Will Pluto ever collide with Neptune?
No, they will never collide. Despite Pluto’s orbit appearing to cross that of Neptune when viewed from directly above, the two objects’ orbits are aligned so that they can never collide or even approach closely. Several factors contribute to this. At the simplest level, the two orbits do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune’s orbit as viewed from above, it is also the farthest above the ecliptic. This means Pluto’s orbit passes about 8 AU above that of Neptune, preventing a collision. Pluto’s ascending and descending nodes, the points at which its orbit crosses the ecliptic, are currently separated from Neptune’s by over 21°. However, this alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) would alter aspects of Pluto’s orbit (such as its orbital precession) over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of the
