Supposing we wanted to create a Saturn-like ring system around the Moon. Could we do it with today’s technology? If so, then what’s the lowest cost solution?
How visible would the rings be and what might it look like? Could we get fancy and do multiple rings at different inclinations? How stable would it be (i.e. how long before it all comes crashing down?)
Here’s how I see it working: we shoot unmanned rovers to land on the Moon like the Mars Pathfinder. They scoop up regolith and throw or shoot it into lunar orbit. Repeat.
The tricky part is getting the rocks into a stable selenocentric orbit. We would need a course correction, perhaps using a free gravitational assist from a seed object that we already put into lunar orbit?
I suspect we would need to use one of the four frozen orbits (or perhaps all four for a fancy four-ring configuration). Assuming we can somehow get a rock there, then it’s just a matter of time before we can get enough rocks to form a ring system.
If this runs continuously, how long until we get a fully grown, visible to the naked eye Lunar Ring System?
To make a ring system, first establish an industrial infrastructure on the Moon. Use this to construct solar power collectors and mass-drivers, then start launching material into orbit, while also continuing to build more power collectors and more mass-drivers. Eventually you’ll get your ring system.
But note that it is the construction of power collectors, mass drivers and manufacturing capacity in general that is the important part of this process; the more mass drivers and power collectors you have on the Moon, the faster you’ll get the job done.
And of course you could use the launched material to make more useful things than a ring system - this strategy is basically the same as that described by Gerard K O’Neill in The High Frontier, except he was describing how to make orbiting habitats. A ring of orbiting habitats might be worth doing; a ring of interplanetary spacecraft ready to carry colonists to Mars might be even more useful.
I have heard it stated (but can’t cite) that there are no stable lunar orbits. By the time you are orbiting closely enough to the moon’s surface to avoid perturbation from the earth, you suffer perturbation from lunar surface irregularities.
No lunar rings would be possible, if this is the case.
(The Apollo command module orbited the moon, but only for a very short period of time; the orbit would have degraded in a few months.)
The OP mentioned the four ‘frozen orbits’ that are reasonably stable; see
this causes a problem, since the stable orbits are so thin they will barely be visible from Earth. You’d need to spread material into the unstable orbits as well, resulting in a banded structure to the artificial ring - something similar to Saturns’ rings, which have gaps of instability caused by the influence of the various moons.
I’d guess you could create visible rings in a few hundred years of determined productivity; but the rings would probably be very short lived, and only last a few hundred years themselves.
To do this efficiently and in an affordable manner, we need a form of technology that is theoretically possible but not available today. Eric Drexler, back in his youth in the 1970s, read books and journals, including some by O’Neil. He realized that in order for this to be practical, you need to be able to put a factory on the moon that can make all of the parts used in itself.
That way, you only have to pay to land one factory on the moon, and it can self-propagate after that. For various reasons, the factory would need machinery that rearranges matter with atomic precision for this to work, and the subject is now known as “molecular manufacturing” or more commonly, “nanotechnology”.
Without such technology, which is decades away, to do what you are describing would cost a phenomenal amount of money. Basically, it currently costs $10,000/Kg to get anything into Low Earth orbit. Landing on the moon is even more costly, I recall that the fuel : payload ratio was about 6 :1 for the lunar landers. This means you need to pay for $60,000 worth of fuel tanks for every Kg you place on the moon.
Even the smallest quenchgun, if you had to manufacture every part on earth and then land them on the moon (and the only way you would be able to avoid that is if you could put a high precision-factory on the moon. This would require a massive plant with today’s technology, because quench-gun coils are made with high temperature superconductor wire) it would weigh 213,000 kilograms.
That would cost 2 billion dollars just for the launcher, and realistic costs would be far higher.
Note also that if you throw stuff up in the air (ok, sky from a moon perspective) it will fall back down again.
The only way to achieve orbit is to accelerate it in a horizontal direction once it has gained some altitude. I would venture that this would be highly impractical if your goal was to make small particles orbit.
The only way I see of achieving this would be to insert some kind of craft into a stable orbit and from there release a payload of particles. The problem then would be getting the particles to disperse into some kind of ring-like configuration. Left alone they will merely orbit in a clump with the craft. Any force that causes them to disperse would at the same time cause them to move away from the orbit in which you placed them. Ie, no ring.
And I may be wrong, but I think that someone will be along soon to tell us there is no such thing as a stable lunar orbit. (The three body problem and all that.) I am pretty sure that craft like the Lunar Prospector needed to make continual adjustments to stay in orbit and that the reason for the LP’s short operational life was in part related to limited fuel load.
How did we get the moon then? Horizontal powered acceleration does not seen necessary as the current theory of how the moon came about avoided this, and what created the moon could similarly cause a ring instead. And after all this is exactly what the OP wishes to create.
Which may be possible, for a short time, if a large enough asteroid could be nudged into such a collision.
The event that caused the Moon presumably did create a ring at first, with a whole bunch of chunks of debris interacting with each other. These interactions would have caused most of the chunks to either crash back into the Earth or go flying off into the deep, but some stayed in orbit. Then, those of them which were above the Earth’s Roche limit coalesced together, and formed the Moon.
In order to get a persistent ring system that doesn’t just coalesce into a single moon, you need to be below the parent object’s Roche limit. If your orbiting material has about the same density as your parent object, this is about 1.5 times the parent object’s radius (or half the radius above the surface).
What happens if material is put into all the stable orbitals at the same time? Do you get a fancy looking ring system with multiple inclinations? What would that look like from Earth?
If you already have a partially formed ring, you could throw stuff into the air at faster than escape speed, aimed a the ring. At least some of your objects will collide with the ring and slow down, and some of them will enter orbit.
(Of course your statement is generally true. If you launch an inert object from the surface at below escape speed, it immediately enters an elliptical orbit which goes through the launch site. So it must intersect the surface of the moon/planet it was launched from. If you launch it at faster than escape speed, then it will never fall back down, but it won’t stay in orbit either. It will fly away.)