A space mirror is a satellite that reflects sunlight toward a specific target on the ground. The concept is a century old. The first experiments flew in the 1990s. The first commercial attempt is underway now.
A space mirror — also called an orbital mirror, solar reflector satellite, or space reflector — is a spacecraft whose primary payload is a large, lightweight reflective surface. Rather than carrying cameras, communications equipment, or scientific instruments, its job is to catch sunlight and redirect it toward a chosen point on Earth.
The reflective surface is typically a thin metallic film, most commonly aluminised Mylar or a similar material, deployed to a large area in the vacuum of space. Because the material is extraordinarily thin — a few micrometres — a large mirror can be launched in a compact form and unfolded once in orbit. A mirror with a surface area of hundreds of square metres can fit inside a small satellite launch package.
In low Earth orbit (LEO), at altitudes of 400–600 km, a well-pointed space mirror can add measurable light to a ground area during the minutes it takes to pass overhead. The reflected beam cannot be focused more tightly than the angular diameter of the Sun (approximately 0.53°), which sets a minimum ground footprint of around 5–8 km across. See our How Bright page for the full geometry and brightness calculations, and our Glossary for definitions of the technical terms used throughout this article.
The fundamental principle is specular reflection — the same physics as a household mirror, applied at orbital scale. A highly polished flat surface reflects incoming light at an angle equal to the angle of incidence. By precisely controlling the orientation of a mirror satellite, an operator can direct reflected sunlight toward a chosen point on the ground.
Three physical constraints govern what a space mirror can and cannot do:
The beam cannot be focused below the Sun's angular size. The Sun subtends 0.53° of arc as seen from Earth. No mirror, regardless of quality, can produce a reflected beam narrower than this — the beam is a geometric projection of the light source. At 500 km altitude this translates to a minimum ground spot approximately 4.6 km across. A space mirror delivers sunlight spread over a large area; it cannot concentrate it to a point. See Solar Economics for the energy delivery implications.
The satellite is always moving. In LEO, orbital velocity is approximately 7.6 km/s. A mirror satellite passes over any given ground point in 3–5 minutes before it moves on. Providing longer illumination over a fixed location requires a constellation of many satellites handing off coverage sequentially — each entering the geometry as the previous one departs. Reflect Orbital's proposed constellation of 57 satellites is designed around this requirement.
Geometry constrains when a mirror can operate. The mirror satellite must be in sunlight while the ground target is in shadow — otherwise there is no contrast between mirror light and ambient daylight. This limits the operational window to the hours around dawn and dusk, when the satellite is illuminated but the ground below is dark. The terminator — the day/night boundary — is where space mirrors have their effect. For the full brightness calculations, see How Bright?
Four distinct applications have been proposed across the history of the concept, each with different requirements and different levels of current interest:
Redirecting sunlight to ground-based solar farms during the hours before sunrise and after sunset, extending their effective operating window. This is the application Reflect Orbital is pursuing with its Eärendil-1 mission. The economics depend on the value of electricity at dawn and dusk — times that often coincide with peak grid demand. See Solar Economics for the full analysis.
Providing temporary light to large outdoor events, sports competitions, or emergency situations. The 2018 Chengdu proposal from China framed its mirror primarily as a way to illuminate the city at night, functioning as an "artificial moon." The practicalities — 3-to-5 minute pass duration, diffuse beam — make this application limited without a large constellation.
Illuminating disaster areas or search zones during nighttime operations, providing temporary light for rescue teams without requiring ground infrastructure. The Soviet Znamya programme in the 1990s included humanitarian illumination as part of its stated rationale. Pass duration and footprint area limitations remain the primary constraints.
Providing battlefield illumination during night operations without satellite communications or ground-based lighting infrastructure. Various military space programmes have studied the concept since the 1960s. Whether a space mirror deployed for military use falls within or outside the Outer Space Treaty's prohibitions is a matter of ongoing legal debate. See Controversy for more detail.
The objections to space mirrors are not primarily technical — the physics works, and the engineering is achievable. The disagreements concern what those mirrors do to the sky, to ecosystems, and to international norms.
Astronomers are the most organised opposition. A bright space mirror during a pass can saturate the detectors of telescopes working on faint objects, and a large constellation could raise the background brightness of the night sky measurably. The International Astronomical Union has raised these concerns formally. For the scientific community's specific objections, see Astronomy Impact.
Ecologists raise concerns about the effects of introducing artificial illumination to nocturnal ecosystems — disrupting the circadian rhythms of animals, affecting plant flowering cycles, and altering insect behaviour. Unlike a streetlight, a space mirror's light falls on wilderness areas and dark-sky preserves that have no local lighting at all. See Light Pollution for the research on biological impacts.
Regulatory sceptics argue that the governance frameworks — the FCC, the ITU, COPUOS — were not designed with space mirrors in mind and have not adapted to assess them properly. The FCC's categorical exclusion of satellites from environmental review is the most criticised gap. See FCC Regulation for how the licensing process works in practice.
For the full range of positions, see our Controversy page and Night Sky Future.
Reflect Orbital is the only company with an active commercial space mirror programme as of 2026. Its Eärendil-1 mission — a single demonstrator satellite — is the first attempt to commercially validate the solar-farm-extension business case. The company, founded by Ben Nowack, has filed with the FCC and is proceeding through the regulatory process. See the Eärendil-1 mission page for current status and the launch date tracker for the latest schedule.
The 2018 Chengdu proposal from Chengdu Aerospace Science and Technology Microelectronics System Research Institute in China attracted substantial media attention but did not advance to deployment. It proposed an "artificial moon" satellite large enough to replace street lighting over a city. See the Chengdu page for what was proposed and why it did not proceed.
The Soviet Union flew the only previous hardware: the Znamya series of experiments in 1993 and 1999, using Progress cargo spacecraft retrofitted with a deployable aluminised film mirror. Znamya 2 (1993) successfully reflected a beam across Europe and North America; Znamya 2.5 (1999) failed to deploy fully. See the Znamya History page for the complete record.
Konstantin Tsiolkovsky described orbital mirrors in his theoretical writings on space travel. The concept appears in his broad vision for the long-term use of space, predating any hardware capability to realise it by decades.
Hermann Oberth proposed a "space mirror" (Weltraum-Spiegel) in his book Wege zur Raumschiffahrt (Ways to Spaceflight). Oberth's design envisioned a large orbital mirror capable of illuminating ground areas and even melting ice in northern sea lanes. This was the first detailed engineering conception of the idea.
The Soviet/Russian Znamya 2 experiment deployed a 20-metre aluminised film mirror from a Progress spacecraft and reflected a 5-km-wide beam across Europe and North America at night. It was the first demonstration that the concept was physically achievable. Full Znamya history →
A second Russian experiment, using a larger 25-metre mirror, failed when the film snagged on an antenna during deployment. The programme was not continued. No further hardware experiments flew until the current decade.
A Chinese research institute proposed an "artificial moon" satellite for the city of Chengdu, framed as a replacement for street lighting. The proposal generated significant international media coverage. It did not advance to hardware. Chengdu page →
Reflect Orbital's Eärendil-1 is the first attempt to commercialise space mirrors for solar energy augmentation. It is proceeding through US regulatory review as of 2026. Eärendil-1 → · Full timeline →