A nuclear powered space tug was line itemed in last year's Congressional budget, apparently as a sop to Los Alamos. Given the relative simplicitiy of designed a crude nuclear rocket - take one small nuclear reactor & attach to one solar electric thruster-derivative system - it's likely that the US may indeed have some form of nuclear propulsion in its space artillery; but that's along way from saying you could scale a crude, military-class unit of the kind i've just described to something capable of powering a manned flight to Mars anytime soon. Frankly the report that sourced this - which is now known to have been based on a garbled press conference transcript - is basically laughable under current technological & budgetary limits - no wonder Bob Zubrin supported it so fulsomely.