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

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

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Anonymous 0 Comments

Well you are already travelling at a velocity that escaped one planet’s gravity before you even start.

Then you calculate it so that the boost to speed caused by the gravity well as you approach is higher than the cost to speed caused by the gravity well as you depart.

No fuel should be required at all, except for minor course corrections.

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