Sensors and Countermeasures

There are numerous ways to guide a missile or drone to the target. But for every potential way to guide an autonomous payload to the target, there is a countermeasure, and possible counter-countermeasures, and so on.

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Close up of photodetectors of a telescope.

These techniques remain relatively unchanged from Earth, but there are a few significant differences in space. One is that there is no horizon.

Two, there is no significant intervening medium (air or water). This first makes sonar and acoustic homing irrelevant, but it also vastly reduces the effectiveness of any gaseous countermeasure like smoke. Another consequence is that exhaust plumes fade out very quickly. However, the biggest change this makes is that it reduces the noise floor heavily, making targets stand out much more against the background.

Three, there is no GPS or satellite network for additional guidance help, and if there is one, only the defender would have access to it.

Before we discuss guidance techniques, a quick primer on sensors. Sensors are built as Photodetectors for a specific wavelength likely with a telescope lens. While they can be expensive, especially if you want them diffraction limited, they require so little power that power use is assumed negligible, especially in comparison to all the high power systems of a spacecraft, missile, or drone. Sensors actually give you power, but this will be less than any rotators or computing system utilizing the sensor.

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Photodetectors being installed on the HESS telescope.

Unlike lasers, sensors can be made very close to being diffraction limited because they are so low power (if kept very cold).

What can they see? As mentioned in Stealth in Space, you can detect the radiators and the exhaust plume from far away, so there’s no doubt you will see them up close. At combat ranges, the ship itself will be visible from light reflecting off the hull. Less sensitive sensors will be needed at that close of ranges, lest you burn out your sky-scanning sensors.

But what is the visual resolution of these sensors? Although you can see the exhaust plume billions of kilometers away, it will show up as a single pixel, which is not helpful. Note that you can not achieve stealth in this way. You can’t hide multiple identical ships in a single fleet via pixel resolution. Careful study of the spectrum of this single pixel over time will reveal multiple overlapped exhaust plumes.

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Data produced from a single pixel can be studied and differing emitters can be determined.

Visual resolution of a diffraction limited optic can be easily calculated using the angular diameter (follow the links for the relevant equations). Here are a few example calculations to give sense of range:

Given a detector 10 cm in diameter (reasonable for a drone, missile, or capital ship) looking for visible (550 nm, green) wavelengths, what’s the visual resolution?

At 1000 km away (orbital distance, close missile launching distance), each pixel is about 7 m in size.

At 100 km away (very long range projectile combat), each pixel is about 70 cm in size.

At 10 km away (close range projectile, drone, and laser combat), each pixel is about 7 cm in size.

At 1 km away (only missiles about to hit and maybe close ranged drones would ever get this close), each pixel is about 7 mm in size.

In Children of a Dead Earth, capital ships tend to be at least 50 m in length, often around 100 m long, or twice that for the flagships. Drones and missiles tend to be 10 m long at most, and usually much shorter.

This means that at missile launching distance, enemy ships are a tiny blob of a pixels while missiles are single pixels on screen. At very long range projectile combat, the enemy ship might be distinguishable as a shape with radiators, while missiles will be a few blurry pixels. At close projectile range, you’ll get a nice view of the enemy and their missiles as your shots tear them apart.

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Of course, taking into account enemy countermeasures, your view screen will likely look as clear as this.

Larger sensors will give you better feedback though larger sizes tends to be a problem for missiles and drones. Additionally, infrared sensors will have half the resolution stated above at best (since pixel resolution decreases as wavelength increases).

Visual resolution is rather important for missiles and drones, however, as their guidance systems need to be able to distinguish what they need to hit versus decoys. The better the visual resolution, the further away decoys need to be launched to fool the missiles.

Let’s go into the guidance techniques available in space.

Infrared (IR) Homing is a passive technique where the missile or drone chases infrared wavelengths. Often it would chase the greatest heat source, though this is easily fooled with decoys. Infrared Homing is the most important of all guidance techniques in space, principally because all ships give off tremendous amounts of heat from their engine and their radiators.

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The eye of an IR homing missile.

Ultraviolet Homing chases after the ultraviolet part of the spectrum instead of the IR portion of the spectrum. This is beneficial because UV decoys are much more difficult to produce and field. On the other hand, low heat radiators like life support radiators emit very little UV light, which means turning engines off and retracting powerplant radiators is an effective countermeasure.

