# Can Pluto crash into Neptune? **URL:** \ **Category:** Factual Questions\ **Created:** [August 18, 2006, 7:02pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265 "2006-08-18T19:02:20Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Malienation](https://avatars.discourse-cdn.com/v4/letter/m/4af34b/32.png) [@Malienation](https://boards.straightdope.com/u/Malienation)\ **Post date:** [August 18, 2006, 7:02pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/1 "2006-08-18T19:02:20Z")
Pluto, apparently, periodically crosses paths with Neptune. Doesn’t this mean that eventually they’ll collide when things line up just right (wrong?)? :eek: If they _almost_ collide, will gravitational effects on each other effect their orbits? 😕 ---
**Author:** ![Monstre](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@Monstre](https://boards.straightdope.com/u/Monstre)\ **Post date:** [August 18, 2006, 7:10pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/2 "2006-08-18T19:10:54Z")
[QUOTE=Malienation] Pluto, apparently, periodically crosses paths with Neptune. Doesn’t this mean that eventually they’ll collide when things line up just right (wrong?)? :eek: If they _almost_ collide, will gravitational effects on each other effect their orbits? 😕 [/QUOTE] Pluto’s orbit is not on the same plane as Neptune’s. The portion where it becomes closer to the sun doesn’t actually interect Neptune’s. That only appears to happen when you look at it from “overhead” on a 2-dimensional drawing. [Here’s a cite](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970326c.html) ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [August 18, 2006, 7:12pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/3 "2006-08-18T19:12:00Z")
The paths only appear to cross when seen on paper. Actually Pluto’s orbit is inclined to the plane of the ecliptic, and Neptune’s is very close to lying in it so the orbits are not very close at all. In addition they are in a 3:2 resonance so that they don’t ever approach each other as closely as the orbits would allow. In fact Pluto’s closest approach to Neptune is almost twice as distant as its closest approach to Uranus. > **[Dynamics of Pluto](https://nineplanets.org/plutos-orbit/)** > > ... Read more ---
**Author:** ![Jackknifed\_Juggernaut](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jackknifed\_Juggernaut](https://boards.straightdope.com/u/Jackknifed_Juggernaut)\ **Post date:** [August 18, 2006, 7:12pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/4 "2006-08-18T19:12:58Z")
I’m sure that someone more knowledgeable in astronomy will answer the question definitively. However, I thought that Pluto was kind of an oddball planet because it revolves on a different plane. A related question, how often is Pluto closer to the sun than Neptune? ---
**Author:** ![Monstre](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@Monstre](https://boards.straightdope.com/u/Monstre)\ **Post date:** [August 18, 2006, 7:14pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/5 "2006-08-18T19:14:46Z")
[Here’s another nice info page with diagrams](http://pluto.jhuapl.edu/science/everything_pluto/16_plutoOrbit.html) ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 18, 2006, 7:17pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/6 "2006-08-18T19:17:38Z")
Wow, there’s actually a good exposition on this over at Wikipedia: [Pluto](http://en.wikipedia.org/wiki/Pluto) > [@](#): > > **Consequences of the orbital resonance** > > The impression of Pluto’s orbit crossing Neptune’s orbit given by the polar view is dispelled by the next diagram (a view from around 10° above the ecliptic). Pluto’s nodes (points where the orbit crosses the ecliptic) are both situated outside Neptune’s orbit (illustrated by red segments becoming darker). The diagram shows also the closest points of the two orbits, separated there by ~ 6.4 AU. However, Pluto and Neptune never get so close. Due to the orbital resonance Pluto executes 2 full cycles while Neptune makes 3 and their relative positions are locked near these points. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [August 19, 2006, 7:50am UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/7 "2006-08-19T07:50:32Z")
[QUOTE=Jackknifed Juggernaut] A related question, how often is Pluto closer to the sun than Neptune? [/QUOTE] For about 20 years out of every 248 year orbit. Last time was in 1979-1999. It should be noted that Pluto is not the only KBO (Kuiper Belt Object) that crosses Neptune’s orbit. There’s a fair number of others that do. A certain subset of KBOs are also in 3:2 orbital resonance with Neptune. These are called plutinos. ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [August 19, 2006, 1:59pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/8 "2006-08-19T13:59:37Z")
