What principle do solar sails utilize for propulsion?

Solar sails operate based on the concept of momentum transfer from light photons. When sunlight hits the sail, the light photons exert pressure on it. This pressure arises because photons, while massless, carry momentum. When they collide with the surface of the sail, the photons reflect off the sail, changing their momentum, which in turn imparts an equal and opposite momentum to the sail itself, causing it to accelerate. This propulsion method is fundamentally about the transfer of momentum rather than any thermal effects or direct wind resistance.

The principle does not involve the heat generated from sunlight affecting the sail directly; instead, it is the momentum from the photons that is essential for propulsion. Thus, while some might think of wind resistance or the mass of the sail, solar sails specifically harness the interplay of light and momentum to achieve propulsion through space.