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The Question

(Submitted March 26, 1997)

Pluto's orbit crosses that of Neptune's, on what date will they eventually collide?

The Answer

Pluto "crossed" Neptune's orbit on January 21, 1979, and temporarily became the 8th planet from the sun. It will cross Neptune's orbit again on Feb. 11, 1999 to resume its place as the ninth planet from the Sun for the next 228 years.

Despite the fact that Pluto and Neptune temporarily change places in their distance from the Sun, they will never collide. This is due to two reasons: First, Pluto's orbit is inclined to the ecliptic. by 17 degrees. (To see an illustration of this, take a look at http://nineplanets.org/overview.html.) So even though we say their orbits "cross", Pluto is actually quite a distance "above"Neptune. Secondly, Pluto orbits the Sun twice for every three orbits of Neptune. The two planets are said to be in a "resonance orbit". For such orbits, the two bodies never get close to each other. In fact, the closest the two planets come to each other is 2 billion kilometers.

Jim Lochner & Karen Smale
for Ask an Astrophysicist

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