Solsegel

Solsegel, or solarsegel, is a method of spacecraft propulsion that uses momentum transfer from photons in sunlight to generate thrust on a large, lightweight, reflective membrane. Because photons carry momentum, a sail positioned to capture sunlight experiences a continuous, though very small, push. At 1 astronomical unit from the Sun, the radiation pressure on a nearly perfect mirror is about 9 micro-newtons per square meter, so the resulting thrust scales with sail area and the angle of incidence.

Sails must be extremely large and very light to be effective. Typical designs use thin, flexible films such as Mylar or Kapton, often with aluminized or dielectric coatings to maximize reflectivity. Modern sails are designed to unfold and deploy in space and to maintain a stable, controllable attitude. Attitude control is achieved by tilting the sail, adjusting its shape, or shifting onboard mass. Materials must tolerate ultraviolet exposure, micrometeoroid impacts, and long-duration environmental stresses.

History and development: the idea of using light pressure dates back to early theories of radiation momentum, but the solar sail concept gained modern attention in the 20th century. Notable milestones include Japan’s IKAROS mission (2010), which demonstrated a functioning solar sail in interplanetary space, and The Planetary Society’s LightSail demonstrations (2015–2019), which validated deployment and basic control in Earth orbit.

Applications and challenges: solar sails offer propellant-free propulsion for long-duration, deep-space missions and rapid exploration of the solar system. They provide continuous thrust but at extremely low accelerations, requiring long mission times and precise navigation. Major challenges include sail deployment reliability, material durability, deployment and control in microgravity, and the management of varying solar radiation with distance from the Sun.