Space Mirror
Glossary

The apparent size of an object expressed as an angle rather than a physical dimension. The Sun has an angular diameter of approximately 0.53° as seen from Earth. This figure is critically important for space mirrors: because any reflected beam cannot be narrower than the angular diameter of the Sun, a mirror in low Earth orbit produces a ground footprint of at least 5–8 km across — there is no physical mechanism to concentrate sunlight into a tight point. See our How Bright page for the worked geometry.

A logarithmic scale measuring the brightness of celestial objects as seen from Earth, without correcting for distance. Lower numbers indicate brighter objects: the Sun is −26.7, the full Moon −12.7, Venus at its brightest −4.9, and the faintest naked-eye stars around +6.5. Each whole-number step represents a brightness factor of approximately 2.512. Reflect Orbital's Eärendil-1 is projected to reach magnitude −1 to −3 during a pass — comparable to Venus or, at peak, Jupiter.

The bending of light as it passes through the atmosphere, caused by the varying density of air at different altitudes. Atmospheric refraction causes celestial objects near the horizon to appear slightly higher than their true geometric position, and it affects the intensity and colour of light from low-angle sources. For space mirrors, atmospheric refraction is a minor but calculable factor in predicting where the reflected beam will actually fall on the ground, particularly during low-elevation passes.

The horizontal angle of a celestial object or satellite measured clockwise from north, expressed in degrees from 0° (north) through 90° (east), 180° (south), and 270° (west). Azimuth is one of the two coordinates used in the horizontal coordinate system to describe where an object appears in the sky; the other is elevation (altitude). Satellite tracking applications report azimuth and elevation together to help observers point telescopes or cameras at a passing satellite.

The rate at which a beam of light spreads as it travels through space. For reflected sunlight, beam divergence is set by the angular diameter of the Sun (0.53°) and cannot be reduced below this physical limit regardless of mirror quality. At 500 km altitude, this angular limit produces a minimum ground spot diameter of approximately 4.6 km. Laser systems can achieve much tighter beams, but space mirrors using reflected sunlight are fundamentally constrained by solar angular diameter. This is one of the core reasons space mirrors cannot function as precision illumination tools.

A nine-level numeric scale developed by amateur astronomer John Bortle in 2001 that measures the darkness of the night sky at any given location. Class 1 is the darkest possible sky; Class 9 is an inner-city sky. The scale is a practical tool for comparing observing sites and for quantifying the degradation caused by light pollution sources — including satellite constellations and space mirrors. A significant space mirror illumination event could temporarily push a Class 3 rural sky toward Class 5 conditions. See our Light Pollution page for detail.

A coordinated network of satellites operating together to provide continuous or near-continuous coverage of a geographic area. Unlike a single satellite, a constellation is designed so that as one satellite passes out of range, another is already in position. SpaceX's Starlink is the largest operational constellation; Reflect Orbital's proposed network of 57 mirrors would function as a constellation to deliver near-continuous illumination over a target area. See our SpaceX Satellites page for context on the regulatory precedents that large constellations have set.

The United Nations committee responsible for international cooperation in space. Established in 1959, COPUOS operates by consensus among member states and produces guidelines and legal principles rather than binding treaties. It oversees the five international space law treaties including the Outer Space Treaty of 1967. Because COPUOS cannot compel nations to deny satellite licences, astronomers pushing for international governance of bright satellite constellations and space mirrors must work through a slow consensus process. See our FCC page for how US-specific regulation works in parallel.

The moment when a celestial object or satellite reaches its highest point above the horizon during a given pass — the point of maximum elevation angle. For a satellite, culmination is usually the moment of closest approach and therefore peak brightness. A mirror satellite at culmination is nearest to the ground observer, which means both the brightest illumination and the shortest slant range through the atmosphere. Pass prediction tools report culmination time and azimuth to help observers know exactly when and where to look.

A designated area where outdoor lighting is controlled and restricted to protect the natural night sky. Dark-sky preserves are typically established by national parks, municipalities, or international designations (such as those from the International Dark-Sky Association). They are increasingly vulnerable to satellite brightness: unlike terrestrial light pollution sources, a mirror satellite passes over all dark-sky areas impartially, and no amount of local lighting control can shield a preserve from an overflying reflector. See our Astronomy Impact page for the scientific community's response.

