A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Friday, 1 April 2016

Stealth in Space is Possible IV

Stealth in space is possible... but it will not resemble our conventional understanding of it.

We will now consider all elements discussed in previous parts of this series to paint a general picture of how stealth would be applied in a plausible setting.





First of all, you must understand that stealth is not an absolute. That means that 'stealth' is actually a smooth transition between low and certain detection.

This leads to sorting a detected spacecraft into one of four categories: 

  • Soft Detection
  • Hard Detection 
  • Identification
  • Target Lock 

The F117 Nighthawk, designed for radar and infrared stealth.
Soft Detect

In space, the spacecraft is fighting for its place among the meaningless static and noise generated and picked up by normal sensor operation. An insulated hull or cold-plate design does this by trying to match the background temperature of 3K as closely as possible. Directional radiators try to reduce their irradiance until they fall under the detection threshold of a sensor. Spacecraft on a departure burn try to hide in the brightness of a planetary background.


A soft detect happens when a spacecraft emits enough energy in the direction of a sensor that the signal generated rises above the noise floor. This sort of detection is generally the job of wide-angle scanners that sweep the entire sky, searching for above-average levels of photons. 


Looking at a planet and measuring a spike in brightness, or watching empty space and detecting a handful of high-energy photons, will reveal that something is emitting energy. However, the same characteristics that allow a soft detect by a sensor prevent it from establishing a precise location or velocity of the emitter. They can only say that 'something in this direction is hotter than empty space'.



How the sensor data would likely look from one direction. Several directions give a 3D image.
Cross-referencing the data from several sensor platforms can narrow down the location of the stealthed spacecraft, but it will still encompass billions of cubic kilometers.  

Hard Detect


Once the wide-angle sensors have piked up a statistically significant signal, the defenders' next step is to try to obtain a hard detect.


A hard detect is a precise and certain localization of the stealthed spacecraft. This is achieved with narrow-angle sensors that focus on a small slice of the sky. Once they narrow down the source of the energy emissions to a small enough area, the amount of data obtained on the spacecraft rises quickly. You could reasonably say that the spacecraft is not 'stealthed' anymore. 


By watching a time-lapse of the spacecraft's location, the velocity and heading can be obtained. Even more sensitive sensors can be set to track the spacecraft instead of scanning huge areas of the sky, leading to a 'hard detect'.


However, transitioning from soft to hard detection is not a simple feat. The wide-angle sensors and the soft detect only provide a cloud of likely positions of the stealthed spacecraft. Over time, the cloud becomes smaller and denser. A narrow-angle sensor would still have to be run over millions of cubic kilometers, if not billions, of potential positions before the emissions are caught in its field of view. This process can be lengthy, and months can be spent trying to chase down an accelerating spacecraft. 



A 10-degree vs 3-degree field of view comparison.
Our reference 1GW spacecraft with its cold 208K radiators and 'stealth' 151kW propulsion could change its position by up to 6.5 million kilometers in a single day. This is a volume of 1.15 million million billion cubic kilometers to hide in, even after a soft detect has been achieved. 

Identification

After a hard detection has been achieved, and your spacecraft is being tracked with great accuracy, there are still ways to fool the sensors.



A Q-ship: The USS Anacapa hid guns behind hinged flaps
One method is to hide your spaceship inside a voluminous shroud. Once visual surveillance becomes available, you will be hard-pressed to hide the exact size of your radiators, the shape of your propulsion bell and the width of your primary laser lens... Hiding all this in a metamaterial cloak that shrouds or obscures the exact features of your spaceship probably won't hide your purpose (an attack fleet would probably be travelling along deltaV-expensive or otherwise unusual trajectories), but it will reduce the accuracy of your opponent's estimate on the composition and strength of your forces.

The downside is that if this technique is permanently deployed, it will interfere with your stealth (catches incoming sunlight and outgoing waste heat), and if deployable, requires you to know when a passive sensor has detected you... which is impossible.

Another option is to bundle several spaceships together. This way, your opponent's mass estimates cannot be relied on. Yet another is to place your radiators on extremely long booms, so that they do not correspond to the position of your spaceship. If they move or rotate, it will further confuse opponents into over or under-estimating your forces.

