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The title is intentionally provocative, but let me try to justify it.

Let's take a common science fiction setup of two interplanetary civilizations with a hand-waved FTL technology that are at war (something involving either warp-bubble or hyperspace so that FTL travel doesn't involve relativistic speeds which causes even worse narrative problems for space war). Nearly all of their population live on planets or moons, with a small fraction living on space ships or space stations. Naturally, warfare focuses on the planets.

In common science fiction, an attack on a planet is done through roughly the process that Twilight Imperium uses to resolve combat:

  1. Have a space battle in around the planet to establish orbital superiority
  2. (Optional) bombard military forces on the planet
  3. Send down ground troops

Every now and then, you have a faction construct a superweapon and they will blow up a planet with it to show off their military might.

But wait! Planets are at the bottoms of huge gravity wells. You don't need to shatter the planet into fragments to effectively destroy it. You just need to drop enough stuff into it's atmosphere to render it uninhabitable.

For example, the entirely of humanity's current nuclear arsenal (2022) has an estimated yield of 3 gigatons of TNT. The 10km asteroid that made the Chicxulub crater (and likely killed the dinosaurs) had an impact energy of 100,000 gigatonnes of TNT. Even a 2km or 3km asteroid could do the trick, and at that point, we're looking at the likely mass of some of the larger scale spaceships in science fiction.

But this leaves us in a predicament. If you have such large spaceships, you don't even need to establish orbital superiority to blow up a planet. First, you can start towing big asteroids into collision courses with the target planets. If they have defenses for massive asteroids, just show up in orbit with one of your super massive battlecruisers; if you're outnumbered in battle, you can crash it into the planet. If your enemy blows it into pieces, then the pieces still crash into the planet and impart a similar amount of kinetic energy.

Given this, it seems like it is much easier to destroy a planet than to assault it (a similar argument is presented in this video). There's a good discussion for why a faction would want to assault a planet rather than just blow it up here, but it seems to me that logic would only apply to the faction that is stronger/winning and has the resources to establish orbital superiority and assault the planets of the other. The faction that's weaker/losing just wants to not lose, which they could achieve by striking at the industrial/population/economic centers of the other faction by blowing up their planets.

So, given this setup, how does warfare not lead to the total annihilation of both sides? I see two potential angles here:

  1. There is some conceivable technology that makes blowing up planets hard and this starts to look more like a "conventional" space war. In this case, what is it (and why is there not also some conceivable technology that beats it)?
  2. One faction will get blown up but not the other. In this case, at the point at which one faction has started smashing planets, why does the other not respond in kind?

Some answers I want to rule out:

  • Mutually assured destruction doesn't help if war has already broken out.

  • A space Geneva convention requires some sort of international community working together such that there are other parties present to prosecute violators. It doesn't work if there are just two civilizations.

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    $\begingroup$ The goal of a war is rarely the destruction of the other side. The US had troops in Afghanistan for a decade. They didn't destroy Afghanistan. Russia is attacking the Ukraine right now. But aren't trying to annihilate it. What is the goal of the wars you're talking about? Is it destruction? Or is it not? Because that will probably hint to an answer to your question. $\endgroup$
    – VLAZ
    Sep 24, 2022 at 9:24
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    $\begingroup$ I think you need to step back and reevaluate the premise of your question: why would an interstellar/interplanetary species ever embark upon a path of "Warfare"? Excepting weird casus belli like a religious drive to indiscriminately purge all xeno scum, a policy of annihilation would actually be counterproductive. I mean, traditionally wars are fought over control of resources, and those resources aren't worth jack if they are all atomized into radioactive particles. The goal of most military forces is not to destroy the enemy, it is to destroy the enemy's capability of fighting back. $\endgroup$
    – Dragongeek
    Sep 24, 2022 at 14:29
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    $\begingroup$ Never mind how many gigatons of TNT equivalent you have; if you can accelerate things to near-light-speed, then the kinetic energy of pretty much any projectile could blow the whole planet to smithereens, as long as you can aim accurately enough. $\endgroup$
    – kaya3
    Sep 25, 2022 at 2:45
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    $\begingroup$ @kaya3 Yes, that's why I specified warp bubble or hyperspace as the method of FTL, because otherwise anyone with an interstellar spaceship can blow up a planet. $\endgroup$
    – Zags
    Sep 25, 2022 at 3:20
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    $\begingroup$ @seldon In the Lensman series, that's how the good guys smash a fortress planet in the end. They take two planets with opposite velocities and warp them in either side of the target planet. $\endgroup$
    – user86462
    Sep 25, 2022 at 18:22

