Thoughts on planetary combat

[Lord Revan]

I think fixed fortifications would a good analogy for planetary weapons, in modern days fixed forts are obsolite due to much more mobile targets (including but not limited to "man-portable" AT-weapons) being able to destroy them with relative ease. But if the "armor" in the "forts" was strong enough to take those weapons the whole scenario would alot different.

[Formless]

Firing millions of weapons at a single ship multiple AUs away? Don't make me laugh; if that's necessary than you're bound to lose in an even-strength situation, because the enemy can spam enough ships to overwhelm your defenses. Not to mention the fact that he can simply use a planet or moon for cover.

Millions of weapons for a measly few targets is over the top. However, a planet can still probably afford to outgun most fleets on just the resources on the planet itself, not to mention the surrounding resources. Plus the ability to throw guided nuclear missiles lowers the number of guns you have to point at any given target to make it go away.

If you're trying to defend a planet and you don't have crazy-powerful planetary shield technology like Star Wars, your best solution is an old solution: use constant patrols and scouts in order to identify and attack enemy ships before they can attack you. Drone swarms, for example. Trying to fight them off with weapons limited to the planet and its immediate area is madness.

Personally, I see the planet bound weapons as a supplement rather than a replacement for space bound defenses. Rather than using them as weapons of last resort for a planet whose defense fleets are gone, the planet would use its large weapons to proactively neutralize targets. Not only can they have lots of weapons, but they can also have much LARGER weapons thanks to having the room and heat sink to install them, such as free electron x-ray lasers that can rad kill targets from light hours away.

Basically, a planet should be thought of as a MASSIVE spacecraft with both military and civilian sections, tons of guns, tons of stored resources, a few special properties (eg. weather), but no engines to speak of.

[Stark]

Is it worth spending resources and time building planetbound defences instead of more ships to provide defence in depth? A credible local defence may be so difficult to build it's not worth trying (if for instance less than 'millions of missiles' is a waste of time, that's a big drone swarm on perimeter).

[Ford Prefect]

Is it worth spending resources and time building planetbound defences instead of more ships to provide defence in depth?

Personally, I would think not. Ultimately you must be able to police more of a solar system than just your favourite planet, otherwise an enemy could exploit the bountiful resources of gas giants and their moon systems.

[PeZook]

Yeah, that's what occured to me, too: what if the enemy simply ignores your planet and builds up an orbital industry?

And if you can churn out millions of missiles, add some basic AI to them for minimal cost and turn them into giant drone minefields constantly orbiting around likely entry points, to be activated when an enemy fleet tries to breach the system.

Though all in all, general rules for this kind of combat will be few and far between, since sci-fi is so varied. What if the enemy can jump into the atmosphere at will, nBSG style? Does defence in depth make any sense when all you need to do in order to genocide an enemy is build a suicide ship loaded with a continent-cracking nuke?

We'd be better served by categorizing sci-fi technologies by their game-changing nature and then trying to come up with some basic rules for each of them.

[Samuel]

But then an enemy can potentially cut you off from your supply.

Not if the bases was self-sufficient. Once you build the population up into the millions there is no reason not to be. Otherwise you have to import from planets and that is expensive.

And if most of the population is on the planet, putting industry in space would increase not decrease costs; and that is the kind of population distribution that planetary shields could easily encourage. W

You can't build as effective spaceships on the surface of a world.

And if the planet calls their bluff? If the goal is conquering the population, then killing off the population isn't an option. For that matter, they could threaten planetary suicide; use nukes/bioweapons/nanoweapons/whatever to render the planet useless if the enemy fleet actually does something genocidal like that. After all, at that point what do they have to lose?

Don't bluff? And if the planetary leaders are willing to destroy their planet and kill the population in order to deny it to you, broadcast that information to the populance.

Oceans, glaciers, the air; all of those are far better heat sinks than space.

I know- my point was this would mess up the planets climate. If you pump enough heat into the world you can completely mess up the biosphere. Even with indefinate shielding there comes to be a point where it just doesn't work anymore.

Well; with a planet's resources you could be talking about millions of weapons. And if we are talking about energy weapons you can arrange the beam to spread quite a bit; you don't need to hit with enough concentrated power to burn a hole in the ship. You just need to overheat them.

Than shoot from farther away or hide behind worlds between attacks.

It's also a way to get them to show up at your worlds and start making genocidal threats in turn. Or just killing your planets right off.

