It is possible to build a ring base around a moon or a planet. Not familiar with Starship Troopers' Luna Base — not sure how much things you like to place on the base. But as a basis for a base 30×30 m may be enough.
How to implement
Active supporting structures are the answer: Isaac Arthur, Megastructures series covers the concept of active supporting structures in different applications, as one of the key elements which make them possible.
Basically, the concept is used in Launch loop as a thing which makes it theoretically possible — some kind of rotor moving inside of the structure at high velocity and its centrifugal force counteracts the gravitational force of the structure, which surrounds it.
The ring is the same, but just significantly bigger as a construction.
It does not have to act as a rigid structure which counteracts all forces (gravity, warping and other things) — those forces have to be counteracted by a rotor.
The rotor itself does not have to be a single core or one continuous string. This has to be understood and having it as separate short pieces may be important to neglect some stress effects which may have a place in this construction, by slowing or accelerating those parts at different points of the ring.
Probably there should be different tracks inside this structure, so those pieces of the rotor may change tracks, and be able to run inside this construction at different speeds, to counteract possible resonance events, uneven forces acting on the ring etc.
A bit simpler structure in the wiki article Orbital ring, and according to the article it was proposed by Nikola Tesla in 1870, so he probably the Father of them all, as a concept.
The main problem is the same as with the launch loop — reliability.
Thus answer for “What would need to happen for it to be feasible?” technology should happen to allow to run those active supporting structures flawlessly, without a single issue in the time of its use, especially for a permanent structure like planetary rings. Technology which is used in my answer about moving the planets is one of such; it allows active supporting structures to be compact and fail-free. The answer is here(relevant part is Note about Venus scrap, snake elephant).
Another problem is maintenance. It is heavenly correlated with reliability, but also the ability to eliminate the results of external events, such as hit of meteorites or a debris hitting the structure, or internal ones such as wear of the construction). Basically, any smart matter, advanced enough, will solve the problem. It does not have to be nanomachines or gray goo or nano assemblers and such — other types may be good enough.
What is the lowest tech to build one of such.
- or do we already have the tech to build one of such
- or other problems to solve before to build one of such.
It makes not much of a sense and it is dangerous to build such a ring if the technology does not allow to build it reliably if the construction is not 100% reliable. The only reason to build a not reliable structure is the need to solve a temporary problem if it is suitable to be solved by such a construction. There is not a lot of such problems, but possible one is to organize an escape for the humanity from the gravity well in a short time (few years), as the structure may be used as a base for multiple, 100 km long space lifts with may have way much more intensive throughput per single lift then usual space lift, and cables can be made out of more regular materials.
The good thing about the ring is we probably can build it with our current technologies and material we know and use on regular basis.
Not entirely related, but few examples of space lift masses for the earth:
A kevlar made space lift (3.2 GPa, 1440 kg/m3) for the earth will have astronomical mass (not so astronomical but 0.004% the mass of earth) and be able to lift 100-tonne capsule (maybe once per few hours).
Some kind of CNT-MMC (Carbon nanotube metal matrix composites just to define, no good data there) with something like 15 GPa tensile strength and 2600kg/m3 density — same payload (100-tonne capsule per few hours) will have more reasonable mass 254'000'000'000 kg (254 million tonnes) not bad.
And just to include CNT, something like 100 GPa, 2000kg/m3 (same capsules) total mass about 44'000'000 kg.
Capsules - vehicles which travel along the cable and where we place our payload, they apply an additional force which acts on the cable, and their mass determines the diameter of the cable at the bottom, and define the thickness of the cable at all points. In examples above, additional force is a weight of 100 metric tonnes of mass.
The Ring may have a mass about 36'000'000'000'000 kg (36'000 million tonnes), maybe a few times more, but let's say the overall density of the construction is about 1000 kg/m3.
It is a lot, but not even close to astronomical values, but a lot. It is equivalent to about 1000 big dams on Earth by mass of materials used for the construction. It is a lot for space too, but not really that a lot. To launch all those metric tonnes from the Moon just in one year, we may need 3.3 TW of power generation which is an 110×110 km field of solar panels(or equivalent technology) on the surface of the Moon. (40% efficiency of energy generation use, half time night, half time day)
- Sure the launch of materials have to be done by some kind of mass driver(maglev and such).
Those materials have to be launched in some orbit around the body. There is a problem with our tech, we can't do that easily if the body do not have the atmosphere, so now on and farther it is about building the Ring from the Moon around the Earth.
Good aiming, aircapture, take orbit 1000×100 km, delta about 250 m/s to make it circular (see this answer for Hoffman transfer formulas). There are also other ways to do that as an example https://en.wikipedia.org/wiki/Electrodynamic_tether, but the atmosphere is useful to have.
It possible to build such construction from an airless body, if it has all materials needed (the moon does) it just harder to build the ring around another airless body from the body.
Chunks can be 1 km long, so it is possible to launch the construction in 1 km chunks into the orbit around the earth. In about 46000 chunks later we may have a ring of independent parts orbiting the Earth. KSP game will probably calculate this situation at 0.001 FPS but generally if each part have ION drives, energy sources(and they should for future use to have — solar panels or other ways to convert solar energy or other sources of energy) — they can to couple in a single orbiting ring, relatively easy.
Low tech solution ends almost there, not quite but about there. To implement it we have to use:
- SpaceX, BFR — to establish moon base.
- automation to make big energy source on the Moon, big energy storage system on the Moon from materials available there.
- Build a pretty big mass driver for the project capable of launching 1 km chunks of the construction (basically 1 km long train).
- perfect the VASIMR drive for the purpose of the project, mass assembling it on the moon.
- Develop the ring project in details, and define what you actually like to do with it. (there are different goals and different types of construction to achieve them — no need to imagine it as a single big tube around another space body).
- Mass produce and mass launch parts (about 100 per day)
- Connect the ring.
Now the goodies and fun part
The ring may do not have a rotor like a launch loop do, but instead it may be the rotor itself with an external shell. The shell may have same angle velocity as earth have, as its goal is to isolate (as in Hyper Loop project idea) this train from the atmosphere at that altitude.
The ring can change the size of its orbit, and shrink it to equatorial 100×100 km orbit from a 1000×1000 km equatorial one. It will be Hyper Loop train but kinda upside down floating in the air at 100 km height above earth surface, with speed of the train inside it at about 8 km/s.
The pressure at that height is about 1/128000 of 100 kPa, thus if somehow external shell will be corrupted (leak or something), nothing super exciting will happen, and there will be plenty of time for repair.
The good thing about the construction is: it does not have to change orbit as whole construction, it can have just some parts of it closer to earth.
Mad picture, made by me, but better one bad picture than 1000's of words:
The good thing about having the Ring is that compared to the conventional space elevator, it can have multiple lifting points, and those lifting points may lift more per same amount of time compared to the conventional space elevator.
120 km of glass fiber+aluminum matrix cable (1.7 GPa, 2600 kg/m3) will weigh about 9000 metric tonnes, for lifting 1000-tonne payloads+crawler with safety factor 1.3, and will have 1:3 taper.
We are pretty close to be able to build such a ring, but we probably won't until we have a reliable technology of building it.