Re: landing at an angle that denied its solar panels vital sunlight
Pu-238* is about 8,000,000 USD per kg**, and for some years now there isn't that much of it to go around until someone makes some more. Which isn't trivial as you need Neptunium-237 and a nuclear reactor, hence the price tag. Right now a couple*** of kg Pu-238 is produced every year in the USA, after a break of 25 years.
The School of Nuclear Science and Engineering at OSU is working on a better method: Rebuilding the supply of Pu-238. Still in the theoretical / computer simulation phase, though.
* Other isotopes work too (Curium-244, Strontium-90, Polonium-210, Promethium-147, Caesium-137, Cerium-144, Ruthenium-106, Cobalt-60, Curium-242, Americium-241, Thulium), but if you want a small unit that lasts long, Pu-238 is the way to go. There once even was a series of cardiac pacemakers powered by Pu-238 RTGs (some of them still in use), that's how small you can make them.
** Probably not the deceiding factor. Cassini carried 32.7 kg of Pu-238 dioxyde.
*** Some sources suggest 1.5 kg, others 15 kg per year.
Bootnote: 1950ies Doc Brown was wrong. It's 2016, and we still can't buy Plutonium at the local drugstore. Time to invent Mr Fusion, so get cracking!