So? Surrender or die is perfectly legitamate. I don't see why they would start killing your planets- after all, if there is no option to surrender their responce just insured you fight to the death.

So are all the mass destruction tactics you've been talking about.

No, shielding military targets with civilians is explicatly a war crime. Bombarding the resources an enemy needs to survive is not a war crime. If targeting their farms counts than we start bombarding power planets instead although that would probably kill more people.

I think you have matters reversed. The planet will likely see the attacking fleet coming, while the ships won't know where the planetary weapons even are until their fire arrives. As is often pointed out, stealth is if not actually impossible a lot harder in space.

No, the planet will see where the attackers were- we are talking about incredibly large distance here.

And if they are firing missiles, random motion isn't likely to do much at all.

Except you can shoot them down and if you are far enough away you have a long time to do so.

If they are located underground, they'd be quite hard to find and identify from space.

Than we start bombing highways, railyards, transport ships, etc. Factories are nothing without supplies.

On a planetary surface there's all sorts of heat sources.

Yes, but if you have the power grid redirected to your guns whenever they fire, they stand out. It depends on the difference in heat emissions from these defenses compared to civilian power sources.

However, a planet can still probably afford to outgun most fleets on just the resources on the planet itself, not to mention the surrounding resources. Plus the ability to throw guided nuclear missiles lowers the number of guns you have to point at any given target to make it go away.

Why? Fleets can be built with the resources of entire star systems. As for countering using nukes, what makes you think they will get past point defenses?

We'd be better served by categorizing sci-fi technologies by their game-changing nature and then trying to come up with some basic rules for each of them.

Lets see...

Shields- if you don't have them raids against worlds are easy.
Type of FTL- ones with fixed points lead to choke points, ones that can go anywhere allow you to get under shields.
Heat removal technologies (do you have a magic alternative to heat sinks?)- if you have these planets ability to soak up heat becomes less relevant. On the otherhand it allows worlds to take truely enormous amounts of punishment.
Power generation levels- if your guns can burn through asteroids than using them to hide behind doesn't work.
Transporters- set up relay stations and you can invade worlds in another solar system. Stargates make this easier- mandatory if you don't have spaceships.

What else am I missing?

[Oskuro]

Not really; you can bombard them from really far away. The planet can't maneuver or hide behind something, while you can do both. The only way to deal with this is to have defense weapons that can destroy incoming projectiles and shields of some sort which can block incoming weapons. And the shields have to be ridiculously strong.

Or you can build Wolrd Engines.

Now, seriously, the only reason to send in troops, be it to a planet, or to an station, is if you have some McGuffin motivation that forces you to do so, like shutting down defense systems, rescuing hostages, stealing the station, or dropping a torpedo down a ventilation shaft. I tend to value sci-fi that bothers to give a reason for unreasonable behaviour, rather than just do it because it's cool.

[Darth Wong]

Now, seriously, the only reason to send in troops, be it to a planet, or to an station, is if you have some McGuffin motivation that forces you to do so, like shutting down defense systems, rescuing hostages, stealing the station, or dropping a torpedo down a ventilation shaft. I tend to value sci-fi that bothers to give a reason for unreasonable behaviour, rather than just do it because it's cool.

What if it's one of your own colonies and it's suffering severe civil unrest? You send in troops to quell the unrest, and they have to stay on a semi-permanent basis to maintain order. That's not a MacGuffin motivation, and it actually seems realistic. What if you're suppressing a rebellion? What if you want control of a planet's natural resources? You wouldn't have to blanket the entire planet with troops, but you would need to provide security around specific facilities.

[Knife]

I think attacking and invading something like a whole planet is so far outside any frame of reference we have (as humans) that it is hard to make our analogies fit. I mean, sure if your invasion of the planet is to capture the half dozen population centers on the small sub continent, then yeah we can comprehend that; but to make WWII analogies to invading a populated planet is just... well staggering.

You'd need hundreds of millions of troops, if not a couple hundred billion. If you need that many troops, you might as well have millions of ships to get them there, overwhelming defending weapons ability to kill them all before they land. That said, once on the ground, the troops would need immediate coverage from orbit or ground based defenses. What is the use in landing a million troops on a plain if the enemy can fly a bomber over you and wipe you out with a nuke? Even spreading out the 'beach head' will have millions of troops concentrated enough that WMD would put huge dents in manpower, all done with easily deployed weapons that don't need huge, fixed installations to field that can be targeted from orbit.

