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Pretty simple question. Whether you're launching a spy satellite, building a top secret orbital missile weapon, or investigating a mysterious alien artifact in orbit around Earth heralding the return of the Starborn ones, could a large satellite remain hidden from modern humans long enough to do anything interesting with it (from a narrative perspective at least) or has our information-gathering technology become too sophisticated to hide from?

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  • $\begingroup$ no stealth in space. Also, you ask about remaining hidden as if it could ever become hidden in the first place. Large ones cannot. Everyone knows everyone else rockets and huge difference between declared cargo and max would be spotted. $\endgroup$
    – Mołot
    Nov 18, 2016 at 22:54
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    $\begingroup$ It's probably not a good idea to try to hide a satellite from your orbital neighbors, lest they go to maneuver and discover your satellite's existence the hard way! $\endgroup$
    – Shalvenay
    Nov 19, 2016 at 0:57
  • $\begingroup$ In your story did we have a klingon (other race here) cloaking device? It would have to absorb all energy, and let our transmission pass through it, but an alien race might be able to do it. $\endgroup$
    – cybernard
    Nov 19, 2016 at 1:18

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Space debris is well-tracked by both public space agencies (NASA, ESA, presumably the Russians and Chinese too), and military and intelligence organizations (such as....haha you thought I was going to tell you?).

Here is the reference on Wikipedia. According to this bulletin, there are over 17,000 distinct things in orbit being tracked by NASA. Here is a paper about the capabilities of the Space Based Visible satellite.

In general, any object over 10cm in LEO to Geostationary orbit is being tracked. Keep in mind, those are cold specks of debris. There is extensive IR monitoring of space by military and intelligence groups, to determine maneuvering capabilities of potentially hostile satellites. If a satellite was energized and operating, it would be radiating heat which would certainly be detectable by multiple modern nations at least out to the moon, maybe further. It might be harder to detect satellites over the poles; I'm not sure where each countries monitoring stations are, but the US/UK/Canada/Australia joint sites certainly have 100% global coverage.

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  • $\begingroup$ Tracking does not imply identification. An advanced (though relatively small) secret satellite could easily be in the mix collecting data without generating enough heat to be detected (or directing that heat away from the Earth). Also worth noting that space agencies are tracking over 21,000 of 500,000 estimated pieces of space debris according to NASA. $\endgroup$
    – Samuel
    Nov 18, 2016 at 23:31
  • $\begingroup$ @Samuel From your link, 'There are more than 20,000 pieces of debris larger than a softball orbiting the Earth.' A softball is about 9 cm in diameter. So when I said, any object over 10cm in LEO to Geostationary orbit is being tracked, I was correct. Not generating enough heat to be detected is only possible if you have an entropy-violation device onboard. Turning energy into any work generates waste heat, and any waste heat will make you very noticeable on the shady side of earth where debris temperatures will drop to 200 kelvin or below. $\endgroup$
    – kingledion
    Nov 19, 2016 at 0:26
  • $\begingroup$ That the number tracked and number over 10cm are similar does not mean those objects are in the same set. So you may be correct, but it's statistically unlikely. Entropy violation is not required. I said it could generate little enough great to go undetected. I doubt NASA is even monitoring temperature with enough precision or frequency to notice if something was active. You also missed my point about waste heat directionality. It's feasible to operate with low enough waste heat to avoid detection, especially when radiating it away from the Earth. $\endgroup$
    – Samuel
    Nov 19, 2016 at 1:04
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    $\begingroup$ IIRC, the tracking of orbital debris (and satellites that aren't yet debris) is done by radar. To make a satellite invisible to radar, use stealth technology. I doubt that IR emissions could be detected from Earth surface, since the atmosphere strongly absorbs IR. Optical can be handled with flat black paint. Which leave the only means of detection the occasional occultation of stars. If your satellite's not that big, and in a high orbit, that will be quite difficult. $\endgroup$
    – jamesqf
    Nov 19, 2016 at 4:42
  • $\begingroup$ @Samuel You can argue, but you'd be wrong. This is just something I know about. We can spot all space objects larger than a few centimeters, and IR signatures in space are of high interest to certain people in large 5-sided buildings in the DC area. There are satellites that do the IR monitoring, even if you direct your waste heat away from the Earth (good luck) the co-orbital observatories can still spot it. Just take my word for it. You can't hide a satellite in orbit. $\endgroup$
    – kingledion
    Nov 19, 2016 at 5:24
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Hide in plain sight.

Call it a weather satellite, or a communications satellite, or something boring and mundane like that. Actually include some of that hardware so that it can pretend to play the part. Launch it with two other satellites and a six-pack of cubesats as part of a standard commercial launch. And then, oops, a "malfunction" after launch causes it to wind up in an "unexpected" orbit. Some sort of navigational or computer error. Announce that, not to worry, it's in a safe orbit. And since you don't want to lose your investment, you're going to keep monitoring it to try and nudge it into a useful orbit.

Now you have a satellite up in whatever orbit you actually want, that everyone dismisses as "that lost satellite", and if someone notices that it's maneuvering, well, no big deal, its all just part of the painstaking satellite recovery process.

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  • $\begingroup$ This is the best way to hide anything. $\endgroup$ Nov 19, 2016 at 22:52
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Although stealth technology doesn't work in space (according to the first comment on your question), you might be able to piggyback on an existing satellite or previously-identified piece of space junk. That is, you could bring your new satellite (through handwavium) close enough to an existing satellite to dock with it. That would create a single blip on radar screens. So long as your satellite didn't interfere with the operations of the existing satellite, it may go undetected.

Plus, as Truman said in the movie Armageddon, "[We can] track about 3% of the sky, and begging your pardon sir, but it's a big-ass sky."

EDIT

You would also need to launch your satellite into orbit as a secret component of another, legitimate launch.

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    $\begingroup$ Truman in Armageddon lied to you, man. We can track a lot more sky than that. $\endgroup$
    – kingledion
    Nov 18, 2016 at 23:02
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    $\begingroup$ @kingledion What? A Jerry Bruckheimer film wasn't factually accurate?? I'm stunn. $\endgroup$ Nov 18, 2016 at 23:03
  • $\begingroup$ To piggyback, you need 1) put it in space and 2) maneuver. Both are highly visible and well tracked. $\endgroup$
    – Mołot
    Nov 18, 2016 at 23:05
  • $\begingroup$ @Molot You're right. I'll edit my answer to make it somewhat more feasible. $\endgroup$ Nov 19, 2016 at 0:08
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A lot depends on how far an orbit you accept. This is especially significant because its an asymmetric situation - a satellite very far from earth becomes harder to detect, but might only be little affected by the extra distance, or the physical size needed to allow good angular resolution.

Example - imagine a spy satellite used to video some specific part of earths surface (a military zone for example). Space-based optics are routinely able to be extremely high quality because there is neither distortion from gravity nor from atmosphere. Think Hubble telescope in an orbit some millions of miles from earth, photographing periodically when the orbit is favourable. Harder to detect.

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