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You have a spaceship, covered in steel armor. The enemy is shooting a laser at you in pulses intense enough to vaporize the steel. But you have a brilliant idea: float a big glass lens in front of your ship, positioned so it will bend the laser slightly to the side so it misses your ship.

In theory, if the lens is transparent enough, it should let the vast majority of the energy pass straight through it without heating up the glass too much.

Could this work? Or would the laser always destroy your lens more easily than an equal mass of steel?

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    $\begingroup$ What sort of sensors do the ships use? If it's based on electromagnetic radiation in the same frequency range as the lasers, the attacking ship could just aim where the sensor says the defending ship is. That position will be offset due to the lens deflecting the light before it reaches the sensor, but that offset exactly compensates for the lens's deflection of the laser shot. I don't have the equations for heat dumped into the lens handy, but I believe dumping a cloud of sand into the likely path of the laser is considered a more popular theoretical defense. $\endgroup$ Sep 29, 2023 at 8:37
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    $\begingroup$ This would only work if the lens was perfect. No imperfections. No contaminants. The curvature is perfect. While I doubt individual atoms would be a problem, pretty much anything else will build up heat and, sooner or later, destroy the lens. Otherwise it's a cool idea... except how are you moving/positioning the lens? Whatever that mechanism is, it's wholly susceptible to the laser. $\endgroup$
    – JBH
    Sep 29, 2023 at 9:00
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    $\begingroup$ Even if the lens was nearly perfectly transparent when it was new, it will quickly encounter micro-meteoroids, space dust, and other debris. Even if it's not shattered (and it may very well be, some of those objects have a lot of energy!) the abraded surface will tend to be less and less transparent. $\endgroup$
    – Cadence
    Sep 29, 2023 at 9:26
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    $\begingroup$ @JBH Yeah the lens will eventually be destroyed, the question is whether it will be destroyed more quickly or more slowly than an equal mass of steel armor. (So you can decide whether to pack the lens, or just more steel armor.) Positioning the lens can be done with a little rocket-thruster utility bot that stays safely in the "shadow" of the lens and only touches it at the shadowed edge. The bot might get burned while touching the lens if the laser is aimed right there, but it doesn't have to touch it for very long before it can retreat into the lens shadow. $\endgroup$
    – causative
    Sep 29, 2023 at 14:46
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    $\begingroup$ @bificommander If the lens deflects incoming laser light to the side of the ship, then the ship is invisible from the perspective of the laser. There is no image in the attacker's lens, nowhere to shoot at that would hit the ship, until the lens is destroyed. Martin, no, assuming only extrapolation from current tech. $\endgroup$
    – causative
    Sep 29, 2023 at 18:47

9 Answers 9

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I used to work with high-power pulsed lasers. There is a limit to how much power you can stick through a lens. Beyond a certain power density, the refractive index depends on the power, and a flat wavefront will focus itself into lines, which ends up breaking the glass. The same might happen for your lens.

However, that isn't the real answer. The laser that is attacking you may be made of lenses. Unless they have a one-shot weapon, those lenses survived, and yours could too. The real answer is that lasers are a kinda shit space weapon. A high-power laser is a massive piece of kit. It is hard to get more than a few percent of your energy into the beam. And a nice, shiny mirror (which could be a really light, thin sheet of aluminium on a plastic film less than a micron thick) could reflect a lot. It would also hide your ship from being seen, as most of the time it would be showing a reflection of more space. If the beam is too powerful for this mirror, it will be vaporised, but the resulting plasma will be opaque to most light. If your craft is rotating (as it might to generate artificial gravity) it would be hard to hit it in the same spot.

Space is big, so any laser beam will spread out a bit. The Apollo laser rangefinder experiment managed to land one photon in 25 million on the mirror, and even less got back. Most of that loss was from beam distortion by the Earth's atmosphere, but there is still something like a 10:1 loss on the way back just from the optic finesse.

