Space is a really big place. Even the solar system is too large for exploration using conventional rockets. But we manage to get there - the trick is gravity assisted flight - a technique developed by Michael Minovitch in the early 60s.. A spacecraft passing close to a planet can gain momentum from the planet giving a nice acceleration. The planet, in return, loses a tiny amount. As a graduate student Minovitch worked out about 200 planetary fly-bys.
Up until this time it looked like a brute force form of space flight was necessary and NASA was working on nuclear rockets and nuclear powered ion propulsion systems. The technology didn't work out at the time and gravity assist has seen great use.
a thorough book chapter on the subject... (pdf)