What Is a
Heliostat?
A heliostat is a mirror system that continuously tracks the Sun's position and reflects sunlight toward a fixed target. The same principle underlies concentrated solar power plants on the ground and Reflect Orbital's proposed space mirror satellites in orbit.
Tracking the Sun to a Fixed Point
The word heliostat combines the Greek helios (Sun) and statos (standing still). A heliostat does not stand still itself — it moves constantly — but it makes the Sun's reflected image appear to stand still at the target. As the Sun arcs across the sky from east to west throughout the day, a heliostat adjusts its orientation so that its reflected beam continues hitting the same fixed point despite the source's motion.
This is harder than it sounds. The Sun's angular position changes continuously in both azimuth (horizontal direction) and elevation (height above horizon). A heliostat must track both axes simultaneously, maintaining a pointing accuracy typically within 1–3 milliradians (approximately 0.06°–0.17°) to keep its reflected beam on target. The required update rate depends on how tight the target must be held — CSP tower receivers are large enough to tolerate several minutes between adjustments, while an orbital mirror must adjust continuously at rates of several cycles per second.
Concentrated Solar Power Plants
Concentrated solar power (CSP) tower plants are the largest operational deployments of heliostat technology. In a CSP tower system, hundreds or thousands of individual flat mirror panels surround a central tower, each independently tracking the Sun so that all reflected beams converge on a receiver at the tower's top. The receiver heats a working fluid — molten salt, steam, or oil — which drives a turbine to generate electricity.
Located near the California–Nevada border, Ivanpah is one of the world's largest CSP tower plants. It uses approximately 173,500 heliostat mirrors across three generating units, with a combined capacity of approximately 392 MW (gross). Each heliostat at Ivanpah consists of two mirror panels totalling approximately 15 square metres, mounted on a two-axis tracking drive. The mirrors reflect sunlight onto boilers atop three 140-metre towers, producing steam to drive conventional turbines. Ivanpah's heliostats must be repositioned every few minutes to maintain beam concentration on the receiver.
Gemasolar, operated by Torresol Energy, uses 2,650 heliostats each with approximately 120 square metres of mirror area, surrounding a 140-metre central tower. Its distinguishing feature is molten-salt thermal storage, which allows it to generate electricity for up to 15 hours after sunset — extending generation well past the sun's departure. Gemasolar's nominal capacity is approximately 19.9 MW. It demonstrated 36 consecutive days of 24-hour electricity generation in the summer of 2013, validating the molten-salt storage concept at commercial scale.
Both installations illustrate the core heliostat engineering challenge: each mirror must independently know its own position and the Sun's position to millisecond accuracy, compute the correct orientation to redirect the beam to the tower, and actuate that orientation with sufficient precision — all while withstanding wind loads, thermal expansion, and years of outdoor exposure. Ground-based CSP heliostats solve this with GPS, astronomical ephemeris tables, and motorised two-axis gimbal mounts. The individual unit cost of a modern CSP heliostat is in the range of $150–$250 USD per square metre of mirror area.
Reflect Orbital's Application
Reflect Orbital uses the term "heliostat" explicitly in its FCC filings and technical materials to describe the Eärendil-1 satellite. The conceptual parallel is exact: Eärendil-1 is a heliostat operating from orbit rather than a ground mount. It tracks the Sun's position relative to itself and to a fixed ground target, adjusting its mirror orientation continuously so that reflected sunlight falls on the target solar farm during its orbital pass.
The differences from a ground CSP heliostat are significant in engineering terms, even though the optical principle is identical:
GROUND CSP HELIOSTAT
ORBITAL HELIOSTAT (EÄRENDIL-1)
What Makes Orbital Heliostats Hard
Pointing accuracy. A ground CSP heliostat directs its beam onto a receiver a few hundred metres away — small pointing errors produce a small miss. An orbital heliostat at 500 km altitude is directing its beam onto a target a few kilometres across. The angular budget is actually comparable (a few milliradians), but the reference frame changes continuously at orbital velocity. The satellite must know its own attitude, the Sun's direction from its position, and the ground target's direction — all simultaneously, all in a moving reference frame — and combine this to compute the correct mirror orientation. This requires a star tracker or horizon sensor for absolute attitude knowledge, plus an onboard ephemeris of both the Sun and the target.
Mirror quality and flatness. A flat mirror that is not actually flat produces a blurred, distorted reflected beam. Ground CSP heliostats use rigid glass or aluminium mirrors held flat by structural mounts. An orbital thin-film mirror is held in shape by the tension of its deployment structure — lightweight booms or inflatable frames. Achieving the flatness required for a well-defined beam from a large flexible film in orbit is a primary engineering challenge. Surface deviations of even a few millimetres across a 10-metre sail affect the reflected beam quality measurably.
Thermal stability. In LEO, a satellite alternately faces direct sunlight (~1,361 W/m² solar irradiance) and passes through Earth's shadow every ~94 minutes. The temperature swing on the mirror surface can exceed 150°C between sunlit and shadow phases. A thin metallic film expands and contracts with temperature, affecting its flatness and reflectivity. The mirror material must maintain acceptable optical properties across this cycle for the lifetime of the mission — typically targeted at 2–5 years for an orbital demonstrator.
Radiation pressure disturbance. Sunlight exerts a small but continuous pressure on any surface it strikes — approximately 4.56 μN/m² in full sunlight at Earth's distance. For a large thin-film mirror, this is a significant disturbance torque that the attitude control system must continuously counteract. The same force that makes solar sails propulsive makes heliostat pointing control more difficult. Careful modelling of radiation pressure torque — including the effect of partial illumination as the satellite moves in and out of shadow — is essential to the pointing budget. See Orbital Mechanics for more on attitude control challenges.