Space Mirrors Meant to Beam Sunlight to Earth Could Brighten Night Skies for Miles, Study Finds

A company that plans to beam sunlight from space to Earth on demand might unintentionally brighten the night sky for many more people than intended, according to a new study accepted for publication in the Astrophysical Journal Letters.

The US company Reflect Orbital plans to launch a test satellite called Eärendil-1 later this year, extending an 18-by-18-metre mirror in orbit, to test the feasibility of its plan to eventually launch up to 50,000 larger satellites measuring 54 by 54 metres that would reflect sunlight to Earth on demand. The launch was approved by the Federal Communications Commission (FCC) in July. Reflect Orbital has said the goal is to prolong hours of sunlight for uses including solar panel charging, emergency response and military activities.

Miroslav Kocifaj, an astronomer at the Slovak Academy of Sciences, and colleagues calculated the extent to which light beamed to the ground by the satellites would scatter. They found that within the beam's intended target area, a circular patch five kilometres across, a single Reflect Orbital satellite would appear about 40 times brighter than the full moon; as far as 14 kilometres from the centre, it would still be as bright as the full moon. Combining the beams of 400 satellites, which Reflect Orbital eventually plans to do, would produce light as bright as 10,000 full moons within the five-kilometre area, or 2.4% as bright as the sun, and would be visible as a glow above the horizon up to 80 kilometres away.

"Deploying these mirrors would be seriously damaging for astronomy and for the nighttime environment," Kocifaj said. "The damage extends far beyond the target area." Reflect Orbital chief executive Ben Nowack disputed the findings, saying "some assumptions are simply inaccurate" and that the company's safeguards, including exclusion zones, account for scattering; Kocifaj said the company has not provided data, models or assumptions to support that claim.

Olivier Hainaut, an astronomer at the European Southern Observatory, had previously modelled the broader effect of Reflect Orbital's beams and found they could brighten the sky by up to 300% worldwide; he said the two studies "really cover most of it."