What I'm curious about is how fast a solar sailing craft could go before the thrust of the photons is balanced out by the drag of the interplanetary medium. Fi

What I'm curious about is how fast a solar sailing craft could go before the thrust of the photons is balanced out by the drag of the interplanetary medium. Five particles per cubic centimeter isn't much but I suspect it wouldn't be irrelevant for a square mile sail moving at 400km/s.