Well a 1 g spacecraft at .2 c has roughly 10^12 J kinetic energy, so it just loses something in the order of 10^-3 of its energy due to collisions with the ISM.

Well a 1 g spacecraft at .2 c has roughly 10^12 J kinetic energy, so it just loses something in the order of 10^-3 of its energy due to collisions with the ISM. (Which should have an order of magnitude less density in the solar neighborhood.)