This is not to say you cannot use a laser as a weapon. But in the battle between a massive laser and a soap-bubble thin mirror, the mirror has all the odds.

The attacking ship has an engine that chucks matter out of the back to make the ship go forward. It can also be a weapon that fires matter at other ships. A lump of something doesn't spread out like light does, and at typical space encounter speeds it will get though most defences.

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    $\begingroup$ It's my understanding that with a large enough aperture for the laser (multiple km size aperture), dispersion of the beam can be made as low as you like. Then it would be possible to use a lens at the aperture to focus the light onto a small point at any distance. This overcomes the heat dissipation problem for the attacker as well as the dispersion problem. Second point: mirrors have limited reflectivity, like 99.9% is very good, leaving 0.1% of the heat to burn the mirror. It's my understanding that it is possible to make a lens that is much more transparent than that. $\endgroup$
    – causative
    Sep 29, 2023 at 14:10
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    $\begingroup$ +1 just for the first paragraph, which is cool. So... if you were daydreaming, how could a laser be focused when glass (physical) optics aren't available? Are we stuck with creating a "gravitic lens?" I'm thinking the gravitational version of what a rail gun does for a ferrous projectile. Or is there a simpler way? (not that such a tube or ring wouldn't be cool, the precursor to artificial wormholes, methinks.) $\endgroup$
    – JBH
    Sep 29, 2023 at 14:38
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    $\begingroup$ @JBH I know you said "physical" optics, but curved mirrors (not glass) can also focus light. IMU that you might use metallic optics that take advantage of fresnel reflection for really short wavelengths. (What those would look like, idk, but probably kms long.) Though glass tends to be much less absorbing than mirrors. $\endgroup$
    – BMF
    Sep 29, 2023 at 15:09
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    $\begingroup$ You could carry an awful lot of sacrificial sheets of aluminised mylar for the weight budget of one lens, in other words $\endgroup$
    – Chris H
    Sep 29, 2023 at 15:44
  • $\begingroup$ +1 for mirrors. then you can name you ship Perseus, and have a whole mythological side to it. $\endgroup$
    – njzk2
    Sep 30, 2023 at 16:03
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Frame challenge

In a setting I'm working on, space battleships go up against aliens who use lasers religiously. To combat their effectiveness, human ships employ a long & narrow cone-like structure made of strong tensile material as a shield. Imagine taking a pencil and shaving it into a cone without reducing length. That's the sort of aspect of these structures. The upshot of doing this is two-fold:

  • incident laser light is spread over a much greater surface area.
  • narrow-angle effects cause much of the incident light to bounce off harmlessly due to fresnel reflection.

As long as the shield tip is pointed directly at the enemy, you can avoid >90% of the beam energy this way. You can further reduce it by making the circular base of the cone star-shaped, increasing the presented surface area (as opposed to the cross-sectional area).

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  • $\begingroup$ Why not mirrors? $\endgroup$ Sep 30, 2023 at 11:11
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    $\begingroup$ @MatthieuM. Mirrors degrade easily and the shield is expected to endure a heavy thermal load, being shot at by lasers and all. Plus, the narrow-angle effects basically turn the material into a mirror, albeit at very specific angles. $\endgroup$
    – BMF
    Sep 30, 2023 at 17:36
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    $\begingroup$ There's a case to be made for mirrored surfaces, but in my setting the aliens use nanosecond pulsed lasers which deal mostly mechanical damage, not thermal like a continuous beam. Which necessitates a shield material with high dynamic tensile strength (some carbon allotrope, probably). Mirror materials generally aren't as strong in that department. $\endgroup$
    – BMF
    Sep 30, 2023 at 17:41
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"transparent enough" is not enough.

With a transmittance of 99.9%, your lens would absorb 0.1% of the light hitting it.

