[rcthompson]

That's not how sailing works. The only reason you can sail upwind in a sailboat on the water is because you have a keel pushing against the water to prevent you from blowing downwind. Essentially, a sailboat sits on the interface between two fluids (air and water) that are moving relative to each other, and abuses that fact to go wherever it wants to. If you couldn't push against the water, or in the extremely unlikely case that the water and wind had identical velocities, you wouldn't be able to sail upwind. (Actually, if the water and wind had identical velocities, you wouldn't be able to sail at all. It would feel like no wind.)

To tie this back to solar sailing, there's obviously no fluid interface in space, so to the extent that solar sailing is possible, solar sailing "upwind" toward the sun is not.

[michaelsbradley]

There is a heliospheric current sheet[†], but it's not clear to me if/how it could be practically employed to achieve a sailing effect.

[†] http://en.wikipedia.org/wiki/Heliospheric_current_sheet