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I wanted to create a main destination/headquarters for each galaxy in my story but a single planet or megastructure didn't seem grand enough.

My idea is to have a vast number of structures from rotating space habitats to non-rotating structures used for storage, computers etc. All structures will be joined together by strong cables to keep them from wondering off or needing to use too much fuel to keep its position.

The arrangement of joined space structures will not be orbiting anything, they will be stationary in a set location far from any celestial bodies that could gravitationally influence them.

Carbon nanotubes, steel and aluminium will be used for the structures and possibly the cables. Nanotechnology is used within the structures for different purposes.

The majority of the structures will have masses between 100 million and 10 billion tons with possibly smaller structures of less mass along the cables. I haven't decided on the size of the whole arrangement of structures, for starters I was thinking that it can be an astronomical unit in diameter but if that is possible then it could be larger.

Could a collection of tethered space structures like this exist if they were spaced at a distance to keep them from collapsing due to gravity? If so what would be the minimum distance between structures and how large could a collection be?

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    $\begingroup$ Important moment, are they orbiting something, like some star or a planet, or it is an infrastructure which is far from any star, at least half ly from any star or any other massive body. Also as note 10 billion tons is not a megastructure for space, it more like casual size of few km's structure, 100'000 is more like tiny/small stuff. The whole thing 1 a.u. yes that is a megastructure. $\endgroup$
    – MolbOrg
    Dec 3, 2021 at 0:23
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    $\begingroup$ It better to add this information to the body of question, as comment section is expendable, and this information is more useful there. // yes, cylinders aren't megastructures, they are just regular structures, like a typical house on earth is not a megastructure. How about technology level in the setting? Nanotechnology is it available, or it is a steam punk or, add this information as well, it not super important but can be useful. $\endgroup$
    – MolbOrg
    Dec 3, 2021 at 0:41
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    $\begingroup$ Have at least one galactic culture that believes that their administration lives in the central supermassive black hole. They throw all statistics and reports to the hole and wait for replies that never come back, of course. It's a happy civilization :-) $\endgroup$
    – Edheldil
    Dec 3, 2021 at 11:01
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    $\begingroup$ I would suggest a geodesic polyhedron (which is essentially a geodesic dome that has been completed into a sphere) as a shape for this, that you could then also nest inside each other. This would make it similar to a Dyson sphere, except, with enough space between the node, more reasonably possible. And it would be pretty impressive in appearance. (Answers should feel free to steal this idea) $\endgroup$ Dec 3, 2021 at 11:05
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    $\begingroup$ what-if.xkcd.com/127 be careful when dealing with tethered cables that can snap $\endgroup$
    – Anketam
    Dec 3, 2021 at 13:54

6 Answers 6

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Spin them.

Trivial case: two structures

Consider the trivial case of two megastructures tethered to each other. Their gravity (which may be almost non-trivial, with the masses you're listing) will tend to pull them together--but if you have them rotating around each other, their inertia would easily overcome that pull. You just need to find a sweet spot (probably a very slight rotation) where the acceleration from their gravity nearly neutralizes the centripetal acceleration.

If sufficiently balanced, then in theory the cable holding them need not be very strong. In practice, you would want a big margin for error - especially if there are gravitational fields from nearby bodies that might cause tidal forces.

A wheel of structures

Now, if you want more than two megastructures, this scheme can still work as long as the structures and the cables are assembled in a planar and radially symmetric configuration. The simplest cluster to visualize would be a "wheel" of megastructures - a four of them connected by cables to form a square; or five of them connected as a pentagon; or six of them as a hexagon. As long as they have sufficiently similar mass, the same perfect balancing is, in theory, achievable.

Limitations

If you want to have irregularly shaped clusters, however - or 3-dimensional layouts, or superstructures with very different masses - these factors would all add complication to the system that will make it more difficult to achieve a "perfect" balance, and you will probably start to be limited by how strong your cables are.

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  • $\begingroup$ Small note: if they're moving at the right velocity away from each other to balance out their gravity, then they're co-orbiting, not spinning. You're essentially making an artificial Pluto-Charon system. $\endgroup$
    – Cadence
    Dec 2, 2021 at 21:53
  • $\begingroup$ @Cadence: Right! I mean spin the cluster of megastructures, not the structures themselves. I'll edit to clarify. $\endgroup$
    – Qami
    Dec 2, 2021 at 21:55
  • $\begingroup$ That makes sense having them in a plane rotating, I had originally thought of a 3D grid like arrangement but that may not be stable. $\endgroup$ Dec 3, 2021 at 0:06
  • $\begingroup$ "Their gravity (which may be almost non-trivial, with the masses you're listing)" - downvoted for this one(maybe a bit too rush), really plug the numbers, 10 billion tons it is not that much, two such structures 1km apart it about 2h15min rotation period. As for more than 3bodies and 3d structure of such cluster - you have typical 3body problem here and instabilities which would be good to mention $\endgroup$
    – MolbOrg
    Dec 3, 2021 at 0:08
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Tubes rather than cables

There is no wind. In space, in 0-g condition, with near-zero gravity of the construct itself, it may be feasible to use a web of rigid connections (tubes) to keep it together, that is lots of metal tubes say 2-4 meter in diameter, between the megastructures, forming a web, or cocoon-like assembly. It is stable, you'd have predictable behavior of the whole construct, the relative position of your megastructure components will be constant, and you won't need to arrange orbits, or handle megatons of centrifugal tension forces. You can use these tubes for pneumatic transport of goods, or even as corridors, for transport of personnel.