When you are talking about power fluxes of several kW or even MW, like those involved with a laser capable of vaporizing steel, even that 0.1% is enough to damage the lens and make your defense system moot.

In real life system where intense light goes through lenses (e.g. lithography scanners for IC production), those lenses need to be cooled to prevent them from overheating due to the absorbed light, and they use nowhere close to the power flux used in weapons.

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  • $\begingroup$ Would a hypothetical "100% transparent" lens still have refractive capabilities? $\endgroup$
    – komodosp
    Sep 29, 2023 at 9:17
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    $\begingroup$ @komodosp Yes, refractive capabilities come from the speed of light in different materials, not absorption IIRC $\endgroup$
    – Or4ng3h4t
    Sep 29, 2023 at 9:43
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    $\begingroup$ Under some conditions, glass can do much much better than 99.9% transparency. There are fiber optic cables that lose only a few percent of light per kilometer. opg.optica.org/jlt/abstract.cfm?uri=jlt-36-1-44 Although these can't deal with very high intensities. So the question is how transparent the lens can really be under high intensity. $\endgroup$
    – causative
    Sep 29, 2023 at 14:31
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    $\begingroup$ @causative fiber optic cables don't use refraction though, they use total internal reflection. Different phenomenon. That could be an interesting answer though! But it's not a lens. $\endgroup$ Sep 29, 2023 at 19:49
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    $\begingroup$ @Blueriver That total internal "reflection" is really refraction at work. The cables are actually a cylindrical lens, any photon that strays from the central core gets bent back into the core. $\endgroup$ Oct 2, 2023 at 0:32
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Mist (frame challenge)

Instead of using precious resources for perfect lenses that are fragile to so many other things, why not just dump a cheap gas or other particles in front of the predicted targets? You can even envelop a whole side of the ship in danger. Either by blocking or by dispersing the beam it will reduce or prevent damage. It isn't a passive solution, but waste to disperse can come from anywhere. Excrement of the crew or dust from meteorites, the supply should be easy in most cases. It is also more easy to leave behind in a pinch, while mirrors are harder to collect again or produce again.

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    $\begingroup$ A gas placed in front of the ship will rapidly disperse and cease to be effective. Excrement from the crew can be vaporized far more quickly per kg than steel. Sure, you'll put whatever you have available in front of the beam, but it's all about the mass: the enemy is trying to burn through all your onboard mass as quickly as he can. If you run out of mass before getting away or killing him, you lose. Whatever your shield material is, you want it to be efficient at resisting vaporization per unit mass. $\endgroup$
    – causative
    Sep 29, 2023 at 14:19
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    $\begingroup$ @causative, This is a good answer, as you could use millions of tiny glass beads to disperse the laser's energy harmlessly. The cloud of beads wouldn't maneuver with the ship, but it would be hard to get a lens to do that, either. $\endgroup$ Sep 29, 2023 at 16:45
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    $\begingroup$ "Excrement of the crew or dust from meteorites, the supply should be easy in most cases.", "Ah yes, the poop wars of 3050, may we never forget them." Haha $\endgroup$
    – Ian Smith
    Sep 29, 2023 at 18:34
  • $\begingroup$ But wouldn't they likely scatter like micrometeorites and cause hull damage? Friendly fire: ON $\endgroup$
    – Someone
    Oct 1, 2023 at 23:25
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Can a laser make for an effective space weapon to begin with?

First I will address some of the frame challenges that say a laser makes for a useless space weapon to begin with, since that seems to be a major point of contention. Depending on what technology you put in your setting, this could either be true or false; so, first, let's design a setting where lasers are a threat worth trying to counter.

