Mr. Day was my 8th grade science teacher. I remember him talking about the orbits between Neptune and Pluto overlapping. He once said (if I heard him right), "Pluto will once cross into Neptune's orbit, and there will no longer be a nine planet." Well, his prediction came true about no longer having nine planets in our solar system with Pluto's degradation to a dwarf planet, but apparently he was wrong about Pluto and Neptune colliding. Here's what I found.
Pluto will never ram Neptune
Q: Will Neptune and Pluto collide? Because their orbit is getting closer--Orglab, Iowa
A: Neptune and Pluto will probably not collide, at least for the next 750 million years or so. Gerald Jay Sussman, MIT designer of a special computer called the Digital Orrery, worked with MIT astronomer Jack Wisdom to simulate the celestial mechanics of the solar system over the next billion years or so. They found that Pluto is in a chaotic orbit but one that would not take it closer than tens of millions of miles from Neptune.
Pluto goes around the Sun twice for every three times that Neptune orbits the Sun. So, Pluto and Neptune's position relative to each other repeat every 497 years. They will never, however, be close to each other when Pluto is crossing at the same distance from the Sun as Neptune is. They can't collide as long as Pluto maintains its present orbit. Pluto's orbit is high above Neptune in the orbital plane and that keeps them apart.
By the way, Neptune's largest moon, Triton, is gradually getting closer to Neptune and will collide with the planet in about ten to a hundred million years.
For those of you too lazy to read the fine print, Pluto will most likely never collide with Neptune (the possibility is 750 million years away), but Triton, the largest moon of Neptune will be gone in about 10 to 100 million years. Now, you know something new.