space travel - How are interplanetary trajectories found?

How are interplanetary trajectories that involve gravity assist maneuvers found?

Examples:

• the MESSENGER spacecraft made flybys of Earth, Venus, and Mercury before getting into orbit


• the Juno spacecraft will flyby Earth (returning after launch) before continuing to Jupiter



• many more...

Are these found using a brute force search of virtually all possible paths or is there a more direct method for finding these helpful alignments?

If I decided to leave for Neptune today (using gravity assist), how would I find a good route and are such tools available on the desktop?

Answer

If I decided to leave for Neptune today (using gravity assist), how would I find a good route and are such tools available on the desktop?

I don't know about the 'find[ing] a good route' stuff, but my understanding is that the main tool used for these calculations is the SPICE Toolkit, and if I understand correctly, it's no longer ITAR controlled, so anyone should be allowed to download it.

I've also never used it myself, so I have no idea what sort of user-interfaces there are to it; we just use it for computing the location of spacecraft via an API.

(and this reminds me -- in the movie 'Starship Troopers', if the computer's able to confirm that the new route is the 'optimal path', why did they rely on humans putting in the path in the first place?)