40 year old energy proposal
Solar power satellites are one of perhaps 2 techniques that scale into the size needed to replace fossil fuels.
They have been held up on either the high cost to ship millions of tons of materials into space or the decades long effort to build up a space industry to supply them.
The critical number for power satellites to make sense is about $100/kg. In the last year there have been two proposals that look like they would reach this goal. One of them uses a small rocket (300 tons) lifting off every 15 minutes with a laser propulsion stage. The laser stage goes goes to GEO delivering 100 tons per hour.
It's a big project, but we either solve the energy problems or we starve and freeze in the dark.