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A Bishop ring is a space habitat, in my project about 100 km long and with huge walls 10 km high to keep the atmosphere in.

enter image description here

The ring rotates about once every 20 minutes to simulate 1 G, and is built from woven carbon nanotubes. All it needs is a source of lighting.

The structure is too small to encircle its sun like a Niven Ring, or to be tilted so that certain regions are periodically illuminated like a Banks’ Orbital. As there is no roof, lights cannot be placed on it. Any system should ideally be able to function for long periods without maintenance, such as nanotechnology or self-repairing systems.

Image from Orion's Arm

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    $\begingroup$ Have you considered lamp posts? $\endgroup$ Dec 22, 2022 at 17:29
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    $\begingroup$ I don't think 10km will be sufficient to keep the atmosphere in. There will be major "leakage" over the top at what would be roughly the tropopause on Earth at a low but still significant pressure. $\endgroup$
    – Slarty
    Dec 22, 2022 at 23:31
  • $\begingroup$ Really I think at the point where a civilization can build this, you can also just grant them the ability to build an amazing light source. "And up there is our hyperspace dark matter reactor which you can see creates something like sunlight." Or if "the builders" are gone, just put a light source there and never explain it, lol. It could just be one of the many mysteries they left behind. No one knows how it works because it's too hot to get close enough to find out. $\endgroup$
    – JamieB
    Dec 23, 2022 at 20:27
  • $\begingroup$ Can your wheel have spokes? $\endgroup$
    – g s
    Dec 24, 2022 at 22:30

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The simple way would be a conical mirror ring (not spinning) anchored at the spin axis. The ticklish bit is that the axis of your Bishop ring needs to point at the local star, and gyroscopic forces mean it can't readily precess its axis with its orbit (even over the course of a year or more).

Fortunately, O'Neill solved this in the original Cylinder colony design: build two rings that rotate in opposite directions and anchor their axes together (ideally, they'd be concentric, but then the lower ring would be in the dark without artificial lighting). Angular momentum cancels, and you can rotate the spin axes as needed to keep the mirrors pointed at the star.

You'll also have (probably unacceptably) high air loss over walls only 10 km high, though -- Niven's Ringworld had walls 1600 km high, and you'd need at least 100 km for the air at the top to be thin enough that losses could be made up by ongoing importation of gasses. O'Neill's design solved this by completely enclosing the colony, but that works better with only a few kilometers diameter and some tens of kilometers length along the axis.

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    $\begingroup$ Good call on the wall size. I think we see this a lot in worldbuilding: "some large number" sounds large but really a 10km wall is "only" about 32,000 feet -- less than the maximum cruising altitude of a 747. Big as a structure, but not for holding in an atmosphere. $\endgroup$
    – JamieB
    Dec 22, 2022 at 18:19
  • $\begingroup$ Since the ring center is in microgravity, using a fairly spindly armature to orient the mirror and even displace it off of the axis a little bit when the ring eclipses the sun should be plausible with present-day technology (or in fact medieval technology plus a couple of guys with space suits). $\endgroup$
    – g s
    Dec 24, 2022 at 22:37
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It's not that the structure is too small to be tilted like a Banks Orbital, it's too wide. I can tilt my wedding band to allow about a third of it to be entirely illuminated by the sun.

All you need to do is design the orbital to accommodate basic optical physics. That means the diameter of the orbital, the ratio of the width to the diameter, and the height of the walls must all pan out to guarantee that an angle exists to allow illumination.

You can improve this my mirroring the inner surfaces of the walls, at least on the side that would receive the light.

However, if you have the technology to build such a structure, why wouldn't you have the technology to line the top of the walls with fusion light sources? Basically large engines that are constantly banging out photons that are magnetically reflected toward the ground. The generators come on or go off to simulate night along the ring.

Another solution is to line the outside of the ring with photon capturing panels that redirect the photons (not unlike through fiber optics) to specific areas for illumination via the inside surfaces of thew walls.

And finally, you could simply have a central illumination source (insert favorite word for powering it here) that simply cycles on and off to provide daylight and night to the entire ring simultaneously.


You might be disappointed with the ambiguity of my answer.

It's tempting to try and create a realistic solution for this problem — but we're not being presented with a realistic problem to solve. We can model the possibility of megastructures, but it's a breathtakingly long journey from that to practical engineering applications for construction. You want to design the ring to overcome obvious problems that reflect science we understand: like the wall heights and the ratio of the width to the diameter. But let's take an example from the Encyclopedia Galactica, which you link to in your question, Ceres is "filled with air and artificial light sources...." That's it, no explanation of how they work. Ambiguity is your friend when writing scifi. The more you try to explain, the more you're giving people to rip apart because we don't actually have any idea how to do any of this.

I'll leave you with a direct example, you suggest the ring is made of "carbon nanotubes." That's a phrase that doesn't have sufficient meaning in cases like this. It's a phrase that means "Clarkean Magic Happens Here" to overcome the "how" for things that need massive strength, like space elevators and multi-km-tall buildings and megastrutures. In reality, the odds of carbon nanotubes having the properties needed to construct any of that is pretty slim and the likelihood of discovering a better material by the time we can overcome the engineering limitations of a megastructure are pretty good, which makes the phrase so much technobabble. Remember, the transistor is only 75 years old. A lot is going to change by the time we can build a megastructure.

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