On Earth you need a keel to oppose the force on the sail, which has a component in the direction of the wind. In orbit, the equivalent radial force is mostly pe

On Earth you need a keel to oppose the force on the sail, which has a component in the direction of the wind. In orbit, the equivalent radial force is mostly perpendicular to the direction of motion, and so does very little work. Radial forces in orbit are much less efficient at changing orbital parameters than the equivalent axial (prograde/retrograde) acceleration. Moreover, if your orbit is roughly circular the applied force mostly averages out over each orbit.