Can we build an enormous umbrella to dim the sun? Such a feat would be a megaproject on a scale like no other. It would take at least 400 dedicated rocket launches a year, for ten years (There have been 172 rocket launches by all nations so far in 2022). The project would weigh in at 550,000 tons: at its lightest. And it would be an ecological experiment that puts us all—the entire planet—in the petri dish, with high risk and high reward. But could such a project actually reverse climate change and bring us back from the brink of global disaster?