It’s been estimated that just one kilogram of sulphur could offset the warming effect of several hundred thousand kilograms of carbon dioxide. In the following years, Earth cooled by a few tenths of a degree. It happened as recently as 1991, when the eruption of Mount Pinatubo unleashed thousands of tonnes of sulphur dioxide gas. The particles stick around for years, increasing the density of the atmosphere and reflecting sunlight back into space. Angel’s solution is so outrageous, for years it was thought to be impossible because it defied the laws of physics: a giant electromagnetic gun embedded in a mountain. In all we’d need to fire 20-million-tonnes-worth into outer space – still too heavy to be feasible by chemical rocket. “If you leave them alone they’ll drift off and eventually fall back to Earth,” he says. After that they’d need regular nudges from ‘shepherd dog satellites’ to stop them crashing into each other or being blown off course by the sunlight they’re deflecting. The robots would steer themselves into orbit by solar-powered ion propulsion, a technology already used by the European Space Agency's Smart-1 Moon orbiter, to form a cylindrical cloud 60,000-miles wide. “You can’t stop sunlight with anything thinner than that,” he says. To keep the burden low, the lenses would be less than a hundredth of the thickness of a human hair. Each would weigh about a gram – the same as a large butterfly – and deflect sunlight with a transparent film pierced with tiny holes. To be a viable option, the technology would need to be wafer-light and it would have to be assembled here on Earth.Īstronomer Roger Angel believes he has the answer: 16 trillion flying space robots. It currently costs at least $10,000 (£6,930) to launch a pound of payload into orbit, and we haven’t put a man on the Moon since 1972. By far the greatest challenge is getting the sunshade into outer space.
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