Trying to hide as a civilian vessel is a fantasy often perpetuated in science fiction. First of all, it is impossible to convince a space military force that a spaceship heading along an unusual trajectory, with an insulated hull and 'cold' radiators is anything but hostile. Secondly, civilian spaceships travelling at several kilometers per second are weapons of mass destruction in the wrong hand, so all will need to be equipped with transponders that continuously report on their positions from launch to destination. Your spaceship would have to build up an entire paperwork trail to be plausibly found at that location in space, and that fall into spy work outside the scope of this blog. 

In any case, you would have to follow a trajectory that would take you straight through the most heavily defended volumes of space. Thirdly, a military spaceship will not be build like a civilian craft. For example, civilian craft might take the performance hit of using water as propellant, but military craft would require the higher performance of liquid hydrogen to catch up with targets. Their larger, hotter reactors and more powerful propulsion systems would create thermal signatures and exhaust trails that are quite different from those of civilian craft. The radiators would be quite different too. Trying to fit military systems inside a civilian craft will leave you with a spaceship that is hard-pressed to pass as a civilian craft, and would get torn to bits when facing a dedicated military opponent.    

In practice, identification will be performed using active scanners. Once your position is established, the power output of a RADAR or LIDAR can be focused on your position for good return signals. This creates a requirement for a set of countermeasures quite different than those for thermal imaging.

RADAR countermeasures include radar-absorbent surfaces and cool-looking angular shapes. LIDAR defenses include meta-materials that can modify the light bounced off. 

These techniques can help fool identification, but immediately flag your spaceship as a hostile target. 

Target lock

So, your spaceship has been caught in a hard detect, has been identified and you are unlikely to escape the enemy's sensors for the foreseeable feature. Is that the end of your mission?

Hardly.
For the enemy to do anything, they must establish a much tighter feedback loop. A hard detect can be achieved with as little as two positive pings on a radar. Getting weapons systems to fire at you requires a way to report on your position, bearing, velocity and changes in those values with much less delay.

In some scenarios, this is trivial. Your spaceship might be quite visible on a radar, and your propulsion might be easily picked up after a narrow-angle sensor is told to follow you and report on changes in your vector regularly. In other scenarios, it might be very difficult. With countermeasures for active scanners and an undetectable propulsion system, a laser beam might not be able to follow you because of lightspeed lag, and you might accelerate out of the way of incoming missiles.  

Now let's discuss further aspects of stealth.

Strategic movement

Why bother with stealth at all?

Movement of the WWI Baltic fleet across the globe
Stealth on its own does not achieve anything. Your spaceships WILL be eventually detected, and the enemy will not jump in surprise. The thermal signatures increase in number, become statistically significant, are narrowed down then identified as spaceships, with sensors attached to track each of them days, weeks or months before the come close. So what's the point?

Stealth allows for strategic movement. If spaceships are launched on an 8 month trip, and are only detected in the last week, then you can launch multiple fleets from several directions, and have them insert into various orbits for a multi-pronged or staged attack, before any are detected.

Similarly, stealthed spaceships can choose to engage or break off from an upcoming encounter. 

Stealth allows for first-attack advantage. In its purest form, a fleet can fire upon an opposing fleet twice its number, and immediately destroy half of it. This means that even if your are immediately spotted, identified and targeted after firing, you'll be able to wield a decisive advantage going into any engagement.

Stealth also ties into the capabilities off various weapons systems. If lasers are effective from a distance of 100000km, and you are spotted incoming from 80000km, then you can strike first. You can launch missiles from closer ranges, too. This means your missiles will not need as much deltaV to reach the target: as a result, they can be smaller, and you pack more of them into the same ship, which is important when facing laser defenses

The home advantage

While this might not apply to all settings, the 'home advantage' is an important aspect of space war, and stealth plays a major role.

The 'home advantage' is an extension of how battles are won: an objective is set, and two opponents fight to complete it or stop the other from completing it.

