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Someone claimed to me that they could see the actual shape of the ISS as it passed overhead without use of any optical device. I would think this isn't possible since it is only about 100 meters wide but over 370 km in altitude. I read through this question already but I wanted to know if it was possible to discern the solar arrays or general shape of the station (assuming you're in a decently remote area).

Edit: This was claimed to have been observed recently, so it wasn't a case of a shuttle rendezvous with the station.

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    $\begingroup$ It's perfectly possible to see the shape with a telescope, though. $\endgroup$ Nov 27, 2017 at 7:50

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Having observed the International Space Station numerous times passing over the early night skies, on a clear night and when my eyes are rested (I stare a lot at monitors like I'd guess most of us here) having normal visual acuity (20/20 vision), I can assure you that your friend's claims are quite impossible even for an experienced amateur astronomer with many tricks up his sleeves, such as knowing how to use averted vision and observing the station when it barely reflects any light (usually farthest East during its early night sky when it would most likely start entering the Earth's shadow), to reduce atmospheric diffraction of high-contrast close bright sources on a dark background reducing into a single point of light:

                                                 enter image description here

           Airy diffraction patterns generated by light from two points passing through a circular aperture, such as
           the pupil of the eye. Points far apart (top) or meeting the Rayleigh criterion (middle) can be distinguished.
           Points closer than the Rayleigh criterion (bottom) are difficult to distinguish. Image and quote: Wikipedia

But before we even consider what other limitations to discerning features of a distant object might there be, let's first see what's still reasonably plausible for a human eye. So the question is, what's the human eye's resolution and the minimum spacing between two bright objects at the ISS orbital altitude (370 km or 230 mi)?

According to Human Photoreceptor Topography, Curcio et al., 1990 (PDF) that lists several sources as well as own measurements of the spatial density of cones and rods in whole-mounted human retinas, the greatest cone density recorded was 324,100 cones/mm2. That gives us acuity (or row-to-row spacing we need to discern at least two individual features) of 86.3 cycles/°. So for our best case, with a great eye, neglecting any atmospheric effects, the ISS right above us when it's closest, and optimal contrast with the background sky, we get minimum separation of objects of 74.83 m. If there was no air between the observer and the station!

So while 74.83 m seems just about right with the ISS truss length at 109 m (and solar panel arrays stretch a bit further than its truss, so their two sides' center spots would indeed be separated about that much), we shouldn't forget that there is about as much matter in between the observer and the subject that the observer is looking through, as if he was looking through roughly 10 meters of water. So indeed, the two sides of the ISS would be quite impossible to discern, even from a high altitude observation point, no air or light pollution, and extremely clear night.

It is however still possible that your friend isn't making anything up, saw an iridium flare of two closely following satellites flying in formation, and confused them for the ISS. Those are not that rare, I've seen two such double iridium flares just last month, and I can assure you that I was not that bored to constantly look at the night skies. Those double (or sometimes triple, quadruple,...) flares of satellites flying in formation can look something like this (long exposure taken with a zoom camera):

   enter image description here

Inexperienced observer could easily confuse these for a single object, since they appear to move as if they were connected to each other, like the ISS solar panels are via its truss. They move exactly like you'd expect of Low Earth Orbit satellites (like the ISS) to move, which is of course because they are. And they can be as bright as the ISS. But they do keep a bit larger distance to each other than the ISS's roughly 100 yards. Actually, there's a lot less space between the ISS solar panel arrays, since that's approximately its total length.

