From a bird's eye view of the ecliptic, it is clear that Pluto's orbit is more eccentric than any other planet's. Pluto's orbit appears to intersect Neptune's, and for a period of about 20 years Pluto is slightly closer to the sun than Neptune. The next viewgraph shows that the apparent intersection of orbits is an illusion.
Here we see the z-component of each planet's position plotted against the simultaneous distance from the sun. Most planets only make small excursions in the vertical and radial directions, but Pluto executes a large, tear-shaped figure. This vertical projection shows that Pluto is far from being Neptune-crosser.
Pluto and Neptune are in a well-known 3:2 resonance which prevents their collision or even any close approaches, regardless of their separation in the z-dimension. This figure shows the orbits of the four giant planets and Pluto in a reference frame rotating synchronously with Neptune's mean motion. The minimum Pluto-Neptune separation is 17 AU, compared to the minimum Pluto-Uranus separation of 11 AU. The libration of Pluto's orbit is due to an exchange of angular momentum with Neptune. When Pluto is "catching up" to Neptune, it gains angular momentum at Neptune's expense. Pluto moves into a higher orbit, but falls behind Neptune, since higher orbits have slower tangential velocities. Thousands of years later, as Neptune seems poised to overtake Pluto, the opposite exchange takes place. Pluto loses angular momentum, falling to a lower - but faster - heliocentric orbit.
This page is the work of Renu Malhotra and appears in her book "Pluto and Charon" (University of Arizona Press, 1997) For more on this topic on line, see her Scientific American article.
Bill Arnett; last updated: 1999 Sep 20