[MKSheppard]

I award this thread the Medal of AWESOME. And thanks for reminding me of Star Legions.

There's also another reason for invading a planet -- it's part of the campaign to seize the solar system it's in.

AS long as the planet isn't neutralized, it's a thorn in your backside from which the enemy can sally forth to threaten your hold of the system, unless you either

1.) Station a lot of units over the planet
or
2.) Bombard it to the stone age.

[Gil Hamilton]

I've heard the school of thought that states the Shep Solution is the inevitable conclusion to any sort of interstellar or interplanetary battle, because with reasonably realistic technology, actually invading another planet or moon from yours would be a sufficiently expensive endeavour to build enough ships to transport troops and material that fighting any war less than a nuclear one isn't worth it. If a war is to be fought at all, it is for all the marbles because interstellar war is so costly anyway that fighting one for lesser reasons isn't cost effective.

[MKSheppard]

I've heard the school of thought that states the Shep Solution is the inevitable conclusion to any sort of interstellar or interplanetary battle, because with reasonably realistic technology, actually invading another planet or moon from yours would be a sufficiently expensive endeavour to build enough ships to transport troops and material that fighting any war less than a nuclear one isn't worth it.

Consider from the point of view of a human being in 1900 if you told them that amongst the battlefields of the future would be one nearly four miles high; in which 10,000 men in nearly a thousand or more aircraft (each one costing a stupendous $50 to $100 grand each ) battle for control of the air.

Then tell them about 4.5% or so of those aircraft don't make it back from each mission.

It would be inconceivable.

[Stark]

And that tells us... what?

Anyway, in most settings nuking the site from orbit is always going to be an option in war, just like it is now. I hardly see how it would be necessary, when once you have control of orbit you can totally interdict all their starports, but if someone was losing they might as well drop a few thousand bombs before they retreat... unless it was in an actual setting and not a thought experiment where there are a couple of good reasons why not.

[MKSheppard]

And that tells us... what?

Considering that in 1884, Aluminum (the primary component in aircraft manufacture in WWII) was as expensive as silver; and that Petroleum production was only just taking off in the 1900s (consider how much high octane aviation gasoline you need to run a bomber fleet); and it shows how what was considered inconceivable becomes commonplace.

Right now, we have the technical ability to zip around the inner solar system in reasonable amounts of time -- the big problem is that there is no industrial infrastructure to support this technical ability -- e.g. you can get to Mars a lot shorter than the current minimum energy orbit; it's just that you will need a large industrial infrastructure near mars to replenish your propellant reserves once you get there.

[GrandMasterTerwynn]

I've given some thought to a somewhat controversial aspect of sci-fi warfare, that of planetary combat. Different sci-fi polities do it to differing degrees, but the main controversy is whether or not planetary combat is actually worth doing. Why go to the trouble, one may ask, when even a basic interstellar travel capability would allow for devastating bombardment from space, by redirecting asteriods if nothing else (yes, I have sat in the blue chair).

In a hard-ish sci-fi universe, invading planets isn't worth the trouble because only Luddites, convicts, other non-contributors or invalids, and eccentric people live on them.

The simple answer is that one invades a planet because one wants the planet intact for some reason. You can trash it from orbit if you want, but the best you'll get is a pile of rubble (or a sea of molten lava in some cases).

Not true. You could severely affect a heavily technological world simply by shooting down all its satellites. No more GPS. No more satellite telecommunications. If that level of chaos isn't enough, you could conduct pinpoint orbital strikes against launch facilities and major railway, small vehicle, and air vehicle hubs. Then you spam low planetary orbit with lots of mines, or a sufficiently thick cloud of debris. Then you can safely ignore the planet until you've captured everything worth capturing (i.e. the orbital/deep space infrastructure.) If, for some reason, everything of merit is on the planet, then you can laugh at them from high-orbit while you begin construction of your own deep space infrastructure. If the important political figures that you'd like to capture have taken refuge on the planet, then you simply start threatening to commit various and sundry kilo- or megadeaths, or things which could result in kilo- or megadeaths, like turning the agricultural regions into cratered wastelands, until the planetary authorities hand them over. To hold a starsystem, it's not necessary to hold the planets.

Capturing a planet intact allows the faction in question to capture at least some of the planet's infrastructure and resources intact, making it a boon to their war effort.

