I was considering a story based on a version of Earth that was being observed from the beginning of civilisation (Around 3000-4000 BC in my understanding "Early civilizations arose first in Lower Mesopotamia (3000 BCE), followed by Egyptian civilization along the Nile River (3000 BCE)").

This version of Earth/Humanity follows the exact path our has up until the discovery they are being observed. Assuming they're being observed by a 21st era satellite, at what point in history would the humans notice it? (And if it wasn't noticed until the 21st century, would it ever be discovered as being out of place as long as it was close enough in design?)

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    $\begingroup$ How large is the satellite? Was it designed to prevent detection? $\endgroup$ – 79037662 Nov 7 '19 at 16:10
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    $\begingroup$ Around the size of a bus, and it was not specifically designed to be hidden. $\endgroup$ – KM.Thomas Nov 7 '19 at 16:15
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    $\begingroup$ The distance from Earth is also an important factor. Further away objects naturally appear smaller, and are more difficult to see without higher magnifications. $\endgroup$ – overlord Nov 7 '19 at 17:30
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    $\begingroup$ It is essential to indicate how far away it orbits. For example, none of the satellites currently on the geostationary orbit can be seen with the naked eye, because they are not big enough to reflect enough light. $\endgroup$ – AlexP Nov 7 '19 at 17:32
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    $\begingroup$ One thing, though: assuming that satellite is in LEO and people can actually see it, mankind very likely starts to concentrate on seeing it better! Answers here state that it will take until Galileo for mankind to have telescopes, but that assumes nobody in 4000 years before him got triggered by that thing/'deity', that follows hourly/daily/yearly regular patterns. It might have changed the course of human tech history slightly to allow for something close to telescopes to be available before the 16th century! $\endgroup$ – t.ry Nov 8 '19 at 10:35

Artificial satellites orbiting around Earth are pretty easy to spot, since they appear as bright fast moving objects across the night sky. This even in a light polluted environment like a modern city. I remember I spotted satellites moving in the sky as a kid looking up at the sky with no knowledge of astronomy. Spotting them was even more exciting than seeing a shooting star.

In a farm based civilization which relies for its own wealth on marking the seasons by observation of the sky, such an object would be immediately noticed.

The civilizations inhabiting Mesopotamia, Egypt, Indus Valley, would be aware of it as soon as they start to systematically look up in the sky.

It will require technology such as Galileo's telescope to better observe it and understand its shape, and then probably radio technology to detect its communications.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monty Wild Nov 11 '19 at 1:39

As many other comments have suggested, orbit altitude is a key parameter in determining how visible the recon satellite would be. However, orbital altitude also determines how long the object will stay in orbit—specifically the perigee (lowest-point) altitude.

Let's say the recon satellite was in a roughly-circular¹ orbit of 400km, which is approximately the orbit of the International Space Station. Let's also assume that the recon satellite was shaped like a white school bus. This would would make it much more dim than what the ISS would be (ISS is huge!), but I think should occasionally be visible in the night sky when it is broadside. Ultimately I think it would be noticed, but, due to atmospheric drag, it would deorbit within a year or two. You could, of course, use thrusters to maintain your orbit, but eventually the fuel will run out. The most amount of time you could expect it to maintain such a low orbit would be on the order of a decade or two, and even that is pushing modern technology².

If you doubled the perigee to, say, 800km, you've given yourself about 100 years if the orbit is somewhat circular. The recon satellite would be just 1/4th as bright when directly overhead, making it significantly more difficult to notice. But, as the orbit decays, it would slowly start to become more visible until everyone would notice. And then plop, it would suddenly disappear as it drops into ocean somewhere out of human view.

Past a perigee of 800km, the time your recon satellite stays in orbit dramatically increases. At a 1000km roughly-circular orbit, you are talking around a thousand years. If the eccentricity of the orbit is high, then it could be in orbit for several thousand years.

Geosynchronous orbit is around 35,780km. Those birds are going to be practically eternal, lasting for eons. However, they would likely be invisible to any society without early 20th century optics: being 8000 times less bright than at 400km.

While I really have no idea when a society might realize that it was not a natural satellite, if I had to guess I would say it would require something on the order of mid-20th-century optics. This answer shows a great example of the kind of image of an object at 400km with a 0.64m telescope. You can see a picture of the space shuttle in that picture, so you can use that for scale: a school bus could easily fit in the cargo bay.

Anything at a geosynchronous orbit would likely remain unnoticed or of little interest until the 1960's, when it would eventually be detected using radar and catalogued. It would at first be assumed to be natural, but eventually scientists would likely want to examine it more closely (late 1970's?), and then the jig would be up.

