A comment below Which celestial body is able to come closest to Pluto? says that

Technically, many, many small Kuiper belt objects enter Pluto's Hill Sphere (or sphere of influence SOI) every day.

Pluto is small, but it is very far from the Sun, so maybe its Hill Sphere could be surprisingly large, and if we called each particle of dust out there a "small Kuiper belt object" then this can be true.

Question: But realistically, how many solar system objects that can really qualify as a KBO are likely to enter Pluto's Hill sphere each day?

For the purposes of this question let's see if there is some way to ascertain what a KBO might be, and not focus on the helpful comment too much.

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    $\begingroup$ Afaik almost all TNOs are so large they're considered 'possible dwarf planets'. Arrokoth is the only clear asteroid beyond Neptune I know of. But if a tiny meteoroid's orbit is fully within the Kuiper belt, I don't see a reason why it shouldn't be considered a KBO. $\endgroup$ Commented Jan 19, 2021 at 14:47
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    $\begingroup$ @PlutosLoyer Did you look at Wikipedia's List of trans-Neptunian objects ? The vast majority aren't considered dwarf planet candidates. $\endgroup$
    – notovny
    Commented Jan 19, 2021 at 17:59
  • $\begingroup$ @notovny Thank you. $\endgroup$ Commented Jan 19, 2021 at 18:20

1 Answer 1


The Hill sphere radius of Pluto is about $r$ = 6 million km. Most of the Kuiper belt is in prograde motion around the Sun (like Pluto). Pluto's average speed is under a lazy 5 km/s for an orbital period of about 248 years. If the difference in orbital speed between Pluto and an average KBO is just 1 km/s, then Pluto will "sweep out" $\pi r^2 *86,400s *1km/s = 9.8 \times 10^{18} km^3= 2.9 \times 10^{-6} AU^3$ of the Kuiper belt per day.

The Kuiper belt is described as a donut extending from 30 to 55 AU. The volume $V$ of a torus is $V=2\pi^2Rr^2$, where $R=42.5$ and $r=25$ are the major and minor radii, so the approximate volume of the Kuiper belt is $5.24 \times 10^{5} AU^3$. Of course, KBO's are distributed more densely near the center of the torus, and exist increasingly rarely outside the torus.

So, Pluto might sweep out about $V_1 = 2.9 \times 10^{-6} AU^3$ of the $V_2=5.24 \times 10^{5} AU^3$ volume Kuiper belt per day. That means there would have to be approximately $V_2/V_1 \approx 1.8 \times10^{11}$ (or 180 billion) total objects in the Kuiper belt for one to enter into Pluto's Hill sphere each day, on average. It might even need to be much higher since Pluto's orbit is more inclined then the distribution of the Kuiper Belt, as shown here:

enter image description here

We only know of about 2000 KBOs according to this NASA site. Pitjeva and Pitjev postulate only millions of objects in the Kuiper belt, so it may be unlikely that many KBOs enter Pluto's Hill Sphere radius each day.


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