The game by itself has a poor areodynamic modle, if you want something more realist try FAR P.S. it becomes verry hard to make rockets fly well so you will want a mod that adds farrings, I use Procedural Fairings
So lately like a space enthusiast i am i was looking at space missions and noticed the RTG on New Horizons look like the ones in ksp. is this a common shape for them? http://en.wikipedia.org/wiki/File:New_Horizons_1.jpg
I would love this so much and with the procedural generation i think these could be done, though to be honest i can do when it come to programing is something that does math for me on my calculator. But any ways i would love to see this!!!
I am not a modder myself and do not know what is feasible to do and not to do, but i think it would be cool if modders can add planets. I also do understand though it is probably a lot of work.