The space battles in the Expanse universe look very much like ship-to-ship (or plane-to-plane) fights from the late 20th and 21st centuries; they rely on expensive, guided torpedoes (that need to be shot in large quantities to get past the Point Defense guns), classical kinetic bullets, and occasionally a rail gun.

Yet the most popular sci-fi weapon type— lasers—is missing. The technology is there: in the Slow Zone, when all kinetic weapons are neutralized, Belters manage to turn a communication laser into a weapon able to destroy other ships and (in theory)

even the alien station.

I remember, that engineer warns that this laser will melt to slag after a few seconds, but it is because this weapon is made basically from scraps. There is also no problem with power, since each ship is equipped with a large fusion reactor. If such a thing was properly developed, it would be much more viable - maybe not as a primary but at least as a decent secondary weapon. Yet they are not present at all.


  • 3
    James Corey said "...talking to my buddies who design high energy weapons for a living. We've been making weapons for thousands of years now, and so far we've never come up with a more efficient way of killing stuff at a distance than accelerating a chunk of metal to high velocity. Lasers are useful as point defense here on Earth where gravity is factor, but in space where a bullet will travel in a straight line basically forever, lasers are just not a better choice." CONTINUED BELOW Jun 28, 2018 at 3:20
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    James S A Corey CONTINUED: "A chunk of tungsten traveling several kilometers a second imparts as much energy in a nano-second as even our most powerful lasers would in several seconds on continuous contact." Jun 28, 2018 at 3:21
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    That would be a good answer @JamesfromNZ
    – Yasskier
    Jun 28, 2018 at 3:27
  • I can't be bothered to type it. Someone else can have it. Jun 28, 2018 at 3:34
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    @JamesfromNZ One advantage lasers would have over kinetic or explosive projectiles (bullets or missiles) in space is that for most purposes they're (nearly) instantaneous, point and shoot weapons. Given the distances that may involved in space battles this could be a huge advantage for the first group to start using them.
    – Xantec
    Jun 28, 2018 at 15:12

6 Answers 6


Within the books, anti-personnel lasers exist in the Expanse universe and are sufficiently common that there are standard defences against them, notably gas grenades that fill the air with "anti-laser smoke" and "ablative" armour.

Anti-ship lasers are far less common and are basically a known technology but one that is used infrequently since they're a) energy-hungry and b) largely useless against anything other than an unarmoured civilian ship (which you could just blast with a railgun and save yourself the trouble.).

The service corridor they’d cut into was narrow and dim. The schematics the Tycho engineers had worked up suggested they wouldn’t see any real resistance until they got into the manned parts of the station. That had been a bad guess. Miller staggered in with the other OPA soldiers in time to see an automatic defense laser cut the first rank in half.

“Team three! Gas it!” Fred snapped in all their ears, and half a dozen blooms of thick white anti-laser smoke burst into the close air. The next time a defense laser fired, the walls flashed with mad iridescence, and the smoke of burning plastic filled the air, but no one died. Miller pressed forward and up a red metal ramp. A welding charge flared, and a service door swung open.

Leviathan Wakes


The first thing that went on was what the grunts called the full-body condom. It was a thick black bodysuit, made of multiple layers of Kevlar, rubber, impact-reactive gel, and the sensor network that kept track of his injury and vitals status. Over that went the slightly looser environment suit, with its own layers of self-sealing gel to instantly repair tears or bullet holes. And finally, the various pieces of strap-on armor plating that could deflect a high-velocity rifle shot or ablate the outer layers to shed the energy of a laser.

Caliban's War


While lasers are inherently cool, their practicality as a weapon is greatly curtailed by several factors

First, applying a reflective coating to a ship, or plating the hull in a high melting point metal such as tungsten, would reduce their effectiveness greatly.

Second, lasers are a line of sight only weapon.

Third, unless you make a laser extremely powerful, it has to stay focused on a small area for a prolonged period of time to actually do damage. This would be difficult if the laser was fired from a moving platform at a moving platform over the distance of several kilometers.


Ultimately lasers are not present, as large scale weapons because the military's decided their flaws outweighed their virtues.

