How Do Solar Sails Propel Spacecraft?

Although photons have no mass, they carry momentum. When a photon reflects off a mirror-like sail, it transfers twice its momentum to the sail. The force is tiny — about 9 micronewtons per square metre at Earth's distance from the Sun — but it acts continuously with no fuel cost. Over weeks and months, this adds up to significant velocity changes. Solar sails are made from ultra-thin reflective material, typically aluminised Mylar or Kapton just a few micrometres thick. JAXA's IKAROS, launched in 2010, was the first spacecraft to demonstrate solar-sail propulsion in interplanetary space. The Planetary Society's LightSail 2 (2019) successfully used a 32 m² sail to raise its Earth orbit.