Lee, I wrote: >OK, now you've done it. You've made me mad. >-| > >Sail size = A square m >Sail mass: M = A * 0.0001 kg/m^2 > >gamma at 0.9c = 2.29 >Momentum of the sail at 0.9c = gamma*M*v = A*0.0001*3E8*2.29 = A * 6.87E4 kgm/s There should be a 0.9 in the line above: = A*0.0001*0.9*3E8*2.29 It won't make much of a difference for the final outcome though. Timothy