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Two fleets from opposing factions are approaching each other at sublight speed at the edge of our Solar System. The Earth is in a position to affect the outcome of the battle, (i.e, superweapon, etc.) but must correctly identify the fleet friendly to them. Unfortunately, Earth is temporarily unable to identify them by any means other than visually. That means we cannot send a hail to the friendly fleet, nor can we send a scout or fleet of our own to confirm at close range. By the time Earth is able to get these capabilities back online, the battle will already be decided.

Would we be able to identify the allied fleet, and if so, to what granularity?

Could we distinguish between something such as this: enter image description here enter image description here

Or this, where the design methodologies would look very similar from afar? Or would it take two entirely different ones, like UNSC designs vs Borg style cubes? Would we be able to differentiate between fleets that share the same class of ship, but have different insignia stenciled on them, i.e, a large rebel flag or imperial flag?

A few details to hammer out first:

The viewing conditions for both fleets are considered the same. One does not have sunlight shining on it while the other doesn't, for example.

Earth still has its modern day space telescopes (2018), but only ones within it's own orbit.

The opposing fleets are roughly the same size, as well as their individuals. So finding a standout such as an obviously distinct capital ship such as a Super Star Destroyer or Death Star style weapon is out of the question.

The baseline distance where both fleets start at and move roughly parallel to the Earth is about the distance of Neptune.

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    $\begingroup$ Do I understand this correctly that you are not asking if this can be done in principal but with contemporary telescopes? "Modern day" could mean today or whatever is state of the art in your scenario which is at least a couple of centuries in the future $\endgroup$ – Raditz_35 Jul 13 '18 at 17:17
  • $\begingroup$ I am asking in regards to 2018, modern technology. $\endgroup$ – Raznarok Jul 13 '18 at 17:18
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    $\begingroup$ Here you can find a picture of the very large moon Triton of Neptune as seen from Earth with contemporary equipment: solarviews.com/cap/nep/tritonhst.htm Imo the question should be how you even detect any fleet approaching $\endgroup$ – Raditz_35 Jul 13 '18 at 17:24
  • $\begingroup$ A further question I would have is, what level of resolution would we need to even have a chance of identifying the fleet? $\endgroup$ – Raznarok Jul 13 '18 at 17:30
  • $\begingroup$ Space is big. The Empire's Death Star would probably go unnoticed at that distance. Anything we saw would be 8 to 16 hours after the fact, and be mistaken for noise artifacts if seen at all, Any help we sent would arrive 20 years after the fact. $\endgroup$ – pojo-guy Jul 13 '18 at 18:12
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Nope. Read this answer to What is the resolution in megapixels of the Hubble Telescope? on Astronomy SE
For Hubble space telescope, the entire dwarf planet of Pluto is 2*2 pixels.

Hubble is not cutting edge anymore, though. That honor belong to adaptive optics, e.g. the Very Large Telescope
The best resolution is 0.001 arc-second, and Pluto is 0.1 arcsec, so we have 100 pixels for a Pluto, which is 1400 miles in diameter, so you have 14 miles per pixel.

You might be able to tell the color of a really large ship, or count the number of large ships if they are far apart, but that's about it.

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  • $\begingroup$ A further question I would have is, what level of resolution would we need to even have a chance of identifying the fleet? $\endgroup$ – Raznarok Jul 13 '18 at 17:32
  • $\begingroup$ So it would need to be an enormous capital class slugfest or the battle would have to be closer to the Earth to even have the vaguest idea then. $\endgroup$ – Raznarok Jul 13 '18 at 17:44
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    $\begingroup$ @Raznarok, "enormous" is a wholly inadequate word in this case. The capital ships would need to be about half the diameter of Pluto, 750 miles, just to get 50-100 pixels. I remember the good old days of computer games. There's not much you can identify with 100 pixels. $\endgroup$ – JBH Jul 13 '18 at 17:57
  • $\begingroup$ Hi Bald Bear. I added some clarification to your links. By all means feel free to Edit further. $\endgroup$ – a CVn Jul 14 '18 at 6:04
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If I read into the question properly, we know nothing about either fleet.

Then technically, neither fleet are our allies.

Odds are that whichever fleet we shot down, the other fleet would BECOME our allies quite quickly.

As for identifying who the "good guys" are, about the only thing I could imagine is possibly looking at combat tactics. Especially brutal tactics, as opposed to surgical strikes intended to incapacitate might suggest a more brutal (and possibly "evil" society.