The celestial equivalent of geographic latitude, measured in degrees north (+) or south (−) of the celestial equator. Declination is one of the two coordinates in the equatorial coordinate system, used alongside right ascension to specify a fixed position on the celestial sphere. For satellites, declination is less used than azimuth/elevation, but orbital inclination — which sets the range of declinations a satellite can reach — determines which latitudes on Earth the satellite can pass directly overhead.

The angle between a celestial object or satellite and the observer's horizon, measured in degrees. 0° is on the horizon; 90° is directly overhead (zenith). Satellite visibility, brightness, and pass duration all depend strongly on elevation: a satellite at low elevation has more atmosphere to pass through, appears dimmer and moves more slowly across the sky, while one at high elevation appears brighter and passes more quickly. Observers generally find 20° or higher to be the threshold for useful satellite observation.

A table or data set giving the computed positions of a celestial object or satellite at regular intervals of time. For satellites, ephemerides are derived from TLE data and propagator software (most commonly SGP4). The ephemeris is what satellite-tracking websites convert into the "rise at 22:14, culminate at 22:17" format visible to observers. Ephemerides have a limited accuracy window — satellite positions drift from their predicted paths within hours or days, and TLEs must be regularly updated to maintain forecast accuracy.

A defined angular area around a major observatory that satellite operators agree to avoid pointing their satellites or antennas at, to prevent radio frequency interference or, in the case of space mirrors, direct illumination of sensitive optical instruments. Exclusion zones are a voluntary or negotiated instrument — they require cooperation from the satellite operator and have no legal enforcement mechanism under current international law. Their practical utility for optical observatories threatened by space mirror illumination is limited, since a mirror in LEO passes over an observatory whether or not it is "pointed" at it.

The section of the US Code of Federal Regulations governing satellite communications licensing. Part 25 sets out the rules for applying for, maintaining, and transferring satellite licences, including requirements for orbital debris mitigation, spectrum coordination, and technical specifications. Any US-based company seeking to operate a satellite — including Reflect Orbital — must obtain FCC Part 25 authorisation before launch. See our FCC page for a detailed walkthrough of the licensing process.

A brief, intense brightening of a satellite caused by sunlight reflecting off a flat, highly polished surface — typically a solar panel or antenna — at precisely the right angle to direct a specular (mirror-like) reflection toward an observer. The original Iridium satellites were famous for producing predictable flares reaching magnitude −8 or brighter, briefly visible in daylight. Space mirrors are specifically designed to produce sustained specular reflections rather than accidental ones. The Znamya experiments in 1993 and 1999 were the earliest intentional demonstrations of this principle.

A circular orbit at approximately 35,786 km altitude above the equator in which a satellite's orbital period exactly matches Earth's rotation, making the satellite appear stationary from the ground. GEO is the standard position for television broadcast, weather, and communications satellites. Despite appearing "fixed," GEO is impractical for space mirrors: at 35,786 km the Sun's reflected beam spreads to a ground footprint hundreds of kilometres across — far too diffuse to deliver useful illumination intensity. Space mirrors must operate in LEO to produce any meaningful ground brightness.

The area on the Earth's surface illuminated by a space mirror at any given moment. Because reflected sunlight cannot be focused more tightly than the Sun's 0.53° angular diameter, the minimum footprint of a mirror in LEO is approximately 5–8 km in diameter. Larger or poorly-aligned mirrors produce proportionally larger footprints with lower intensity. Reflect Orbital's Eärendil-1 targets ground-level solar farm arrays, relying on the footprint overlapping continuously with a fixed installation during the 3–5 minute pass window.

The path traced on the Earth's surface directly below a satellite as it orbits. A satellite's ground track is determined by its orbital inclination, altitude, and the rotation of the Earth. Because the Earth rotates beneath a satellite in LEO, successive passes of the same satellite occur roughly 1,500–2,500 km westward of the previous pass. Ground tracks are used in mission planning to determine which cities or solar farms a constellation will overfly and at what intervals.

