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This alien spacecraft would be orbiting around earth at 300-400 km, it has a similar size to the ISS, and it has to be undetectable with our current technology. I assume that it would prevent emitting heat, maintain radio silence, absorb radio waves, and have to bend light around it to be invisible to passerby satellites and surface telescopes. could there be another way to detect it?

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  • $\begingroup$ A heat pump. Life as we know it has to have a high level of heat. To be undetectable, spaceship would have to pump heat from the side facing us to the other side to radiate it into space. $\endgroup$ Jun 7, 2022 at 13:27
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    $\begingroup$ If it absorbs radio-waves it is easily detectable by radio-telescopes as it would obscure astronomical radio-sources. It would need to bend radio-waves around itself just like light. $\endgroup$
    – Tonny
    Jun 7, 2022 at 14:36
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    $\begingroup$ The ships could be really, really small. $\endgroup$ Jun 7, 2022 at 18:44
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    $\begingroup$ "Undetectable" as in "unlikely to be detected" or as in "invisible"? For the former, blackened heat shield and radio wave absorption is all that is needed. For the latter, we would need light and other electromagnetic wave bending. $\endgroup$
    – Alexander
    Jun 7, 2022 at 19:08
  • $\begingroup$ @shawnhcorey good point about heat signature, although if it did that then it would be detectable from our space satellites. $\endgroup$ Jun 7, 2022 at 20:08

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I assume that it would prevent emitting heat, maintain radio silence, absorb radio waves, and have to bend light around it to be invisible to passerby satellites and surface telescopes.

Radio waves can be deflected to reduce the radar return - the technology is old news on Earth, too.

Light bending is unnecessary; at any reasonable distance, it wouldn't cause significant occultation of anything. It would need to keep track of the position of Earth and any manned vehicles, and never come between them and either the Sun or the Moon. Other satellites aren't really equipped to image other satellites.

So-called transits do exist and are seeked by amateurs and professionals alike, but they rely on accurately fixing and focusing a known point. If the ship doesn't follow a predictable orbit, at any reasonable zoom there will never be enough time to focus and detect it after the occulted star has blinked. Sun and Moon transits are far more dangerous, and to a lesser degree planets will be -- but the number of such large background objects is manageable.

In the case of Jupiter, its angular size is approx between 30" and 50". An ISS-sized object in ISS orbit is about a third of that, so at best 20". Now our ship is in a ~90' orbit, that is to say it covers 360°, or 1296000", in 5400" - a speed of 240 arcseconds per second. A partial Jupiter occultation (otherwise you'd just see Jupiter "blink") will then last about a tenth of a second and you need a special setup to catch it reliably, and even there you know in advance when it's going to happen.

Meanwhile, the ship just needs to keep track of its "Jupiter" shadow (as well as the others), which is a 100-m wide track on the ground, and steer it over water or high-light-noise areas.

Another possibility to muddle detection would be to camouflage using counter-illumination (easier than light bending). 1945 technology, and pretty naive at that, proved able to decrease detection by a factor of six. This would also help against visible imagery being taken by remote sensing satellites in higher orbit.

The same tracking would detect "windows" in which an angle of more than 90° (but less than 170° to avoid a possible Gegenschein-like interference) exists between the ship and any (or, in emergencies, "most") potential infrared observers. Whenever that happens, accumulated waste heat can safely be radiated from the vehicle at that angle.

The big problem here, though, is how to identify potential infrared observers. Unless equipped with Culture Effectors or the like, your ship cannot be sure that any satellite in an higher orbit, up to geostationary, might not carry IR imagers. Yes, transmissions can be detected, decoded and compared with IR images, but still. And, once you catalogue all the various IR-imaging satellites continuously mapping the Earth (and do that before overloading your internal thermal sump), you might discover that they achieve a 200% coverage and you have no angles to radiate over.

Which would leave you with the awkward option of radiating at a tangential angle, to stay over the horizon for earthbound imagers and hopefully below the horizon of higher-orbit imagers (but at a height comparable to the ISS, this is bound to cause a detectable secondary heating in wide enough swaths of the upper atmosphere. The anomalies will be tiny, but they'll accumulate, and they'll look exactly like somebody was radiating heat in a stealth fashion; so it'll attract exactly the wrong sort of attention).

Another possibility is to employ a very inefficient "porcupine" radiator, with cylindrical emitters each directing heat in very narrow cones. In space, you have little atmospheric dispersion, so the waste heat ray shouldn't attract the attention of satellites out of line of sight.

