I don't think that's right. Modeling Voyager's interaction with Jupiter as an elastic collision with the planet gives an incorrect answer. The spacecraft's spee

I don't think that's right. Modeling Voyager's interaction with Jupiter as an elastic collision with the planet gives an incorrect answer. The spacecraft's speed with respect to Jupiter doesn't change. As the post discusses, Voyager's speed (scalar) in the Jupiter frame is essentially unchanged. Its velocity (vector) of course changes quite a lot because the spacecraft's direction changes as a result of the encounter, but its speed with respect to Jupiter (and therefore its kinetic energy) isn't significantly different. It approached Jupiter with enough energy that it couldn't be captured and left with pretty much the same energy with respect to Jupiter. What matters is that Voyager's speed with respect to the sun is greatly increased during its interaction with Jupiter's gravitational field. And since the point of the mission is to escape the sun, what matters is the spacecraft's energy state with respect to the sun, not to Jupiter.