&gt; Is the 2-body mechanics just a question of simulation speed?<p>No.<p>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.<p>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&#x27;re only interested in the <i>restricted</i> n-body problem, ie. movement of one massless particle (the spacecraft) in the gravity field of the planets and moons, so it&#x27;s O(n) complexity, not O(n^2).<p>Ultimately this was a game design decision.<p>&gt; Or why wouldn&#x27;t they compute based on the top N nearest-biggest objects?<p>For such a small number of bodies, this wouldn&#x27;t save any time but would add a lot of complexity.