[QUOTE=Monstre] Pluto’s orbit is not on the same plane as Neptune’s. [/QUOTE] Unless there is some definition of “plane” unique to astronomy, this argument is irrelevant. Two planes which are not parallel always intersect somewhere. It is therefore possible that two planets with orbits in different planes will eventually collide. (Of course, this also requires the circles or ellipses of the orbits to intersect, not just their planes.) ---
**Author:** ![Kamino\_Neko](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kamino_neko/32/34_2.png) [@Kamino\_Neko](https://boards.straightdope.com/u/Kamino_Neko)\ **Post date:** [August 19, 2006, 2:15pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/9 "2006-08-19T14:15:16Z")
[QUOTE=psychonaut] Unless there is some definition of “plane” unique to astronomy, this argument is irrelevant. [/QUOTE] No it’s not. It’s what allows Pluto’s orbit to go inside Neptune’s without actually crossing it, as Monstro says in the very next sentence. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [August 19, 2006, 3:55pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/10 "2006-08-19T15:55:39Z")
[QUOTE=psychonaut] Unless there is some definition of “plane” unique to astronomy, this argument is irrelevant. Two planes which are not parallel always intersect somewhere. It is therefore possible that two planets with orbits in different planes will eventually collide. (Of course, this also requires the circles or ellipses of the orbits to intersect, not just their planes.) [/QUOTE] Yes, it is possible, However, if their orbits are in different planes and their distances from the primary do not usually occupy the same range, then I’d say that the chances that the actual orbits would intersect are vanishingly small. The statement that the planes are not the same is an important piece of information for the most astronomy newbies, who aren’t used to thinking of such things in three dimensions, just simplified drawings in a book. (if ever.) It helps to explain how pluto can go from being further out that neptune, to further in than Neptune, without actually crossing its orbit. It is not, in itself, an ‘argument’ that their orbits do not cross. ---
**Author:** ![Monstre](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@Monstre](https://boards.straightdope.com/u/Monstre)\ **Post date:** [August 19, 2006, 4:12pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/11 "2006-08-19T16:12:10Z")
[QUOTE=psychonaut] Unless there is some definition of “plane” unique to astronomy, this argument is irrelevant. Two planes which are not parallel always intersect somewhere. It is therefore possible that two planets with orbits in different planes will eventually collide. (Of course, this also requires the circles or ellipses of the orbits to intersect, not just their planes.) [/QUOTE] Please pardon me terribly for not writing every individual sentence as a mathematically complete answer to the OP’s question – I was attempting to keep to a more brief answer. I will endeavor to be more wordy, verbose, and pedantic in the future. More specifically, if they **were** on the same plane, then any such crossing of orbits would **guarantee** an intersection point. An ecliptic orbit inclined this much (like Pluto’s), makes this intersection a lot less likely in the _general_ sense, and in this _specific_ case, I believe I did in fact provide a [cite](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970326c.html) or [two](http://pluto.jhuapl.edu/science/everything_pluto/16_plutoOrbit.html) that explain the situation in more precise detail – links I felt were sufficient to answer the question clearly and simply on their own merits, without the need to copy their contents onto these pages. From the first cite, in fact: > [@](#): > > So even though we say their orbits “cross”, Pluto is actually quite a distance "above"Neptune. But thank you for taking my initial sentence out of context and presenting it as if it were my sole answer to the question. > [@tengu](#): > > No it’s not. It’s what allows Pluto’s orbit to go inside Neptune’s without actually crossing it, as Monstro says in the very next sentence. Thank you. _psst!_ and it’s Monstre. monstro is a female, and last time I checked, I didn’t fit the bill… 😉 ---
