Ion Drives for Spy Sats

DARPA is looking at ion-drive spy satellites. Truth be told, this is not surprising.

One of the limiting factors on many spy satellites is the amount of propellant available, and if you can use a very high impulse (rocket fuel economy) for station keeping and scheduled moves, you can either make a lighter vehicle, which means more for the same price, or one with much hhigher flexibility and operational life.