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

If you approach the planet at the correct angle, you would get pulled by the planet’s gravity, but your trajectory wouldn’t change enough to crash into it. You would approach, gain speed from gravity, and “miss” the planet if you aimed your spacecraft correctly.

Here is a depiction of the Juno probe sent to Jupiter.

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