Can a mirror in orbit light up the night?

Model. A flat square mirror of side s sits at altitude h, straight above the spot, under a clear sky. Sunlight comes from the Sun’s disc, 0.533° across, so every point of a flat mirror sends out a cone 0.533° wide and the spot is D = s + h · tan 0.533°: 5.6 km from 600 km whatever the mirror’s size (the mirror adds only its own 18 or 54 m). The light at the spot is E = n · E_☉ · R · cos 45° · s² / (πD²/4), with E_☉ = 100,000 lux (direct sunlight at the ground), reflectivity R = 0.9 (aluminised film) and cos 45° because at dusk the sunlight arrives sideways, so a mirror that sends it straight down is turned 45° to it. For Reflect Orbital’s 18 m test mirror at 600 km that is 0.84 lux; for its planned 54 m mirrors, 7.4 lux. A full moon high in a clear sky gives about 0.26 lux, a lit residential street about 7.5 lux (European road-lighting class P3), a cloudy day about 1,000 lux.

Passes. A mirror can light a spot while it is high in that spot’s sky; the tool counts the time it is more than 30° above the horizon. At 600 km the satellite moves at 7.6 km/s and laps Earth in 96.5 minutes, and it stays above 30° for 4.1 minutes, so light without gaps needs a new mirror every 4.1 minutes: 24 spaced around one orbit. Earth turns under the orbit, so lighting a place for hours takes many such orbits.

Why the spot doesn’t shrink. A flat mirror makes an image of the Sun, as a pinhole does. The Sun is not a point but a disc half a degree wide, so its reflected image spreads at half a degree on the way down. A bigger mirror sends more light into the same image. Only a lower orbit could make the spot much smaller: from 600 km no mirror can beat about 5.6 km, and a curved one can trim no more than its own width.

The company and its critics. Reflect Orbital, a startup in Hawthorne, California, says it wants to sell sunlight after dark, for example to solar farms, rescue crews and events. The US Federal Communications Commission approved its demonstration satellite, Eärendil-1, an 18 × 18 m mylar reflector at about 600–650 km, in July 2026; as of late October 2026 it had not launched. Reflect says each pass lasts up to about five minutes within two hours of sunset, about as bright as a full moon, that it will avoid observatories and other protected sites and can switch the light off quickly; it plans thousands of satellites and eventually up to 50,000 of 54 × 54 m. Astronomers and dark-sky groups object. Kocifaj, Bakos and Kundracík (2026) estimate about 4 full moons for the 18 m mirror and 9.5 lux for a 54 m one, a sky brighter than a full-moon night out to about 14 km from one 54 m beam, and a glow visible from 80 km for 400 mirrors on one spot; the American Astronomical Society warned of eye damage through telescopes and brighter skies far from the target. The model here gives 0.84 lux (3.2 full moons) for 18 m: between the company’s figure and the astronomers’.

The pictures. Everything on the stage is drawn in code from the model; the valley is an invented desert map, and the cone is drawn 40 times wider than 0.53° so it shows. In the film the angles in the Sun-disc diagram are exaggerated too; the Earth cross-section and the orbit are to scale. Three short real clips appear only in the film, credited on screen, picture only: Reflect Orbital’s hot-air-balloon mirror test, flashing sunlight into a camera on the ground and seen from a drone with the mirror hanging under the basket (video by founder Ben Nowack, @bennbuilds on X, posted March 2024; a low-altitude test from a balloon at twilight, not from orbit), and satellite trails over Mt Taranaki, New Zealand, a two-hour stack of 578 photos of ordinary satellites (@blarsenphoto on TikTok). Only one mirror has lit the ground from orbit so far: Russia’s Znamya 2, a 20 m film mirror unfurled near the Mir space station in February 1993, which swept a spot about 5 km wide across Europe (Znamya).

Limits. No clouds, haze or atmospheric loss beyond what direct sunlight at the ground already includes; a perfectly flat mirror (a film that wrinkles or sags spreads the light wider and dimmer); the Sun on the horizon (deeper after sunset the mirror turns further and sends a little less); the satellite straight overhead (lower in the sky it is farther away and the spot stretches and dims, to about a sixth at 30°). Real passes, pointing errors and scattered light are left out.

See also Satellite Dusk: why satellites still shine after sunset.

Sources: Reflect Orbital, Reflect Orbital seeks collaboration with dark skies (Oct 31, 2026); M. Kocifaj, G. Á. Bakos and F. Kundracík, Atmospheric light pollution by proposed Reflect Orbital space mirrors (2026); Space.com, Reflect Orbital just got permission to launch its 1st space mirror; Astronomy, FCC approves Reflect Orbital filing for mirror in space; MIT Technology Review, Space mirrors and the night sky; Payload, Reflect Orbital responds to its critics; Wikipedia, Reflect Orbital, Angular diameter and Lux; EN 13201-2 road lighting classes, summary.