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.


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    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 – James from NZ Jun 28 '18 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." – James from NZ Jun 28 '18 at 3:21
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    That would be a good answer @JamesfromNZ – Yasskier Jun 28 '18 at 3:27
  • I can't be bothered to type it. Someone else can have it. – James from NZ Jun 28 '18 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 '18 at 15:12

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. – James from NZ Jun 28 '18 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 '18 at 3:07
  • Lazers usually require massive power sources , so a space platform or a planet having lazors is easier then a ship – Himarm Jun 28 '18 at 3:44
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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. – Klaus Æ. Mogensen Jun 28 '18 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 '18 at 15:17

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 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.

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