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The space borne planetary invasion force is a staple of many Sci-fi genres and most space invasions face the need to destroy some, or all, ground based infrastructure from orbit in order to take over a world. One of the things that often puzzles me is the depiction of lasers or particle beams for targeted ground attacks. These weapons would seem to tend to be dispersed or deflected by atmospheric gases and planetary magnetic fields.

Assuming:

  • that orbital dropped munitions are at least as accurate as modern smart bombs.

  • that you have access to "dialable damage" kinetic weapons for attacking ground side infrastructure.

  • that you want the planet you've gone to the trouble of invading largely intact, so no cobalt bombs or other long-term area-denial weapons.

  • that these weapons, while extremely useful for hammering planetary targets, are too slow moving for ship-to-ship combat and as such are not used as combat vessels' primary weapons.

  • that such weapons are not excessively large and can be mounted on/carried by warships without compromising their primary mission capability.

  • that planetary bombardment is rare in other circumstances but almost always necessary to support invasion forces.

For what practical reasons would weapon systems which are more suited to ship-to-ship combat be used in this planetary mass destruction role when they appear to suffer issues of accuracy and loss of power due to atmospheric effects and possibly cause excessive/secondary damage effects as a result of either heat bloom or, as in the case of nuclear or particle beam weapons, the nature of their operation?

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    $\begingroup$ Lasers and orbital bodies: what-if.xkcd.com/13 $\endgroup$ – CaM Jul 9 at 16:12
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    $\begingroup$ And one more that describes what lasers and water vapor can result in: what-if.xkcd.com/119 $\endgroup$ – CaM Jul 9 at 16:14
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    $\begingroup$ Why do you assume that lasers are great for ship to ship combat and will kinetics won't be? Considering that lasers can be proactively doged by drunk-walking and require the vessels to carry around huge, woundable radiators to deal with all the waste heat they produce, slow but heat efficient kinetics look quite appealing to me. Lasers need to be stupidly powerful to deal sufficient damage instantly, assuming the enemy sits cooperatively still, does not use mirror-granates or has heat-sinks with secound sound thermal superconductors in the shield or hull to decrease laser efficiency further. $\endgroup$ – TheDyingOfLight Jul 9 at 16:28
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    $\begingroup$ @TheDyingOfLight I didn't say kinetic rounds can't be used for ship-to-ship combat simply that kinetic rounds large enough to fulfill a mass destruction role after going through atmosphere are too slow for the job, in this scenario. $\endgroup$ – Ash Jul 9 at 16:31
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    $\begingroup$ @Ash "kinetic rounds large enough" the size of the round does not matter, the kinetic energy it carries does. Ekin = 0.5 * m * v^2 this shows us that speeding up the slug is vastly more efficient than increasing its mass. As for the "through the atmospere" part using a speer shaped, thus aerodynamic, tungsten rod with some stabilisation fins will deliver all the kinetic energy you want. Of cause even aerodynamic shaping has its limits, but it will allow you to create decently sized craters. $\endgroup$ – TheDyingOfLight Jul 9 at 17:30
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Because they are the most efficient way to deal damage to your specific target

An (arguably) non-kinetic bombardment weapon would be an antimatter beam, which is a great opportunity to commit genocide..., no!!! cleanse heresy, no!!! strategically secure the target yes!!!. You did say that the planet should be captured intact, but there are many levels of intact. Especially if you are dealing with a hostile biosphere or some biological enemy like the Tyranids or the Zerg, keeping the atmospere at a toasty 500 C for a few days before landing should help. Antimatter beams would probably be best for this, but lasers whose energy will be absorbed by the atmosphere will do great as well.

Because Ammunition is more (mass) expensive than reactor fuel and radiator time

Remember the rocket equation? Everyone in space does. Maybe it is just more efficient to carry around the reactor fuel to power the energy weapons than it is to carry missiles and uranium enriched tungsten speers. While in a space battle radiator time is a valuable resource, during a bombardment taking care of heat is simple.

Kinetic weapons are energy weapons

Maybe your military avoids many issues (waste heat management) shipboard energy weapons bring with them and uses missiles with nuke-pumped-x-ray-laser or casba howitzer warheads (nuke-pumped particle beams). For planetside bombardment they are used as an energy and kinetic weapon. They fire the energy-weapon component to blind the target and hit it then with the high speed shrapnel the missiles husk has turned into.

Because energy weapons are what the vessels carry

This is based on the rocket equation again. Every gram counts on a spacecraft. The military may have access to decent kinetic weapons, but if they are as you say useless in vessel to vessel combat (I do not think the are, but it's your universe) carrying them around is simply too costly delta-vee wise.

