So I was thinking about this... with modern thin film cells (which can be made as thin as 2-3 micrometers) how good of a specific power could you get for really powerful SEP systems (like the VASIMR 39 days to Mars stuff)?If you could put thin film solar cells on a solar sail type backing, and deploy it like a solar sail, it seems like you could get enormous areas at very low mass (if density is silicon's 2.5 g/cm^3, 3 micrometers thick means about 7.5 tons per square kilometer, unless my math is wrong).Could you really get tens to hundreds of megawatts of thrust out of VASIMR or other electric propulsion systems? (1 square kilometer of solar cells at even 10% efficiency should be about 136 megawatts at Earth orbit, maybe 50MW at Mars).