# Thread: Pluto and Neptune: do they really cross orbits?

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## Pluto and Neptune: do they really cross orbits?

In a 2D, 'overhead' view, the object we call Pluto crosses the orbit of Neptune. (Personally I think we should have waited for New Horizons before deciding what to call it, so wait I shall)

But do they cross in 3D? In other words, if one could cause Pluto to stop in its orbit, is there any location in that orbit that would allow Pluto and Neptune to collide? (Disregarding, of course, such frivolous considerations as normal physics, effects of gravity, etc. -- I really just want to know if there is a 3D intersection.)

If not, can these two objects still be accurately described as having "crossing orbits"?

Or is there a difference between "crossing orbits" and "intersecting orbits"?

2. The don't intersect in three dimensions: Pluto's orbit is well away from the plane of Neptune's orbit when the two are at the same distance from the sun.

Grant Hutchison

3. Oh, in answer to your other question, I think it's only possible to say that the orbits appear to cross in certain projections (in this case, most notably in the classic view of the solar system from ecliptic north). But it's perfectly possible to adopt a viewpoint of the solar system from which the orbits of Neptune and Pluto don't appear to cross each other.
However, if they truly intersected each other it would be impossible to find such a viewpoint, I think.

Grant Hutchison

4. Bob
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5. Originally Posted by Bob
http://www.seds.org/nineplanets/nine.../plutodyn.html

The minimum Pluto-Neptune separation is 17 AU, compared to the minimum Pluto-Uranus separation of 11 AU.
Even disregarding the 3:2 resonance, the smallest distance between two orbit is, IIRC, 2.7 AU. But as the above article explains, when Pluto is at that "point of closest approach", Neptune is far away and vice versa.

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Originally Posted by grant hutchison
Oh, in answer to your other question, I think it's only possible to say that the orbits appear to cross in certain projections (in this case, most notably in the classic view of the solar system from ecliptic north). But it's perfectly possible to adopt a viewpoint of the solar system from which the orbits of Neptune and Pluto don't appear to cross each other.
However, if they truly intersected each other it would be impossible to find such a viewpoint, I think.

Grant Hutchison
Since their distances overlap, I don't think you can find a vantage where they would appear to not cross. But since they don't actually intersect, the places where they appear to intersect will not line up between different vantages.

Here's a graphic of the classic overhead view, and a view from in the ecliptic. Notice where they appear to intersect in the overhead view is not lined up with where they appear to intersect in the ecliptic view.

In this next graphic, I adjusted Pluto's orbit so it did intersect Neptune's orbit at one point. In each view it still appears to intersect twice, but the position where it actually intersects (~4 o'clock position in the overhead view) will line up in different vantages.

7. Originally Posted by tony873004
Since their distances overlap, I don't think you can find a vantage where they would appear to not cross.
Does this qualify?

plutoorbit.gif(Click to enlarge.)

It took me about 30 seconds to find with the NASA Near Earth Oribit Program Orbit Diagram applet for 2003 UB213. I just set the view tilt and spun the system around until the orbits didn't overlap. (The mostly arbitrary date was chosen for making the object names legible.)

Also, in the other direction, it wasn't hard to find a vantage point from which Pluto's orbit appeared to "cross" that of Jupiter -- and if scale wasn't an issue, probably even Mercury as well. I think that an extremely oblique view of Pluto's orbit, projected, can appear to come arbitrarily close to the Sun.

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Thanks everyone!

9. If a pair of orbits interlink, like the links of a chain, there won't be a viewpoint from which they appear not to cross.
But for any other pair of non-intersecting orbits, I think the following process will provide a viewpoint that shows them uncrossed.
1) Draw the line that marks the intersection of the two orbital planes. This line will intersect the orbits themselves at two points for each orbit. If the orbits don't interlink, as described above, then one orbit will be the "outer" and one the "inner": the order of intersection will go BAAB, for instance, with orbit A being the inner orbit at the line of intersection.
2) Draw the tangents to the inner orbit (A in this case) at the two points at which the orbit intersects the line drawn in 1) above.
3) Go to the point at which these tangents cross (or, if they're parallel, follow them to infinity).
This point will give you a view in which the whole of orbit A is seen as a line segment bounded by the tangent lines, while orbit B lies outside the tangent lines. A region around this point will give you views of both orbits as non-crossing loops. The larger the separation between orbits along the line of intersection in 1), the larger will be the region from which they are seen not to cross.

Grant Hutchison

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