I think there are some interesting ideas here.
I found the switch from planet physics to space physics to be jarring. I think I would have preferred space physics except when I was actually touching the surface of a planet, but then there'd have to be some kind of gravity in the space mode and that's a bit tricky with all the different planets. Dunno. It also felt odd that I maintained momentum after landing, just increasing the friction as some others have suggested might fix that.