In my sci-fi universe, spaceship shields are generated by capturing plasma between two electromagnetic fields. Projectiles are evaporated by the heat of the plasma, and charged particles are deflected by the magnetic fields of the particles and the containment fields.

The problem arises when laser weaponry is considered. In theory, a dense plasma field should be capable of deflecting EM radiation (as it happens in the ionosphere). The denser the field, the higher frequencies it can deflect. But the problem is that enemies can use a wide variety of rays, from microwaves to gamma rays. Therefore, there has to be a way to detect the lasers coming in from afar, and dynamically adjust the shield's density.

Is there any way to detect an incoming laser without actually having it touch the ship, because that would defeat the whole purpose of the shield?


Thank you all for your amazing feedback! On the basis of your inputs, I have come up with a new shield design:

First level ablative/reflective/thickened body armor

Second level, powerful magnetic field with surrounding plasma layer generated by picking up from solar winds and vents from the fusion engine. Thickness can be varied by releasing some plasma to vacuum (thinning) or venting some from the engine (thickening).

Third level, active defenses, like interceptor weapons and defensive drones. Also, radio noise makers to throw off missile guidance systems (in space, a missile off-target is a missile destroyed).

I would like your thoughts on this. Also, a question:

Is there any way to curve the shield in specific ways to leave certain parts like the viewports and the weapon batteries undefended, at least temporarily, so that the pilot can actually see and fire his own weapons?

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    $\begingroup$ Re, "Projectiles are evaporated by the heat of the plasma," That's all right if the projectile is an explosive warhead or other device that is disabled by being vaporized, but if it's just a dumb slug that delivers a huge wallop of kinetic energy, then vaporizing it a millisecond or less before impact won't really change the amount of damage that it does to your hull. $\endgroup$ Dec 17, 2018 at 2:06
  • $\begingroup$ If you have the technology to create gamma-ray lasers, shouldn't you also have the technology to shield from and divert them as well? $\endgroup$
    – user44399
    Dec 17, 2018 at 3:19
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    $\begingroup$ You only really need to handle the highest frequency energies with shields. Visible spectrum can be reflected away with mirrored plates, and any large low-band heat gain is either small enough to handle with your heat pumps, or your armor HAS to ablate to prevent people inside from cooking. If you shield lower frequency emissions, your heat rejection probably isn't working very well either. Don't let me discourage you, though, just pointing out that this gets handwaved a lot for a reason. $\endgroup$
    – user8827
    Dec 17, 2018 at 7:34
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    $\begingroup$ Note that if your shields deflect All EM radiation, your ship is effectively blind while shields are up - no light in the visible spectrum, no radar, no radio transmission etc. etc. $\endgroup$
    – G0BLiN
    Dec 17, 2018 at 9:44
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    $\begingroup$ @SolomonSlow On the contrary, a projectile would go through plasma nearly unharmed. There would be no time for the plasma to heat the projectile, and not enough mass to significantly impact it. Plasma shields are strictly for stopping particle beams (and, if opaque at the given wavelength, lasers) $\endgroup$
    – Eth
    Dec 17, 2018 at 10:40

7 Answers 7


Don’t block the shot: block where they’re aiming.

Basically: you can’t block the laser once it’s been fired, as the moment you know about the laser is when you’re hit by it. What you can do is monitor the emissions/light bouncing off of your enemy prior to the laser being fired.

For this to work you need two things on your ship and one further thing to be true:

1: You need stupidly good sensor packages that can track nearby threats and their emissions/any light bouncing off them. Using LIDAR (like radar but with lasers) to actively paint potential threats would be useful, but kinda defeats the point here...

2: You need stupidly good threat analysis software that can identify ‘they are pointing their guns at me’ and ‘their gun is about to fire x kind of laser’, and bring up the appropriate shielding faster than they can aim and fire at you.

