The orbits of Neptune and Pluto cross each other, so will they collide?

Mar 21, 2016

A collision between Neptune and Pluto can't happen at present.

Explanation:

Although Pluto's orbit crosses that of Neptune the two never get close together.

The reason for this is that Pluto is locked into a stable orbital resonance with Neptune. Orbital resonance is when two bodies are gravitationally linked such that their orbital periods are relation by the ratio of two small integers.

Pluto and Neptune are in a 2:3 resonance. Neptune's period is 165 years and $3 \cdot 165 = 495$. Pluto's period is 248 years and $2 \cdot 248 = 496$. So, their orbital periods are almost exactly in the ratio 2:3.

The effect of this resonance is that when Pluto is inside Neptune's orbit, Neptune is about a quarter of its orbit away, so they never get close.

If the resonance were to become unstable and did get close to Neptune, Pluto would probably get ejected by Neptune.