Residents near Lone Pine, California, looked up from the Owens Valley one night this month and saw a blinding glare pouring down from the Sierra Nevada, lighting up a wide stretch of desert. One local described it as "blinding". It was, in fact, a rehearsal for a business model that does not yet exist. The glare came from large mobile lamps set up near Mount Whitney by Reflect Orbital, a US startup that wants to launch 50,000 satellites over the next decade.

Each would carry a mirror to bounce sunlight onto chosen places on the dark side of the planet. In July, the Federal Communications Commission approved a first demonstration satellite, due to launch next year. Its 59-foot mirror is designed to throw beams about three miles wide, with a strength that can be adjusted from moonlight to the brightness of the midday sun. The company says search-and-rescue crews will be its first customers, and it ran the test with a local rescue team.

Other proposed uses include lighting crops, replacing broken streetlights and powering military operations, all ordered from an app. Environmentalists and astronomers are far less enthusiastic. They argue that the company and the regulator are risking a deliberate change to the boundary between night and day, with possible effects on human sleep, on wildlife that depends on darkness, and on telescopes that need a dark sky. Light pollution already hides the Milky Way from much of the world, and satellites are already streaking astronomers' images.