Can a mirror in space light up the night?

In three steps, and the third one sets the size of the spot.

1. Still sunny

   After sunset your ground is in Earth’s shadow, but a satellite 600 km overhead stays in sunshine until the Sun is about 24° below your horizon. That is why you see satellites shine after dusk.

2. Tilt

   At dusk the sunlight reaches the satellite sideways. A flat mirror turned about 45° sends it straight down.

3. Spread

   Sunlight comes from a disc about 0.53° wide, not from a point. So the reflection spreads at 0.53° on the way down and lands as a spot.

A flat mirror works like a pinhole: each point of the Sun’s disc sends its own ray, and after the mirror those rays arrive at slightly different places. The spot is an image of the Sun. From 600 km it is 600 km × tan 0.53° ≈ 5.6 km across, plus the width of the mirror itself, a few dozen meters at most. A bigger mirror makes the image brighter, not smaller. Only a curved mirror or a lower orbit could shrink it.

The light that lands is the sunlight the mirror catches, spread over the spot. Direct sunlight at the ground is about 100,000 lux. An aluminized film reflects about 90%, and a mirror turned 45° to the sideways dusk sunlight catches about 71% of what it would face-on. Divide by the spot’s 25 km² and an 18 m mirror gives about 0.84 lux. A full moon high in a clear sky gives about 0.26 lux, so that is about 3 full moons.

The 54 × 54 m mirrors Reflect plans to build toward have nine times the area, so about 7.4 lux, close to a lit residential street (European road-lighting class P3 asks for 7.5 lux). A cloudy day is roughly 1,000 lux or a few times that: in the model, 400 of the 54 m mirrors on one spot give about 3,000 lux. Matching direct sunlight would take about 13,500.

Not everyone agrees on the first number. Reflect says its test mirror would give about the light of a full moon. Kocifaj, Bakos and Kundracík estimate about four full moons, and about 9.5 lux for a 54 m mirror. Our model, a perfectly flat mirror straight overhead, falls between the two.

A few minutes. At 600 km a satellite moves at about 7.6 km/s and laps Earth every 96.5 minutes. It is more than 30° up in a spot’s sky for only about 4 minutes; lower than that it is farther away, so its light spreads thinner. Reflect says each pass of its demonstration would last up to about five minutes, within two hours of sunset.

For light without gaps, a new mirror has to arrive every 4 minutes: about 24 spaced around one orbit. Earth turns under that orbit, so lighting a place for hours takes many such orbits. That is why the company talks of thousands of satellites, and eventually up to 50,000.

Reflect Orbital, a startup in Hawthorne, California, says it wants to sell sunlight after dark: to solar farms after sunset, to rescue crews and disaster zones, and to work sites. The US Federal Communications Commission approved its first demonstration satellite, Eärendil-1, in July 2026: an 18 × 18 m mylar mirror, meant for an orbit about 600 km up. As of late October 2026 it had not launched. The company has tested a small mirror under a hot-air balloon by day, and in October 2026 staged a night mock-up near Mount Whitney with ground spotlights, not mirrors.

Air scatters part of every beam, so the glow spreads beyond the spot. For one 54 m mirror, Kocifaj and colleagues estimate a sky brighter than a full-moon night out to about 14 km, and for 400 mirrors on one spot a glow visible from 80 km. The American Astronomical Society and dark-sky groups also warn of eye damage to people looking through telescopes and of bright moving points across the sky. Reflect says it will light only approved sites, keep light away from observatories and protected habitats, switch the light off quickly and turn idle mirrors edge-on, and it disputes some of the study’s assumptions.

1. 1 See

   Open Space Mirror Lab and tap Reflect’s 18 m test mirror. Spot width reads 5.6 km and Brightness 0.84 lux, about 3 full moons. Each pass lasts 4.1 min.

2. 2 Change

   Tap Planned 54 m mirror. The spot stays 5.6 km wide; the brightness rises to 7.4 lux, like a lit street. Now tap Higher orbit: 1,200 km: the 18 m mirror’s spot doubles to 11 km and the light falls to 0.21 lux.

3. 3 Understand

   Tap 400 mirrors on one spot: about 3,000 lux, a cloudy day. Then Daylight? 13,500 mirrors reaches sunlight. For non-stop light shows how many mirrors that needs in orbit when each one passes in 4 minutes.

Space Mirror Lab assumes a perfectly flat mirror straight above the spot, a clear sky and the Sun on the horizon. A thin film that wrinkles or sags spreads the light wider and dimmer. Lower in the sky the mirror is farther away and the spot stretches: at 30° up it gets about a sixth of the light. Later after sunset the mirror has to turn further and catches a little less. Haze, pointing errors and the light scattered into the sky are left out.

The valley is an invented desert map, and the side view draws the cone 40 times wider than 0.53° so it shows; the map from above is to scale. The film uses short, credited excerpts of Reflect Orbital’s balloon test (@bennbuilds on X; a daytime test, not from orbit) and satellite trails stacked from two hours of photos over Mt Taranaki (@blarsenphoto on TikTok); the valley, Earth, mirrors and numbers come from the model.