Orbits

celestial bodies

Positions and velocities of celestial bodies are not calculated by the simulation. Instead, a trusted almanac is used to determine a body's position & velocity at a given time epoch in the solar system coordinate frame.

almanac

ANISE or Nyx.

spacecraft

Spacecraft trajectories are calculated on-the-fly using n-body physics, but the spacecraft is only influenced by potential fields of celestial bodies (not other spacecraft).

algorithm

Brute force pairs via particular.