One way to terraform Venus would be to create a massive sun shade enveloping the planet. How much actual material would be required for this? And where would you get it from?
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About 53-ish cubic kilometers of material, assuming that it is only 0.1mm in thickness.
Of course, how you would keep it on station or keep the solar wind from blowing that uber thin 13,000km disk into the planet would be a different problem.