¹ My understanding is that modern spy satellites often have highly eccentric orbits with perigee altitudes of 250km-600km.

² There is little chance that any orbital machine built using 21st century techniques would remain functional in any capacity past 100 years without maintenance. Space is a very harsh and unforgiving environment.

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  • $\begingroup$ I think geosynchronous would be spotable for the fact that it doesn't move relative to the other stars, which could be pretty weird. But that's specifically geosynchronous and not just that altitude. $\endgroup$ – Andon Nov 26 '19 at 23:59
  • $\begingroup$ @Andon: Hard to say. An object at GSO will be 8000x dimmer than the same object at 400km. That's 13 f-stops, or a difference in apparent magnitude of +9.8. ISS@LEO has an apparent magnitude of -5.9, so if it were to suddenly be transported to GEO it would have an apparent magnitude of +3.9. The limit of human vision is around +6.5, so ISS@GSO would be about as visible as the Orion Nebula. However, an object the size of a bus would have a visible cross section about 100 times smaller than the ISS, making the apparent magnitude of the bus@GSO around +8.9, which would at least require binoculars. $\endgroup$ – darco Nov 27 '19 at 23:03

I think you have to ask (and answer) multiple questions.

  • When will it be detected as a recurring object in the sky?
    L.Dutch suggests the naked eye will do; that might depend on the size and height of orbit.
  • When will it be recognized as an artificial object?
    Here you should keep in mind that it has always been there. People will come up with theories of the solar system which incorporate the recon sat. Speculation that zero-G will lead to "interesting" patterns of crystal growth that gives small moonlets regular shapes, for instance. Or theories that it is a fragment of a basalt column from a long destroyed planet.

I would lean out of a window and say that it takes 20th century telescopes observing the Mars' moons and various asteroids to really nail down that Earth's smaller moon is not natural.

Follow-Up: Imagine a planet with two moons. One is a cratered sphere, several thousand kilometres in diameter, the other is a relatively smooth octagon with two panels and one big lens, among others. How could the cosmology develop?

  • "All moons start out octagonally, but big ones tend to be hit by meteorites and get smashed smooth. When little ones get hit, they just get blown apart. The lone surviving little moon was never hit, or it wouldn't be surviving."
  • "Moons come in a variety of geometrical shapes. Spheres, cubes, cylinders, octagons, ... We have a sample of two, with two different shapes."
  • "One of our moons is natural, the other is an artifact of a vanished high-tech civilization."

Occam's Razor doesn't necessarily suggest the third bullet point.

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  • $\begingroup$ @KerrAvon2055 Oops, slipped some decimal points there - stupid me! Will delete my comment then. $\endgroup$ – Graham Nov 8 '19 at 14:10
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    $\begingroup$ "That's no moon..." $\endgroup$ – Harper - Reinstate Monica Nov 8 '19 at 15:38
  • $\begingroup$ The flip side of this is that observers who want their satellite not to be seen as artificial will disguise it as a small captured asteroid. Think Mars' moons Phobos & Deimos, but smaller, or the hordes of small rocks around Jupiter & Saturn. $\endgroup$ – jamesqf Nov 8 '19 at 16:34
  • $\begingroup$ occam's razor may rule out the third bullet point, but it definitely doesn't meant it wouldn't be on people's minds. think how excited we all get every time people discover anything in space that doesn't have an immediate explanation $\endgroup$ – user371366 Nov 10 '19 at 7:07
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    $\begingroup$ @dn3s, Occam never "rules something out." The complicated explanation may be the true one, it just makes sense to look at the simple explanation first. $\endgroup$ – o.m. Nov 10 '19 at 11:17

NASA has no reason to try to hide a satellite and all the current answers are working from a basis of a non-stealthy satellite.

If someone is trying to hide a recon sat they certainly could do so until the late 20th century. Build the satellite with all Earth-visible surface (other than the minimum needed for whatever sensors it uses) as black (both to light and to radio) as possible, the solar cells are shielded from the Earth. (The satellite will need a substantial black barrier to hide those cells behind. Obviously, if it uses some internal power source that's not as much of an issue.)

At this point detecting it will require a high power radar or an infrared camera (which won't work from the surface) or by occultation. Eventually it will be picked up in the quest for identifying all orbital debris.