In interplanetary space war, the attacking fleet's objective is to destroy all space defenses so it can move onto pressuring ground objectives. To do that, it approaches along a Hohmann trajectory, during which it drifts through space after a departure burn.

The second step of a Hohmann trajectory is an insertion burn. The attacking spaceships perform a retro-burn that puts them in orbit around the destination planet.  

The spaceships defending the planet can win by destroying the incoming spacecraft. However, they can also perform their own departure burn, and attempt to meet the attacking fleet in deep space. If they can stop the attacking fleet from performing a retro-burn, they will force them to be flung back out into interplanetary space. This is a second win condition, and constitutes the home advantage. 

In practice, the defenders don't really have to send out their own spaceships. They can shoot projectiles, launch missiles or send off drones into the path of the attacking fleet, and home to defeat them weeks or months before they approach the planet. If the attacking fleet is then too damaged to face the remaining defenders, or expends too much propellant dodging the projectiles and so on, then it will be forced to abort the mission and perform a fly-by.

Not-to-scale diagram of home advantage. Defenders can shoot along the transfer trajectory.
Where does stealth come into play?
If the attacking fleet completely forgoes stealth, then the defenders will be able to fire projectiles and missiles at it for months. Sending a missile into the path of an incoming spacecraft is much cheaper and faster than sending another spaceship, so defenders will have a great advantage in terms of resources and efficiency.

With stealth, the attacking fleet is detected closer to the planet. This reduces the amount of weapons fire that it has to dodge, and considering the fact that a soft detect only gives a fuzzy location with lots of room to hide in, the defenders would have to shoot huge volumes of fire to hope to catch and destroy an attacking spaceship from far away.

With stealth and stealthy propulsion, the attacking fleet can come from a variety of trajectories that are close to the Hohmann trajectory, but can deviate by millions of kilometers from the most efficient route. This vastly reduces the 'home advantage' of defenders.  

Worldbuilding

How stealth affects your setting depends on the technology level of the setting, its level of development and ultimately, where you want the balance to lie.

Remember, this is ToughSF, where we give options, not restrictions.

Submarine launching ICBMs
If you want to recreate submarine warfare in space, you can. Restrict the sensitivity of sensors, increase the effectiveness of stealth techniques and the mass devoted to them, and you'll have spaceships traversing the solar system unnoticed until they attack. You have to realize the consequences, though: If 'space submarines' are capable of invisibly launching missiles and streams of kinetic projectiles without being detected, then your opponents will try to counter it with more sensor platforms, and in return, you'll build sensor hunters to keep your 'space submarines' undetected and safe.

Similarly, you can try to find a sweet spot that gives stealthy spaceships some level of effectiveness, but make the requirements great enough that fleets are regularly composed of both stealthy and unstealthy spaceships. For example, you might build a setting where the solar system has been explored and settled for a long time, and tension between the warring parties have been building up gradually. Sensor platforms will litter the solar system, above, below and around your planet. In such a situation, the only way to escape detection is with a 'hydrogen steamer' - a spaceship with large volumes of liquid hydrogen that it boils off to reduce its emissions to zero. However, such a spaceship could not compete with armored, high-powered warships in direct combat. As a result, you'll build some of both.

Sensors are what really make or break stealth.

If you want spaceships to accelerate into faster trajectories than multi-month Hohmann missions, then you'll need directional stealth: cold plates, angled radiators and so on. For that to work, you'll need the enemy's sensors concentrated into one area of the sky - so maybe during peacetime, opposing factions will spend their military budget creating spaceships equipped with powerful sensors, LIDARS and small lasers. Their only job is to hunt down enemy sensor platforms and shoot them down at the start of the war, paving the way for the main fleet to attack undetected.

Replacement sensors take time to reach the far-away but advantageous watchpoints, and those who try to do it quickly will be detected, so as the war goes on, sensors will be concentrated near the enemy, where they can be replaced faster than they can be shot down.

Similarly, you've justified a 'destroyer' class, a 'cruiser' class and a 'submarine' class, allowing you to use all the naval tropes scifi is famous for.

