A US startup has booked two Indian rocket launches to test a planned service no company has sold before: sunlight at night.

California-based Reflect Orbital plans to place giant mirrors in low-Earth orbit, and use them to steer reflected sunlight onto selected locations on the ground.

The company wants to sell space-based illumination for a wide range of purposes, from keeping solar farms generating energy after sunset to search-and-rescue operations. Its first orbital demonstrator, Eärendil-1, will carry an 18-by-18-metre reflector.

Using space mirrors to deliver useful illumination will require overcoming formidable engineering challenges. But there’s another pressing question: who will decide where sunlight redirected from space is allowed to hit the ground?

The light does not stop at the edge of the beam

You might think a space mirror could illuminate a customer’s solar farm or a rescue site while leaving everywhere else untouched. That’s not quite how it works, because Earth’s atmosphere doesn’t respect sharp boundaries.

A peer-reviewed scientific study published last August modelled what happens when reflected sunlight passes through our atmosphere. Molecules and aerosols scatter some of the light beam across the surrounding sky. Light reaching the ground is reflected upwards and then scattered again.

This means the light can affect a much wider area........

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