Have a look here: https://en.wikipedia.org/wiki/Orbital_mechanics https://en.wikipedia.org/wiki/Orbital_mechanics The short version is - for the most part thes

Have a look here: https://en.wikipedia.org/wiki/Orbital_mechanics https://en.wikipedia.org/wiki/Orbital_mechanics The short version is - for the most part these trajectories can be calculated with a decent degree of accuracy with relatively simple equations. Rocket engines tend to give acceleration in short bursts (which simplifies the computations a lot as well), so New Horizons was launched out of Earth orbit at about 16km/s and since then has only fired its own engines for short correction burns.