3: The enemy weapons need to be visible from your ship so you know where the laser will hit you. The exactness of these measurements will depend on how precise your lasers/shields need to be. More precision means better info on laser gun position is needed, but if your shields cover whole arcs of the ship then you can get away with knowing ‘my enemy is that way’.

Once you have those things you can bring up shields in the instant you get the ‘ they're Aiming at me and firing’ em radiation from your enemy, thus blocking the laser that will arrive shortly thereafter.

This method can be blocked/jammed/messed with in any number of wonderful plot-hooky ways, from your own shielding temporarily blinding you to your enemy deploying drones that spoof active laser signatures so your defences can’t be focussed, but if you can see your enemy you can use this as a pre-emptive defence method.

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    $\begingroup$ So in summary ... fire faster, that's a lot easier? $\endgroup$
    – DonQuiKong
    Dec 17, 2018 at 9:13
  • $\begingroup$ @DonQuiKong : Watch what they're doing, and fire just before they do. Kinda like boxing: Watch for any sign of their next blow and block exactly as they throw it. $\endgroup$
    – Joe Bloggs
    Dec 17, 2018 at 12:55
  • $\begingroup$ I was thinking more like We can't block their shots if the sun is shining because it blocks our sensors and we can't block if the sun is not shining because we can't see our enemy -> Shoot anything that moves $\endgroup$
    – DonQuiKong
    Dec 17, 2018 at 13:04
  • $\begingroup$ @DonQuiKong Well, yes, but that presupposes that the first shot is lethal all the time. If it isn't then being able to spot your enemy is about to fire at your aft/fore section (and thus focussing your defences appropriately) will be the difference between life and death. It also assumes that everyone is your enemy all the time, which isn't necessarily true. You could imagine a space-Mexican-standoff with multiple ships maintaining an active LIDAR lock on each other, just waiting for the first high-powered laser cannon to twitch in the wrong direction but not actually firing yet. $\endgroup$
    – Joe Bloggs
    Dec 17, 2018 at 14:21
  • $\begingroup$ Seems like an easy workaround for this would be a single laser capable of firing multiple types and cycling through them quickly. If the weapon changes from type 1, to type 2 (and fires), then type 3 your shield would either: not be able to keep up with the speed of change in type and defend against type 1, or would be able to and defend against type 3, in either case still being hit (by type 2). $\endgroup$
    – Centimane
    Dec 17, 2018 at 15:18

A laser beam is made of photons, and photons travel at the speed of light. Therefore whatever they emit will reach the target together with the laser, and there is no much use for it.

The only way is to detect the fingerprint of the device used to emit the laser, which will be necessarily working before the emission of the laser.

If the X-ray laser has a different fingerprint than the IR laser then the target can adjust the shield accordingly. Mind that the fingerprint can precede the shot of just few millisecond or even less, therefore the shield has to be able to quickly adjust itself.

However I also assume that the attacker will also try to shield or alter the fingerprints, so that the target is lured into using the wrong shield.

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    $\begingroup$ Or you look at the shield of your opponent. Any wavelength or fingerprint that can pass through their shield is a potential wavelength they can use, or that you can use against them but at the cost of them being able to shoot through yours as well. Checking this is simple by aiming low-intensities of different fingerprints and watching how it scatters at the shield. $\endgroup$
    – Demigan
    Dec 16, 2018 at 18:45
  • $\begingroup$ Also, detecting the "fingerprint" doesn't mean that the device is aimed at your ship. Or it could be a continuous beam that is turned towards your ship... $\endgroup$
    – jamesqf
    Dec 16, 2018 at 18:49
  • $\begingroup$ @ Demigan - The problem is that they drop their shield just long enough to fire and instantly reinstate it. I think this probably happens in Star Trek. The time to react is microseconds even if you can detect their shield being switched off. They could even selectively switch it off just in front of their laser but nowhere else. $\endgroup$ Dec 16, 2018 at 19:36
  • $\begingroup$ @chaslyfromuk if they have microseconds then so do you. Also the shield cant protect against everything, so why would you make a hole to shoot through? Lastly nothing mentions how fast the shield can adapt. It could be microseconds it could be minutes. $\endgroup$
    – Demigan
    Dec 16, 2018 at 21:19

If you don't detect enemy ships and their laser-capabilities you just do not have any time for detecting the laser-shots as they go at the speed of light. For your ship moment of you see the shot is the same moment it hits you.