Boon? Planets are boondoggles. A habitable one requires many tens of megajoules per kilogram exported worth of energy expenditure just to get things off them.

Capturing a space station will be no simple matter, depending on its size, resilience, and purpose, with different varieties posing different problems. A space station built with real-life technology would be rather flimsy, and a battle going on inside would almost certainly result in its destruction.

Space stations will be easy to capture, since they tend to have miniscule delta-Vs and, thus, can't dodge, and can't impart any starting velocity (beyond whatever the station's orbital velocity happens to be) to whatever missiles they might have onboard. Assuming the station isn't crewed by would-be Jihadist martyrs, capturing it will be as simple as threatening to poke holes in the station's vital bits until the occupants surrender.

A large and fairly sturdy space station built by an experienced space-faring civilization, such as DS9, a Golan-2 (Star Wars) or a Ramillies (WH40k) would not face this particular problem. The problem instead would be actually capturing the station.

A softer sci-fi station suffers much the same problem as a harder one. It has no delta-V to affect real course changes with. If it's got lots of reactor capacity and bunkerage, an invading fleet will simply make it expend it all shooting down mass-driver accelerated projectiles and the odd anti-ship missile, or reduce its shields through long-range bombardment. Then we're back to the whole threatening to shoot holes in the vital bits thing. Or else, you launch a swarm of missiles at it from long range, and give them the option of either evacuating, and having the missiles go inert . . . or staying and being converted into a large cloud of incandescent gas when the missiles arrive a week from now.

With enemy starships and orbitals out of the way, the threat comes primarily from the ground (outside reinforcements aside). This comes in the form of ground-based weaponry and small starships (one of the few realistic applications for starfighters). The likely response depends on the extent and capability of the weapons. If the ground-based defences are sufficiently powerful to seriously threaten even the attacker's warships, then bombarding them would be ill-advised.

If the planet's using that tremendous heat-sink capacity to adequately cool weapons that could threaten starships, then you simply drop the missiles from higher up, and drop more of them.

If theatre or planetary shields enter the equation, such as in Star Wars, then bombardment would be completely ineffective, at least if Echo Base's shield is anything to go by.

Incorrect. Assuming you're not talking about magical invincible shields, the shields will have a certain amount of sink capacity, or immediate energy-handling capacity that can be defeated with a bombardment delivering enough energy. Why else do you think they invented Death Stars?

Captured orbitals, if in any workable condition, can be of help in this respect. The top end of a Space Elevator would be a particular boon, so long as the bottom end can be secured and the enemy can be prevented from destroying it. Warships can provide fire support and telemetry, while also ensuring that enemy starships do not interfere.

You don't begin thinking about capturing an enemy planet until after you've completely taken control of the host system. Planets are easy to isolate and ignore until after you've captured everything else of consequence and hold the system firmly enough to begin moving in the tens or hundreds of millions of soldiers needed for the occupation effort.

But even this stage has its complications. Unless the attacker's civilization is capable of mass-teleportation, then the transports must still get the necessary assets down to the surface, either by landing themselves or by sending down smaller vessels.

By the time you start thinking of troop landings, the planet's either already surrendered, or the place has been Shep-Solutioned into submission. The only worlds where you might consider landing troops on before the surrender of the planetary government are worlds where the population is tiny to begin with, and the military infrastructure needed to shoot down your troop transports is all located across the planet from where you're actually planning on landing.

However, this could lead to complications if enemy warships are abroad. A concentrated landing would be preferable in this case, since it limits the amount of orbital space the attacking warships must secure and defend. This refers to large-scale invasion landings, not small-scale raids, which could be carried out at the same time.

If enemy warships have turned up, then that means you fucked up somewhere. At this point, you'd resume ignoring the planet and turn your attention to repelling the new enemy attack. The only soldiers you'd be landing on a planet while the space around it is still contested would be Special Ops people whose job it would be to sow Fear, Uncertainty, and Doubt, while capturing or assassinating whatever enemy Important Persons are hiding on the planet.

Once on the ground, the area under the attacking forces' control effectively becomes a 'state', which intends to conquer the rest of the planet. This 'state' is not merely a glorified spaceport, but also part of the attacker's infrastructure.

Why do it piecemeal this way? A drawn out ground war of planetary conquest is good for your enemy, because it gives him time to gather a fleet together to chase you out. You don't want to give him time to do that. Ergo, you will simply lock the planet's occupants onto the planet and ignore them, or Shep Solution them into surrendering, or not being around to resist anymore.