Once the radar return footprint falls below that of a 15-cm aluminum sheet, one trivial way of reducing interception chances is not to be on a fixed orbit. Small debris is tracked using large radars (in Europe we have the FGAN Tracking and Imaging Radar or TIRA), and those radar use the knowledge of any given orbit's parameters to integrate and refine position over time. So, a return from point X1 in orbit family Y(1) followed by a return from point X2 in orbit family Y(2) will slowly coalesce into a solid acquisition of a physical object in the one well-defined orbit that best satisfies all detections. If there is no one orbit that satisfies a group of sightings, those will be ignored as spurious. Also, this so-called target-directed mode relies on a priori knowledge about the debris and how it came to be, so staying away from existing debris will further decrease the chances of being spotted.

Another, opposite trick once the existence of ground-based debris tracking stations has been deduced would be to hide behind one of the higher-orbit (thus, longer-lived) pieces of space debris. The radar return would mostly be that of the piece of debris, and its reflected luminosity would be enough to disturb even direct imaging.

This latter option could also be used to explain how suspicions on the ship's existence might come by: that particular piece of space debris, being large and in a safe orbit, would also be an ideal target for a junk-retrieval test mission.

Something like this happens, if memory serves, in Ted White's Secret of the Marauder Satellite (1967).

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    $\begingroup$ It absolutely needs to bend light. There are a ton of people looking at objects in the night sky. Spotting and identifying satellites (and photographing them) is an actual hobby. Even that won't be good enough probably. We do detect light bent around black holes. $\endgroup$
    – ventsyv
    Jun 7, 2022 at 20:01
  • $\begingroup$ 300 - 400 km orbit is kind of low. There are earth imaging constellations above that so sooner or later it will be imaged from them. $\endgroup$
    – ventsyv
    Jun 7, 2022 at 20:05
  • $\begingroup$ Yes, light bending is a must. I mean, most if not all current UFOs that are monitoring the Earthlings (I mean, UAPs) have been detected exactly by chance encounter in visible spectrum by fighter jets, random individuals etc. $\endgroup$ Jun 7, 2022 at 20:11
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    $\begingroup$ Fair enough, that might work. (I've no knowledge about it.) The orbital insertion burn is another matter, of course. $\endgroup$
    – N. Virgo
    Jun 8, 2022 at 4:23
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    $\begingroup$ @N.Virgo you're quite right, and that had completely escaped my notice too. You can't do an insertion with a dynamo thruster. The only "stealth" alternative I can think of would be... to expel large, cold masses -- use a sort of flash-suppressed minigun or railgun as propulsor. Of course, you can't point anywhere near other satellites or the lower atmosphere, to avoid very visible fireworks. $\endgroup$
    – LSerni
    Jun 8, 2022 at 8:37
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Distance is all that's really needed.

A little bit of a frame challenge here, they don't need to be that close, and don't need any special shielding tech to go unnoticed if they're not.

We can't even spot inbound asteroids that might hit the planet until their really close unless we get extremely lucky, so just don't come in that close and they'll be very unlikely to be spotted .. and they don't need to be that close if it's a recon and observation mission so why do that.

If they've good telescopes (presumably if they're interstellar travellers they'll have pretty good ones) they can just sit out way beyond the orbit of Mars look at us through telescopes and eavesdrop on our broadcasts.

In addition to that all the stuff in the solar system orbits the sun on a single flat plane and as that's the only place there's anything interesting to look at that's where all our telescopes are pointed .. so they can increase their 'invisibility' by simply not being anywhere in that plane.

"Where all our telescopes are pointed" well .. the great majority of them.

Add a few simple precautions like engines off, radio silence, having the side of the ship facing us painted matt black and making sure any heat dissipation only happens on the other side of the ship (or better yet storing it and releasing it when their course passes on the other side of the sun to us) and for all practical purposes they'll be pretty invisible to us .. unless we're both actively looking for them (and unless we know they're there why would we) and get really lucky.

"better yet storing it and releasing it when their course passes on the other side of the sun to us" which is when they'll switch the engines on for any adjustments to their orbit as well.

Any active detection that rely on bouncing anything off them (not that anyone's going to be pinging them if they don't already know both that they're there and roughly where they are) can be deflected by the same methods already used in stealth bombers of course.

A good computer with 3D modelling of a decent star chart to ensure you don't accidentally eclipse any star or other distant object someone on earth just might be observing could also be useful.

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You won't be able to, short of ridiculously advanced super-science like access to other dimensions to dump waste and the like.