But there are ways to combat lasers.

  1. Every ship would want to have good detection capabilities and some measure of stealth to be able to detect enemy ships first or push range from where you can be sniped at far enough . Long distance would allow to employ an evasive patterns in the movement of your ship. For example, at the distance of Earth-Moon enemy ship would need to "guess" where your ship will be in 1.3 seconds. They try to predict you or get closer and both ships detect each other.

    Lasers may look like they are a lines, but they are more cones, so extra distance spreads it's energy over more area.

  2. Passive defense. Your hull can be covered in layers of reflective surfaces with a gap with a metal foils, that when hit by a high-energy laser beam evaporate and produce plasma clouds, that absorb even more of the energy from the lasers.

  3. Hybrid one. Use floating sheet drones around your ship, that cover up the ship if there is a danger of laser assault or any other methods that will give you time to detect enemy ships and return fire.

For more effective shield you may use them in layers. Each layer is optimal for different type of laser. After analysis of attack or scan of detected attacker you convert most of your layers to optimal version. Methods above may be used to give you more time for the preparations.

  • $\begingroup$ "and some mesure of stealth" nope though there may be one possible type of submarine-style dedicated stealth platform $\endgroup$
    – Eth
    Dec 17, 2018 at 10:48
  • $\begingroup$ Good point, I may have explained how hard it is to try not to be a beacon in the sky. Tricky, but it is possible to make it hard to spot you or confuse enemy. Sure, no movie stealth mid-battle and mostly no situations when you can cover up your use of propulsion. But that is a question in of itself. $\endgroup$ Dec 17, 2018 at 11:43
  • $\begingroup$ Ah, you mean tactical jamming, with ECM, flares and such, to make the ship harder to target - yes, my bad, in that scenario "punctual" and/or directional stealth makes sense. $\endgroup$
    – Eth
    Dec 17, 2018 at 16:55

There are two potential ways to have your shields adjust. One from remote sensing, one from direct sensing.

Remote Sensing: For a light-speed weapon, if the first time it is fired in a battle is against you and if it hits on that shot then the first notice (outside having a "shell" of FTL sensing somethings) you will get of that is of it actually hitting you. See direct sensing for that case. However, if it is ever fired other times prior to the shot that hits you (against other targets, or against you and missing) then you have a chance to determine its frequency. Lasers have one very telling property that makes them relatively easy to discern, and that is they are extremely monochromatic. This is what makes it relatively easy to make laser "radar detectors" (really lidar detectors) for cars on earth. The laser of a police lidar is scattered somewhat by dust and by whatever else it hits and that scattering combined with the fact it is extremely monochromatic makes it relatively easy to distinguish from other EM sources. For space-born laser weapons, well, even in space there are some particles. The Earth's observable exosphere extends to at least 10,000km. For some planets this could conceivably extend much, much further. Even far away from a planet with an atmosphere, there is always some dust, some hydrogen - in some areas much more than others. And, presumably, in an area where there is a space battle, there could be a lot of "stuff" around that might cause some scattering of a laser weapon. Gas and particulate from engines and thrusters, debris, vented ship atmosphere, and the hulls of other ships being hit with the same weapons. And with laser weapons, which are presumably quite powerful, it wouldn't take much to create enough scatter for another ship to detect it. In fact, there may be enough scatter just from the laser's own focusing lens, since no lens is perfect. Ships' combat and target threat management systems and operators can be set up to look for that monochromatic scatter and trace it back to the firing ship in order to determine the best shield parameters to use for that ship's weapons. While the firing ship may have more than one laser weapon, it's unlikely that any one weapon will be able to fire many different frequencies. Lasers are something, at least with technology we can reasonably foresee today, that you can't really make to be frequency agile.

