According to another Martin Co fact sheet that I have; they already had nuclear power in 2 defence satellites in the early 60's.
The SNAP-9A Nuclear Generators . The info states that they have a service life of 5 years; but the that the satellite should stay in orbit for 900 years, by which time the nuclear material would be mostly decayed!
It is not a fission system; but converts heat from plutonium -238 directly into electricity by thermocouples.