That wouldn't actually go against what I was saying, and what you are suggesting is probably possible, although would require some careful flying and plotting to make it happen. The reason is thus, when you are on one planet, and another collides with it, the other planet will pass right through you if you are standing there, there is no physics there, no collision map. You can see the same with planetary rings, and if you drop out in one frame of reference and fly into a planetary ring, not only do the rocks fly past at insane speeds (because they are not in your frame of reference) but they are also intangible.
So, let's imagine a really carefully set up scenario, where two players land on colliding planets, and they are positioned so at the start they are are in separate instances, but on a course so the two players will directly hit each other.
If my hypothesis is correct, then both players would simply pass through each other as the planets pass through each other. Each is within their own physics grid, but neither grid is aware of the other and no collision detection takes place as they pass by each other. The only thing that might get collision detection is the players themselves (or their ships if in ships), because i think FD specifically code for that to happen (it happens when players from different instances merge instance bubbles for example), so you might have a funny situation where the grids are passing each other, but the ships/players gets sandwiched together, possibly resulting in either something going boom or being forced through each other or the terrain.
