It's not Rocket (limited super)Science

[Fred Brackin]

Originally Posted by jeff_wilson

However, it is not the direction or the same speed relative to the sun, since the planet is orbiting the sun at numerous miles per second itself, and it is not the same speed relative to the other planets which have their own speeds and orbits of course, so the slingshot maneuver can effectively add or subtract as much as twice the difference in speeds between the slinging planet and the other planets, or the sun. The velocity change of the spaceship can be considerable, but since the mass is so much smaller than the planet, conservation of momentum makes some invisibly small alteration in the planet's orbit that will be lost in the noise of everything else zipping past it.

Yes, this is most of the stuff I didn't have the time to explain before going to watch Dr Who last night. :)

There's also stealing some of the planet's angular momentum by traveling in the same plane and direction as its' rotation and getting pulled along by its' gravity as it spins. With planets like Jupiter (angular momentum king of the solar system) this can be significant.

Still, I stand by my statement of "singshots don't work the way they do in Star Trek". The possible benefits are limited and specific circumstance only. It's not a general purpose way to gain velocity.

If you're in danger of hitting a planetoid you need to change your vector enough to not hit and this may or may not result in a graceful looking arc around the planetoid. Particularly from a planetoid you're unlikely to gain any significant velocity by stealing some of its' orbital or angular momentum. There's too little gravity involved.