Spectral Seeking is a little more sophisticated, where a specific spectra is targeted. For instance, the spectra of methane at 3000 K is unique among spectra, and only exhaust plumes of methane at 3000 K would be targeted. This means in order for decoys to throw this type of guidance off, the decoy needs to match the spectra exactly (which is difficult). On the other hand, the target needs only change the spectra of the engine or the radiators to throw off the guidance system.

Passive Radar is another passive technique where the missile chases after radio signals bouncing off of the target from third party sources (likely civilian). In space, due to distance, the signals are significantly weaker than on Earth.

Active Radar Homing is the same technique, but with an active radar system specifically illuminating the target. This is much more effective, and doesn’t rely on weak third party civilian signals. However, all radar systems can be fooled very easily by chaff. Additionally, Radar Absorbent Material is another effective countermeasure.

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A radar display affected by chaff. The left side is jammed by chaff, and produces no usable data.

Semi-active Radar Homing is also the same technique, but with the radar illuminator being separate from the radar homing device. In this case, the capital ships would have the illuminator, while the missiles would chase the illuminated targets. This yields a somewhat cheaper option, but it still has the same limitations as previously mentioned.

Laser Guidance swaps out radar for lidar, radio waves for Visible, IR, or UV light. The target is illuminated by a laser either with a separate device (Semi-Active Laser Homing) or with the homing device itself (Active Laser Homing). It can be fooled by paint absorbent to the particular wavelength, and such paint is much cheaper to produce than Radar Absorbent Materials. On Earth, this sort of countermeasure can be fooled by simply aiming near the target instead of directly at the target, but in space, no such solution is possible, since there is no nearby “terrain” to aim at.

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Schematic of a laser guidance system. The laser is “painted” on the moving target, and the missile chases the dot like a cat.

Beam Riding is a technique which uses either a laser beam or a radar beam to illuminate the target. Then, the missile “rides” the beam down, using the beam as a guide. On Earth, this restricts the missile to line-of-sight attacks, which is problematic, but in space, this is not a problem at all. Unlike laser guidance, it is immune to absorbent materials. The main issue, however, is diffraction. Any laser or radio beam will diffract, meaning the beam gets increasingly inaccurate as range increases. This restricts missile usage to very close combat, similar to lasers themselves.

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Diagram of a missile riding a beam to the target.

Neutron Homing is a guidance technique that does not currently exist on Earth, but is likely to gain prominence in space. The nuclear reactors on a capital ship dump out an extraordinary amount of neutron radiation (both fast and thermal) in every direction, and these neutrons more or less pass through any material short of radiation shielding. Trying to shield this radiation in every direction requires a huge amount of mass, so weak unidirectional shielding in a single direction (towards the crew compartment) is used instead in addition to long distance.

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Shielding against neutron radiation is rather costly in terms of mass.

This means that the neutron radiator of the reactor will be dumped out in a sphere shape, attenuated with the inverse square law. The background neutron of space is essentially zero in the absence of an atmosphere. This means very even trace quantities of neutrons can be picked up and used by homing missiles. The main countermeasure besides tons of expensive shielding would be launching neutron decoys: cheap, highly radioactive waste materials spewing off plenty of neutron radiation. Alternatively, a highly directional beam of neutrons can be generated as a way to throw off such missiles.

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Schematic of a Neutron Generator.

Command Guidance dispenses with all those sophisticated tracking methods, and relies solely on the launching ship to guide the missile with manual controls. It is immune to all countermeasures except for communications jamming and spoofing. This is a common technique for drones, since they do not need to collide with the target, they only need to aim at the target. In Children of a Dead Earth, this is the go-to method for aiming missiles if the enemy has countermeasures for every other technique tried.

Inertial Guidance is the ultimate “dumb” missile method, which rejects all attempts at actual guidance and homing, and simply follows a preset trajectory based on the target’s initial velocity and position. It is the least accurate of all homing techniques, but it is completely immune to all countermeasures. It can be defeated by simple acceleration or dodging, however.

Whew! That’s a lot of different guidance techniques and countermeasures and counter-countermeasures. So what is employed? All are employed to a small degree, but the primary homing technique used is one of the simplest: Infrared Homing.

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A salvo of missiles and a fleet of three ships in the background dropping numerous decoy flares.

This is because of the heat radiators on ships, and the extremely bright exhaust plumes. Countermeasures can be developed for every other homing technique, but for IR Homing, countermeasures are much more expensive.

