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Kuiper belt is 50 AU thick....correct? Oort cloud touches Keiper belt....correct? Neptune is 31 AU away. That means Voyager 2 at 153 AU is beyond the Kuiper belt So where is it??

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Keiper belt is 50 AU thick....correct?

Not really. Objects are classified as Kuiper Belt Objects if their semi-major axis length is greater than that of Neptune and less than or equal to the semi-major axis needed for a 1:2 orbital resonance with Neptune. The latter means a semi-major axis length of about 48 AU. To be in a 1:2 resonance with Neptune, those objects need to dive to less than 45 AU at perihelion, which in turn means that objects spend most of their time beyond 50 AU from the Sun.

Moreover, that boundary is a bit arbitrary. There are objects in orbital resonances with Neptune with much larger semi-major axis lengths. For example, 2015 KE172 is believed to be in a 1:9 orbital resonance with Neptune. It ranges from as close as 44 AU from the Sun to as far as 222 AU from the Sun -- further out than Voyager.

Oort cloud touches Keiper belt....correct?

No. 2015 KE172 is a scattered disk object. The detached objects lie beyond the scattered disk, at least in terms of perihelion distance. The detached objects don't come close enough to the Sun to be perturbed by Neptune.

That said, there are some astronomers who argue that the three most distance objects found to date are Inner Oort Cloud objects rather than detached objects. (There are other astronomers who disagree.) What this means is that the Kuiper belt objects, the scattered disk objects, the detached objects, and possibly the Oort Cloud objects are do not occupy distinct regions of space. They are instead classifications of objects that orbit beyond Neptune, and these different classifications overlap in terms of the regions of space in which they orbit.

Graph showing trans-Neptunian objects. The horizontal axis is perihelion distance while the vertical axis is eccentricity. See text for a detailed description.
Source: https://en.wikipedia.org/wiki/File:Extreme_trans-Neptunian_objects_eccentricity_vs_perihelion.svg

The above diagram depicts trans-Neptunian objects (TNOs) based on their perihelion distance (horizontal axis) and eccentricity (vertical axis). Objects with a semi-major axis length less than 250 AU are plotted in pink. Three colors are used for objects with a semi-major axis length greater than 250 AU. (These are the extreme TNOs. Other astronomers use a boundary of 150 AU to distinguish between vanilla TNOs and extreme TNOs.) Objects with a semi-major axis length greater than 250 AU and a perihelion distance less than 40 AU are green circles; these are the extreme scattered disk objects. Objects with a semi-major axis length greater than 250 AU and a perihelion distance between 40 and 50 AU are yellow triangles; these are the extreme detached objects. The 40 AU boundary is a bit arbitrary. Far on the upper right, there are three objects with a semi-major axis length greater than 250 AU and a perihelion distance greater than 60 AU are black diamonds. These are the sednoids, named after Sedna.

I'll look at three objects, two of which are labeled in the diagram and one of which is not.

  • 1996 TR66 is the first discovered "twotino", an object that is in a 1:2 orbital resonance with Neptune. This makes the object a Kuiper Belt Object. It's perihelion and aphelion distances are 28.63 AU and 66.61 AU, respectively.
  • 2014 FE72 is considered to be an extreme scattered disk object. It's perihelion distance is 36.3 AU and its aphelion distance is 3060 AU. The aphelion distance takes 2014 FE72 well into what was considered to be the Oort Cloud. On the other hand, the perihelion distance takes it well within where 1996 TR66 orbits.
  • 2015 TG387 is one of the three known sednoids, and the one with the highest aphelion distance. It's perihelion distance is 65.16 AU, just within the aphelion distance of 1996 TR66. It's aphelion distance is 2106 AU, again within the inner bounds of what was considered to be the Oort Cloud.

What to make of these three objects? Nobody classifies 1996 TR66 or even 2014 FE72 as Oort Cloud objects. There are some astronomers who do want to classify 2015 TG387, along with the other sednoids, as Oort Cloud objects. There are others who do not. (Aside: To me, science where scientists agree to disagree represents science at its most interesting points in time.) One thing both groups do agree on: They hope very much that the James Webb Space Telescope will successfully launch, deploy, make its way to its planned halo orbit about the Sun-Earth L2 point, and collect data throughout its ten year expected lifespan.

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  • $\begingroup$ Your last paragraph is bit hard to understand for me. Do you mean that the answer to the "Oort touches Kuiper" question is not clear-cut yet? (meaning there can be Oort objects that can be considered "almost Kuiper", by some astronomers). $\endgroup$
    – Ng Ph
    Aug 14 at 8:03
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    $\begingroup$ @NgPh I'm saying that it doesn't make sense to think of the Kuiper belt and the Oort Cloud as regions of space, which is what the OP appears to want to do. $\endgroup$ Aug 14 at 11:20
  • $\begingroup$ That's an AHA moment for me. Thanks! And this is not an obvious conclusion, reading only tutorial materials on the Web, IMHO. $\endgroup$
    – Ng Ph
    Aug 14 at 12:59
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    $\begingroup$ @NgPh I added quite a bit to my answer to better address your comments. $\endgroup$ Aug 14 at 13:35
  • $\begingroup$ Actually, it is legit to view the Kuiper belt and the Oort cloud as regions in space, albeit they are not strictly defined by their dimensions (and shape). With this view, the "regions" are disjoint. To reconcile with your counter-examples, if a space probe is at 152AU can it (still) encounter a Kuiper object? The short answer: Yes, depending on the direction. Similar for an Oort object (but independent of direction). The key concept is that these objects can wander out of their "home region", yet retaining their ID. This is how we detect & ID them and hypothesize on their "home regions". $\endgroup$
    – Ng Ph
    Aug 15 at 9:17
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Voyager 2 is indeed outside the usually understood limits of the Kuiper belt. (About 50 AU, but typically defined as being in resonance with Neptune).

That puts it in the scattered disk, but not as far as the Oort cloud. The beginning of the Oort cloud is typically set to 2,000 AU.

All these regions are sparsely populated with small icy bodies, which the Voyagers are not able to observe.

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  • $\begingroup$ "The beginning of the Oort cloud is typically set to 2000AU". I am not sure that this "typically" is correct. According to NASA, it could be somewhere between 2000AU to 5000AU. The OP seems to believe that the inner limit of Oort cloud is much closer (although it could be a rhetorical question from the OP). $\endgroup$
    – Ng Ph
    Aug 13 at 21:26

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