Originally Posted by StuM
The direction of travel (final vector result) will depend on the angle of incidence of the light on the sail at any given point of time. By adjusting the satellite's attitude it can effectively be steered in any direction.
Not exactly. You need a second force working in opposition to the wind
in order to be able to sail upwind. On a sailboat, that force comes from the resistance of the water
. Without that force, you can't tack a lightsail upwind like you can a sailboat.
What you can do, however, is use the lightsail to change your orbital height if you're orbiting the sun. While orbiting, slowing your orbital movement will decrease your orbital height, while speeding it up will increase it. You're using the sun's gravity as the second force. This gives you something sort of like sailing upwind in that you can move towards the sun by changing your orbital height, but you still can't sail in any arbitrary direction.