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When Neptune and Pluto are closest, about 100 million mi (160 million km) from each other, would an observer on Neptune (or rather on one of its moons, since Neptune is gaseous) be able to see Pluto, and maybe even Charon, with the naked eye? If not, could Pluto be seen in average binoculars? I think Pluto would appear a bit smaller than Mercury from Earth (but at much lower apparent brightness because of the distance from the Sun).

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    $\begingroup$ Hi, this is not an answer to your question, just an idea. If you have Stellarium, you could change the planet in location window to Neptune and watch Pluto from there. It gives you an endless opportunity to tweak and try things. All the best! $\endgroup$ – lordparthurnaax Jan 18 at 12:42
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    $\begingroup$ @lordparthurnaax Thank you. I do have Stellarium, but I dunno when Neptune and Pluto came/come closest. I think it was in the 1970s/80s when Pluto was closer than Neptune. I'll research their intersection dates. $\endgroup$ – Plutos Loyer Jan 18 at 12:51
  • $\begingroup$ As a rule of thumb, halving the distance to an object decreases its apparent magnitude by about $-2.5 \log_{10} 4 ≈ -1.5$. To get Pluto into the visible range, you'd have to decrease the mangitude by about 7.5 relative to its magnitude on Earth, so you'd need to be about 32 times closer to Pluto than the Earth is to (barely) see it with the naked eye. If I've worked my numbers right, this very conveniently works out to be about 1 AU away (maybe a bit less.) $\endgroup$ – Michael Seifert Jan 18 at 20:50
  • $\begingroup$ Pluto and Neptune never get within 160 million km of each other. More like 2.5 billion km at their closest. See: astronomy.stackexchange.com/questions/22304/… $\endgroup$ – Connor Garcia Jan 19 at 20:11
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No, it cannot. Far from it.

The closest approach between both planets is roughly 16 AU due to the 3:2 orbit resonance. Pluto will even then be a tiny dot among many with a brightness around 14 mag.

You can try that with Stellarium yourself, placing the observer on Neptune and looking for Pluto. You just have to find the right time. One such time is approx. in the year 2877.

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    $\begingroup$ I found an 18.9 au approach in 1896. $\endgroup$ – Mike G Jan 18 at 18:42
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    $\begingroup$ @MikeG Indeed, 18.88 au from me in fall 1896. Still not visible at 70x magnification. $\endgroup$ – Plutos Loyer Jan 19 at 7:24
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    $\begingroup$ 14 mag? Isn't it the same brightness as what we see from Earth? $\endgroup$ – Eric Duminil Jan 19 at 10:25
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    $\begingroup$ In Stellarium, I found a 9.8 AU distance between Pluto and Neptune, in -2527. Not sure if it's really reliable, though. With a 12.9 mag, as seen from Neptune. Oooooh. And 10 mag at 3 AU in October 99861. XD $\endgroup$ – Eric Duminil Jan 19 at 13:08
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    $\begingroup$ @EricDuminil Pretty sure this is fake news. $\endgroup$ – ProfRob Jan 19 at 18:41
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According to the NASA Pluto fact sheet, the brightest that Pluto gets is an apparent visual magnitude of 13.65 when it is 28.6 au from the Earth and (presumably) about 29.6 au from the Sun.

To work out how bright that would be from Neptune we could work out how close Neptune can be to Pluto when Pluto is at perihelion. This is complicated by the fact that the semi-major axes of the orbits are not aligned, that the inclination of Pluto to the ecliptic is quite high and that the orbital periods are in a 3:2 resonance. The closest they can actually get is about 16 au and this will not occur when Pluto is at perihelion. And, when Pluto is at perihelion it will be further than 16 au from Neptune and will not be fully illuminated (or even as illuminated as we can see it from Earth) as seen from Neptune.

Therefore an upper limit to the brightness would be to scale how bright it can appear from Earth to a distance of 16 au, which gives an apparent magnitude $>12.4$.

For the reasons stated above it must be fainter than this because it is not fully illuminated as seen from Neptune at 16 au and it is not at perihelion at that time, and so it cannot be anywhere near visible to the naked eye.

EDIT: A paper by Williams & Benson (1971) puts the closest approach of Pluto and Neptune at 16.8 au, but crucially, this occurs when Pluto is near aphelion. They also show that this is true for a timespan exceeding 1 million years.

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  • $\begingroup$ Do you happen to know how reliable the planets positions according to Stellarium in ~100000 years are? Because in 99861, the distance between Neptune and Pluto would be only 3 AU, for a brightness of 10 mag. $\endgroup$ – Eric Duminil Jan 19 at 13:11
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    $\begingroup$ @EricDuminil that would make another good question. I think the 16 au is based on the current orbits. The closest the lines of the orbits ever come to each other is 2.4 au. AFAIK they are in a stable configuration, so I can only assume that there is some numerical accuracy problem in Stellarium (which contains no physics remember), but I'm surprised by that. $\endgroup$ – ProfRob Jan 19 at 13:27
  • $\begingroup$ Stellarium uses VSOP87 unless the user has downloaded DE430 or DE431. $\endgroup$ – Mike G Jan 19 at 18:30
  • $\begingroup$ @MikeG: VSOP87, then. According to en.wikipedia.org/wiki/VSOP_(planets)#VSOP87, it should still be accurate for Neptune in -2527 ("over 6000 years before and after J2000"). So is it unreliable for Pluto? $\endgroup$ – Eric Duminil Jan 19 at 20:41
  • $\begingroup$ @EricDuminil Dunno. Certainly no more reliable than for Neptune. $\endgroup$ – Mike G Jan 19 at 21:25
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That's great question! ... I'm just thinking that just for comparison, Pluto is roughly 70% the diameter of our Moon and Pluto would be far away from Neptune than the Moon is from Earth. Moon is also sort of "fixed" to Earth, but Pluto is in a different orbit than Neptune. I think that Pluto will not be visible with the naked eye given its size and distance from the observer.

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    $\begingroup$ Welcome to the site, Daniel! Yes, as mentioned in the comments on planetmaker's answer, even the closest approach distance is rather large. By way of comparison, the mean orbital radius of Uranus is 19.19 au. Also, it's rather dark way out there. Neptune's mean orbital radius is ~30.1 au, so by the inverse square law, the intensity of sunlight is ~1/900 of its intensity near the Earth & Moon. $\endgroup$ – PM 2Ring Jan 18 at 19:50
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    $\begingroup$ This isn't a correct way to think about the problem. The surface of Pluto is much less "luminous" than that of the Moon because it is much further away from the illuminating Sun. $\endgroup$ – ProfRob Jan 19 at 12:47

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