Recent advance in solar concentrators and lightweight solar sails mean we can use solar power rather than nuclear to power plasma engines. Then we already have the capability to make manned flights to Mars at travel times of weeks rather than months: Sunday, March 16, 2014 Short travel times to Mars now possible through plasma propulsion. Please Register or Log in to view the hidden image! Developmental Status of a 100-kW Class Laboratory Nested channel Hall Thruster. Table 1, Example of concentrically NHT specific mass and footprint savings, p. 5. http://exoscientist.blogspot.com/2014/03/short-travel-times-to-mars-now-possible.html Bob Clark