While lasers do spread out some in space, they will still likely be the best option for long ranged attacks in most near-future settings. No, they will not hit something from several light seconds away, but that is not necessary. It just needs to out perform your kinetic weapon options to be your best option. As one other answer pointed out, a laser that fires at something 360,000km away loses 90% of its energy due to optical limitation... but that also means that anything within about 30,000km is still going to hit with most of it's energy. While some settings assume that space battles will take place in deep space at relativistic speeds and ranges of several light seconds, other more conservative settings posit that battles will only take place in the areas of space directly surrounding targets of importance where ships are required to match speeds enough to become part of the same general reference frame so that they can both make an attempt to "hold the ground" where the planet, space station, etc. that they are fighting over is. When you envision space battles being most fought primarily between planet orbiting fleets, two ships in a low orbit around an Earth size planet can accurately and nearly instantly hit any target it can achieve line-of-sight on meaning that lasers have plenty of range to be a sensible weapon choice.

In real life, unguided munitions (cannons, railguns, etc.) become pretty inaccurate at ranges of more than a few km; so, even with computer assisted targeting, minor material imperfections can cause a shell to drift several meters off target after just a few km. So, even if you can fire a shell that can deliver a more powerful, more concentrated strike than a laser, odds are it will not be able to hit anything small enough to matter at ranges of greater than a few hundred km. So, a laser ship could cook a gunship from way beyond its effective combat range.

Missiles can theoretically do better, but depending on the tech available in your setting, this may or may not be the case. If ships in your setting can not both accurately and instantly move at relativistic speeds, then this puts a cap on how fast you can make your missiles while still giving them the "reflexes" required to hone in on a target. So while your engine tech may be able to get up to relativistic speeds, it could lack the acceleration to get missiles up to speed fast enough to overwhelm point-defenses. Low Altitude Hypersonic missiles for example are hard to shoot down on Earth only because you have so little time to react from the point when a missile crests over the horizon to the time it makes impact, but in space, the same missile fired from >1,000km away will have no horizon to hide behind on its approach as it builds up speed. This will give computer systems and point defenses plenty of time to identify and respond to the threat.

As for suggestions about cheap reflective/refractive armor. There is already a known solution for this. Yes, there are material's out there that can reflect over 99% of light in the visible light spectrum, but there is no material that is super reflective at every possible frequency in the EM spectrum. If you make mil-spec lasers able to adapt thier light frequencies based on the enemy's paint job, you can adjust your attack to be absorbed by your enemy's specific defenses. Also, most super reflective surfaces burn at relatively low temperatures turning them black; so, even a low efficiency on the front-end could potentially turn into a higher efficiency as it starts to burn the target. Adaptive frequencies also allow the laser to find gaps where things that should stop the laser become transparent allowing the beam to go right through smoke screens that block visible light.

As for the damage potential of a laser, you do not need to actually do a lot of damage to incapacitate a space ship. Giant sci-fi lasers that cut ships in half causing glorious explosions is totally unnecessary in space. All you need to do is make a hole big enough to vent the atmosphere or cut a small hole in a vital system. A modern 25 kilowatt laser is adequate for burning a hand sized hole into a light armored combat vehicle AFTER passing through a significant amount of atmosphere. Since weight is a major concern in space, it is likely that armoring against a laser significantly more powerful than this would be grossly impractical; so, laser systems no more powerful than already exist that are small enough to fit on a truck will be all the firepower a warship needs. Assuming any sort of future tech, there is no reason you should not be able to manage even much more powerful lasers than this.

So now that we've established a workable laser META, will your lens idea counter it?

Unfortunately, there is no setting in which this would make since. A lens can not bend light without also heating up and absorbing some of the energy of the light passing through it. In school, many of us learned that light passes through a medium, refracting due to photons following some weird orbital or bouncing pattern through the medium, but this has been experimentally and mathematically disproven. Refraction requires light waves to stimulate vibrations in a medium. These vibrations create a wave pattern of constructive and destructive interference that form the apparent change in the speed and direction of light. So, if your medium absorbs no energy, it creates no vibrations and there is no refractive wave pattern... and vibrating atoms are the very definition of heat.