In the middle, you could have a megastructure "central station" where most tubes are connected to.

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    $\begingroup$ Would rigid tubes be able to take the tension of a large construct? What distances do you think would be needed between megastructures to ensure it doesn't collapse ? $\endgroup$ Dec 3, 2021 at 0:00
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    $\begingroup$ You're in zero-g with no wind. What would cause the megastructures to "collapse", or move anywhere ? Small, slow alignment errors can be corrected by bending the tubes from the inside. And I'm thinking of many of these tubes (hundreds), not 4 or 5. Of course when an asteroid would collide, it's gone.. but.. rare circumstance.. $\endgroup$
    – Goodies
    Dec 3, 2021 at 0:04
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    $\begingroup$ Good one, not bad. Gravity should be accounted btw, two 10billion ton structures at distance 1km attracted with force of about 6 billion Newtons, or 600'000 tons $\endgroup$
    – MolbOrg
    Dec 3, 2021 at 0:10
  • $\begingroup$ I had a similar idea of using tubes in the past for a smaller arrangement of structures but because this version could contain up a thousand or even far more joined megastructures I thought rigid joining structures would have too much force put on them. $\endgroup$ Dec 3, 2021 at 0:21
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    $\begingroup$ @Goodies "There will be no other forces on the structure, " - do not forget gravity of the structure itself. Few things of mass of billion tons is not much, but few million or more of those - it needs to pay attencion to that. OP wants grand and epic, so I guess millions is not the limit, to occupy volume of size measured by astronomical units $\endgroup$
    – MolbOrg
    Dec 3, 2021 at 0:46
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Gravity wouldn't be a big issue.

If you calculate it, assuming 10 billion ton objects paced at 10 kilometers, the force is about the thrust of the Thrust of Saturn V rocket at 60 million Newtons.

Cables aren't that useful.

Cables don't make force disappear, and that's enough force to snap a lot of cables. You can cable them together if you want, but it won't counteract the forces.

You could make the stations into a ring or a sphere for additional stability

If you have a ring of mass, the net attraction to any one object is fairly low, because it's counteracted by other objects. It's a bit unstable, but it'll be easier to manage.

If you have a sphere, it'll be very stable.

This could reduce fuel costs to that needed to stabilize parts.

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  • $\begingroup$ Thanks for the calculator for later use but as i will have thousands of joined structures I am not sure how to calculate it. I did envision it being sphere like or a 3d grid of some sorts, why do you say a sphere of this type will be very stable? $\endgroup$ Dec 3, 2021 at 0:34
  • $\begingroup$ If you place objects correctly, there's no net gravitational attraction for a sphere other than inwards, which you can counteract with them orbitting it. That's how a dyson sphere works in terms of stability. $\endgroup$
    – Nepene Nep
    Dec 3, 2021 at 14:26
  • $\begingroup$ "You could make the stations into a ring". When you first saw Halo, were you blinded by its magesty? $\endgroup$
    – ARC
    Dec 3, 2021 at 22:39
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You mention Ring World but forget, the Puppeteers Fleetworlds.
Arrange your structures into a Klemperer rosette. The Rosette will require station keeping mechanisms, but at this scale that's not an issue.

A Klemperer rosette is a gravitational system of heavier and lighter bodies orbiting in a regular repeating pattern around a common barycenter. It was first described by W. B. Klemperer in 1962, and is a special case of a central configuration.

Your objects will also be required to have enough mass to exert some gravitational force over the other objects in the rosette. Scaled down enough tubes/cables etc between them is possible.

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Ring

orbital ring https://www.reddit.com/r/IsaacArthur/comments/8ku618/orbital_ring_cable_thickness/

You can have your vast number of megastructures in orbit around a sufficiently large mass. Megastructures can be connected by cables or perhaps they can just have magnetic bumpers to allow some movement relative to one another. You have built an orbital ring. You will still need your strong cables. These have made many appearances in SF and there is a lot about artificial rings on the WB stack.

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OK, you don't have tidal forces pushing them around relative to each other, but will have the momentum transfer due to arriving and leaving different modules, and this could systematically built up over time (e.g. visitors always arrive at module A, with the shuttle bay on the side facing away from the cluster; they do their tour and then depart from module B.)

My solution is to not use simple tension in cables to keep them together. The cables — or better yet, conduits that carry utilities or even allow transit — have expansion joints to either slide apart a bit or straighten out a bit. Meanwhile, this acts as a sensor that will trigger active correction.

The correction is not a rocket throwing propellent out into space to be lost forever! Rather, since you have these conduits, you can use them to shift ballast around, or very gently pushing and pulling on each other.

If A and B are moving apart, this will be sensed and even though the cable between them is still slack, it can be jerked to produce an attraction. This could be done by contracting a small section of the cable at a time in a wave that moves from one end to the other; this will transmit the force even though the cable is slack, without having to thrash it around. This can be used to push as well as pull.

If the connecting tubes carry water or feeds for replicators or other material stuff, imbalance in supply and demand might be what causes the difference in momentum in the first place. Intentionally shifting material can correct it. Perhaps water is pumped between A and B to exactly oppose the daily cycle of commuters who go from A to B in the morning and B to A in the evening. In general, such routine activity, whether people commuting or the flow of goods produced and consumed by different modules, can be planned and anticipated, as well. Logistics can try to reduce the net imbalance by scheduling things properly, and avoid having to transfer ballast for no reason other than to make a correction. Just plan the delivery schedule for something that you needed to transfer anyway.

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