Different sci-fi universes handle planetary combat in different ways, while some either minimize its role or ignore it altogether. The most likely reason why some sci-fi polities choose not to engage in planetary combat is the difficulty of creating, transporting, and supplying the necessary planetary assets, along with the difficulties I have described. Real-life Earth, for example, has a population of over six billion. Its active military personnel number over ten million, out of over two billion suitable for military service.

Here's how to invade and conquer modern-day Earth.
Step One: Destroy all the satellites.
Step Two: Mine low-Earth orbit so we can't launch more.
Step Three: Since Earth doesn't have an effective central planetary government that you can extract a trustworthy surrender from, drop a meteorite on Australia and demand that Earth comes up with one. The blast should be large enough to send the planet into an impact autumn, giving the local militaries something to do.
Step Four: Insert Special Ops forces onto the planet to capture human beings to make sleeper agents, or to mindrip to extract the movements of notable political figures from.
Step Five: Help Earth along in its planetary unification by dropping meteorites on the heads of/using native guerillas to assassinate especially recalcitrant national political figures.
Step Six: Accept the surrender of Earth's planetary government. Hopefully you've colonized the rest of the Solar System by this time, and have moved in the 100 million soldiers needed to ensure Earth plays fair.

[Stark]

Considering that in 1884, Aluminum (the primary component in aircraft manufacture in WWII) was as expensive as silver; and that Petroleum production was only just taking off in the 1900s (consider how much high octane aviation gasoline you need to run a bomber fleet); and it shows how what was considered inconceivable becomes commonplace.

Amazing! How is this relevant to the discussion?

Right now, we have the technical ability to zip around the inner solar system in reasonable amounts of time -- the big problem is that there is no industrial infrastructure to support this technical ability -- e.g. you can get to Mars a lot shorter than the current minimum energy orbit; it's just that you will need a large industrial infrastructure near mars to replenish your propellant reserves once you get there.

So you're saying it depends on the setting, just like everyone else?

[MKSheppard]

Amazing! How is this relevant to the discussion?

It shows how the feasability of something changes with the technology/infrastructure available. What's wildly unfeasible becomes feasible with the march of technology.

For example, waging a war on say, the Mooooooon (TM) wouldn't be workable with the present level of technology. But if we invested in something like the Rockwell Star Raker concept from 1979, then costs to orbit would become cheap enough to at least support a limited level of warfare on the moon.

[Stark]

No, don't say it again; how is it relevant. Saying 'once upon a time, expectations fell short of reality' doesn't mean fuck all about this situation unless you can show that this is always true. If you attempt to do so, I'll just pull up 50s-60s attitudes toward the amazing rocket-future of the 90s. It all comes down to what nonsense the 'world building' author decides to throw out.

In your example are you talking about combat on the lunar surface (Space Vietnam) or combat between spacecraft in lunar orbit?

[Samuel]

Here's how to invade and conquer modern-day Earth.
Step One: Destroy all the satellites.
Step Two: Mine low-Earth orbit so we can't launch more.
Step Three: Since Earth doesn't have an effective central planetary government that you can extract a trustworthy surrender from, drop a meteorite on Australia and demand that Earth comes up with one. The blast should be large enough to send the planet into an impact autumn, giving the local militaries something to do.
Step Four: Insert Special Ops forces onto the planet to capture human beings to make sleeper agents, or to mindrip to extract the movements of notable political figures from.
Step Five: Help Earth along in its planetary unification by dropping meteorites on the heads of/using native guerillas to assassinate especially recalcitrant national political figures.
Step Six: Accept the surrender of Earth's planetary government. Hopefully you've colonized the rest of the Solar System by this time, and have moved in the 100 million soldiers needed to ensure Earth plays fair.

Eh, that is too expensive. Just park a warship with an extinction level asteroid in Lunar orbit and demand Earth's surrender. I don't see why you need to bring in troops- just let them know that if they kill the orbiting warship a fleet will pop in and make their species extinct.

[MKSheppard]

In a hard-ish sci-fi universe, invading planets isn't worth the trouble because only Luddites, convicts, other non-contributors or invalids, and eccentric people live on them.

