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My seven-year-old son loves astronomy-- so much so that we read space books before bed time. One of our books talks about Pluto's orbit crossing over Neptune's orbit and will be (or is) closer to the Sun than Neptune for a period of years.

I'm assuming the orbits actually never have a collision path, but what will happen when Neptune and Pluto get as close to each other as possible-- will Neptune's gravity do something to Pluto's orbit?

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The Wikipedia article on Pluto has a good explanation.

Basically, the inclinations of their orbits are such that they never approach each other more closely than about 17 AU (more than 1.5 billion kilometers). In fact, Pluto comes closer to Uranus (11 AU) than it does to Neptune. And since Pluto and Neptune are in a 3:2 gravitational resonance, their orbital relationship should be stable over millions of years.

... will Neptune's gravity do something to Pluto's orbit?

It already has.

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