A device consisting of a mirror or set of mirrors that continuously tracks the Sun and reflects its light toward a fixed target, regardless of the Sun's changing position in the sky. Heliostats are the core technology in concentrated solar power (CSP) installations. A space mirror functions as an orbital heliostat: it must continuously adjust its pointing to redirect sunlight from its position in orbit toward a fixed ground target. The difference is that an orbital heliostat is moving at 7.6 km/s relative to both the Sun and the target, requiring high-frequency attitude adjustments. See our dedicated Heliostat page for a full comparison.

The international body responsible for defining astronomical nomenclature and standards, including the official definitions of planets, dwarf planets, and the magnitude system. The IAU has taken an active role in advocating for the protection of dark skies from satellite constellations, convening working groups and issuing recommendations to national regulators. The IAU's Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (IAU CPS), established in 2022, monitors satellite brightness and maintains liaison with operators including SpaceX.

The angle between a satellite's orbital plane and Earth's equatorial plane, measured in degrees. An inclination of 0° is an equatorial orbit; 90° is a polar orbit that passes over both poles. Inclination determines which latitudes a satellite can overfly: a satellite at 53° inclination can pass over every point on Earth between 53°N and 53°S latitude. Reflect Orbital's proposed sun-synchronous orbit has an inclination of approximately 97°–98°, which gives it a retrograde (slightly westward) orbit that precesses in a way that keeps the orbital plane aligned with the Sun.

The United Nations specialised agency responsible for coordinating the global use of radio frequencies and satellite orbits. Any satellite using radio frequencies — including command-and-control communications for space mirrors — must be coordinated through the ITU's filing process to prevent interference with other operators. Unlike the FCC, which covers US operators, the ITU's Radio Regulations are binding on all 193 member states. ITU orbital slot filings are also used strategically: some countries file satellite orbital positions years in advance to establish priority rights, even if deployment never follows.

A theoretical cascade scenario, first described by NASA scientist Donald Kessler in 1978, in which orbital debris in a given altitude band becomes dense enough that collisions generate more debris than they destroy, creating a self-sustaining chain reaction that renders the affected orbital region unusable. A large-scale space mirror constellation — consisting of thin-film satellites with large surface areas — would represent significant debris-generation risk in the event of a collision. The thin-film material, though lightweight, would fragment into many small pieces difficult to track. This concern appears in regulatory filings objecting to large mirror constellations.

The orbital regime from approximately 160 km to 2,000 km altitude. LEO is the operational environment for the International Space Station (approximately 400 km), Starlink (approximately 550 km), and proposed space mirrors including Eärendil-1. In LEO, orbital periods range from roughly 90 to 127 minutes, and satellites complete 11–16 orbits per day. LEO is the only altitude at which a space mirror can produce ground illumination intense enough to be useful — higher orbits spread the reflected beam too wide. See our Eärendil-1 page for the mission's specific orbital parameters.

The introduction of artificial light into an environment where it causes adverse effects. Terrestrial light pollution from cities and infrastructure has progressively degraded the visibility of the night sky for most of Earth's population over the past century. Space mirrors represent a novel, orbital source of light pollution that is qualitatively different from ground-based sources: it cannot be blocked by local shielding, affects all latitudes globally, and can temporarily illuminate locations during astronomical twilight or full darkness. See our dedicated Light Pollution page for a detailed treatment.

See Apparent Magnitude. The term "magnitude" without qualification generally refers to apparent magnitude in the context of satellite brightness discussions. Absolute magnitude — which corrects for distance — is less commonly used for near-Earth objects like satellites.

A polyester film, originally developed by DuPont, that is extensively used in space applications due to its combination of low density, high tensile strength, and excellent reflectivity when aluminised. Space mirror proposals from Znamya onward have relied on aluminised Mylar or similar thin-film materials as the reflective surface. The material can be compressed into an extremely small volume for launch and then deployed to large dimensions in the vacuum of space. Its primary limitation is susceptibility to micrometeorite damage and the generation of debris if it fragments. Reflect Orbital's Eärendil-1 uses a thin-film reflective sail of similar material.