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  • $\begingroup$ I'm wondering if it could have been an instance where something like the Shuttle was approaching the ISS too. $\endgroup$
    – PearsonArtPhoto
    Apr 20, 2014 at 12:56
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    $\begingroup$ @PearsonArtPhoto It is a possibility, if it happened years ago, but the solar panels of today's resupply vehicles and Soyuz are really small compared to the ISS ones and they would have to be on final approach for them to appear as if they were two parts of the ISS. E.g. for yesterday, here's the ISS: flickr.com/photos/11113385@N02/13940060613/in/… and the Dragon about 3 minutes behind it: flickr.com/photos/11113385@N02/13916922912/in/… As the source says, the Dragon resupply vehicle was faint but still visible to the naked eye. $\endgroup$
    – TildalWave
    Apr 20, 2014 at 13:04
  • $\begingroup$ The Shuttle could be seen, but I agree, everything else it just too dim to have much of a chance. Remember that the Shuttle stayed near the ISS for about 6 hours before docking, so... $\endgroup$
    – PearsonArtPhoto
    Apr 20, 2014 at 13:05
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    $\begingroup$ That said, even with $20 Walmart binoculars you can see the shape of it. $\endgroup$
    – Skyler
    Oct 3, 2017 at 18:48
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    $\begingroup$ As mentioned by Skyler, it's perfectly possible to see the shape of ISS with binoculars or a telescope. It's an incredible sight, and it's not that hard to do with a dobsonian. I could even distinguish the 16 photovoltaics arrays and the distinct modules in the middle. More info here. $\endgroup$ Nov 27, 2017 at 7:50
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Human vision has an unaided resolution (typically) of around 0° 4'.

The ISS orbits at 370 km. It is approximately 73 m wide, 109m m long.

3.7e5 * sin(4') ≅ 430.5

Each "pixel" is roughly 430 m. That the station is visible at all is a simple matter of brightness.

To get a visible shape reliably would require a roughly 10x scope, at which point the image would be between 1 x 2 pixels and 3x3 pixels, depending upon where exactly it falls - sufficient, over time, to infer shape.

Even at the fovea, where peak individuals have been noted to have as narrow a discrimination as low as 0° 0' 21" of arc resolution, the peak resolution would be about 21 m per pixel. Still only about 4x5 pixels worth, at the very peak.

Note also: human shape discrimination isn't a continuous field of uniform resolution, but a variable field ranging from about (typically) a ≤1° zone of 1' resolution through an outer band of ≥1° per cone at the very edges. To make out the station at peak is barely doable, but only if looking right at it and then only while it remains within the narrow view of the central fovea.


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    $\begingroup$ Do you have a source for your human vision resolution figure? $\endgroup$
    – RossV
    Apr 21, 2014 at 13:05
  • $\begingroup$ I've "known" for years that typical good human eyes have an angular resolution of about 1 minute of arc, or 1/60 of a degree. This is surprisingly hard to find an official source for, though! $\endgroup$
    – dotancohen
    Apr 21, 2014 at 15:38
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    $\begingroup$ I verified my recollected 0° 4' via wikipedia, and yes, I am aware that peak human vision can get (in the sweet spot) down to as fine as 0° 1' in exceptional individuals. That's still not quite enough to get to the level of resolving to more than 6-9 pixels. $\endgroup$
    – aramis
    Apr 22, 2014 at 19:57
  • $\begingroup$ Best corrected visual acuity in adults is usually 1 minute of arc when measured with a Snellen letter chart en.wikipedia.org/wiki/Visual_acuity , Many people (particularly myopes wearing their correction) are significantly better than this. When measure as Vernier acuity, most people are significantly better than 1 arc minute, some up to 0.13 arc minute. $\endgroup$
    – Woody
    Apr 28 at 2:38
  • $\begingroup$ It is not particularly useful to model the retina as a field of pixel-like photoreceptors. As well as photoreceptors, the retina has several layers of neurons which analyze inputs from surrounding photoreceptors, in the same manner as the neural networks used in AI. The neural networks form center/surround units which achieve Vernier acuity which exceeds the "pixel spacing" of the photoreceptors. $\endgroup$
    – Woody
    Apr 28 at 2:45
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In addition to the other answers: you could actually test this yourself.

Build or print a 5 cm model of the station. Make sure it's white. The scale would be 5/10900 = 1:2180

If you assume an average distance of about 370km, that would scale down to roughly 170m.

  • Find a straight road with no traffic. Check with any map that it's long enough.
  • Take a bicycle, measure the circumference of the tyre and mark one spoke. Calculate how many turns you need to get to 170 meters.

  • Take black cardboard on a sunny day, put the model in front of it and then slowly move away and count revolutions. A good gps will work, too but it's less fun.