That said, a HEL's focusing lens can take a large lens surface perhaps a meter or more across and focus it down to just a few square mm. It is the focusing of energy that allows a laser to project an amount of energy that is low enough not to destroy the focusing lens, but then hit high enough to melt steel. When you focus this energy into another lens of similar quality near the target, it will rapidly heat just the tiny little point that it touches and create a thermal shock that will shatter your attempted refracting lens.

There is also the issue of positioning the lens. Lasers fire at the speed of light meaning that you can not know where it will hit until it hits you. This means that your lens would have to be big enough to protect your whole ship, but tough enough that every square millimeter of it can withstand the thermal shock of an HEL laser.

So what is the best defense against a space laser?

If you want to make a good defense against lasers, you want to use Starlite, or something like it. It is a light weight material that can coat your ship's armor and protect against lasers well in excess of strong enough to melt steel. Starlite based armor will significantly increase the needed power it takes for a laser to put a hole in a ship, but even if this forces you to put your lasers into the megawatt or gigawatt range, the addition of any future tech that would allow interplanetary warships to exist in the first place would inherently make the heat and power concerns of such lasers on a spaceship achievable... so you just need to assume that such armor is thick enough to matter, but the lasers are strong enough to still be useful: whatever that balance may be. Otherwise there will be no laser weapons, and you are back to ships using missiles and cannons anyway.

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    $\begingroup$ The biggest Frame Challenge is how the ship firing the laser is radiating all that extra energy from the laser without turning white and exploding, IIRC lasers arent 100% efficient, some energy is lost as heat, and enough energy to vaporize metal, the energy lost as heat will certainly kill the whole crew inside the craft $\endgroup$
    – Or4ng3h4t
    Sep 29, 2023 at 16:08
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    $\begingroup$ @Or4ng3h4t ... the energy lost as heat will certainly kill the whole crew inside the craft That's why you build a thermal management system for the job... If you're contemplating mega/gigawatt nuclear engines, you're contemplating huge thermal management systems. Same thing with a mega/gigawatt laser. $\endgroup$
    – BMF
    Sep 29, 2023 at 17:45
  • $\begingroup$ With jitter control systems you could expect to get aiming accuracy of micro- to nano-radians, which IIRC is good enough for engagements in the tens of thousands of km. Space telescopes have a comparable accuracy, and advancements would be expected. $\endgroup$
    – BMF
    Sep 29, 2023 at 17:52
  • $\begingroup$ @Or4ng3h4t I've updated my answer to address this concern, honestly, it did not even hit my radar as a problem to worry about since the same energy it takes to raise a handful of steel a few thousand degrees, if spread over a whole ship, will raise it less than a single degree. $\endgroup$
    – Nosajimiki
    Sep 29, 2023 at 18:18
  • $\begingroup$ @BMF The apollo laser range finder project that Richard cited is an example of a laser that can hit a ~0.3m wide target from 360,000km. I figure accuracy should be a non-issue for lasers at ranges into the millions of km which is why I focused more on dissipation than accuracy. $\endgroup$
    – Nosajimiki
    Sep 29, 2023 at 18:27
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Instead of lens, you can just use a mirror at the very slight anlgle to the ray, so that the area of reflection is increased and it works for longer. It will eventually burn through, though. Ideally a moving mirror strip.

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  • $\begingroup$ I'd love to see the Death Star versus Superman. Maybe not a wise idea to fire your planet-killing laser at my giant space mirror (the Phantom Zone). $\endgroup$
    – DrSheldon
    Oct 2, 2023 at 21:57
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[FrameChallenge]

Create a barrier of Superfluid helium-4 between you and the enemy, every shot will heat up the slime which will disperse the heat through its whole surface instantaneously, the more spread it is the more heat it can radiate in a low amount of time, allowing it to absolve more heat, and protect you from more shots.