A US law requiring federal agencies to assess the environmental impact of their decisions before taking major federal actions. The FCC has historically granted satellites a categorical exclusion from NEPA review — a presumption that satellites have no significant environmental impact — meaning no full Environmental Impact Statement (EIS) is typically required for satellite licences. Critics of large constellations and space mirrors argue that this categorical exclusion is outdated and that the cumulative impact of thousands of bright satellites on the global night sky constitutes a significant environmental effect. See our FCC page for how NEPA has been raised in the context of space mirror regulation.

The curved path of an object in space around a central body, maintained by the balance between the object's forward velocity and the gravitational pull of the central body. For Earth-orbiting satellites, an orbit is characterised by its altitude, inclination, eccentricity (how circular or elliptical it is), and several other parameters collectively described in a two-line element set (TLE). Different orbital regimes — LEO, MEO, GEO — have distinct properties affecting visibility, communication latency, and coverage.

The time a satellite takes to complete one full orbit around the Earth. Orbital period is determined by altitude: at LEO (around 500 km), a satellite completes a full orbit in approximately 95 minutes. At GEO (35,786 km), orbital period is exactly 24 hours. As altitude increases, the satellite must travel a greater distance but also experiences weaker gravity, so orbital speed decreases and period increases. Knowing a satellite's orbital period is essential for predicting when it will next overfly a given location.

The foundational international space law document, formally the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies. Signed in 1967 and now ratified by 114 countries, it establishes that outer space is the "province of all mankind," prohibits weapons of mass destruction in space, and assigns international liability for damage caused by space objects to the launching nation. The treaty does not explicitly address space mirrors or other orbital illumination technologies. There is ongoing scholarly debate about whether a space mirror deployed for military battlefield illumination could constitute a prohibited weapons system under the treaty's provisions.

The point in a satellite's elliptical orbit where it is closest to Earth. The opposite point — farthest from Earth — is called apogee. For a perfectly circular orbit, perigee and apogee are equal. Most operational satellites maintain nearly circular orbits where perigee and apogee are within a few kilometres of each other. Atmospheric drag at low perigee altitudes (below approximately 300 km) causes rapid orbital decay, requiring either propulsion to maintain altitude or acceptance of a limited operational lifetime.

The angle between the Sun, a satellite, and the ground observer — analogous to the lunar phase angle that produces the different phases of the Moon. Phase angle critically affects satellite brightness: a satellite is most visible when the Sun is behind the observer and the satellite is "fully illuminated" (small phase angle). Space mirrors are particularly dependent on phase angle: they must be positioned and oriented so that sunlight reflects directly toward the target on the ground. Unfavourable phase angles mean the mirror's reflected beam misses the intended target entirely.

A type of reflection in which light is returned directly back toward its source, regardless of the angle of incidence. Retroreflective materials (such as road signs and safety clothing) use microscopic prisms or spherical beads to achieve this. Space mirrors do not use retroreflection — they use specular (mirror-like) reflection to direct sunlight at an angle toward a target on the ground. Retroreflection would send the sunlight back toward the Sun, which is useless for illumination purposes. The distinction matters when evaluating proposals that claim to "beam" energy: only a specular mirror can redirect light to a designated ground point.

Any object placed into orbit around a celestial body. In common usage, the term refers to an artificial object launched by humans into Earth orbit for communication, observation, navigation, or other purposes. The first artificial satellite, Sputnik 1, was launched by the Soviet Union in 1957. As of 2026, approximately 9,000 active satellites orbit Earth, with thousands more inactive objects and debris fragments tracked by the US Space Surveillance Network. Space mirrors are a specialised type of satellite whose primary payload is a reflective surface rather than electronics.

The standard mathematical model used to predict satellite positions from two-line element (TLE) data. SGP4 accounts for the main perturbations affecting LEO satellites: Earth's non-spherical shape (J2 oblateness), atmospheric drag, and solar radiation pressure. Most public satellite-tracking applications use SGP4 to convert TLE data into real-time position predictions. Accuracy degrades over time as the satellite's actual trajectory diverges from the model; TLEs must be refreshed (typically daily) to maintain useful accuracy. SGP4 is accurate to roughly 1 km for a freshly updated TLE.