The real reason why popular media shows this

Hollywood has no clue about Hardish Sci-Fi. Lasers look cool and cool weapons are better war-maschines than realistic ones. Visually colourful laser beams are more impressive and easier to produce than depicting missiles and kinetic slugs. Star Wars and Star Trek did it and have influenced most subsequent Sci-Fi. Audiences and lazy writers know it and love it and thus a vicious cycle ensues.

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    $\begingroup$ +1 just for that first point, the fact that cooking the atmosphere to get the planet intact hadn't occurred to me, although it's rather more destructive of the world than I was thinking about. $\endgroup$ – Ash Jul 9 at 17:46
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    $\begingroup$ My answer would have been the visuals and Hollywood. The rest sound solid too, however just a thought: if they can travel from habitable planet to planet, they shouldn't worry about mass that much. Just mine the nearest asteroid for some metal, if you don't want to carry it around. $\endgroup$ – Lupus Jul 9 at 21:55
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    $\begingroup$ your only problem is a laser capable of doing comparable damage to a kinetic weapon uses vastly more energy and thus fuel than a kinetic one. Kinetics is just a much more efficient way of transferring energy. $\endgroup$ – John Jul 10 at 4:42
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    $\begingroup$ and it's not just (directly) mass but volume. If you can fuel a laser cannon for 100 shots with 1 liter of whatever but 100 shots of a kinetic projectile are 100 liters, it's more efficient to use the laser as you can carry 100 times the ammunition in the same volume of ship. If the fuel is only 1% the weight of the kinetic projectiles for the same volume that means 10.000 shots for the same weight of 100 kinetic ones. $\endgroup$ – jwenting Jul 10 at 5:38
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    $\begingroup$ @Lupus In a solar system scale war sending out self replicators to either turn unclaimed space rocks into kinetic kill missiles or to "secure" them with fusion bombs should be an early move byb any side intending to win. That no rocks are available for material collection isn't an implausible scenario. After all, area-denial weapons are already quite common today. $\endgroup$ – TheDyingOfLight Jul 10 at 15:23
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Economies of scale matter on warships too

If you look at naval vessels throughout history, they have always tended to evolve towards specialization in weaponry. The more different kinds of weapons you have, the more complicated your maintenance, logistics, and even design and construction of your ship get. This is why it's very rare to see a warship of almost any era with multiple primary weapons systems. The best designs pick one thing and do it as thoroughly as possible.

In response to your question, the simplest explanation is that the PRIMARY role of the space battleships is destroying other space-based defenses and that is best accomplished with directed energy weapons. Once that has been accomplished, the actual orbital bombardment is a secondary task, and the weapons you're using only have to be 'good enough'. Adding specialized planetary bombardment weaponry would make the ships LESS capable at defeating the space-based defenses, so you're better off specializing your ships for space combat and accepting that your bombardment is going to be a bit less effective.

EDIT:

Assume... that such weapons are not excessively large and can be mounted on/carried by warships without compromising their primary mission capability.

I'm going to challenge this item in your post because you can't have a realistic discussion about military engineering with an assumption like this. Nothing is free in engineering, and there's no such thing as a weapons system that doesn't contribute to the primary mission of the platform that DOESN'T compromise the primary mission capability to some degree. It's always a question of 'how much' does it compromise that mission capability.

Remember we're not JUST talking about mass, or power consumption, or any of those things. Adding extra weapons makes the ship more expensive. It makes it more complicated. It means you're mounting this kinetic weapons system INSTEAD of something else that WOULD contribute to the primary mission and that is never beneficial.

Here, again, if the primary energy weapons systems on the warships are capable of performing the secondary planetary bombardment mission adequately, it doesn't really matter if it's a small compromise versus a large one to mount a separate kinetic bombardment system, it's still unwelcome and makes the ship less effective than it could be.

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  • $\begingroup$ Assume ... "that such weapons are not excessively large and can be mounted on/carried by warships without compromising their primary mission capability" ... while you make a salient point in terms of the probable realities of space warfare the question did consider that point and ask you to dismiss it. $\endgroup$ – Ash Jul 9 at 18:13
  • $\begingroup$ @Ash I guess I'm confused why you would explicitly dismiss the primary answer to your question... There's no such thing as a secondary weapon that doesn't compromise primary mission capability in warship design. Payload is always a compromise, in ANY kind of military engineering. Saying "You can do this for free, why wouldn't you" isn't a rational question... $\endgroup$ – Morris The Cat Jul 9 at 18:18
  • $\begingroup$ You're assuming more mass is always a bad thing, and it almost is, but there are several proposed drive systems (none of which are currently more than extremely theoretical) that thrive on extra mass so it's not necessarily a valid argument, thus excluding it. $\endgroup$ – Ash Jul 9 at 18:24
  • $\begingroup$ "if the primary energy weapons systems on the warships are capable of performing the secondary planetary bombardment mission adequately" but they're not, that's the point of the question, you can't use an energy weapon for a precision strike at the bottom of the atmosphere, unless you can propose an energy weapon that won't scatter disastrously going through the whole atmosphere of an earth-like world. $\endgroup$ – Ash Jul 9 at 18:32
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    $\begingroup$ If your question assumes that energy weapons CANNOT be used for that mission, you should have said so in your question. I have to say I'm confused now what the point of your question is, it sounds like you had already decided what the answer should be when you wrote it. What exactly are you looking for here? $\endgroup$ – Morris The Cat Jul 9 at 18:40
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Particle beams are a nice clean way to eradicate life forms.

