Thrust from photon momentum exchange
Reflection redirects light and transfers momentum to the sail and spacecraft. This component of thrust does not require continuously expelling stored reaction mass, but it still exchanges momentum with the surrounding environment.
Solar panels instead convert light into electrical power for equipment or electric propulsion. Both devices can be present on one spacecraft; deploying power panels does not demonstrate sail deployment. NASA explicitly made this distinction during ACS3 preparations.
Area, mass and controlled attitude
Photon thrust is small, motivating large illuminated area at low total mass. Reflective coatings, membranes, booms and other hardware all contribute, while materials must tolerate stowage, deployment and radiation exposure.
Changing sail attitude changes illumination and momentum transfer, allowing orbital control. NASA’s July 2024 ACS3 deployment-preparation report emphasized lightweight stiff composite booms and maneuvering through sail angle: propulsion requires structure and control as well as membrane area.
Photon sails, electric sails and demonstrations
The word solar does not identify one working medium. NASA’s electric-sail description uses charged wires and fields to deflect solar-wind protons, whereas photon sails act on light. Their environmental and hardware requirements consequently differ.
NASA’s small-spacecraft survey lists demonstrations including NanoSail-D2 and LightSail 2, distinguishing deployment, deorbiting and apogee changes. Demonstrating one structural or control function does not establish a mature large-sail system for every mission; the complete design needs evaluation against its purpose.
References: [2]