As always for Stealth in Space, the major problem is heat: you have to get rid of it, and delaying it by storing and then releasing only changes the problem because you end up being a lot thermally brighter when you eventually do have to dump it. And here is where your altitude really, really hurts. Putting aside whatever classified military birds looking down there might be and what their capabilities are, you've got systems like FIRMS, NASA's Fire Information for Resource Management System, which uses data from several satellites to track wildfires (and, these days, fires set by artillery to track combat in Ukraine) and is freely available. The data for that system comes from Aqua (orbit 703 km), Terra (709-is km), and NOAA-20 (820-ish km) Earth-observation satellites equipped with infrared detectors.

Then you've got Suomi NPP and the upcoming JPSS satellites, which carry the same sorts of sensors, the infrared camera on Landsat 9, and, making it worse for your aliens, the GOES-16, GOES-17, GOES-18, EWS-G1 (formerly GOES-15), Japanese Himarai series, the Russian Elektro-L series, the European Meteosat series, and Chinese Fengyung series geostationary weather satellites and equivalents which have the world (and any lower orbit) under constant 24 hour surveillance and also carry infrared sensors (albeit those on GOES-17 have problems).

Now, you can argue that the more distant birds have lower resolution and the ones in lower orbit narrower fields of view, and you'd be right, but there are more and more satellites going up there looking down, and the instruments that would detect heat emissions are relatively cheap and off-the-shelf, so more and more sensors.

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As a slightly different take on the problem: You don't actually need to be undetectable.

Here is a tip from Information Security point of view - instead of trying to make transfer of sensitive information undetectable, simple hide that needle in haystack of similarly looking information / noise. It is much easier and much more efficient.

So two easy options:

  • be detected like something else (look like regular airplane with transmitting correct-looking codes, or like a weather balloon, or a other nation spysat, or Starlink satellite, or a drone, or a flock of birds, or whatever)
  • actually look like flying saucer, and release to the human datasphere (i.e. the Internet) dozens of thousands of "real sightings of UFOs" over time (many of which are easily detected fakes, some are harder to detect but still provable fakes, and some which are unprovable but still likely fakes. Then just be one of the thousands of those unprovable but likely fakes).
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If you are orbiting earth you likely already have the technology needed.

Getting to another planet using chemical or physically powered engines is incredibly hard, never cheap, and probably not economically sound. Anyone who is sending more than a basic probe is likely using an space folding propulsion system, or another physics based system.

Therefore light bending, heat radiating, and radar absorbing is simple. If you set you set your space folder to curve around you, you can act like you aren’t there, you can then put out instruments small enough not to be seen to see outside.

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I assume that it would prevent emitting heat, maintain radio silence, absorb radio waves, and have to bend light around it to be invisible to passerby satellites and surface telescopes.

This is not feasible: this object as described only absorbs energy and has some working device in it, without letting out any form of energy. Due to basic physics considerations it would end up exploding, as one cannot cram energy in a system forever.

It can be undetectable to some observation methods, but cannot be totally undetectable: it has to emitt enery somewhere, and that will make it detectable.

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    $\begingroup$ Or... You can do it Mass-Effect Normandy style, using heat tanks to store the energy, and remove the excess every once in a while when out of sight. This supposes however that the ship isn't on Earth's orbit 24/24, every day, and for months. $\endgroup$ Jun 7, 2022 at 7:45
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    $\begingroup$ The question does not say that it doesn't emit energy at all, only that it doesn't do it as heat. Neutrinos come to mind as a good way to dump energy undetectably. $\endgroup$
    – AlexP
    Jun 7, 2022 at 7:59
  • $\begingroup$ @AlexP And neutrinos can't be detected, right? $\endgroup$
    – L.Dutch
    Jun 7, 2022 at 8:36
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    $\begingroup$ @L.Dutch: They can be detected only with great difficulty, and they surely can't be focused. Which means that at best we might get a vague indication that there's a source of neutrinos somewhere in a very poorly defined direction. No precision, and defintely no distance. A supernova can explode and we will detect a dozen neutrinos. Neutrinos are extremely reluctant to interact with anything much. $\endgroup$
    – AlexP
    Jun 7, 2022 at 8:54
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    $\begingroup$ @KeithMorrison You pump the waste heat through a 3 cm sheet of handwavium and it comes out as neutrinos on the other side. $\endgroup$
    – EvilSnack
    Jun 7, 2022 at 17:51
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In the real world it has been hypothesised that UFOS (Flying saucers) are surrounded by a bubble of high energy plasma that fulfils several functions.

One of these functions is said to be the ability to absorb or scatter incoming electromagnetic signals such as radar by deflecting it across its surface so that a radar beam that strikes the UFO from the front will pass across the surface of the plasma bubble and exit behind the UFO, rather than being reflected back at an angle that the radar station can pick up again.

This can be found in Unconventional Flying Objects by PR Hill,

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