Or instead, military spaceships could spend their entire time tailing each other. If one fleet breaks off and enters an attack trajectory, the tailing fleet will attack it well within detection range. To complicate things, you can have a fleet of stealthed ships tailing the visible fleet tailing your visible fleet, with the opponent's stealth fleet trying to hunt it down at the same time.... 

On the opposite end of the spectrum, you can apply stealth techniques to the sensor platforms and make the impracticably well hidden. In a setting where you'll always get detected, there is no need for stealth. Since it is cheaper to shoot down a spaceship than to build one, the defenders might simply build orbital defenses to counter fleets rather than using their own. The attackers would then trade in their fleets for massive, interplanetary lasers that require re-focusing mirror drones that are also much cheaper than spaceships, and easier to hide too....

In conclusion, you cannot ignore stealth in space as being possible. If will affect how your fleet is build up, how spaceships look like and even the grand military strategy pursued by opposing factions. At the very least, you must give strong arguments as to why it is not feasible and even then, consider the fact that like many modern military technologies (tank armor, air drones, aircraft carrier fleets...) it will enter into cycles of development and proliferation that have to be matched or countered.

18 comments:

  1. The space warfare in Aliens universe is stealthy kinda like this. I mean, like "almost submarines".
    Is this the end of Stealth is possible series? That was one of best explainations of how would space stealth would work, thanks for your work!

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    1. Yes, the last part. You are welcome.

      This series only considered 'hard sf' methods for achieving stealth. There are many fictional ways to achieve pseudostealth.

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    2. Yeah, there are plenty of them, hyperspace, cloaking devices, directing heat into other dimension... In the Aliens universe "Sulaco" warship has got cooling towers like power plants in real world... But they work only in atmosphere :p
      What are your plans for next posts?

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    3. Probably piracy in space and laser webs.

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  2. If you can stealth missiles to even a small degree then you have a way to square the circle that concerns how to keep back salvoes for future engagements. Less missiles can be used to slip through defences, instead of swamping them with your entire reserve.
    You might even be able to toss them into the path of an unsuspecting constellation, requiring less propellant and thus more of the small-but-many warheads you mention here.

    Done right, stealth in space is not just a strategic utility, but an excellent logistical tool as well.

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    1. Good point. I hadn't thought of using stealth beyond applications in combat.

      I did though mention stealthed projectiles as a requirement for the 'slow projectile' tactics discussed in the Electric Cannon two-part post.

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  3. A short note on detection: there is a physical limit to the amount of resolution possible, even with the highest tech, narrowest field sensors. Spacial resolution is dependent upon angular resolution. This latter is determined by the base-length of an array (or any single sensor) and the wavelength(s) of the energy emissions detected. A detector ar array with a baseline of 300m, such as the Arecibo observatory, provides a resolution for visible light wavelengths of 0.001 arc seconds. At 1 AU, this would provide a spatial resolution of 300m length (90 000 m^2 area) per pixel. In order to reduce this to a resolution on the order of 1 m^2 at 1 AU, you would need an array baseline of 1 000 000 km (I forget if I was still basing this on visible spectrum, or if I was including IR spectrum when I researched this figure).

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    1. How do virtual telescopes (interferometers) affect this requirement.

      In discussions with astronomers on Google+, a prevailing idea is that after initial detection using CCDs, the area the spaceship is in will always be narrowed down eventually to a Hard detect level of precision.

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    2. The baseline requirements are a physical absolute. When I refer to an "array", however, I am refering to the interferometric telescopes that you meantion. Interferometric telescopes allow you to save mass (and money) by creating a virtual aperture, or baseline. If you have two puny 5mm telescopes, one located on the Earth and the other on the moon, you will have the same reolution as a single giant telescope with a 364 000 km aperture. You won't be able to detect much of anything, but anything you DO detect ill have quite good resolution (rough guestimate: 10m^2 at 1 AU, visible spectrum). The actual sensitivity of the array, however, is pretty much exactly as good as the total apperture area of the array's constituents. Your two 5mm telescopes will give you the sensitivity of about a single 7mm telescope.