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  • $\begingroup$ We do know what their ships look like, but its more of a question of can we identify them from this distance. Sort of how it can become difficult to differentiate between allied and axis tanks through smoke and chaos. Although picking arbitrarily could be in our best interest as you said. $\endgroup$ – Raznarok Jul 13 '18 at 17:38
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    $\begingroup$ Slavers would presumably aim for minimal loss of life, anyone fighting forces that never ever surrender might adopt extermination tactics. Though if one side expects Earth's support they probably would be more inclined to stall and maintain separation. $\endgroup$ – user25818 Jul 13 '18 at 17:39
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Forget about insignia. But if we know about ships' design, we could be able to differentiate them.

The idea is that when approaching the inner solar system, and likely even before that, ships will fire their engines for braking down and maneuvering. This is the most likely how they are going to be discovered, by the way. If two fleets' engines are different, we can analyze the exhaust spectrum and determine whether it's a friend of foe.

P.S. This answer assumes that ships use "scientifically plausible" reaction engines.

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  • $\begingroup$ Would we be able to analyze the exhaust spectrum from that distance though? I wouldn't doubt we could simply look at the ships themselves once they got close enough, although I think exhaust is an important part of the IFF. $\endgroup$ – Raznarok Jul 13 '18 at 18:30
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    $\begingroup$ @Raznarok I assume that incoming ships will need to do much more engine burning than our current interplanetary probes do. Engine exhaust will be much brighter than any reflected sunlight. $\endgroup$ – Alexander Jul 13 '18 at 18:36
  • $\begingroup$ Seems more reasonable the more I think about it. Any capital class style ship is going to be putting out a LOT of exhaust to move that amount of mass, and there are dozens of them arriving within a few miles of each other. $\endgroup$ – Raznarok Jul 13 '18 at 18:39
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    $\begingroup$ @Raznarok There's no need to continually fire the engines, at least not with something resembling modern day reaction engines. Most space travel is done without any active propulsion. Typical propulsive maneuvering time is on the order of tens of minutes even for interplanetary missions within our solar system; the rest of the time is spent in freefall. $\endgroup$ – a CVn Jul 14 '18 at 6:06
  • $\begingroup$ @Michael Kjörling - this is when traveling at slow speeds, below or slightly above Sun's escape velocity. Interstellar fleet should be arriving at a speed orders of magnitude higher. $\endgroup$ – Alexander Jul 14 '18 at 6:16
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As pointed out, optical identification of the fleets is effectively impossible unless we postulate improbably large space telescopes, with mirrors measured in kilometres rather than metres.

However, visual identification of targets isn't the only way to go, and even visual identification might be useful if we are looking at something else.

Space is both a vacuum and extremely cold, so the heat energy of the spacecraft stands out like a beacon against the background of space, and isn't absorbed by intervening fluids (at least not on the scale of the solar system. There are interstellar nebula of gasses and dust which can absorb energy on a cosmic scale). This is the reason there is "no stealth" in space, the spacecraft are highly visible in the infrared spectrum.

This includes heat radiating from the ships even when they are drifting, since a great deal of "hotel" power in needed for life support, running the various ships systems and so on. Unless the aliens live in a cryogenic environment, the ships will have to be at several hundred degrees Kelvin just so the crews can survive.

The radiated energy of the ships is compounded when they use their engines. Atomic Rockets points out that the Space Shuttle engines put out enough energy that the small manoeuvring thrusters could be theoretically detected out to the asteroid belt, and the burn of the SSME would be visible from the orbit of Neptune. More detail can be found here.

enter image description here

Exhaust plume of an F-35

So while the ships are still smaller than a single pixel, you will have a large and otherwise inexplicable infrared source in deep space. Assuming you have other information about the aliens, you might be able to determine who is who by carefully looking at the infrared signatures. One set of aliens might have significantly warmer or cooler spacecraft because of their environmental preferences, and the energy used to manoeuvre will be very distinctive, especially if you have information about the actual sizes/masses of the ships (larger ships will require more energy to move, so you have a metric of sorts to determine which fleet is which).