**Author:** ![Monstre](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@Monstre](https://boards.straightdope.com/u/Monstre)\ **Post date:** [August 19, 2006, 4:19pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/12 "2006-08-19T16:19:12Z")
[QUOTE=chrisk] The statement that the planes are not the same is an important piece of information for the most astronomy newbies, who aren’t used to thinking of such things in three dimensions, just simplified drawings in a book. [/QUOTE] And thank you. This is exactly the reason I pointed it out. Many people look at two-dimensional drawings in a book and get the mental image of all of the planets orbiting on a two-dimensional platter. ---
**Author:** ![Kamino\_Neko](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kamino_neko/32/34_2.png) [@Kamino\_Neko](https://boards.straightdope.com/u/Kamino_Neko)\ **Post date:** [August 19, 2006, 4:30pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/13 "2006-08-19T16:30:33Z")
[QUOTE=Monstre] _psst!_ and it’s Monstre. monstro is a female, and last time I checked, I didn’t fit the bill… 😉 [/QUOTE] Oops. My mistake. My monitor sucks, and the e looked strikingly like an o. Apologies to both Monstre and monstro. That’s my story and I’m sticking to it. ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [August 19, 2006, 10:32pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/14 "2006-08-19T22:32:55Z")
[QUOTE=Monstre] But thank you for taking my initial sentence out of context and presenting it as if it were my sole answer to the question. [/QUOTE] I did no such thing. I merely pointed out that the fact that Pluto and Neptune’s orbits are not coplanar does not affect your argument. That is, you could have shown that they will not collide without reference to their orbital planes. It’s easy to imagine orbits of two bodies in different planes such that they will inevitably collide. (For example, consider one satellite orbiting the Earth around the equator, and another satellite orbiting the earth at the same altitude around the poles; unless their oribts are perfectly timed, they’re going to crash into each other.) Likewise, it’s also easy to imagine orbits of two bodies in the same plane (e.g., Mars and Venus), or even in the same orbit (e.g., two satellites in geosynchronous equatorial orbit on opposite sides of the Earth), such that they will never collide. ---
**Author:** ![Kamino\_Neko](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kamino_neko/32/34_2.png) [@Kamino\_Neko](https://boards.straightdope.com/u/Kamino_Neko)\ **Post date:** [August 19, 2006, 11:06pm UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/15 "2006-08-19T23:06:24Z")
[QUOTE=psychonaut] It’s easy to imagine orbits of two bodies in different planes such that they will inevitably collide. (For example, consider one satellite orbiting the Earth around the equator, and another satellite orbiting the earth at the same altitude around the poles; unless their oribts are perfectly timed, they’re going to crash into each other.) Likewise, it’s also easy to imagine orbits of two bodies in the same plane (e.g., Mars and Venus), or even in the same orbit (e.g., two satellites in geosynchronous equatorial orbit on opposite sides of the Earth), such that they will never collide. [/QUOTE] All true. That doesn’t make the fact that Pluto and Neptune don’t orbit on the same plane irrelevant. If Pluto and Neptune were on the same plane, their orbits would cross - because Pluto’s orbit is sometimes within Neptune’s, sometimes outside it - and, thus, they could collide. But, since they are on differing planes, Pluto’s orbit can go within Neptune’s without crossing it. This is not an issue with Mars and Venus, because Venus’ orbit is entirely within Mars’. This doesn’t work with your hypothetical satelites, because both of them cross through the line where the planes meet at the same point. Being on different planes doesn’t make a collision impossible in all cases, but in this case, it would. Being on the same plane doesn’t make a collision possible in all cases, but in this case, it would. ---
**Author:** ![sinjin](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@sinjin](https://boards.straightdope.com/u/sinjin)\ **Post date:** [August 20, 2006, 3:10am UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/16 "2006-08-20T03:10:56Z")
This is the astronomical equivalent of looking at a roadmap and saying "Hey, why is it that the people on interstate 44 going West never crash into people on interstate 270 going North?\* The answer is that, while it looks on the map as though they intersect ([Map](http://maps.google.com/maps?q=Saint+Louis,+MO&ie=UTF8&om=1&ll=38.548501,-90.426718&spn=0.004212,0.008208)), in fact the 270 goes over the 44 ([Photograph](http://maps.google.com/maps?q=Saint+Louis,+MO&ie=UTF8&t=k&om=1&ll=38.548501,-90.426718&spn=0.002106,0.004104)) - Insert your favorite intersecting freeways here. ---
