A google news story about the Soyuz reentry being steeper than usual (8Gs!) led me to the wikipedia page for the Soyuz. From there i went to Hohmann Orbit transfers, which is an interesting bit of physics about the most efficient way to move from one orbit to another. That page led to the Interplanetary Transport Network - a route between each planet that uses the existing gravitational environment as much as possible. It minimizes on-board thrust needs, taking instead a lot of time to wait for the right set of gravitational pulls. It was "first investigated by Jules-Henri Poincaré in the 1890s", such a long time before the results could be implemented.
Interplanetary Transport Network (wikipedia fun)