Do you have any comments or suggestions?
The most developed idea is
https://en.wikipedia.org/wiki/Stratospheric_aerosol_injectio...
to build a new aircraft that can fly somewhere around 80,000-100,000 feet. There is no civil or military airliner today that fits the requirements but suitable engines do exist. Developing such a new aircraft wouldn’t be as expensive as developing a new airliner or fighter jet because it doesn’t face fierce competition in terms of operating costs or military supremacy.
See https://iopscience.iop.org/article/10.1088/1748-9326/aae98d
The budget to get started is the low billions, possibly accessible to smaller and poorer “hot zone” nations such as Bangladesh.
My favorite idea is the L1 sunshade. I made some notes on the issue of how you would build a factory that builds a factory that turns a carbonaceous chondrite asteroid into metallized plastic films. Interesting the chemistry overlaps a lot with petrochemical factories, fuel synthesis from coal, and techniques of carbon capture and utilization. There are the “large” but general problems of reproducing all the unit processes of chemical engineering in zero gravity and also some problems like building a ship in a bottle except you are inside the bottle and the ship is outside.
Two specific but hard problems are:
(1) CC asteroids probably contain a lot of trapped gas which could escape when you disturb them. Ideally you would extract the gas (not only precious but possibly dangerous) first thing but you won’t be able to build anything out with local materials at the beginning (say storage tanks, you are going to be building lots of storage tanks…)
(2) Do you send people or do you control the whole operation remotely? It is pretty obvious that sending people on a several year voyage to something that isn’t even a planet is dangerous and expensive. On the other hand you could have a human outpost in cislunar space with mock-ups of the equipment and somehow deal with 10+ min of teleoperation latency.
I man not so sure at this point if I should try to turn it into a serious proposal or use it as the background for a sci-fi story.