I want to build a big structure that will be visible from space. A object would need to be very large, maybe kilometers across, to be visible from, let's say, the ISS (which orbits at an average height of 413 km above Earth).

How big would I have to build a non-light-emitting object, so that it is visible from space (the ISS) bare-eye?

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    $\begingroup$ Why is this on-topic, really? What it has to do with building a world? not my vtc, but I kinda sympathize. $\endgroup$
    – Mołot
    Sep 11 '16 at 10:34
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    $\begingroup$ @Mołot how it is not on-topic - he likes to have realistic sizes to spot alien buildings, or whatever. sure it could be also asked elsewhere, but that do not makes questions off-topic here $\endgroup$
    – MolbOrg
    Sep 11 '16 at 12:53
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    $\begingroup$ @MolbOrg "Do not know which refinement could help this question to look better." - well, description of downvote has "lack of research effort", and this topic is discussed on so many places on the Internet, like Wikipedia. Also, putting ISS as designated height is hardly useful - fictional aliens might require different (probably greater) distances. Last but not least, most man-made structures emit light. What's the woldbuilding use of excluding this factor? $\endgroup$
    – Mołot
    Sep 11 '16 at 16:30
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    $\begingroup$ @Mołot no needs to exclude, humans over alien planet, where all aliens are dead, nothing works and far far away from home looking for return. As you might see he had answer he wished to post, and OP made question for it, because he had an answer. That's why Q looks like lack of research, would be that research shown would be no answer and no question. Valid reason to close should be is that dup question or not here no WB, if not, op should accept own answer, and we should forget about it and hope it will be useful for someone, for something, sometime. Yhea not super situation, only op had fun. $\endgroup$
    – MolbOrg
    Sep 11 '16 at 16:42
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    $\begingroup$ This question is the subject of a Worldbuilding Meta discussion: Looking for some constructive criticism $\endgroup$
    – user
    Sep 16 '16 at 18:43

The smallest (circular) object that is possible to see from space would have to have at least (bare minimum):

cca. 120 meters

in diameter. But this would work only if the object was distinguishable from the terrain around and the conditions were perfect. It would have to be in a unnatural color - red, for example - and may have to be a strong light source (the ISS and most stars are visible from Earth only because they shine really bright), but making your object shine would be a "cheat", as you said.

The maths:

An average man's eye can distinguish objects that cover up more than about $0.0165$ or $\frac{1}{60}$ degrees. If we define $x$ as the minimal size, we get this image:

This is an image description

If we cut it down, we get:

This is also an image description

That means that $\tan(0.00825°) \approx \frac{\frac{x}{2}}{413}$ and from that we can get that:

$x \approx 2×413×0.000144 \approx 0.120km$

Enough math, let's go real: An object would have to be much bigger than 120m in order to be visible from space. For example, the biggest pyramid of Giza takes up 440×440m up, but has the same color as it's surroundings, thus being "invisible" from space. If people manage to color the pyramid(s) red or brown or a similar visible colors, they would be visible.

Man-made structures visible from space bare-eye:

Another image description

The last image description

  • $\begingroup$ Just for comparison: a 120m object 413km away would be as visible as a 1.2mm object placed 4.13m away. That's in the very limit of my eyesight ability. With such an object you will be able to see a point, in the best case. If you want to see some detail, you will need a bigger object. $\endgroup$
    – Pere
    Sep 11 '16 at 21:34
  • $\begingroup$ I'm sure there are also effects due to atmospheric effects to consider. Turbulence in the troposphere would likely make your resolution much worse than the ideal case. $\endgroup$
    – Snyder005
    Sep 11 '16 at 22:39
  • $\begingroup$ It doesn't need to be 120m diameter. It needs to be large enough to cause an observable colour or intensity change averaged out over a patch this size. In particular, a road (black or grey) across dense vegetation (green) will be noticed even if it is considerably less than 120m wide. $\endgroup$
    – nigel222
    Sep 12 '16 at 13:26
  • $\begingroup$ Further to my last, this means that a strong light source will be seen even if tiny, though you will detect only it's presence not its size. $\endgroup$
    – nigel222
    Sep 12 '16 at 13:32

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