The diffuse brightening of the night sky over populated areas caused by the scattering of artificial light by the atmosphere. Skyglow is the most widespread form of light pollution, making it impossible to see the Milky Way from most cities on Earth. Space mirrors and bright satellite constellations contribute to skyglow differently from terrestrial sources: they add discrete bright moving points rather than a diffuse glow, but a sufficiently large constellation could contribute measurably to the overall sky background brightness. See our Light Pollution page for modelled brightness impacts.

A polar orbit with an inclination typically between 96° and 98° that precesses — gradually rotates its orbital plane — at a rate matching Earth's annual orbit around the Sun. This keeps the orbital plane oriented at a nearly constant angle relative to the Sun throughout the year, meaning the satellite always crosses the equator at the same local solar time. Earth observation satellites favour SSO because they see each location with consistent illumination angles. Reflect Orbital has proposed using SSO for its mirror constellation: an SSO mirror satellite can be positioned near the terminator (the day/night boundary) and reflect sunlight into areas just entering darkness.

The boundary between the sunlit and dark hemispheres of the Earth — the line separating day from night at any given moment. At the terminator, the Sun is on the horizon: locations just inside the night side have just experienced sunset; locations just inside the day side are in early morning. The terminator is critical for space mirror geometry: a satellite in LEO can be simultaneously illuminated by the Sun (because it is high enough to see over the Earth's curvature) while the ground below it is in shadow. This is the window during which a mirror can redirect sunlight toward a darkened ground target. The window is narrow — satellites move through it in minutes.

A reflective surface made from a thin layer of metal — typically aluminium — deposited on an ultra-light polymer film such as aluminised Mylar or similar materials. Thin-film mirrors are the enabling technology for all space mirror proposals: they are light enough to achieve the large area-to-mass ratios required for useful solar reflection while being compact enough to launch affordably. Thicknesses of a few micrometres are typical. The material can be deployed from a compact launch configuration using inflatable booms or other deployment mechanisms. The primary engineering challenges are maintaining flatness in the thermal environment of orbit and surviving micrometeorite impacts over the mission lifetime.

A standardised data format used to convey the orbital parameters of a satellite in two 69-character lines of text. The format was developed by NORAD in the 1970s and remains the universal standard for satellite tracking. A TLE encodes inclination, right ascension of ascending node, eccentricity, argument of perigee, mean anomaly, and mean motion, from which satellite-tracking software can calculate the satellite's position at any moment using the SGP4 propagator. TLEs are published by the US Space Force's 18th Space Control Squadron and available publicly via sources including Space-Track.org and Celestrak. Freshly issued TLEs for active satellites are accurate to within about 1 km; older TLEs degrade in accuracy within hours to days.

The period of partial illumination between full daylight and full darkness, caused by the scattering of sunlight by the upper atmosphere. Three defined categories exist: civil twilight (Sun 0°–6° below horizon — enough light to read outdoors), nautical twilight (Sun 6°–12° below horizon — horizon visible at sea), and astronomical twilight (Sun 12°–18° below horizon — sky still not fully dark for sensitive observations). Space mirrors are most visible and most impactful during astronomical twilight: the satellite is in sunlight but the ground is dark, maximising the contrast of the reflected beam. Once the Sun is more than 18° below the horizon, satellites in LEO typically pass into Earth's shadow and become invisible.

The point in the sky directly above an observer — exactly 90° elevation. The zenith is the highest point any satellite can reach in the sky from a given location, and occurs only for orbits that pass directly overhead. A satellite at zenith has the shortest slant range (distance from observer to satellite), which means it appears brightest and its reflected beam reaches the ground most efficiently. For a mirror satellite at 500 km altitude passing overhead at zenith, slant range equals altitude; for the same satellite at 30° elevation, slant range is approximately double that distance, reducing apparent brightness by a factor of four.