Terraforming Venus

...build a round sunshield of lunatic aluminum, very thin material, only 50 grams per square meter and yet still totaling 3 x 1013 kilograms, the largest thing ever built by humans. Concentric strips give the sunshield flexibility and allow it to tack up into the solar wind to hold its position at the L1 point, where it will shadow Venus entirely. Deprived of insolation, the planet will cool at a rate of five K a year. After 140 years, the CO2 atmosphere will have rained and snowed to the su...