First off, smoke (including thermal smoke) dissipates rapidly in space. Without an atmosphere, smoke expands at a constant velocity, required a huge amount of mass to provide a smokescreen for any extended period of time.

Second, deploying thermal decoys is expensive in terms of mass. Concealing hundreds of megawatts of radiator heat against a black background requires a similar amount of power emitted by the decoys. And these decoys have to be burning for a decent amount of time (10 seconds seems to be the minimum). This means the decoys will be rather massive pyrotechnic payloads.

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These three ships really don’t want to get hit.

Extremely massive payloads are difficult to launch at high velocities away from ones own spacecraft. If they aren’t far enough away by the time the missiles hit, significant damage can still result even if the missiles hit the decoys. Launching very massive decoys at high speeds requires significant amounts of power, which requires even larger decoys, which requires even larger launchers, and so on.

Third, using a high powered IR laser to blind the incoming missiles works great, except these missiles are insensitive to all but the brightest signals. This means the laser needs to produce comparable power to the radiators they decoy, which is costly.

This is exacerbated by the fact that missiles are launched in salvos, not one by one. As a result, the laser needs to widen its beam in encompass an entire formation of missiles, vastly reducing the power. And missile salvo formations can easily reach hundreds of meters wide, which means your beam needs to be hundreds of meters wide too, which is pathetically weak. Either that, or you need hundreds of high power lasers, each focused on tracking and blinding each individual missile, which is prohibitively expensive.

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A salvo of 100 missiles on its way to the enemy defense force around Ceres. Enemy force is roughly 40 km away.

Generally speaking, IR homing is the most effective guidance system, and IR decoys are the most commonly used counter-measure. In particular, dropping radiators and shutting off the engine while launching decoy flares is a common survival technique against missiles, though it is not foolproof. Things are especially difficult for large laser crafts with enormous amounts of waste heat.

In the end, no exact technique trumps all other techniques, and most electronic warfare focuses around IR homing and counter-measures. And when counter-measures are effective, Command Guidance is the usual response.

Stealth in Space

Few concepts of space warfare have inspired as much controversy (and hate mail) as discussing stealth in space, so I figured it’s time to have an article about that.

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The bright Apollo 8 plume observed from Earth, as it makes a Trans-lunar Injection.

For starters, though, I’d recommend checking out Winchell Chung’s website, Atomic Rockets, which has an excellent discussion on this topic, aptly titled There Ain’t No Stealth in Space. I will summarize the main points about stealth here, but for an in-depth discussion of them, see the above link.

  • Carefully scanning the entire celestial sphere takes 4 hours or less.
  • Thruster burns of any drive with reasonable power can be detected all the way across the solar system (billions of km away).
  • Even with engines cold, the heat from radiators attached to life support will be detectable at tens of millions of km away, which is still far too large to get any sort of surprise.
  • Radiating heat in a single direction (away from the enemy) is easily defeated by fielding a number of tiny detector probes which idly coast about the system. Additionally, the narrower of a cone in which you radiate heat, the larger and larger of radiators you need to field. A 60 degree cone of radiation is roughly 10% as efficient, and it only gets worse the tighter of a cone you have.
  • Making a huge burn and then trying to stealthily coast for months to the target is do-able, but as long as your enemy can track your first burn, they can very accurately predict where you’ll be as you coast across the solar system. And you still have to worry about radiating your heat for months.
  • Decoys are only really viable on really short time scales, such as in combat. Over the long term, study of a decoy’s signature over time will reveal it’s true nature. It would need a power source and engine identical to the ship it’s trying to conceal, as well identical mass, otherwise the exhaust plume will behave differently. This means your decoy needs to be the same mass, same power, same engine as your real ship, so at that point, why not just build a real ship instead?
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Anti-stealth detection measures was developed heavily during the cold war for detecting ICBMs. In space, without a horizon or an atmosphere, it’s far easier.

There are a few more points that are not mentioned but I get messaged about them a lot, so I’ll put them here.

  • Hiding behind a planet to make a burn is not really feasible. All it takes is two detectors at opposite sides of this planet to catch this. In reality, a web of tiny, cheap detectors spread across the solar system will catch almost all such cases.
  • A combat-ready ship will require very hot radiators for its nuclear powerplant for use in combat. If these radiators are going to be completely cold for the journey, they will suffer enormous thermal expansion stress when activated. In order to avoid this, very exotic and expensive materials for your radiators will be needed to get from 10 K to 1000 K without shattering. Not only that, your radiator armor will need to be similarly exotic, which means it will likely not be very good at armoring your radiators anyways.
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Rocket exhaust plumes can be uncoupled from atmosphere using modern technology after some study. This step can be skipped in space.

