The Prius of Space

If you drive a car - and you know who you are - you have invariably come upon the dreaded dilemma of refueling. When the needle on your gas gauge wavers over the unseemly 'E,' you have to ask yourself one question - Do I stop at the next gas station or press on, hoping for a fuel oasis somewhere down the road? But what if you need to motor somewhere three billion-plus miles off the beaten path - somewhere where neither regular nor premium unleaded have so far feared to tread?