A single large mirror placed in orbit could make the night sky brighter than a full-Moon night across a significant area.
This estimate concerns the project by Reflect Orbital, which plans to send mirrors into space to redirect sunlight toward the ground. An initial experimental satellite is expected to use an 18-by-18-meter mirror. Ultimately, the proposed project envisions much larger structures, reaching 54 meters on each side.
The principle seems simple: even after sunset, a satellite high enough can still receive the Sun's rays. Its mirror then redirects them toward a selected area on the ground. For the first demonstrator, located at an altitude of about 600 km, the illuminated area is expected to have a radius of approximately 2.5 km.
Yet not all of this light reaches the target alone. Some of it encounters air molecules and tiny particles in the atmosphere. It is then scattered in different directions. The illuminated ground also reflects some of the light back toward the sky, producing a visible glow around the area.
Three researchers calculated this light pollution for different atmospheres and different types of ground, under cloudless conditions. In their model, a 54-meter-wide mirror viewed from the illuminated area would appear about 40 times brighter than the full Moon. The sky background would resemble that of twilight shortly after sunset.
Under these conditions, even the brightest stars would become invisible to anyone located in the beam. But the effect would not stop at the edge of the directly illuminated area: at 14 km, the calculated glow would exceed that of a full-Moon sky across most of the sky.
At 34 km, the direction of the illuminated area would still be brighter than a moonlit sky. The researchers also studied a scenario in which 400 mirrors illuminated the same location simultaneously. The glow would then remain clearly perceptible up to approximately 80 km away.
The study, accepted for publication in The Astrophysical Journal Letters, therefore provides figures that can be used to assess the impact before any potential large-scale deployment. These data could help examine the consequences for astronomical observations and for nocturnal environments around illuminated areas.