Well, since most of the speed is from slingshot gravitational interactions, there's 2 parts to it. * How well are the planets aligned * How well can you ex

Well, since most of the speed is from slingshot gravitational interactions, there's 2 parts to it. * How well are the planets aligned * How well can you exploit that Some of the current probes have done multiple flybys of inner solar system bodies to boost speed to reach the outer planets. Voyager 1 and 2 had a relatively direct route to take advantage of a once in a lifetime alignment of the outer planets.