How do NASA and other space agencies plan missions to the outer planets?

Getting to the outer planets is all about orbital mechanics. Space agencies choose launch windows that line up Earth, the target planet, and sometimes other planets for gravity assists, which can sling a spacecraft faster without using as much fuel. Voyagers 1 and 2 famously used multiple planetary flybys in the 1970s and 1980s to reach the outer Solar System. Because these missions can take years or decades, planners must account for communication delays, radiation, and power needs from the very beginning. That careful design is why spacecraft can visit worlds so far from the Sun.