Now there are plenty of dissenting views (as Atomic Rockets is good to point out, as well as rebuttals to the rebuttals). Certain partial solutions, such as using internal heatsinks, and so on, are pointed out, but they all are very limited.

Ultimately, stealth in space is somewhat possible, but current proposed solutions are either ridiculously expensive, impractical, or require you to accept limitations that defeat the purpose of stealth in the first place. Indeed, rather than consider it a ‘yes-or-no’ question, it’s simply a matter of how close you can get to the enemy before they detect you.

In practice, ‘how close’ generally means halfway across the solar system, with expensive stealth solutions reducing that distance only partially. Given this, Children of a Dead Earth runs with the assumption that stealth is not a reasonable military tactic for near future space warfare.

But let’s look at an example of possible stealth: replacing your main engine (nuclear rocket or combustion rocket) with a solar sail. Your exhaust plume is now nonexistent, but now you have to take decades to centuries deliver a military payload anywhere (troops or weaponry). Your best bet is to keep your payload very small if you want to get anywhere in reasonable time. And you still have to worry about your radiators.

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Concept art of a solar sail. Abysmal thrust, and basically useless in the outer solar system, but it’s stealthy.

Suppose replace your crew module with basic electronics, and do away entirely with the crew and their hot radiators. This is reasonable for any short term space travel, but over the course of months where things can and will go wrong with the ship or the strategic situation, having a human element is necessary. Alternatively, if Strong AI can be developed, this is another possible solution, but this assumes that such an AI won’t require lots of power and heat to radiate as well.

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A different idea to get around this problem is to put everyone in cryosleep and keep the ship basically frozen. Comes with a host of it’s own problems as well, chiefly that the technology does not exist yet.

Given a solar sail and crewless ‘dumb’ ships with miniature payloads, you can build ships that can sneak across the solar system and do very little. Such ships would be unable to respond to complex and unexpected tactical decisions, and would be very easy to outsmart, as well as easy to spoof with electronic warfare. They could perhaps be used as mines, given a tiny amount of a delta-v and a small nuclear payload.

Ironically, this specification of tiny, ‘dumb’ stealth crafts is exactly what you need to build a web of detectors scattered about the solar system. This means the field of cheap detectors you want spanning the solar system can be created stealthily.

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The Hubble Space Telescope. Much smaller and cheaper versions can be scattered about the solar system stealthily if using solar sails.

Defensive stealth in space exists in full force. When you enter orbit of an the enemy’s planet, they might have an inordinate amount military hardware and spacecrafts hidden beneath the surface. But as soon as they launch, the secret is out.

This idea plays a major role in Children of a Dead Earth, as when the enemy drops into orbit around your planet, one must always be wary that the enemy fleet is simply trying to draw out your forces to get a tally on what you actually have. This constantly requires balancing of launching just enough firepower to deal with the enemy without revealing too much about one’s own reserves.

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A Titan Missile Silo from the cold war. Similar silos could be littered across planets, moons, and asteroids with full fledged capital ships, ready to launch when the enemy enters low orbit.

The easiest way to conceal a large amount of military hardware for a long distance invasion is to hide it amongst commercial traffic. Of course, this requires complicity with the civilian traders, either bought with money or intimidation, but it is possible. And such perfidy also plays a key role in Children of a Dead Earth.

With that all in mind, I will admit that at the beginning of my project, I was dead set on getting stealth to work in space warfare. Ultimately, I came to the conclusion that while stealth in space is certainly possible, it is not feasible given mass, cost, and time constraints. If you want stealth, you need to pay the price of decades-long travel times, enormously massive ships, vastly reduced military effectiveness, or all of the above all at once.

At the beginning of the project, I did explore some more exotic solutions to stealth, but I ultimately wasn’t keen on implementing technologies that were not heavily reviewed and published in scientific articles. At some point though in future posts, I will go over all of the more ‘out there’ technologies I considered for all aspects of space warfare (like a hypothetical nuclear rocket which generates an exhaust plume at 30 K, for instance). Stay tuned!