> Is the 2-body mechanics just a question of simulation speed?
No.
2 body dynamics can be predicted from initial conditions to an arbitrary time any time in the past or the future. N-body dynamics have to be numerically integrated timestep by timestep.
But simulating n-body dynamics is not difficult or computationally expensive. Two things to understand: n is a very small number, around 20, the number of planets and moons in the solar system. And we're only interested in the restricted n-body problem, ie. movement of one massless particle (the spacecraft) in the gravity field of the planets and moons, so it's O(n) complexity, not O(n^2).
Ultimately this was a game design decision.
> Or why wouldn't they compute based on the top N nearest-biggest objects?
For such a small number of bodies, this wouldn't save any time but would add a lot of complexity.
No.
2 body dynamics can be predicted from initial conditions to an arbitrary time any time in the past or the future. N-body dynamics have to be numerically integrated timestep by timestep.
But simulating n-body dynamics is not difficult or computationally expensive. Two things to understand: n is a very small number, around 20, the number of planets and moons in the solar system. And we're only interested in the restricted n-body problem, ie. movement of one massless particle (the spacecraft) in the gravity field of the planets and moons, so it's O(n) complexity, not O(n^2).
Ultimately this was a game design decision.
> Or why wouldn't they compute based on the top N nearest-biggest objects?
For such a small number of bodies, this wouldn't save any time but would add a lot of complexity.