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You are assuming that the attacker is at the point in technological development where you can easily move the mass of 2km asteroids around (either an actual asteroid, or a ship with similar mass). But you are also assuming that the defender cannot easily move such a mass around. Otherwise, when you put an asteroid/ship on a collision course with a planet, they would simply nudge it off course again.

Remember: the sort of asteroid impact energy calculations you're talking about are assuming the asteroid has come from outside Earth's gravity well, and so will impact with at least Earth's escape velocity. But this means the asteroids came from interplanetary space, not a near orbit around Earth. You can take an existing asteroid orbiting the Sun and alter its orbit to intersect a planet's trajectory, but letting it passively follow that orbit will take months if not years.

If both attacker and defender have capability to easily play with the orbits of asteroids, then the attackers would need to escort their doomsday rock from interplanetary space most of the way down to ensure the defenders don't show up with their own asteroid tugs and easily moved it off target again. The attacker needs to keep it very precisely on course for months or years; the defender only needs to control the asteroid for a much smaller amount of time to make it miss. They might not even need to control the asteroid; the amount of energy needed to divert it by a small amount is much smaller than the amount of energy needed to make a large change to the asteroid's initial orbit to set up the collision in the first place. So if the much larger energies needed to attack like this are easily available to both civilisations, the defenders might be able to apply the smaller energies needed to deflect the asteroid at range (such as simply throwing smaller rocks at it faster).

After all, real world human technology is nearing the point where we could hope to reliably spot asteroids that might impact Earth and then intervene to make them miss. We're worried about it for dealing with extremely rare natural events, just in case. Any technological civilisation that has reached the point where FTL and exciting sci-fi space battles are serious possibilities almost certainly mastered asteroid deflection long ago. If, in their world, it is a much more likely possibility because their enemies might set it up deliberately, they will have put far more effort into counter strategies and technologies.

TLDR: if in your story's technological level it is a trivial matter to move large heavy spaceships between planets, then it is also not difficult to stop asteroids hitting your planets. To pull it off the attackers would need orbital superiority, for long periods of time (so if reinforcements can FTL in from other systems in days or weeks, they essentially need to have totally and utterly defeated the entire space navy of the defender).

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    $\begingroup$ This is really well thought out. This could even stop someone from crashing a supper-massive spaceship into the planet if you engage it far enough out that your tugs can get there in time. $\endgroup$
    – Zags
    Sep 26, 2022 at 3:09
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    $\begingroup$ Isn't early detection as hard a problem as deflecting it? The attackers also have as much time as they want to build up speed. If the defenders don't detect it before it enters the solar system, they have very limited time. $\endgroup$
    – Mark
    Sep 26, 2022 at 7:16
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    $\begingroup$ @Mark There's no reason the defenders would need to wait until it reaches their front door to react - they'd just need to position early warning posts (or scouts, or radar buoys, or whatever) far out enough that they see the threat coming with enough time to counter it. That's true whether you're talking hypervelocity asteroids, warfleets, or an army on the march - only the details will change based on setting $\endgroup$
    – Pingcode
    Sep 26, 2022 at 8:12
  • $\begingroup$ OP: "something involving either warp-bubble or hyperspace" - so spaceships can actually not move much faster than modern day ones, and certainly can't tow asteroids at near light speed. This further justifies the difficulty and length of time it takes to drag or divert an asteroid to Earth. $\endgroup$
    – komodosp
    Sep 26, 2022 at 8:32
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    $\begingroup$ @Pingcode I agree with Mark that there's a huge asymmetry here. Slowly accelerate 1000 rocks from the asteroid field or even further out, maybe over the course of several months. Paint the rocks black so they show up less in telescopes. The detection problem for the defender is huge, and they almost certainly have much shorter time to correct course than the attacker has to set the initial trajectory. To misquote Douglas Adams, space is unbelievably, mind-bogglingly huge. Effectively surveiling the solar system is impossible $\endgroup$ Sep 26, 2022 at 21:55
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Your sides don't want annihilation. They want victory.