**Author:** ![Monstre](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@Monstre](https://boards.straightdope.com/u/Monstre)\ **Post date:** [August 20, 2006, 5:15am UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/17 "2006-08-20T05:15:34Z")
[QUOTE=psychonaut] I did no such thing. [/quote] Yes. You did. Right here: > [@psychonaut](#): > > > [@Monstre](#): > > > > Pluto’s orbit is not on the same plane as Neptune’s. > > Unless there is some definition of “plane” unique to astronomy, this argument is irrelevant. My statement was not an “argument”. It was a factual statement, about Pluto and Neptune. > [@psychonaut](#): > > That is, you could have shown that they will not collide without reference to their orbital planes. I am unaware of any GQ rule that says all questions are to be answered with the least amount of factual information possible. But if there is such a rule, please allow me to rephrase my original answer more appropriately: - **Q:** Can Pluto crash into Neptune?" - **A:**** No** Interestingly enough, the first [CITE](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970326c.html) I provided, which is an “Ask an Astrophysicist” column on NASA’s “Imagine the Universe” web pages, felt that this information was relevant enough in answering the same qusetion. This cite says they will never collide, and… > [@](#): > > This is due to **two reasons** : First, Pluto’s orbit **is inclined to the ecliptic**. by 17 degrees. … So even though we say their orbits “cross”, Pluto is actually quite a distance "above"Neptune. It would appear that at least one Astrophysicist found that little tidbit to be relevant in the explanation. > [@psychonaut](#): > > It’s easy to imagine orbits of two bodies in different planes such that they will inevitably collide. Feel free to point out where I claimed any different. In fact, feel free to point out where I made **any** argument relating to hypothetical orbits of hypothetical bodies in hypothetical different planes regarding hypothetical collision possibilities. The question was specifically about Pluto and Neptune. My answer was specifically about **Pluto** and **Neptune**. And the question was thoroughly answered, with a variety of cites to good references and diagrams, in the first 7 posts – so what value is being added by all this? ---
**Author:** ![Malienation](https://avatars.discourse-cdn.com/v4/letter/m/4af34b/32.png) [@Malienation](https://boards.straightdope.com/u/Malienation)\ **Post date:** [August 21, 2006, 5:23am UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/18 "2006-08-21T05:23:51Z")
Hmmm…been looking over all of the posts and links and have concluded: Pluto and Neptune can’t collide! I agree with those who say the 3-dimensional aspect is important (the normal 2-d pictures you see in a lot of beginner’s astronomy books certainly don’t help matters). **Psychonaut** , you’re not helping matters, either. Play nice, gentlemen. This includes you too, **Monstre**. You might be right, but snottiness is so…snotty. The matter seems pretty much closed…except for one thing. Half of me is relieved. The other half of me is, oh, I dunno, um, maybe…disappointed. I mean, Pluto and Neptune colliding, wouldn’t that be awesome? Anyone else disappointed? ---
**Author:** ![Monstre](https://avatars.discourse-cdn.com/v4/letter/m/e274bd/32.png) [@Monstre](https://boards.straightdope.com/u/Monstre)\ **Post date:** [August 21, 2006, 6:18am UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/19 "2006-08-21T06:18:11Z")
[QUOTE=Malienation] This includes you too, **Monstre**. You might be right, but snottiness is so…snotty. [/quote] No offense to you or your question. I just felt it would be nice to be able to answer a question in a helpful and friendly manner without getting attacked for it. No good deed goes unpunished, so they say… > [@](#): > > Half of me is relieved. The other half of me is, oh, I dunno, um, maybe…disappointed. I mean, Pluto and Neptune colliding, wouldn’t that be awesome? Well, perhaps if you managed to catch it on film! Although I think the astronomical community right now would be a little upset. They are already having enough trouble figuring out how many planets there should officially be. 😉 ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [August 21, 2006, 6:45am UTC](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265/20 "2006-08-21T06:45:44Z")
[QUOTE=Malienation] I mean, Pluto and Neptune colliding, wouldn’t that be awesome? [/QUOTE] If it ever happens, the next planet (KBO (oh, it is too a planet (yo momma! (_sounds of distant geekfight_)))) we find should be called Velikovsky. (Nitrogen iceball in the corner giant.) [Next page](https://boards.straightdope.com/t/can-pluto-crash-into-neptune/369265.md?page=2)