  • We need a in-universe explanation. The Belters on the Behemoth did it, and the other ships were damaged by their souped-up communications laser, so your points, while true, don't seem to be relevant for some reason. The ships in the Expanse seem vulnerable to laser strike. Jun 28, 2018 at 3:03
  • Hmm, I don't believe they are inefficient, since the mentioned created laser weapon was capable of destroying ship (confirmed in universe) - the only technical difficulty was that it would melt itself after few shots.
    – Yasskier
    Jun 28, 2018 at 3:07
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    Milssiles can be guided or heat-seeking. With lasers, you need line of sight. With realistic distances in space combat (visual media tend to cramp this down unrealistically), targets may move out of the path of a laser beam before it hits. Jun 28, 2018 at 11:34
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    @KlausÆ.Mogensen In order to move out of the path of a laser before it hits you you'd need some manner of prescience, or a way to "see" and move faster than light.
    – Xantec
    Jun 28, 2018 at 15:17
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    The Behemoth was able to damage a ship with its laser because it was in the slow zone at a time when no ship could move "faster than a thrown soft ball" and the range was very short. In normal space the target could too easily just spin or change thrust or otherwise make it impossible to keep the laser trained on a specific spot long enough. Also there is the focusing problem. Lasers loose efficiency at longer ranges because of spread while bullets don't.
    – Daniel T.
    Nov 26, 2018 at 14:55

TL;DR - it is more practical and cost effective to fit a Truman-class ships with 42 x 40mm PDCs vs one or two gigawatt lasers.

In The Expanse, ships travel at hundreds of kilometers per second and torpedoes can travel even faster. If you account for relative velocities between a ship and missing, then the apparent velocity of a torpedo may actually only be a few dozen kilometers per second (from the point of view of the targeted ship). That means a torpedo can traverse a distance of around 300 - 500km in a mere 10 seconds.

Lasers travel at the speed of light but beam quality and diffraction drastically decrease their effective range since the beam will spread out as it travels. Range can be increased by building a bigger aperture (structural/material stress and mount size become an issue) or by increasing the power output (waste heat, thermal stress, melting of components, and power generation become an issue), but you are still looking at a fairly large weapon with an effective range of at best a few hundred kilometers and output of a few hundred megawatts (which would require one or more gigawatts of input power to achieve). Also, the greater the range to a target and/or the more armored a target is, the longer a beam needs to remain on it before sufficient damage is done. This task is made even more difficult by the high velocity of the target and the constant adjusting of the laser to focus on the same spot.

So if you have a small swarm of missiles (say six) that are traveling at 50km/s relative to a target defended by a giant laser that maxes out at say 300km and requires one or more seconds to kill a torpedo and acquire a new target, at least two will get through the defense.

Now let's look at PDCs. While they may have muzzle velocities of only a few kilometers per second, they have several things going for them. The lack of horizon in space means that torpedoes can be tracked the second they are launched. At the extreme velocities involved, torpedoes generally cannot afford to maneuver once they are in the terminal phase of their attack or they risk flying past the target completely. So the closer a torpedo gets to its target, the more "straight and steady" its course becomes. A PDC can track the torpedo and once the torpedo enters its terminal phase, the PDC can lead the target and begin throwing out a stream of shells and expect a hit.

Think back to battle between Donnager and stealth ships. One of the crewman says something like "I've never seen torpedo guidance systems this good before sir, they are pushing our PDCs to to the max." He likely meant that those torpedoes were still able to maneuver to some degree during their terminal approach, which made it hard for the PDCs to predict their course.


Actually, there is a laser-class weapon in books 7 and 8, used by Laconian Magnetar-class battlecruisers.

In book 7, it's firing on the central sphere in the gate space causes an outburst of gamma radiation from all gates, capable of frying entire ships at significant distances. Similarly, in book 8, a gamma radiation outburst of a neutron star turning into black hole, directed at the gate (and consecutively, at the sphere), causes an identical effect, only way stronger (it doesn't just fry ships anymore, it vaporizes them instantly this time, just like the main gun discharge on a Magnetar-class battlecruiser does). So, even though it hasn't been stated explicitly, the conclusion is that those ships' primary weapon is basically a very big gamma ray laser.

