Radiation Pressure
& Thin-Film Mirrors
Sunlight carries momentum. When photons strike a reflective surface, they exert a measurable force. For large thin-film space mirrors, solar radiation pressure is a continuous engineering challenge — simultaneously the source of attitude disturbance and a potential propulsion resource.
How Photons Exert Force
Light exerts pressure because photons carry momentum. This follows directly from Einstein's relation between energy and momentum: a photon of energy E carries momentum p = E/c, where c is the speed of light. When a photon is absorbed by a surface, it transfers its momentum to that surface — creating a force in the direction of travel. When it is reflected, momentum transfer is doubled — the photon reverses direction, and the surface receives twice the momentum change.
At Earth's distance from the Sun, the solar irradiance is approximately 1,361 W/m² (the solar constant). The radiation pressure from this flux on a perfectly absorbing surface is approximately 4.56 μN/m² (micropascals per square metre). For a perfectly reflecting surface, radiation pressure doubles to approximately 9.12 μN/m².
The Attitude Control Problem
The force from solar radiation pressure seems negligibly small — fractions of a millinewton for typical space mirror areas. But several factors make it a significant engineering challenge rather than a trivial perturbation.
The centre-of-pressure vs centre-of-mass offset. The solar radiation pressure force acts at the centre of pressure of the mirror's area — the geometric centroid of the illuminated surface. The attitude control force acts through the satellite's centre of mass — which includes the spacecraft bus, actuators, propellant, and electronics. If these two points do not coincide, the radiation pressure force creates a torque about the centre of mass, causing the satellite to rotate. For a large thin-film sail attached to a small spacecraft bus, this offset can be substantial, and the resulting torque must be continuously counteracted.
The force is always on. Unlike aerodynamic drag (which is negligible at 500+ km altitude) or thruster firings (which are brief and controllable), radiation pressure acts continuously whenever the satellite is in sunlight — approximately 60–65% of each 94-minute orbit at 500 km altitude. The attitude control system must continuously work against this torque, consuming reaction wheel momentum storage capacity that must be periodically "dumped" using magnetorquers or thrusters.
The force direction changes. As the satellite orbits, the direction from which sunlight arrives at the mirror changes. As the mirror is slewed to track a ground target, the incidence angle of sunlight on the mirror changes, altering both the magnitude and direction of the radiation pressure torque. A good torque model must account for the satellite's orbital position, attitude, and mirror orientation simultaneously — a computationally complex real-time problem.
What the Mirror Is Made Of
ALUMINISED MYLAR (BOPET)
CP1 POLYIMIDE (ADVANCED)
Aluminised Mylar (biaxially-oriented polyethylene terephthalate, or BoPET with aluminium deposition) is the most commonly used thin-film mirror material in space applications, with heritage from the Znamya experiments (1993, 1999), early solar sail demonstrators, and numerous thermal blanket applications. Its principal advantages are low cost, well-understood mechanical properties, and manufacturability in large sheets.
Its principal limitations for space mirror applications are UV degradation — atomic oxygen in LEO and ultraviolet radiation slowly degrade PET films, reducing reflectivity and embrittling the material over a mission lifetime of years — and the relatively modest temperature range compared to polyimide alternatives. The James Webb Space Telescope's five-layer sunshield uses aluminised Kapton and CP1 polyimide films rather than PET, precisely because of the superior thermal and UV resistance required for a decade-long mission.
The Wrinkling Problem
A thin-film mirror must be approximately flat to produce a well-defined reflected beam. Surface deformations — wrinkles, ripples, billowing — scatter the reflected beam and degrade beam quality. Maintaining flatness in the thermal environment of LEO is one of the primary challenges of thin-film mirror engineering.
In LEO at 500 km, a satellite alternates between sunlit periods (approximately 60 minutes per orbit) and shadow periods (approximately 35 minutes per orbit). During the sunlit period, the illuminated side of the mirror film absorbs a fraction of the incident solar radiation and heats up — potentially by 100–150°C above the shadow temperature. This temperature difference causes thermal expansion. If the film is tensioned by a deployment structure (booms, inflatable frames, or other mechanisms), the thermal expansion adds to the existing tension in some orientations and subtracts from it in others, causing the film shape to change cyclically.
Over many thermal cycles, thin polymer films can develop permanent creases — particularly at fold lines from pre-launch packaging. Managing these fold lines (placing them where they have least optical impact), selecting film materials with low thermal expansion coefficients, and designing tension systems that maintain adequate pre-tension throughout the thermal range are key aspects of thin-film mirror mechanical design. This is a primary technical differentiation challenge for any space mirror programme.
The Solar Sail Connection
The same radiation pressure that creates attitude disturbance for an illumination mirror is the propulsive force used by solar sail spacecraft. JAXA's IKAROS mission (launched 2010) was the first spacecraft to demonstrate interplanetary flight using solar sail propulsion — a 196 m² aluminised polyimide sail that provided approximately 1.12 mN of thrust at Venus distance. NASA's NanoSail-D2 (2010) and The Planetary Society's LightSail 2 (2019) demonstrated solar sail operation in LEO.
The engineering distinction between a solar sail and a space mirror is primarily one of intent and optimisation. A solar sail is optimised for propulsive efficiency — maximum area-to-mass ratio, minimal structural mass, pointing toward the Sun for maximum thrust. A space mirror is optimised for reflective efficiency and controllable pointing — maximum reflectivity, ability to aim the reflected beam at a moving ground target, and structural stiffness for beam quality. Both must manage radiation pressure; a solar sail exploits it while a space mirror constrains it.
Thin-film technology developed for solar sails is directly applicable to space mirrors. The engineering challenges overlap significantly, and advances in solar sail materials, deployment systems, and attitude control inform space mirror design. See our Glossary for definitions of related terms, and Orbital Mechanics for how attitude control requirements flow from the mission geometry.