Thank you again for posting a well researched article! I guess where possible space mission's will try to use solar power, but when your in the outer solar system where sunlight can be 100's even 1000's of times weaker an RTG is the only practical option.
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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.