Smashing the other side's planets is not victory. It's like going to steal a prized racehorse and killing it during the theft. You want the planets, as the thief wants the horse. That's the entire point of war.

Destroying planets would occasionally, perhaps, be necessary, but you don't want it. Conquest of other planets is, in fact, the only way to recover from the economic damage of going to war.

Plus, MAD can work either if other civilizations are out there, and all agree that a planet-buster requires all powers to unite against that one, or if the people of your own civilization think that busting planets is proof of insanity, and if your own underlings don't overthrow you, you (and the underlings) will be torn to pieces by the enraged mobs.

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    $\begingroup$ This is a reasonable line of reasoning for the aggressor, but what about the other side? If they have no real hope of capturing the aggressor's planets and they just want to not lose their own, smashing some of the aggressor's planets seems like a good way to go about it. If someone is trying to steal your prized racehorse, burn down their stable. $\endgroup$
    – Zags
    Sep 24, 2022 at 12:21
  • $\begingroup$ The MAD effect. Your own people would revolt against you. $\endgroup$
    – Mary
    Sep 24, 2022 at 14:02
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    $\begingroup$ This does depend on the two species being sufficiently similar to be able to occupy the other side's planets. In that situation it works, so I gave you thumbs up. If we were fighting organisms from Jupiter, we might have very different ideas. $\endgroup$
    – BillOnne
    Sep 24, 2022 at 15:29
  • $\begingroup$ MAD only works if the people who are able to to launch planet-destroying attacks are themselves in some sort of responsibility over their own planet. But it doesn't take a whole planet's worth of resources to destroy another planet; it would be totally within the realm of private actors (i.e. space pirates) to threaten a planet with global annihilation if they don't do what you want. "Stick 'em up! Give us all of your galactic credits, or we will fling this baseball towards you at 0.99c". You can't guard against that with the threat of retaliation. $\endgroup$
    – kaya3
    Sep 25, 2022 at 2:52
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    $\begingroup$ What about the other side? They surrender. "Let's murder a few billion of their non-combatant civilians and hope that convinces their superior forces to leave us alone" is not a great strategy. Even if their own civilians don't revolt, that's a guaranteed provocation to get the aggressors to escalate. It's suicide. If the other side cannot take the aggressors planets, and they cannot defend themselves, their only other rational recourse is diplomacy. They get to the table, make some concessions, and live to see another day. $\endgroup$ Sep 26, 2022 at 3:04
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I've got one word for you, just one word... economics

Many worldbuilders ignore economics in favor of physics. It's as if they believe the cost of building a giant battleship magically vanishes as technology increases. Maybe it's because so many people see the future in a post-scarcity context. That would be nice... but it ignores the reality that resources are limited. And there are many resources. Not just the minerals in a planet or the life-sustaining ecology of a habitable world, but the resources of available skilled workers in a pyramid of business dependencies to bring that gigantic battleship to pass.

Economics, my friend, is as responsible for ending wars as being the last soldier standing over the dead body of the enemy's leader. Look at history. You'll discover that while the war turns on the success of one or more major battles... it's lost because the losing side has exhausted its resources and can no longer field a defensive force (much less an offensive force).

And what do economics mean to a future battle between monstrous interstellar forces?

  1. No one in their right mind would destroy a planet. Maybe the galaxy will be filled with habitable worlds — but it's more likely they're an uncommon (if not rare) possession. Since a Genesis Device is unlikely to ever exist to magically terraform barren wastelands into paradisaical worlds, a living world that can be used to sustain your own people is not something anybody would be willing to walk away from. Even if you felt it was necessary to destroy your enemy.

Yes, you could create your universe with so many habitable worlds that it's no longer important to care if they're destroyed in the course of a battle — but that also solves your problem. If there are so many worlds, it's impossible (or, at least, unrealistic) to destroy them all. Attrition will always guarantee there is one planet left. No universal annihilation.

