Glad you enjoyed it! Space missions to the inner solar system (within Jupiter's orbit) will almost always use solar power, as it is simply much much cheaper and there are no nuclear regulations to get around. However, as you said, available solar power drops off as 1/r^2 when you stray away from the sun: At Neptune, for example, only 1/(30^2) = 1/900 of the solar power per panel area available at Earth is available. This is where RTGs become necessary, as no other power source could both function in the dark and function for the decades necessary to run these deep space missions.