunar launcher like this are as low as $1 per pound, assuming 10 years of operation. Source:
A more realistic near future lunar manufacturing facility would probably involve a few (to a few dozen, depending on scale) surface bases at locations rich in various resources that are needed by the people running the operation. On the surface, raw materials are extracted (mostly by robots) and processed into the forms needed for manufacturing. Simple fabrication might take place on the surface, too. Yo us probably have some kind of larger station in lunar orbit, both for manufacturing processes that require microgravity (we are only beginning to understand some of the potential here) and for assembling the less complex parts shipped from the surface into your ships and robots and such.
Most bases would have a bunch of short (ie couple kilometer) runways for cargo and longer runways for passengers, especially non astronaut passengers who can’t handle the kinds of g forces fighter pilots deal with.
You’re probably not going to like this. But that is not particularly explored in this story. That’s obviously short-hand for Author’s Laziness. But on a practical level, it leaves plot based options available for subsequent stories.
A good deal of this story actually revolves around the corporate politics that make this a thing in the first place…
Right. Functionally, the thing is more of a hula hoop than anything else. So, not to change the parameters, but to add significantly…
Yes, there are many and constant adjustments required and completed. As well, the idea is that even it is a ‘false’ orbit, the relative motion of the structure should somehow make that easier. But there again, that in and of itself is one of the details I’d be interested in knowing more about.
Heh… Yep. This is the literal source of most concept fiction.
Very damned close, yes. Sorry if I was not totally clear --also something I need to work on, lol. The tethers hang down from a a set of asteroids arranged in a higher orbit. Essentially functioning like a suspension bridge, writ large.
There specifically is an allowance some amount of flexing --it is even a minor plot point. A main character has a tough time knowing whether a Flex is happening or if she is really fucking hung over. . .
Honest answer? I do not know. It will be explored at some point, but part of the point is that Company running the operation isn’t famous for openness.
That I did know (insomuch as I could have known something like that anyway). My understanding is that it would be easier to at least install the asteroid ring at that point and hang the ring structure itself in geosynchronous orbit.
Unfortunately, the politics of the Story prohibit this. . .
All of that makes a lot of sense. And although the Story does have a lot of surface lunar work going on, I feel that part of what a Ring offers (perhaps eventually) is a level of secrecy more easily enforceable. . .
But like most such stories, any sort of budget to do it at all right would… well, you can see why I’m writing a book instead of a screenplay.
The trouble I see is this: you’re asking about “practical limitations” for something that is impractical. Either embrace the fantastical elements and hand wave the practical concerns for the sake of the story if the ring being in orbit is really so critical to the plot and the themes you want to explore, or ditch the orbital ring and make it something that could maybe be built.
Slightly off topic, but I have heard of the idea of a particle accelerator/collider being placed around the circumference of the moon, but at or below surface level, not in orbit. The idea being that to discover fundamental particles of higher energies/mass (if they even exist) than, say, the Higgs boson, you will need ever larger accelerators capable of achieving ever higher energy collisions.
I don’t think you could accomplish a ring like that by hanging it down from asteroids – not around the moon, at least. Around a planet (perhaps not Earth, because THE moon is actually very big for A moon, relative to us, and would mess up the orbit) you could probably do something like that.
On the moon, you’d be looking at beanstalk towers, as was said – but they wouldn’t just be big towers made of metamaterials, they would be active structures.
Of course, this being a novel, you can do whatever you want – but perhaps you can use the idea of active structures in the tethering to give you a little more fuel for handwaving.
The thing to remember is that it’s really easy to get off of the Moon. Even just with old-fashioned chemical rockets, NASA was able to get two men, their life support systems, and a bunch of rock samples into orbit, with a vehicle the size of a small car. If the characters in your book are routinely going into space from Earth, they’ve either gotten really, really good at making rockets, or they’ve got something even better than rockets, and either way, the same system would make launching from the Moon trivial. Focus instead on how they’re getting off Earth.
Oops, forgot the link. https://en.wikipedia.org/wiki/Orbital_ring
This is a magnetically supported structure, which could be placed at any height and with any velocity relative to the Moon.
But this would be a lot of bother for very little benefit.
Indeed. No part of this structure --simply referred to as The Crown– is not active. In fact, there’s a scene where a battery of particle cannons fail, and a piece of space FOD strikes one of the tether anchors. Some detail about the active compensatory balancing ensues. . .
Just thinking out loud here. . .
It is possible that there would be something along those lines at work. The descriptions of this Crown are vague in macro and detailed up close. This is not uncommon in sci-fi works as it allows for a lot of future development without the burden of a lock-down.
In this instance, I can forsee that the Crown may also be tethered to the surface. Although in those instances, the ground tethers would not be supports, but rather ‘drug’ along an equatorial ditch, where such a Collider would be contained.
Indeed, that would be a ridiculous way to supply power, but it would also open up the possibility that this ring exists for other experimental/developmental reasons…
Heh, yeah I do like Optimism.
I wouldn’t rule out money becoming part of this thing (who wants to work a real job their whole life anyway, right?)
Actually one of the things I was trying to get a good insight on. For the story’s dramatic needs, there are at least two scenes where it is mentioned that there are thousands of km between the ring and the surface.
I’m envisaging a giant hula-hoop of a space station circling Luna. Two nested hula-hoops of different circumferences actually, cross-section 8, with the outer hoop spun to a desirable gravity, and the inner hoop hosting movable ballast weights that are shifted to counter any perturbation. That way, stability needn’t exhaust reaction mass. The obvious question: Would shifting ballast be enough to keep the station from crashing down? Or would interlocked crossing polar and equatorial hoops work?