It has to be taken into consideration, though, that this armament is extremely power-consuming. In book 8, it is stated that the Resistance, up until then, didn't figure out what power source could be powerful enough to feed this weapon. Until, in a raid on a Laconian freighter, Bobbie's team discovers four containment cells, that is. It turns out they contain a kilogram of antimatter either (as far as I remember) in total, or each - and that discovery puts the final piece of the aforementioned puzzle in place. Judging by how The Heart of Tempest made only a few shots and needed resupply, we can assume that an entire containment cell's content is annihilated to power a single discharge.

EDIT: Now I've come, however, to conclusion that while the statement above does somehow answer the question in letter, it does not answer it in spirit. Which I assume was 'why lasers are not widely used by UN/MCR/OPA ships'

In that regard, I recognize the technical arguments presented by other users, though they might not be completely valid. Now, what do I mean by that? In terms of laser as a main artillery weapon, the arguments on power output of a laser (more precisely, it's input/output efficiency) and the mobility of targets seem largely valid. However, as PDC replacements, they do not.

The US navy is implementing such defences on their Ford class supercarriers right now. Those are powered by nuclear reactors, ones with similar output or marginally higher than those powering Zumwalt class destroyers, which are either already equipped or are planned to be equipped with railguns. Ones that accelerate projectiles to hypersonic velocities. Now, from what I read, railguns like the ones on Donnager or Truman class accelerate their projectiles to a measurable fraction of light speed. That hints at their reactors providing more power by at least an order of magnitude (1000x).

Now, what has to be taken into account - in space, power loss of a laser beam due to photon deflection is lower over any measured distance by a lot due to vastly lower particle density (actually, negligible, at least when we are talking about distances measured in light seconds which are many times greater than distances at which battles in The Expanse actually take place). And still, despite higher energy loss and lower available power input, those PD lasers are being implemented here, on Earth, nowadays. Now, would they be implemented if they were inferior to projectile CIWS which are basically a predecessor to The Expanse's PDC? Doubtful.

Let's say that aforementioned is a 'currently used weapon' argument.

It is stated in the books that even communication lasers can do a lot of damage, especially to peripheral devices on a ship. Which means ones that due to their function cannot be protected by a layer of armor while at use. Such as sensors, communications equipment... or perhaps even active PDCs.

One should remember that while these lasers can be weaponized at the cost of shorter lifespan or even melting risk due to power input far beyond the intended one for them... they are not purpose-built weapons. Those would most definitely have been built with higher input and better cooling in mind. And probably not powered from reactor directly, but using capacitors to allow discharging a second or ten seconds of power feed within a milisecond or even microsecond (like Honorverse lasers do).

Now, why I think lasers could be employed in The Expanse universe's reality as weapons? Precisely because of what they can do to critical, unarmored devices. Does a missile have to be destroyed in order to count it off? Or is it enough to fry its' fragile sensors (that just can not be armored or they would not work) and guidance and just let it miss. What good does having a railgun do to a ship that is blind and doesn't know where to aim its' weapon? What good will instantly overheated or maybe even melted PDCs do? Hull intact, ship crippled, devoid of combat capability, out of commission. The beam might even be slightly dispersed on purpose to be able to catch multiple missiles at once or cover an entire ship if the output was powerful enough to begin with.

With a lot more powerful reactors than we have nowadays, the fact PD lasers are already being implemented and the fact we already have a petawatt laser on Earth, purpose-built laser weapons seem absolutely feasible in The Expanse's reality.


Well... if that is the case and lasers were there and they were used like I mentioned above, Roci probably wouldn't survive book 2, namely that assault on Protogen station. In pitched battles, ships would have to launch dozens of missiles to have a chance of a single hit, battles would be won and lost within seconds with no place for skillful maneuvering, just a game of sensors, ECM and ECCM - who shot first, who got more accurate lock on target, who had better decoys and/or stealth. Fleet that got a jump on their enemies would, in most cases, win without a single loss. Without defenses that do not seem scientifically feasible on The Expanse's 'level' of technological advancement like energy shields or adaptive-reflective armor, lasers would break the story.

So, in my opinion, their story-breaking nature resulting from author's goal to make a story without an abundance of 'space magic', like shields is the primary, underlying reason they're not there.

