Fly-By of planets

[airbuzz]

How does fly-by works? How is it possible that after fly-by I have more energy than before? Where do I take it?

[physicsguy]

I thought fly by of planets used the gravity of the planet to "sling-shot" the object past. Stuff like the mission to Saturn/ titan used it to accellerate the craft since the launcher was not powerful enough.

[airbuzz]

I know what it does, but I don´t understand how it works...
When I come closer to a planet i accelerate because some of my potential energy transforms in kinetic. The opposite happens when I get far from it. But with fly-by they obtain a kinetic energy bigger than the initial potential energy. From where arrives this energy? I think from the planet, but how?

[Integral]

The passing satellite picks up the ORBITAL velocity (at least a portion of it) of the body it is passing. You are correct that all energy gained from entering the gravity well of the planetary body is lost on exit. But all the time the satellite is in the gravitational influence of the planetary body it is being drug along with the orbital motion. This is the slingshot velocity.

[airbuzz]

The passing satellite picks up the ORBITAL velocity (at least a portion of it) of the body it is passing. You are correct that all energy gained from entering the gravity well of the planetary body is lost on exit. But all the time the satellite is in the gravitational influence of the planetary body it is being drug along with the orbital motion. This is the slingshot velocity.

Great!!!!
I was thinking something like that. Thank you very much!!