Hitting the headlines quite a lot in the past six months, space exploration systems are one of the more interesting technical challenges for PV devices. Not one but perhaps four probes will be on their way to our nearest planetary neighbor, Mars. The success or failure of these missions will hinge on their on-board PV power sources — providing power for experiments and for getting the data back to Earth. But PV is not the only power source option, and the alternatives present challenges which PV makers must face.