      Initial detection will not always lead to a hard detect. If you have an initial detection relying on a scanning array (say the 4hr scan time suggested by K. Burnside, cited by Atomic Rockets), and/or if the initial detection relies on a time exposure, it might be possible to detect a trace on one scan, but then thee might be zero trace on the next several scans. Intermittant burn times, especially randomly timed and/or short burst duration, can take advantage of such time lapse or scanned detections. If you DO manage to repeat a detection, it can be several pixels away from the first. If you only have one vessel, thise might be enough for a preliminary track (if you are lucky). But if you have several vessls conducting manoeuvers this way, it maight be impossible to determine which repeats correspond with which vessels (especially if they change course), or even if you are tracking a vessel rather than a few large asteroids randomly reflecting sunlight.
      If, however, you have a realtime detector that is imaging the same area of space for a significant amount of time, then, yes, even though you don't have good resolution, you should be able to get a hard lock. The problem here is that if you lose trace (intermittent burns), there is no guarantee that the vessel will not make an undetectable low level burn that takes it on a completely different course.
      In short, for most objects, the astronomers would be quite correct. It does not take excellent resolution to achieve a hard detect (you might find, however, that the big asteroid you are tracking is actually 10 big asteroids). The astronomers would be incorrect, however, in some dealings with objects that are making intentional efforts NOT to be detected. Notably, determining the mass and velocity of a vessel by analysing the characteristics of the exhaust plume in order to determine the thrust would definitely NOT be possible without at least TWO arrays, each with at least a 1 000 000 km baseline... and even then might not be possible if it can make an undetectable burn.

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    3. Thanks! I'm really loving the information you're contributing. Would you mind if I collected and re-shared your writing on Google+?

      I did some searching on the differences between sensitivity and resolution and what they mean for telescopes, and if I understand it correctly, resolution is being able to locate a signal within a smaller and smaller portion of the sky, and sensitivity is being able to detect a smaller and smaller change in the signal.

      Ian Mallet commented on the performance of binary detectors, that are basically photon traps that are triggered when a threshold is reached. Modern CCD sensors are extremely sensitive (10^-9Watts or better) under optimal conditions. Hence, the assertion that every spaceship, whether following a soft detect, a random search or simply probabilistic scans, will trip a few pixels on a binary detector somewhere, leading to a hazy cloud of detections along its trajectory.

      I agree with the remainder of your post.

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    4. Sharing would be fine with me. Full disclosure: I am not an expert, just someone who does a lot of reading and research. I am particularly interested in physics and applied sciences research papers; as well as military tactics, strategy, planning, construction, and design.


      You essentially have the right idea regarding resolution and sensitivity.


      Emission flux is inversely squared with the distance. If the distance increases by a factor of 10^3, the flux willbe decreased to 10^-6. If you have an emitter with an effective flux of 1kW at 1m, at 1000 km your CCD with a 10^-9W sensitivity and a 1m aperture would no longer be able to detect the signal. At 1 000 000 km the same CCD will require a combined actual detector area of 1 000 000 m^2 to receive a signal from a 1 kW emitter. It would need a 1000 m^2 aperture to receive a signal from a 1GW emitter at the same distance. On the other hand, most CCDs are actually much more sensative than this.
      Another issue is that it is not enough to detect the signal. It is even more a matter of receiving enough of a signal for it to be significantly greater than background noise. Such background noise includes the 3°K background radiation; stellar, galactic, and other astronomical radiation; random solar illumination (reflection) off of small asteroidal masses; intrference from anything in between the detector and the emitter, such as any atmospheric particles, solar wind, cometary debris, interplanetary dust, etc; 'legitimate' space traffic; interplanetary radio trafic; etc.