  1. Despite what Hollywood might have you believe, the ability to crank out a gigantic battleship in a short period of time is simply unbelievable. So your second protect-at-almost-any-cost economic treasure is your battleships. Curiously, you're kinda looking at this in terms of Star Wars, despite humanity having left battleships behind shortly after World War II. Missiles and aircraft rendered them obsolete. The same would be true of a massive spaceship. That great big juicy target would be so much simpler to destroy than a distributed fleet of smaller, faster, just-as-deadly ships that can work together to achieve results like moving asteroids into position to devastate a planet. They're cheaper and quicker to build and, best of all, you need to destroy a whole lot more than just one to really hurt the fleet. So, if you have gigantic battleships, the will not bear the brunt of battle directly. They'll serve as platforms for smaller sub-fleets that do the actual dirty work, which means it's a lot harder to fully annihilate anything.

This really bears thinking about. Can a civilization really be so wealthy that it can field so many ships that it can utterly destroy its enemy despite that enemy's ability to scatter to the wind? And if they both have that ability, then who's left to farm the proverbial wheat? The economic dependencies that allow large fleets to exist aren't shaped like a column — they're shaped like a pyramid. The cost of just finding your enemy, who is fleeing planet-by-planet, thinning your resources as they go, and making it easier to counter-attack, will stop almost any advance. Even here on Earth today, the idea that, for example, China could land enough troops in America to hunt down and kill every American, even by virtue of destroying the land (and thereby increasing their logistical dependencies, making them more vulnerable...) is IMO laughable. Look at how much trouble Russian troops are having taking Ukraine. Economics, my friend....

  1. To quote Douglas Adams badly, space is honking big and the name of the defensive game is the ability to detect an incoming enemy action in time to bring in your own fleet from somewhere in the absolutely and mindbogglingly humongous sphere of basically empty space surrounding your solar system. Think about it. The attackers have (in a single-habitable-world system) just one place they can go — the planet. But the defenders can be anywhere so long as they can respond in a timely fashion. That means the defenders will always have a strong advantage because the attackers must find and defeat them first before they can take on the planet (IMO). What does all this have to do with economics? Risk! Hollywood likes to show massive space battles, basically slug fests with lots of bright energy beams and little (basically useless) fighters dodging around huge battleships. The economic consequence of being wrong is so high that battles would be incredibly well researched, well planned, well thought-through ventures that try to anticipate anything the defenders can do.

Conclusion

Why won't the two sides annihilate themselves? Ignoring the most likely condition that they can't — they won't. It costs too much.

Annihilation is something that happens to small or geographically tightly bound groups. The larger and more loosely bound (i.e., more planets) your enemy, the less likely you can ever destroy them all. And if you have any chance at all, you'll need every planet you can capture to do it.

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Your factions all use late 1700s technology.

Except for the space tech, which is not theirs. Your people have come into possession of the Stargates and spacecraft which are artifacts of another civilization. These spacefaring artifacts are used much in the way 99.99% of persons in our world use planes - marvelous, mysterious vehicles to travel in and that is it. Your people have used this tech to spread out and populate their galaxy and the worlds of the prior civilization that built the space tech.

The technology responsible for the spacecraft and Stargates is so far beyond the native tech of your people that they do not even have a place to start as regards understanding and reverse engineering these things. There will be no towing of asteroids. There will be no exploring of new planets or rejiggering the Stargates to connect elsewhere. Halting attempts to do these things have been catastrophic and wasteful both of lives and of the alien tech. They use the space tech for bringing people and things from A to B.

There is no planet destroying tech. There is no obvious way to use the tech they have to destroy planets. They can make bombs in great wads of iron and brick durable enough to withstand re-entry but there is no good way to aim them from space.

Warfare is Napoleon style, with cannons and cavalry.

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    $\begingroup$ I'm thinking of Harry Turtledove's The Road Not Taken. The ETs had discovered a low-tech stardrive. They tried to invade Earth (present day). $\endgroup$ Sep 24, 2022 at 20:04
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    $\begingroup$ While this doesn't work for the setting I had in mind, it's a delightful idea $\endgroup$
    – Zags
    Sep 25, 2022 at 20:01
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Planets have a lot of firepower.

Planets have a huge amount of mass and energy, and can devote a lot of it to smashing enemies. They can make huge lasers that draw on a much larger power grid than any ship that isn't planet sized can draw, and fire missiles from hidden underground silos that can overwhelm any missile screen.