While this won't take atmospheric effects into account, it still should be somewhat accurate.

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  • $\begingroup$ I like your answer! Have you ever tried this? It sounds like you might have. If you can describe what you saw, that could count toward an answer to the OP's actual question. $\endgroup$
    – uhoh
    Mar 21, 2018 at 8:45
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    $\begingroup$ @uhoh I have some knowledge when it comes to basic astronomy and mirror making but my eyes are not up to the challenge anymore. It's also much more fun to do it oneself than to just read it. I might do similar things with a telescope to measure distances.in the future. $\endgroup$ Mar 21, 2018 at 8:58
  • $\begingroup$ OK well I'm going to try it then! :-) I don't have a 3D printer, but I do have a laptop with a copy of Blender! Can you recommend a source for a 3D model that can import like this and then display on a black background? $\endgroup$
    – uhoh
    Mar 21, 2018 at 9:08
  • $\begingroup$ @uhoh While I like it a lot when people bring up Blender, I can assure you, a few pieces of white cardboard and a small roll of paper will do. You can even do preliminary tests with any household item that has decent contrast. $\endgroup$ Mar 21, 2018 at 9:12
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    $\begingroup$ Also: Unless you are certain to be alone or have friend who's guarding the laptop, putting considerable space between you and your valuables can backfire. A cigarette ISS with aluminum foil solar panels will not be missed that dearly. $\endgroup$ Mar 21, 2018 at 9:35
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I looked this up because last night I saw what I was sure was the ISS moving w to e over Maple Ridge BC about 22:00 ( clear, dark, sky). I quickly went in to grab binocs and tracked for a few seconds but couldn't focus satisfactorily, so looked with bare eyes ( n b: 2 intraocular Toric astigm. implants within last 2 months) and was amazed to see clearly solar reflection off the solar arrays and a dark vertical image between them, thus an X shaped body, but clearly the reflected solar arrays. Not 2 streaks (as per image above), but the station itself. I was totally amazed as previously ( thru specs ) the passage was just a very bright light in the early morning or evening sky (much brighter than venus).

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IDK if anyone is still reading this, but great answers to a question that came up two nights ago. I lucked into an especially high and bright overhead pass of ISS with pretty good seeing, and SWEAR I could, basically, see two tiny diffractions instead of a singular one. I thought there might be just enough angular separation between the two panel sections to account for that… tried with one eye, other eye, both eyes. Probably wishful thinking, but it definitely seemed like two points.

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  • $\begingroup$ “Averted imagination,” we call it. $\endgroup$ May 24, 2023 at 16:21
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Yes,one can see the solar array with good vision, assuming at right time & very clear sky. I just watched station pass over here coast NS @ 5am Atlantic. The station was heading into pre-dawn skys here. While arrarys not clearly defined, I could definitely see fuzzy outlines of the arrays projecting out in addition to classic station brilliance in cold clear pre-dawn light. That said, it was first time that had happened outta of numerous middle of night sightings over the years, so not surprised reading others claiming it's not possible. Recommend small quality pair pocket size binoculars. I have large and small pairs, while larger ones give more light I tend use small ones more as they're easier on arms & shoulders for extended viewing. They take lot less room on a kitchen table so always at hand, and simple to leave in coat too, ready to go. Don't have much time with viewing station, and I never plan ahead...just look out and go ohh there it is and scoot outside for a boo. Quite the sight...never gets old.👍 Binoculars - Pentax Jupiter 7x20 - 7.5"

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    $\begingroup$ The question asks about unaided eyes, not using binoculars. $\endgroup$ Apr 26 at 11:23
  • $\begingroup$ As mentioned the question is about naked eye. But I have had a similar experience as yours with 7X binoculars with ISS near zenith, which surprised me as I didn't think at that magnification it would be possible to detect two distinct light sources. Reading some of the answers apparently it is technically possible. But I also wonder if there is something more complicated going on with the refraction of the two neighboring light sources as they travel through the atmosphere than can be explained by just trigonometric resolution calculations. Might explain some of the claimed naked eye sightings $\endgroup$ Apr 26 at 20:43

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