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  4. Your analysis of military vs civilian vessels is not entirely valid. Stealth is founded as much (if not more) on behaviour as it is on tech. Yes, a military vessel will probably have higher performance than civilian vessels as a rule (this will NOT be a universal, since some civilian organisations will pay a premium for faster delivery), but military vessels will seldom be running at their highest performance levels. In stealth operations, military vessels will be matching the performance characteristics of civilian vessels as closely as possible. In many cases, the a stealth vessel or craft will actually UNDERperform civilian vessels, even at their maximum performnce (this is the case of the F-117, for example, which is incapable of anything near mach 1).
    Furthermore, strategic stealth missions might actually incorporate civilian vessels and missions. For example, a civilian transport might include a weapons shipment. The launch of a civilian communications array might have a software patch that monitors and collects all communications for analysis by espionaage networks. A science station might be equipped with a weapons array. Probes built for tracking asteroids and analysing them for mining operations could be fitted with hardware and/or software patches to collect stragtegic data. Etc.

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    1. 'Behavious' reliant stealth falls under spywork and statecraft, which was intentionally left out of the discussion. Your other points are valid.

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    2. Well, my point was that all stealth is behaviour reliant, but I can see where some of the specific examples might be considered more as "spywork". On the other hand, stealth is an important tool in spywork, and has been used more by spies than "legitimate" military. The important feature defining stealth is less about not being detected than it is about not being noticed.

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    3. Agreed. Although stealth is most *effective* when used as an enabler for a first strike capability.

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  5. One of the biggest oversight on the part of Atomic Rockets is that they fail to consider the sheer bulk of space traffic. It is not enough to detect presence. You must also sort through the data to detect unusual activity on the part of that pressence. This means that the activity of ALL detection must be analysed over a frame of time to sort out usual from unusual, and then unusual from potentially agressive.
    This is even more difficult if you only have intermittent contacts. Such intermittent contacts would be unavoidable in high traffic zones.
    Finally, there will not only be legitimate civilian activity, but legitimate military activity as well. Are those battleships attacking? Or are they merely conducting harmless training exercises? Is that fast moving craft attacking? Or is it an emergency response unit?

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    1. I believe that any mature spacefaring civilization will enforce a 'full-transparency' tactic of handling traffic. Every spaceship has to have an active transponder that reports its position and vectors, and malfunctioning/out of place/non-responsive spacecraft are by default flagged as dangerous at best, hostile at worst.

      This is because, as mentioned, any civilian spacecraft can become kiloton yield weapon when impacting an airless moon or asteroid habitat. An analogy would be every single aircarft today carrying a nuclear reactor by default. I also believe that computational capacity will outstrip space traffic for many decades to comes.

      I also has to consider the fact that the settings being developed by autnors and game designers are vast and varied. Some are classic invasion stories set in the very near future, so the limitations of physical telescopes are crucial to identifying a far away target. Others have a solar system bustling with activity, and sensors literally dotted around planets and along interplanetary routes, above and along the planetary plane. I have to keep by conclusions non-restrictive, so more 'you could's than 'you cannots's.

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    2. Such a full transparency policy might actually be rather problematic. First, all the transponder signals add to the noise, making it even more difficult to detect the traffic flying without transponders. Second, enforcement could tie up all of your interception forces, reducing your ability to respond to real hazards. Third, transponder signals are easily manipulated (that is, it is easy to provide a false transponder signal that could hide a hostile mission); even if it makes localising a target easier in 3D, it can make it more difficult to determine the real intent of that target. Fourth, it is not so easy to justify at interplanetary distances, because any threat would be at least several days away, and probably several months away. I could really only image such signals being justified once you enter certain "flight zones"... say 1 000 000 km from a planet.
      In any case, any such measure in effect would be null and void once hostilities start; and if war is declared, not only will this policy not be enforced, such transponders would actually be forbidden because it could illuminate nearby military traffic (this is the reason that black outs were imposed in uch of Europe during WWII).

      There is always a level of flexibility. Many of the arguments will be determined by the environment the author sets out for their sci fi universe. You have valid points there. I was basing an argument more for current trends and possibilities in our actual solar system.
      The entire "there ain't no stealth in space" mantra flies out the window if a civilisation has not developed radio imagery. Some stealth might be possible because the current military tech tends to send out one kind of signature, so all detector tech looks for that signature and ignores everything else as civilian (this is the case with "low observable" RADAR... most military frequencies use a specific band for RADAR, and so all RADAR detectors look for this band; thus some "low observable" RADARs are built to use frequencies outside of this band).

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