Stardust is the name of a small startup with enormous ambitions. The company, which is based in Israel and registered in Delaware, proposes to do nothing less than dim the sun. Its business plan is modelled on volcanoes. In a major eruption, millions of tons of sulfur dioxide get thrown up into the stratosphere. There, the gas reacts to form droplets of sulfuric acid that scatter sunlight back to space. The result is that less energy reaches the Earth and the planet cools. After Mt. Pinatubo, in...