Sunlight carries momentum. A reflective sail tilted at pitch angle α relative to the sun-line
redirects incoming photons, gaining thrust mostly perpendicular to the sail — a component of that
thrust can point along the direction of travel (raising orbit, spiraling outward) or against it
(lowering orbit, spiraling inward), depending on sign of α.
F_photon = (2·S·A·cos²α) / c (S = solar flux at current radius)
S(r) = S₀ · (r₀/r)²
a_tangential = F_photon·sin(α) / m → changes orbital energy → spiral in/out
- Sail pitch angle α — tilts the sail; positive angles add energy (spiral outward), negative angles remove it (spiral inward), α=0 gives pure radial push with no spiral.
- Sail area — larger reflective area intercepts more photons, scaling thrust linearly.
- Craft mass — heavier craft accelerates more slowly for the same thrust.
- Time acceleration — speeds up the simulation so orbital drift becomes visible in seconds instead of years.
- Reset orbit — returns the craft to a circular 1 AU starting orbit.
Real-world relevance: missions like JAXA's IKAROS and The Planetary Society's LightSail have
demonstrated exactly this photon-pressure steering to change orbits without any propellant.