Direct Sensing As noted above, in a scenario where there is a surprise shot, where an enemy ship fires for the first time on you and that shot hits, then the first notice you will have is that shot hitting you. Outside of FTL somethings. However, that doesn't mean the game is over. People tend to think of a laser as either hitting or not hitting. In reality it is, of course, more complicated. The amount of energy actually transmitted by a laser weapon will depend on the power output of the laser (in watts) and the length of time it dwells on target: E = P × t which means that a large factor in your shields protecting you against a laser they are not currently configured for will be their reaction time. The better welding masks, that protect your eyes from the extremely high intensity light of arc welding at close distances, are LCD masks. They are, essentially, just a large single cell of liquid crystal between two panes of glass, connected to a light sensor. When the light sensor goes off, the LCD goes dark and blocks the light. But this means some of that light has already passed the filter by the time it reacts. The key is, was there enough to do any damage to the retina? The answer is no - these type of masks are proven safe and are regulated by many different national standards agencies. So, the lower the length of time, t, that the laser is on you, the less power is actually transmitted to the ship. With a sufficiently rapid shield reaction, there would be essentially no damage. No matter the power, P, of the laser, as t approaches zero, so does the amount of energy transmitted to the ship. Of course, the more powerful the weapon, the faster the shields must react to prevent damage. This might infer an "arms race" trying to make lasers that produce very high energy pulses, and shields that react more quickly.


You look at what weapon is pointed at you.

Lasers are actually not very good as ranged weapons in space combat. The laser will spread out too much and lose the energy it has over toi much surface area to be effective. Also firing a laser generates a lot of heat on the ship itself, which is very hard to get rid off on a space ship. This means lasers are mostly thr CIWS of future space ships. Great for point defense against fighters and incoming missiles but useless against long-range targets.

Now imagine that because you have shields, your fights happen at much closer range. Now that laser is effective right? Unfortunately you have a shield that can deflect laaers. This means you need to alter your shield so that your lasers have as little effect on your own shield or you would lose both firepower and desabilize your own shield a little. But since your enemy can simply measure which wavelengths penetrate through your shield they can simply make their shield opaque to those wavelengths, neutralizing the effect of those lasers.

Ergo: you can react to something that moves at the speed of light by knowing what is necessary to fire something at the speed of light.

  • $\begingroup$ Lasers are not inherently good or bad at long range. Effective range depends mainly on aperture size (bigger is better) and wavelength (smaller is better). Depending on those you might get a range of 10 kilometers or 10 light years. I really don't think you can make any sort of overall, setting-agnostic judgment on how good lasers are as weapons. $\endgroup$
    – Elukka
    Dec 16, 2018 at 20:40
  • $\begingroup$ @Elukka the problem is that wavelength wont get you much extra range and to reach 10 lightyears the focussing array is going to measure in the miles. Better use that for a few more railfuns (not a misspelling), shields and missile batteries. $\endgroup$
    – Demigan
    Dec 16, 2018 at 22:24
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    $\begingroup$ The more obvious problem with firing a laser at a spaceship from 10 light years away is that it will take 10 years for the beam to arrive. Unless the target is going to be cruising passively for the entire period, it is unlikely to still be on the path of the laser a decade later. Even over much shorter distances of a light minute, the smallest change in course, acceleration, or deceleration of the target would result in the beam 'missing by miles'. $\endgroup$
    – Penguino
    Dec 17, 2018 at 1:12
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    $\begingroup$ @Penguino: Not to mention that (in the absence of FTL communication), the people firing the laser only know where the target was 10 years ago - so 20 year total time lapse. $\endgroup$
    – jamesqf
    Dec 17, 2018 at 4:05
  • $\begingroup$ @Demigan 'miles'? Look up Nicoll-Dyson Beam. With Dyson as in Dyson sphere. $\endgroup$
    – Eth
    Dec 17, 2018 at 11:12

You've got it backwards

Stopping solid projectiles with a trapped plasma is much, much harder than you seem to imagine. As an example, consider a re-entry vehicle: that stuff heating up the nose is a plasma. After a few hundred kilomtetres of that, all that is vaporised is the ablation shield.

