Sunlight intensity — and with it, the photon momentum striking the sail every second — falls off as the inverse square of distance from the Sun: I(r) = I₀·(r₀/r)². Solar-sail thrust is driven directly by that photon flux, so it inherits the same 1/r² falloff. A propellant engine's thrust depends only on exhaust velocity and mass flow rate — not on where in the solar system it happens to be — so its thrust readout stays flat while the sail's collapses.
F_sail(r) = F_sail(1 AU) · (1 AU / r)²
F_prop(r) = constant
- Sail area — bigger sail catches more photons per second, raising thrust at any given distance (but not changing the 1/r² shape of the curve).
- Sail craft mass — lighter craft gets more Δv from the same thrust (a = F/m), which is why solar sails are built ultra-thin and ultra-light.
- Chart — the amber curve is the sail's thrust vs. distance; the blue line is the propellant engine's constant thrust. Watch the sail curve dive as the craft's marker moves outward past Mars, the asteroid belt and Jupiter.
Mission-design consequence: solar sails are compelling for inner-solar-system work (Mercury/Venus missions, Earth-orbit station-keeping) where sunlight is abundant, but progressively impractical past the asteroid belt — by Jupiter's distance (~5.2 AU) thrust is already down to about 1/27 of its Earth-orbit value.