They don't have to be microwatts. Galileo, for example, used 17 pounds of plutonium to generate 570W of power at launch, with a half life of about 88 years.
Not that you'd want to carry 17 pounds of plutonium around on an airliner all the time, but power output is not an insurmountable problem by itself.
Not that you'd want to carry 17 pounds of plutonium around on an airliner all the time, but power output is not an insurmountable problem by itself.