eli5 How does a spacecraft slingshotting around a planet increase it’s speed?

Question

Shouldn’t the gravity of the planet force the craft to expend fuel to escape the gravity?

In: 1

Answer 1

The reason that gravitational slingshot works is because the planet move relative to the sun.

If you compared speed with the planet as the frame of reference it does not increase but if you compare it to the sun there is a change.

Look at this animation from the Wikipedia page [https://en.wikipedia.org/wiki/Gravity_assist#/media/File:GravAssis.gif](https://en.wikipedia.org/wiki/Gravity_assist#/media/File:GravAssis.gif)