The most insane idea anyone had ever had was to harvest the sun.
By some measures, solar tech started in the late 1800s.
Nowadays, we have better technologies . . .
By the late 2200s, it was finally realized by general industry that rockets would be lighter without the rockets themselves. Specifically, that external propulsion is (literally) infinitely more capable than onboard propellant. And lightsails are surprisingly practical. I received the best demonstration of this in school. They took a 1cm2 piece of gold foil and put it outside. Then they shone a handheld 5mW IR laser at it. Visibly, it started to accelerate. Neato!
The big UV laser grids on Mercury were built in the 2340s, but before the Hermian daylight could be managed with subterranean active cooling, the first interplanetary laser launch system was built in a 1-lightminute-radius solar orbit in 2322.
I.S.S. Inferno was worthy of the name. The thing was basically a spiderweb of radiators and solar panels built around a single gas-core Argon laser. The laser was chosen for its durability and efficiency, but even when inactive, the radiators constantly, ominously glowed, frantically dissipating the sun’s burning wrath.
The green-blue light stabbing forth against a patterned trellis of orange, yellow, and black, was possessed of a futuristic beauty.
(My modeling skills are bad; so sue me. Absurdly strong laser beam is visible due to Rayleigh scattering within the absurdly thin interplanetary medium, as well as stray dust particles (visible in the foreground).)
Ed. note: I derived this equation for acceleration of orthogonal, mirrored lightsail: a=2*r*W/(c*kg), r=reflectivity, W=intercepted power, c=speed of light, kg=mass.