US startup plans to deliver ‘sunlight on demand’ after dark

A proposed constellation of satellites has astronomers very worried. Unlike satellites that reflect sunlight and produce light pollution as an unfortunate byproduct, the ones by US startup Reflect Orbital would produce light pollution by design. The company promises to produce “sunlight on demand” with mirrors that beam sunlight down to Earth so solar farms can operate after sunset.

It plans to start with an 18-m test satellite named Earendil-1, which the firm has applied to launch in 2026. It would eventually be followed by nearly 4,000 satellites in orbit by 2030.

So how bad would the light pollution be? And perhaps more importantly, can Reflect Orbital's satellites even work as advertised? Bouncing sunlight in the same way you can bounce sunlight off a watch face to produce a spot of light, Reflect Orbital's satellites would use mirrors to beam light onto a patch of Earth. But the scale involved is vastly different.

When you bounce light off your watch onto a nearby wall, the spot of light can be very bright. But if you bounce it onto a distant wall, the spot becomes larger, and dimmer as the Sun is not a point of light, but spans half a degree in angle in the sky. It means at large distances, a beam of sunlight reflected off a mirror spreads out with an angle of half a degree. Let's take a satellite reflecting sunlight over a distance of roughly 800km as a 625km-high satellite won't always be directly overhead, but beaming the sunlight at an angle. The illuminated patch of ground will be at least 7km across.