It’s true that Pluto’s elongated orbit carries it closer to the Sun than Neptune, and then farther away from the Sun than Neptune. However, crossing orbits does not imply these planets will collide. Here’s why:
As Pluto travels in its orbit, Neptune’s gravity causes small “course corrections.” These slight corrections maintain several tens of degrees of angular separation be- tween the two planets.
Pluto’s orbital period shares a 3:2 relationship with Neptune’s. Neptune goes around the Sun three times for every two Pluto orbits. This orbital resonance causes the two planets to move in a dance-like pattern. Moreover, during the part of Pluto’s orbit that lies interior to Neptune’s, Pluto is also near its farthest distance above the solar system’s mean plane. This ensures that, when Pluto is near perihelion, the Pluto-Neptune distance (as well as the Pluto-Uranus distance) remains large — well over 10 times the average distance between Earth and the Sun. — Renu Malhorta, Lunar and Planetary Institute, University of Arizona, Tucson