I read in an astronomy book that Neptune and Pluto cross paths in their orbit. Does this mean that one day will collide?
Highly unlikely. They are not on the same orbital plane.
Nope… for one thing, Pluto’s orbit is tilted pretty far with respect to the other planets. So where Pluto’s and Neptune’s orbits seem to cross when seen from above, Pluto’s actually quite a bit “above” (for lack of a better term) Neptune.
I also seem to recall there’s something about the periods of their orbits which keep them far apart from each other in the solar system, and they’ll never actually both be at that “cross point” at the same time anyway.
No. While Pluto’s orbit is quite eccentric, so much so that it is sometimes closer to the sun than Neptune, it also orbits in a path that is tipped quite a bit with respect to the plane of the ecliptic. This means that the two orbits don’t intersect.
Think of two interlocking steel rings like magicians use. Even though one goes inside the other, you can easily hold them tilted to each in such a way that they never touch. This is the situation with Neptune and Pluto.
drat - I’m a day late to this astronomy thread!
anyway, like bibliophage said, it’s the stable 3:2 orbital period that keeps them apart.
Firstly, a small note - even if they did collide, it wouldn’t be a head-on collision. Neptune would be either rear-ended or sideswiped by the more swiftly moving Pluto.
Now, I do admit that Pluto and Neptune are pretty close to a 3:2 orbit, but I’ve never heard that they’re harmonically locked, although that doesn’t mean that they’re not. However, my sources (which admittedly spell it “Pkuton”) show their orbital periods to be 164.79 and 248.54 years, which is slightly off the 3:2 harmonic. Also, it gives a synodic period of 489.04 years, which is significantly different from that NASA figure, so I’m admitting up front that my figures may be flawed.
Anyway, if they aren’t locked, then the point where the are collinear with the Sun (as seen from above) will process around the Solar System. Eventually, this would occur within one Neptune-diameter (1/571837th of Neptune’s orbit) of any given point, every 571837 × 489.04 = 279,649,794 years.
Of course, as bibliophage pointed out, they’re not in the same orbital plane, but in 280 Million years, a planet like Pluto can get pretty jostled around. So maybe Pluto’s vertical position at that point will also process. Given its orbital inclination, Pluto’s vertical range covers 3649321325 km, or 73735.6 Neptune diameters. Using a similar method as before, I predict one collision every 20.62 Trillion years. You people just aren’t thinking long-term enough.
So if Pluto were driving an SUV and Neptune were driving a Mercedes and they collided, who would you laugh at first?
::ducks and runs::
Hey! I can actually answer one!
As several people have stated, the likelihood of Pluto and Neptune colliding is basically zero due to Pluto being in resonance with Neptune. But it’s actually better than that. The current theories (proposed by Malhotra) suggest that Neptune (and the other planets) actually migrated inward early in solar system history due to gas drag (when there was still gas to drag). On the way in, Neptune captured Pluto (and some dregs) into the resonance. So, Pluto’s definitely in the resonance, not just close. I suspect Archenar’s numbers either aren’t quite right, or reflect the fact that Pluto can librate within the resonance a bit. If Pluto started wandering too far, Neptune’s gravity would change Pluto’s orbit back the other way.