(As a small aside, the plasma isn't trapped between magnetic fields; it's trapped within a magnetic field, between zones of high Z field gradient, which tend to reflect ions.) In contrast, any plasma dense enough to vaporise a projectile during the milliseconds it takes to pass, is going to be pretty much opaque to all EM waves.

There are also lots of other issues with beam weapons. Writers who have thought about the engineering issues seem to assume that space combat will consider 100 km to be "point blank range", and most engagements will be at several thousand kilometres. At these ranges, simply focussing the beam on the target tightly enough to do damage is extremely difficult. It becomes practically impossible if the target starts dodging randomly; a plasma shield just ups the required power threshold for damage even further.

So it seems practically mandatory that weapons have terminal guidance. Once you have that, even fairly simple missiles can destroy all but the most extremely armoured spacecraft. (To complicate evasion, they should, of course, be stealthed, with skin chilled with liquid helium and course corrections by low signature engines such as cold gas, rail guns, or ion thrusters.)

Furthermore, vaporising the projectile isn't enough:

  1. If it's a kinetic kill weapon, it still has the same mass, and is hitting at the same hypervelocity impact speed. The exact penetration depth will change a little, but above a certain velocity the hardness of the projectile doesn't make much difference.
  2. If -- as is more likely -- it's a missile with a warhead, it just detonates when its proximity sensor detects your plasma is close enough. Given the scale of space combat, they will most likely be nuclear. Against most targets, sinply detonating near by will be enough -- plasma shield be damned. For very heavy armour, such as the classic "hollowed out asteroid battleship", you can use the x-ray hohlraum of a fusion device as the compression driver of a Munroe effect charge to project a few tonnes of tungsten plasma forward at several hundred kilometres per second. Due to extreme pressures achieved in this sort of device, the tungsten plasma density is several times higher than the solid metal density! Try trapping that with your magnetic mirror.

BTW, issues of the physics of space combat were explored in quite some detail by the space opera RPG "Traveler" way back in the 1970s

  • $\begingroup$ Yeah, but the earth's ionosphere is a very rarefied layer of plasma, barely enough to deflect radio waves. What we're talking about here is extremely dense and thick plasma layer. But the problem is, if it's too thick, visible light will not go through, and you will be flying blind. Thus, a way is needed to dynamically change density. $\endgroup$ Dec 17, 2018 at 16:00
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    $\begingroup$ And laser weaponry will mostly be used in close-range engagements by snub fighters and the like. Most capital ships will use missiles, railguns, charged particle cannons, magnetic cannons and the like. Even these shields are actually for snub fighters. Larger ships carry gravitational shields, which make ammunition bend around the ship. $\endgroup$ Dec 17, 2018 at 16:03
  • $\begingroup$ Hello Budhaditya, I wasn't referring to the ionosphere; the plasma that decelerates a re-entry vehicle is air strongly heated by adiabatic compression by the shock wave formed by the vehicle. It is an exceptionally dense plasma; much denser than anything that could plausibly be trapped by OP's magnetic mirrors. Hence it illustrates that his plasma shield could not possibly vapourise large projectiles. $\endgroup$
    – Securiger
    Jan 2, 2019 at 1:49

Having the laser touch the ship would not defeat the purpose of a shield. Laser weapons generally work by heating anything they hit. Assuming you're not thinking about insta-kill death rays, it probably takes a while for the laser to do significant damage. Your ship could get hit for a very short while, and then adjust the shields - assuming this process is quick, you'd probably take minimal, damage, if any. Just like when you touch something hot, you immediately take your hand away and suffer minimal burns (or none, if you're lucky) - if you didn't, your hand would end up roasted.


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