Solar sail

Photons carry momentum as well as energy. When sunlight is absorbed or reflected by a sail, momentum is transferred to the spacecraft, producing radiation pressure. A reflective sail can receive momentum from both the incoming and reflected light. The pressure is weak, so a solar sail generally needs a very large area relative to the spacecraft's mass; nevertheless, the force can act continuously while the sail is illuminated. Solar sails therefore trade high instantaneous thrust for long-duration, propellant-free acceleration after launch.

A sail is steered by changing its orientation relative to the Sun. Tilting its surface changes the direction of the radiation force, allowing a spacecraft to alter its trajectory rather than simply move radially away from the Sun. The sail's orientation and the spacecraft's center of mass or other control features must be managed to keep the system stable. Sunlight intensity decreases with distance from the Sun, so available thrust also decreases as a sail travels outward. Solar sailing does not use the solar wind: its primary propulsive force comes from photons, not the charged particles streaming from the Sun.

The sail must combine low mass with adequate strength, reflective performance, and reliable deployment. Large sails are difficult to pack for launch, unfold, and control; their flexible support booms and membranes are exposed to changing temperatures and the space environment. The spacecraft and sail together must have a sufficiently low mass per unit area for solar pressure to create useful acceleration. Missions must also plan around the continuous but low thrust, orbital geometry, and changing illumination.

Japan's IKAROS, launched in 2010, demonstrated solar sailing in deep space and verified solar-radiation-pressure effects and in-flight guidance techniques. NASA's Advanced Composite Solar Sail System (ACS3) launched on April 23, 2024, to test deployable composite booms and sail deployment in low Earth orbit. NASA reports that its roughly square sail measures about 9 metres on each side and that the demonstration is intended to inform the design of larger systems. These missions test different parts of the technology; their results support, but do not by themselves guarantee, the feasibility of every proposed solar-sail mission.