Solar-electric Advancing For Deep-space Propulsion

"Work is underway at advancing the readiness
levels of technologies that covert energy from the
Sun into propulsion by using solar-generated
electricity to force ions out of an engine at high
speeds to produce thrust. It isn't a lot of thrust,
but it can go a long way on relatively little fuel.
A SEP system weighs a lot less than chemical
propulsion. Given enough time it can move a lot
of mass through space. That in-space advantage
makes it particularly attractive for pre-positioning
cargo—supplies, habitats and the like—to keep
human explorers alive after they arrive on a faster
vehicle to explore a distant location."

See:

http://www.aviationweek.com/Article....p21-483700.xml