Ideally, you would want to spread it as thinly as possible to maximize surface area to cool it down faster (depending on how fast the enemy ship can shoot their laser without melting their ship). Making a giant Superfluid helium-4 radiator, that could or not absolve all the heat from the laser, or just partial enough for your ship to not have a problem with it, depending on how opaque you want it to be.

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    $\begingroup$ If you can score a direct hit with slime, you could also score a direct hit with a missile and just blow him up. The usual assumption is that the ships are at least thousands of km apart and it is hard to score a direct hit with anything solid, and if you do, you win. $\endgroup$
    – causative
    Sep 29, 2023 at 14:39
  • $\begingroup$ Yes, but it can also be used as a wall to block the lasers, the slime will become increasingly hot with more shots, but it will most likely hold for some time, if the interval between shots are let's say 5 minutes, it's probable it could radiate excess hit and be ready for a new hit, being a superconductor it spreads the heat, so it can radiate it instead of just exploding, you're not dodging the laser, you're mitigating its heat into space, there really isn't a need to hit the slime, just a bonus, ill edit the answer as per what I said $\endgroup$
    – Or4ng3h4t
    Sep 29, 2023 at 15:22
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    $\begingroup$ Problem with this idea is that superconductors are superconductive of electricity, not thermal energy. thermal conductivity of superconductors I read a white paper that indicated oxygen free copper was about 100x better then NbTi. Perhaps you are thinking of super-fluids such as liquid helium? $\endgroup$ Sep 29, 2023 at 23:27
  • $\begingroup$ The problem with missiles is as they get closer they get far easier for defensive lasers to target them. $\endgroup$ Oct 2, 2023 at 0:57
  • $\begingroup$ @GaultDrakkor yes that was it, super fluids seem to be super conductive for both heat and kinetic energy $\endgroup$
    – Or4ng3h4t
    Oct 2, 2023 at 8:45
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A more general answer, as the question started some debates about the link between refraction and dissipation. Ideally, it seems reasonable to distinguish between refraction and dissipation. But it happens one can not exist without the other. (except maybe for gravitational lens I guess. Even then, I do not guarantee it's not the case, in a twisted way. I would believe some energy goes into the curvature for example) The result is "proved" by the Cramer Koenig relations for the refractive index. Since there is always a non zero imaginary part to any refractive index, there is always dissipation.

Now you could look at a thin sheet of metal oriented such that its surface is almost normal to the propagation vector of the laser. 2 remarks:

  1. You can't have the same angle from everywhere so it could be attacked from a different angle.
  2. If you throw more power than the atomic bounds can handle, no matter the orientation of your reflector it will be destroyed.

As mentioned in another answer, the plasma resulting from the blasted metal would provide a good shield, so you better wrap yourself in aluminum foil.

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Another Frame Challenge: CHAFF

Lenses, even if practical would need to be propositioned between the emitter and the target to work. The calculations and effort needed to first preposition lenses in right vectors and then adjust those vectors in real time as the location of the emitter and target change would be enormous.

However assuming your aware of the frequencies the laser operates at you could perhaps deploy pods containing disbursing clouds of reflective particles in between your ship and the enemy. The downside of this is that as a physical defense it would need to be constantly replenished as the laser burns through previous 'shots' so if the fight goes on for long enough your probably going to run out of chaff canisters before the enemy runs out of laser energy.

You'd also have to launch new pods every time your ship changed it's vector relative to the enemy ship (or vice versa) because every particle in the existing chaff cloud will continue along its merry way on the original launch vector independent of your vessel. Unless of course you up the tech and complexity of your launch pods to give them some limited outmaneuvering power of their own, command and control data links and some method (perhaps magnetic fields?) of corralling chaff clouds in a bubble around their pod. And I have no idea if the latter is even physically possible without disrupting the all important 'cloud' effect your trying to achieve in the first place.

Just beware IF this system could work (and that's a big if) it buys you some time to either get your own shots off or to try and escape. But the clock is ticking.

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