Solar sails represent one of the most elegant concepts in space exploration: using sunlight itself to propel spacecraft through the cosmos without any fuel. But these thin, light giants face a stubborn engineering challenge that has plagued missions since their inception; keeping control while riding the solar wind.
Solar sails work by catching photons from the Sun, much like wind fills a sailing ship's canvas. These ultra thin, reflective sheets can span dozens of meters and weigh just a few kilograms, offering theoretically unlimited propulsion for deep space missions. However, their enormous size compared to their mass creates a control problem.
Model of the Japanese interplanetary unmanned spacecraft IKAROS (Credit : Pavel Hrdlička)
Most spacecraft use spinning wheels called reaction wheels to change their orientation in space. Think of them as gyroscopes that can tilt a spacecraft in any direction. But solar sails are so large and experience such complex forces from sunlight that these reaction wheels quickly become overwhelmed, spinning faster and faster until they reach their limits and can no longer function, a condition called "saturation."
When this happens, spacecraft lose control, making it impossible to point instruments accurately or maintain proper orientation. Previous missions like LightSail 2 required daily "momentum dumps" using magnetic torque rods to reset their reaction wheels, limiting their operational efficiency.
Researchers from the University of New South Wales have proposed an elegant solution inspired by Japan's groundbreaking IKAROS mission: Reflectivity Control Devices, or RCDs. These are essentially electronic mirrors that can change how they reflect sunlight with the flip of a switch.
An advanced composite solar sail system’s unfurled for testing (Credit : NASA)
RCDs are thin, flexible membranes embedded with liquid crystals, similar to those in laptop screens. When electricity is applied, they switch between two states: highly reflective (specular) and diffusely reflective (scattered). Since different reflection patterns create different forces from solar radiation pressure, strategically placed RCDs can generate precise torques to help control the spacecraft's attitude.
The research team developed a clever control strategy that alternates between two operational modes. During normal "Earth-pointing" mode, the spacecraft uses its reaction wheels for precise pointing while conducting science observations. When the reaction wheels approach saturation, the spacecraft automatically switches to "Sun-pointing" mode.
In Sun-pointing mode, the solar sail turns to face the Sun directly, maximising the effectiveness of the RCDs. The electronic mirrors then work in pairs, some highly reflective, others diffusely reflective, creating controlled imbalances in solar pressure that generate the exact torques needed to slow down the spinning reaction wheels.
Through detailed computer simulations using a spacecraft similar to LightSail 2 in a Sun synchronous orbit 700 kilometres above Earth, the researchers demonstrated that RCDs could successfully prevent reaction wheel saturation. Without RCDs, the reaction wheels reached their limits in just 48 hours. With RCDs, the system operated smoothly for over a week, with momentum offloading sessions lasting about 5 hours and occurring roughly every two days.
LightSail 2, a solar sailing cubesat by The Planetary Society, rests after testing its sail deployment system at Cal Poly San Luis Obispo, on Dec 6, 2016. LightSail uses four, 4-meter metallic booms that unwind from the spacecraft’s lower section to deploy its solar sail. (Credit : Jason Davis / The Planetary Society)
The RCDs proved capable of generating torques up to 7.4 micro Newton meters small but sufficient to counteract the gradual momentum buildup. The spacecraft could switch between Earth pointing and Sun pointing modes within 11 minutes, maintaining operational efficiency while preventing control system failures.
Perhaps most significantly, RCDs offer advantages for deep space missions where traditional momentum management tools like magnetorquers cannot function. Magnetorquers rely on Earth's magnetic field, making them useless beyond our planet's immediate vicinity. RCDs, powered only by sunlight, could enable precise attitude control for solar sail missions to asteroids, other planets, or even interstellar space.
The technology also offers mechanical simplicity compared to other proposed solutions. Unlike complex gimbal systems or movable masses, RCDs have no moving parts and add minimal weight to the spacecraft. Their successful demonstration on IKAROS proves they can work in the harsh environment of space.
While the current RCD system can only control two of the three rotational axes, requiring additional systems for complete three axis control, the research demonstrates a practical path forward for making solar sailing more reliable and efficient. The simplicity of the approach essentially smart paint that can change its reflectivity on command could make solar sails more attractive for future missions.
As space agencies and private companies increasingly look toward sustainable, fuel-free propulsion for ambitious deep space exploration, RCDs represent a crucial stepping stone toward making solar sailing as reliable as it is elegant. Sometimes the most sophisticated solutions are also the most beautifully simple.