You need planets for large scale industrial production. Simply producing steel requires a LOT of oxygen; and many of the techniques we use to purify steel and other substances use gravity as a way of separating the impurities out. (thanks for that revelation Mike)

Producing the kilotonnage quantities of basic steel required for a modern economy would remain the province of planetary industrial facilities; with lunar facilities or orbital facilities providing specialist steels for specific applications.

I suppose exo-atmospheric steel production can be done by using captured ice comets for oxygen production, but a better use for that H2O would be for drinking water or breathing water.

Not true. You could severely affect a heavily technological world simply by shooting down all its satellites.

And then watch as they put new satellites into orbit. I know the US Military has been looking seriously into micro satellites that could be put into low earth orbit by battlefield mobile boosters (something a bit smaller than a SCUD); for precisely this reason -- our current satellite network is vunerable to ASATS; which any sane enemy would use in a future war with the US.

If that level of chaos isn't enough, you could conduct pinpoint orbital strikes against launch facilities and major railway, small vehicle, and air vehicle hubs.

And where will the planetary defenses be during this?

Then you spam low planetary orbit with lots of mines, or a sufficiently thick cloud of debris.

Low Planetary Orbit gets cleared of debris routinely, due to atmospheric drag that low. Unless you put effort into maintaining it, your minefield of mines/debris gets cleared out within a few months by drag.

If, for some reason, everything of merit is on the planet, then you can laugh at them from high-orbit while you begin construction of your own deep space infrastructure.

You won't be laughing when they hull your ships using heavy planetary lasers that have heatsinks driven right into the bedrock of the planet so they can keep firing as long as they have the juice to do so.

To hold a starsystem, it's not necessary to hold the planets.

Actually it is. Because the planet is a natural fortification that enemy forces can sally forth to harass your forces and contest control over the system. This isn't like Rabaul in WWII, where we could safely bypass it -- because planets are basically self sufficient in food and raw materials production (minus some really exotic stuff that may be found only in space cheaply).

Assuming the station isn't crewed by would-be Jihadist martyrs, capturing it will be as simple as threatening to poke holes in the station's vital bits until the occupants surrender.

A civilian station, sure; but what of a military station with heavy armor, weapons, and the heat sinks to keep those weapons firing? Simply making military stations double hulled, with the space between the inner and outer hull full of water gives you a lot of armor and heat sinks, and unlike a military spacecraft, you don't have to haul that mass around anywhere.

If it's got lots of reactor capacity and bunkerage, an invading fleet will simply make it expend it all shooting down mass-driver accelerated projectiles and the odd anti-ship missile, or reduce its shields through long-range bombardment.

Or the station simply uses heavy energy weapons to pot the missiles and projectiles at long range.

Simply pointing a laser sufficiently strong enough at a incoming mass driver projectile will cause that projectile's material to sublimate, and become a gas, and ironically, provide thrust that pushes the mass driver projectile off course enough that it misses the station by a large margin.

If the planet's using that tremendous heat-sink capacity to adequately cool weapons that could threaten starships, then you simply drop the missiles from higher up, and drop more of them.

And how will the missiles survive against an energy weapon network powerful enough to cause heartburn to starships?

[MKSheppard]

Cheap, powerful energy weapons really do complicate the offense -- imagine how bad things will be once the US perfects a battlefield portable solid state laser; and the French then design a similarily comparable system, export it worldwide, and then the Chinese begin offering a clone of that French system a bit later?

Aircraft would be driven out of the lower atmosphere, and artillery barriages would become useless, unless you planned them like an airstrike today -- the first 500 shells you fire against that position are dumb unguided shells to soak up the enemy's laser batteries, so that the 50 guided shells you fired manage to hit that command post...

[Oskuro]

What if it's one of your own colonies and it's suffering severe civil unrest? You send in troops to quell the unrest, and they have to stay on a semi-permanent basis to maintain order. That's not a MacGuffin motivation, and it actually seems realistic. What if you're suppressing a rebellion? What if you want control of a planet's natural resources? You wouldn't have to blanket the entire planet with troops, but you would need to provide security around specific facilities.

Well, that's what I meant, really, there must be a motivation to the landing of troops beyond mere defeat of the target. My understanding of a McGuffin is that of an element used to justify the plot (be it rational or silly), and your examples do fit the idea of special circumstances that make troop landing desirable, for all other alternatives, there's nukes.

[Sea Skimmer]

Is it worth spending resources and time building planetbound defences instead of more ships to provide defence in depth? A credible local defence may be so difficult to build it's not worth trying (if for instance less than 'millions of missiles' is a waste of time, that's a big drone swarm on perimeter).