Other sensors are available as well. We use radar on Earth, and the Arecibo radio telescope has been used to do radar scans of Venus, providing surface details even from that distance. The sensitivity of the telescope was great enough that during the Cold War it was used for Electronic Intelligence (ELINT), analyzing Russian radars by gathering reflections from the Moon! With this level of sensitivity, radar imaging of the fleets is also possible, but once again, you would have difficulty resolving individual ships.

enter image description here

Arecibo radio telescope

Even more exotic systems could be postulated using current technology. A satellite could be launched with a high powered laser to conduct LIDAR scans of the fleets, to compliment optical, infrared and radar searches. This would likely be in support of fine tuning any observations, and aiming the hypothetical super weapon in the OP. Neutrino detectors deep beneath the Earths surface could also be used to attempt to detect the neutrino emissions from the ships nuclear reactors, although this would be a very low resolution detector.

So although visual detection would not be possible or readable at this time, there are still multiple systems which could be used to locate and identify the various fleets.

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  • $\begingroup$ Arecibo's major downside in this case is probably its restricted ability to scan the sky. The reflector is fixed, though the feed can presumably be moved (I haven't checked). A more movable feed could likely be built and installed, but you're still dealing with the very narrow beam width of the reflector at any reasonable frequency of interest. And you're assuming that whatever is out there is emitting RF in a frequency range that Arecibo, or any other similar equipment (FAST, for one) can reasonably receive. That may or may not be an accurate assumption. $\endgroup$ – a CVn Jul 14 '18 at 6:16
  • $\begingroup$ I'm speaking of using Arecibo as an active radar emitter, but using it as an ELINT platform is likely more useful in this case. As a spherical reflector, the Arecibo facility has the ability to scan a somewhat larger area than you might think, but it is still fixed, limiting the overall ability to scan. $\endgroup$ – Thucydides Jul 14 '18 at 6:34
  • $\begingroup$ @Thucydides So if we can detect the SSME from Neptune theoretically, then a fleet of a dozen plus Star Destroyer class vessels is going to be like firing a gun in a quiet library in terms of subtlety on the infared spectrum then? $\endgroup$ – Raznarok Jul 14 '18 at 8:51
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    $\begingroup$ Not a gun, but a cannon. A great big, Krupp K-5 railway cannon type cannon..... $\endgroup$ – Thucydides Jul 14 '18 at 21:46
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A massive fleet is the size of the pointy tip of a pin compared to a planet, much less the expanse of open space visible from the planet. Bald Bear's answer is very important (I upvoted it). If we knew exactly where to look (three dimensions!) we'd be hard pressed to see the fleet to recognize it before 4-5X the distance to our own moon. And that might be an optimistic assumption.

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    $\begingroup$ @Raznarok, Can you clarify your request? The problem Bald Bear is talking about is that our resolution technology is good enough if the optics allow you to focus in closely enough (they can't) and you know exactly where to look. If the question is, given let's ignore optics and view 180° in both polar planes, what would it take? The answer is petapixels-per-arc° and that assumes the detector is sensitive enough and filtered enough to capture so little light amidst a field of so much. $\endgroup$ – JBH Jul 13 '18 at 17:54
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    $\begingroup$ @Raznarok, yes, and the consequence of better optics is that the further out the battle, the more you need to know ahead of time where to look. Think of using binoculars. An entire battle could be going on a mile to your left but you're admiring the bird a mile to the battle's right. Better optics is good, better light detection is better (and the whomping large amount of processing power to go along with it to analyze the images). $\endgroup$ – JBH Jul 13 '18 at 18:07
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    $\begingroup$ @Raznarok, what keeps me up at night is that there could be space battles going on all the time right now and we wouldn't know about it unless we got very lucky. Considering galactic time scales, space battles are instantaneous events, like a flash of lightning. What would make more sense is a sphere of satellites at the orbit of Mars that detected the flashes and directed us to "look here." That would require much less tech in terms of optics & detection resolution. But, yes, for your plot, more tech than we have. $\endgroup$ – JBH Jul 13 '18 at 18:15
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    $\begingroup$ @Raznarok, from the perspective of a story, Mars is believable. In terms of reality, you'd likely need detecting spheres at the orbits of Mars and Saturn. The problem is cost-vs-value, the further out you go, the more space you're trying to cover, and the more cost (in tech or # of satelites) is involved. $\endgroup$ – JBH Jul 13 '18 at 19:01
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    $\begingroup$ @MichaelKjörling, An "it feels right" guess based on my own use of a telescope, the relative size of the moon, the estimated size of an incoming fleet. I figured we could identify the fleet at 20% of what we could see at the distance of the noral moon. $\endgroup$ – JBH Jul 14 '18 at 6:57

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