Space Mirrors Could Outshine Full Moon by Four Magnitudes

Space mirrors meant to light up chosen spots on Earth at night have their first detailed accounting of the consequences. Miroslav Kocifaj, Gaspar Bakos and Frantisek Kundracik calculated how much light a single satellite from the California company Reflect Orbital would add to the atmosphere. The paper, titled “Atmospheric Light Pollution by Proposed Reflect Orbital Space Mirrors,” was posted to arXiv on August 6, 2026, under the identifier 2608.06433, and has been accepted for publication in Astrophysical Journal Letters. Until now the constellation had only been discussed in general terms. Now every kilometer of distance comes with a number attached.

The company’s plan is no secret. The pilot satellite Earendil-1 carries a mirror measuring 18 by 18 meters and flies at an altitude of about 600 kilometers, with the job of lighting a circular patch of ground with a radius of 2.5 kilometers. That satellite is the preview for a constellation of roughly 50,000 larger mirrors, each measuring 54 by 54 meters. The paper runs its numbers on that production version.

What Minus 16.7 Magnitude Means

Apparent magnitude measures how bright an object looks from Earth. The scale runs backward from intuition: the smaller the number, the brighter the object, and a difference of five magnitudes means a hundredfold difference in brightness.

For an observer caught inside the beam itself, a 54 meter mirror would reach an apparent magnitude of minus 16.7. That is roughly four magnitudes brighter than the full Moon. The diffuse background of the sky at that moment would resemble dusk right after sunset, too bright to see even the brightest stars through it.

How Far Does the Glow Reach?

From a distance of 14 kilometers, the glow of a single mirror would outshine a full Moon’s sky brightness across most of the sky. At 34 kilometers, the sky in the direction of the beam would still be brighter than under a full Moon. If 400 mirrors lit the same spot at once, a scenario the authors specifically model, the glow would be noticeable even from 80 kilometers away.

The paper’s conclusion is that a single satellite changes the night environment out to roughly 30 kilometers from the illuminated point. The beam itself is narrow, but what it scatters into the air is not.

How the Calculation Was Made

The model tracks two paths by which light reaches an observer standing outside the beam. The first is scattering of the incoming beam in the air, both Rayleigh scattering off gas molecules and aerosol scattering off dust particles and droplets. The second is light reflected by the illuminated ground itself and bounced back up toward the sky.

The calculation was repeated across different atmospheric conditions and different values of albedo, meaning ground reflectivity, since snow and asphalt do not send back the same amount of light. Every value in the paper assumes cloud free conditions, a limit the authors state themselves.

Who Decides What the Night Sky Looks Like

The US Federal Communications Commission approved Earendil-1 in July 2026. The approval went through despite more than 1,800 public comments and formal objections from the American Astronomical Society and dark sky protection groups. The commission’s position was that light pollution and interference with astronomical observation fall outside its jurisdiction.

No other body has picked up that responsibility, so the question of what the night sky looks like above someone’s house currently has no regulator handling it at all.

What This Paper Does Not Prove

This is a calculation, not a measurement. The numbers come out of an atmospheric model and assumptions about ground albedo, not from observing a satellite already in orbit, so the first real measurements could turn out different in either direction.

The minus 16.7 magnitude figure does not apply to Earendil-1 itself, but to the larger 54 meter mirror from the announced constellation. The pilot satellite is smaller, and the paper does not equate its brightness with that number. The paper also says nothing about how many mirrors will actually be launched, nor does it measure the effects on animal behavior or human health. It measures one thing only, how much light such a satellite adds to the night atmosphere, and gives that measure per kilometer of distance.