Between the planets and moons are gravitational corridors, tubes that intersect at lagrange points, which can be used to travel between celestial bodies using hardly any fuel, only for course correctional purposes. These corridors can be likened to ocean currents. The trade-off is now between speed and fuel consumption: you can use almost zero fuel between a planets’ moons because the distance allows it, but should you want to drift to another planet using just the corridors, this would take years.

Scientists unveil plan designed to cut cost of space travel – Telegraph.