  • The two cases you state at the beginning are not using lasers as weapons, but as triggers for massive point-source radiation bursts. The only targeting involved is hitting a large, stationary target that's not attempt to evade you.
    – chepner
    Jun 7, 2021 at 19:58
  • @chepner "it doesn't just fry ships anymore, it vaporizes them instantly this time, just like the main gun discharge on a Magnetar-class battlecruiser does" I'm quite certain you read this part that indicates the weapon had not been used solely against immobile targets, but in a ship-to-ship combat as well ;) Jun 9, 2021 at 19:21

Actually it might have some benefits, but it could not replace missiles or kinetic energy weapons. The other comments have contributed a lot to the answer, so I'll limit myself to things that have not been elaborated as much.

The most important factors at play in the (drag-less, atmosphere-free) space environment, become 1) Destructive power, which could be modeled as energy per second per surface area of the target (plus, in the case of massive projectiles, momentum, which becomes extremely important) and 2) Accuracy.

As far as -1- is concerned, some important aspects have already been mentioned. Kinetic energy weapons (such as PDCs or rail guns) will impart a large amount of energy with every hit. In space, there is no drag to slow down projectiles and thus no energy loss as the projectile travels. The potential of the laser to achieve a similar result depends on the available laser power. Theoretically one could build a more powerful laser, or a particle-accelerator weapon (shooting high energy protons for example -effectively a plasma weapon), the question is whether that is feasible with the technological resources available in the Expanse universe. The kinetic weapon delivers its energy on impact, giving extremely high power per surface, making it effectively destructive. The impact will create debris, that in turn will continue the destructive propagation of energy further, as momentum is conserved. A laser beam imparts very little -insignificant- momentum to the target. Even a plasma weapon would impart insignificant momentum. The after-effects of momentum are a crucial aspect of the weapon system.

Divergence, diffraction, are subjects I can't really talk about. Different lasers (or energy weapons) would potentially have different characteristics, in principle it's possible to achieve minimal divergence over ranges where the weapon can be accurately fired.

For the weapon to be effective, the energy must be transferred to the target, and even better to a specific location on the target when possible. Spreading the energy on a large surface is undesirable, since it will minimize the destructive effect. The rail gun seems ideal in that respect. Bullets usually hit in different places, similarly it is very difficult -probably impossible- to keep the laser beam on the same spot for long, given that ships rotate and accelerate in different ways. Very short powerful bursts would be more effective.

If the target's range is below 10 km, it is most likely possible -always depending on relative acceleration- to aim the laser on the target for short bursts (to maximize power/surface). As the range increases, it becomes harder. At a range of 100 km budging even for 1/100 of a degree would bring the laser 17 meters off target. So it depends on the ability of the engines and the weapon system to achieve and maintain an exact orientation and target alignment in combat conditions, the error margins of the navigational instruments etc. Shutting off the engines before targeting and firing would put the ship in free-fall, achieving its maximum accuracy.

I am not aware of the capabilities of current thrusters to achieve and maintain orientation, so I'd avoid making an educated guess. Maybe it can be done with arc-second accuracy, but under what conditions (and given how much time to perform the alignment). Using an autonomously targeted laser (ie one with a target acquisition arc, on a mount that can accurately track the target in this arc), would eliminate the need for accurate thrusters -but only during free-fall.

As ranges increase, active or semi-active guidance homing missiles would be preferred.


Tl;dr: storytelling.

With the unreasonably good fusion powerplants in The Expanse, a reactor pumped x-ray laser would be quite feasible. This would be a weapon of unparalleled devastation, which moves faster than any detection method possible, can punch holes through ships at light minutes of distance, will deliver lethal radiation flux throughout an entire ship, and has extremely low beam divergence.

This presents a problem with narrative structure. You would have most battles be frantic random dodging and firing until someone gets lucky (or their battle computer out-performs the other one) and the ship sees a massive white flash followed by a giant hole being in the ship, and the entire crew dying of radiation poisoning.

Not terribly compelling. Turns the situation into an “all or nothing” fight every time, which would make readers question why in the hell the mother countries of these navies would be insane and cruel enough to put people on these weapons platforms that already are controlled by a computer. And with no people, readers/show watchers lose interest.

Projectile weapons allow for purposeful dodging, a battle the readers/TV audience can follow, and are more familiar conceptually to most people. Thus I imagine this was a pragmatic choice by the author.

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