Neutrons are uncharged particles and so will not be blocked by a planetary electromagnetic field or shields relying on that sort of thing. This is true for heavier particles too. In this question What subatomic particle is best for a particle accelerator gun? I figured that radon particles would be the best for delivering energy that would penetrate intervening matter (here atmosphere) and drop off the energy within a calculable area (where your enemies and their pets live).

The neutron bomb claimed some fame because this type of radiation would supposedly kill everything but leave buildings, weapons, sports cars intact. Rods from god do not leave infrastructure intact. If you want to reclaim your holy sites, or take over the phlogiston mines, or drive around in sweet enemy sports cars you can't turn them into craters. Uncharged particle radiation would be a fine way to eradicate life from the area. A beam instead of a bomb also reduces residual contamination from radionuclides generated in the explosion.

Note: you might need to give the particles time to work. They probably will not kill everyone outright but should within the next couple of weeks.

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    $\begingroup$ You can't use neutrons in a particle beam weapon though... physics.stackexchange.com/questions/136018/… $\endgroup$ – Morris The Cat Jul 9 at 17:16
  • $\begingroup$ @MorrisTheCat - yes, you would need new tech for a neutron ray. I wanted to invoke the neutron bomb thing in the context of particle beams. Radon would be a good particle for a beam. Accelerating uncharged particles is also a little tricky but less tricky than generating bare neutrons. $\endgroup$ – Willk Jul 9 at 17:25
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    $\begingroup$ Yeah neutron radiation causes secondary radiogenic cascade, Neutron Bombs actually destroy nearly as much infrastructure as other nukes they just do it far slower. Um how do you fire a particle beam, especially large neutralised particles, into the bottom of an atmosphere without dispersion scattering the beam disastrously? $\endgroup$ – Ash Jul 9 at 17:40
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    $\begingroup$ @Ash I am glad you asked. You would take a page from the lightning playbook and superheat the air along your path to plasma using a laser. Once the laser is going you fire your particle beam down the same path while the density of gas molecules in the way is low. The particle beam would heat up the air further (to glowing) and cause a thunderclap. Good if you needed to depict this with special effects. You listening, SyFy channel? $\endgroup$ – Willk Jul 9 at 20:13
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Kinetic weapons can be intercepted.

If you have lasers capable enough to engage in ship-to-ship warfare and punch through atmospheres to destroy ground installations then surely those ground installations have lasers capable of destroying falling rocks. What would be the point of constructing planetary defenses if they weren’t capable, at the very least, of defending against the most effective and economical method of attack? Lasers and particle beams, on the other hand, cannot be actively intercepted.

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    $\begingroup$ shooting a falling rock with a laser does not do much, unless you can vaporize a significant portion of the rock instantaneously, in which case you are using orders of magnitude more energy than throwing the rock took. lasers are a lot easier to intercept, all you need is an opaque cloud of particles. $\endgroup$ – John Jul 9 at 22:38
  • $\begingroup$ @John The question posits lasers powerful enough to destroy ground infrastructure from orbit. Ablating a significant portion of a kinetic round seems like a small feat in comparison. The energy asymmetry isn't as significant as you suggest. My laser uses more energy than your rock in the moment, but you had to bring your rock to my world in the first place. Defensive counter-measures against lasers are not so simple. Your cloud is temporary and must be continually replenished and only makes the laser less efficient. Laser fire would ablate the cloud. It will just take longer to get through. $\endgroup$ – Mike Nichols Jul 9 at 22:54
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    $\begingroup$ But given that kind of power they can throw a larger mass much faster, given a mass driver and a laser of the same energy input the mass driver will win in direct opposition. $\endgroup$ – John Jul 10 at 4:04
  • $\begingroup$ @John Absolutely, but then we rapidly leave the domain of precision bombardment intended to take out ground installations and enter the realm of Dinosaur Killers. This leaves room for lasers as a more precise alternative. $\endgroup$ – Mike Nichols Jul 10 at 4:27
  • $\begingroup$ Not really the energy used to deliver one dinosaur killer can deliver hundreds if not thousands of precision kinetics, but you need a dinosaur killer powerplant to stop each one of those kinetics. The mass driver has orders of magnitude better energy efficiency. $\endgroup$ – John Jul 10 at 4:46

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