Light carries momentum. Photons striking and reflecting from a surface transfer a small push. A light sail exploits that pressure with a large area and low mass. A solar sail uses sunlight; unlike an ion engine, this thrust mechanism does not require expelling a stored gas supply.
The principle has been demonstrated in space. In 2010, JAXA confirmed photon-driven acceleration of IKAROS after sail deployment. Sail orientation, area and system mass influence the resulting trajectory. A small force can produce useful orbital changes when applied continuously.
Engineering remains demanding: a thin surface must deploy successfully, remain controllable and operate under sunlight that weakens with distance. Auxiliary propulsion may still support control. Proposals for laser-driven interstellar sails face additional unresolved challenges beyond solar-sail demonstrations. This technology connects advanced propulsion, lightweight materials and directed energy without violating momentum conservation.