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  • $\begingroup$ a satellite doesn't always point the same face towards earth, as it proceeds on its orbit. $\endgroup$ – ths Nov 9 '19 at 12:51
  • $\begingroup$ Or not black but with angled mirrors so that you see the darkness of space when you look at it from beneath. Pretty hard to spot too. I don't know which one would wear out sooner - the black paint or the mirror. $\endgroup$ – Vilx- Nov 9 '19 at 19:02
  • $\begingroup$ @Vilx- Until the mirrors reflect the sun. Also, can we do a mirror that works properly in both the visual and radio spectrum? (Without radar stealth you'll be picked up a couple of decades earlier.) $\endgroup$ – Loren Pechtel Nov 9 '19 at 22:45
  • $\begingroup$ @ths yesterday: No? Looked at the moon lately? Not to mention that communication & observation satellites wouldn't be very useful if they didn't keep antennas & sensors pointed at Earth. $\endgroup$ – jamesqf Nov 10 '19 at 17:05
  • $\begingroup$ @jamesqf But the sensors can be much smaller than the satellite and thus reduce the energy that can be detected. You can also do things like putting sensors in black tubes so they can't catch the sun. Even now there's a limit on how small an object can detect, get below that and it won't be seen. $\endgroup$ – Loren Pechtel Nov 12 '19 at 0:35

Spotting the satelite and observing its orbit would have been possible since ancient times. Understanding what they are is a different thing. They are too fast to discern by telescope so they will be dismissed as fast-moving asteroids at a low orbit. With a good telescope you can have a good detailed image as well. However, they are too fast to allow us to "follow the object" by looking through the narrow-angled lense. I doubht 19th century astronomers could have done that without having a blurred image. Some amateurs have successfully done it. Geostationary satellites have better chances to be discerned.

On the other hand, radio signals would have happened towards the end of the 19th century. Radio waves were predicted by James Clark maxwell only in the 1860's. As radio astronomy evolved, the unique radio signature emanating from an Earth orbit object will be hard to dismiss as background noise.

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  • $\begingroup$ Why would there be radio emissions from the satellite? If it needs to communicate with something further out, it could easily use something highly directional, like a laser. $\endgroup$ – jamesqf Nov 8 '19 at 16:36
  • $\begingroup$ @jamesqf You are right. That would make a huge difference if our attempts to study the objects are based on using radio waves. Otherwise, we must rely on visual identification. Back to 18th to late 19th century we would be using whatever telescopes and object tracking technology available before the radio technology became available. $\endgroup$ – Christmas Snow Nov 9 '19 at 7:43

This turned in to a little longer than a comment could handle, but here are some thoughts I have as an amateur astronomer:

  • It's not unusual to see a dozen or more satellites on a clear, dark evening. Just a few nights ago I was walking across the street and noticed two at once. If it's dark enough to see the Milky Way and you're paying attention to the sky, the movement of a satellite is very eye-catching. It's not just Iridium satellites (although they flare to startling brightness -- far brighter than Venus). The X-37B "secret space plane" is about Magnitude 3 (quite bright) and one imagines that the DoD didn't go out of their way to make it visible. (Although I suspect they don't particularly care how visible it is.) As others have said, if the satellite were way up in geosynchronous orbit, it would likely be more like Magnitude 12 (well below the limit of the human eye).

  • Pre-industrial societies almost universally paid great attention to the night sky. Imagine the societal investment necessary to predict solar eclipses, which several cultures did with good accuracy! The satellite's nature would be a matter of entire lifetimes of scholarship and argument. I think it's reasonable that only the Sun and Moon would be of more interest. The satellite would be a central enigma that would need to be explained in any account of the world.

  • Satellites are visible when the sun is reflecting off them (most commonly, off their flat solar panels). So you see satellites in the evening and pre-dawn, and nearer the middle of the night, you can actually see them wink out quickly when they enter the Earth's shadow. Would astronomers/astrologers use that to know the world was round sooner and with more accuracy?

  • Satellites move in natural ballistic fashion except when maneuvering. So 99.99% of the time, it would move in a way that would support the kind of mathematically periodic models that apply to other celestial bodies. But if it did maneuver to change orbits, that would confound those models.

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The biggest barrier is figuring out that they're being observed. Even if the satellite isn't stealthy, ancient civilizations would assume it's no different from a distant star that somehow fails to produce much light.

If the satellite gives off radio transmissions, then you can expect people on the ground to start asking questions once they invent radio communications and detect a transmission source in low Earth orbit that shouldn't be there.

If the satellite is purely observational and maintains radio silence, then it will likely be uncovered once people on the ground start building satellites of their own, after which they will recognize they are being watched.

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