Something to consider, in 1940 the US found that it could upgrade its entire coastal artillery system, building some 200 new batteries (not all completed) to defend 27 ports and anchorages, for less money then the price of one new battleship. Each of the resulting port defence systems meanwhile was strong enough to repel an attack by a whole enemy battle fleet. Obviously a single extra battleship would not have provided anything like equivalent defence.

Fixed defenses are also desirable for the simple reason that they don’t move. That means you don’t have to worry about someone ordering them off to another system on some ill planned operation and leaving the planet completely open to attack. Because of this factor alone some level of fixed defence would exist, even if it cannot defeat much more then an enemy armed merchant ship trying to drop propaganda leftlets from orbit.

I think fixed fortifications would a good analogy for planetary weapons, in modern days fixed forts are obsolite due to much more mobile targets (including but not limited to "man-portable" AT-weapons) being able to destroy them with relative ease. But if the "armor" in the "forts" was strong enough to take those weapons the whole scenario would alot different.

The whole fixed fortification are obsolete thing is not really true. They’ve certainly become less beneficial and aren’t so great for use as fighting positions as opposed to shelter positions, but they aren’t even close to being obsolete. Just look no further then all the concrete walls around US bases in Iraq, or ICBM silo spam or the Korean DMZ with its 60 foot thick anti tank walls or Iran’s underground nuclear enrichment plant. Future weapons developments will yet further reduce the protection you can gain from concrete and earth, but on the other hand they’ll also open the way to use of other materials, and the growing wealth of civilization will make those more advanced designs (like imagine a bunker with a layer of DU inside of the concrete) practical to build.

Even if a bunker can be easily destroyed, it still is a point target, and the enemy will always have a harder time destroying a number of point targets then he will have destroying an area target. Certain kinds of smart weapons are also surprisingly easy to defeat too. The Swedes for example found they could make all their turreted coastal artillery positions immune to laser guided bomb attacks by installing a simple water spray system. This also blocked many kinds of IR and optical guided weapons.

The growing lethality of weapons and use of things like hoards of drones will also make secure command and control functions more important then ever, and those functions are certainly going to seek shelter in very deep bunkers.

[Samuel]

You need planets for large scale industrial production. Simply producing steel requires a LOT of oxygen; and many of the techniques we use to purify steel and other substances use gravity as a way of separating the impurities out. (thanks for that revelation Mike)

Producing the kilotonnage quantities of basic steel required for a modern economy would remain the province of planetary industrial facilities; with lunar facilities or orbital facilities providing specialist steels for specific applications.

You can fake gravity in space by rotating the station, but air would be trickier. If the cost of oxygen is higher than the cost of export per ton of steel than the steelworks willl be planetside- otherwise they will be produced in space.

I suppose exo-atmospheric steel production can be done by using captured ice comets for oxygen production, but a better use for that H2O would be for drinking water or breathing water.

Technically this isn't mutually exclusive- you just need a reserve of oxygen for steel production. It isn't like it is used up- you can have plants or scrubbers return it to CO2. Of course that adds more costs- the cost of having excess oxygen capacity to provide the steel works.

[Stark]

Something to consider, in 1940 the US found that it could upgrade its entire coastal artillery system, building some 200 new batteries (not all completed) to defend 27 ports and anchorages, for less money then the price of one new battleship. Each of the resulting port defence systems meanwhile was strong enough to repel an attack by a whole enemy battle fleet. Obviously a single extra battleship would not have provided anything like equivalent defence.

Fixed defenses are also desirable for the simple reason that they don’t move. That means you don’t have to worry about someone ordering them off to another system on some ill planned operation and leaving the planet completely open to attack. Because of this factor alone some level of fixed defence would exist, even if it cannot defeat much more then an enemy armed merchant ship trying to drop propaganda leftlets from orbit.

I know we all love WW2 trivia, but how effective would those batteries have been without air superiority?

Obviously some fixed defences would exist (if only to stop navigational errors turning into extinction events) but the idea that planets would invest defence to the 'millions of missiles' level when it's more important to keep the enemy from getting anywhere near the planet in the first place or intercept anything fired at it. Of course, this is scenario-dependent, and I think the best scifi example of 'coast defence batteries' are probably orbital weapon platforms and not ground-launched missiles.