It's also pretty common to build a lot of the infrastructure underground. You need to really pound a planet to destroy everything underground, especially with lots of anti earthquake tech in each facility. To pound a planet this hard you need to get lots of ships close to the planet which leaves them vulnerable to planet side defenses.

And any planet which has survived to the space age knows how to deflect massive rocks you fling at them from afar.

Ground troops can stop planet side coordination.

They can see what's going on on the ground, and can cut links, take command posts, and stop orbital grids from functioning.

It's standard practice to warp a ship in, blast any obvious big military targets of opportunity, and then dump a bunch of troops on the planet. Then, the troops can disrupt the planet's defenses, using occasional orbital support to blast any armies they come across.

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Sci Fi Energy Shields

Since we're assuming FTL ships, I think it's fair game to propose some other sci fi tropes to add in. Every advanced civilization has deployed some sort of anti-ballistic defense systems on every planet they control of any relevance. This can range from anything from energy shields which unauthorized ballistics bounce off of, mass drivers to shoot falling objects back into space, space ships with nets, etc...

It might be interesting to show this off by showing a battle where one desperate side tries to overpower/overwhelm an anti-ballistic system instead of disabling it first. Perhaps they have some secret technology or computer hack that allows them to get some bolts to get through (with reduced energy), but not enough to get through the planet. At that point, planets start upgrading to the next generation systems.

If you do manage to take control of the anti-ballistic, you no longer have an incentive to destroy the planet: you now control the planet, and so it's no benefit to the enemy. Even if you don't technically control the entire planet, you can now threaten anyone who doesn't comply with destruction. Only if you're really evil would you destroy a planet you control because you hate the inhabitants (although saddeningly plausible).

An interesting edge case is what happens if your conventional forces disable the defense system without taking control of it. Other mechanisms (like M.A.D. or international law) might be able to help, but I'll lead that edge case up to you ;).

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"Mutually assured destruction doesn't help if war has already broken out." - doesn't it?

Let's assume that both sides have hidden fleets that can destroy the other side's planet(s).

If either side wants, they can do that. If they do, the other side will probably take revenge.

Given this situation, why would either side be willing to take that step? Even if they're losing a war, they'd be better off negotiating peace than blowing up the enemy and then dying.

(This assumes there's no major 'first strike advantage' where you can launch a surprise attack so devastating that the other side can't retaliate.)

So if this didn't prevent all wars in the first place, we might develop a situation where war is more like a sport - one side wins an engagement, and the other side agrees to concede a bit of territory, and both get to consider existing.

Or we might develop a situation of dangerous brinkmanship and bluffing where clashes break out and both sides try to use the threat of annihilation to get what they want, and the leader who acts like the biggest madman usually gets his way.

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It is that way for widely known stuff because no one did a proper modeling of what actually can happen in space, tropes were formed for purposes of snowing a conflict and then it was towed this way since then basically unchanged, and the meand and goals like a copy from old naval fleets of ww2 time.

Another reason is that on different technological levels, or even on the level close to ours space battles can be done in countless ways, different ones as a result what they actually have, not necessarly a difference in hkw advanced levels are. Small difference of the same tech level can turn strategy from head to toe, a small difference described in specs like missiles having 1-10-100 km/s delta-v - each of those is totaly different game as the strategy to be formed.

Or that ftl - some assume they can flash pretty much freely, some limit them by influence of stars, which we understand is for reason to shorten the space between star systems but leave everything else to be a more predictable fashion - but it also has huge difference in battle strategies.

  • for reasons that we can't say energy difference between moving a planet mass from system to system in traditional way and do the same with ftl. Moving such mass (which may be a basic startkng requirement for a different star invasion) in a reasonable time it requires quite an energy, so which energy output provided by one star is not necessarly an adequate power source. (And which realistically can be a reason behing absence of mad large scale attacks of other planets - they just do not have enough energy and resources to deliver what it takes for success of that missiin - just one ship is far from being enough)

So, there are objective difficulties in imagining space battles and their strategies, so for that reasons most ppl (as creators, so as viewers) go along linear point blank ships shooting each others, planets, stations, whatever.

Most of depicted space civilisations not crossing even a trillions people and that less than a trillion number lives on many planets which make their space empire - it is laughable - a trillion in a village like star system looks a bit too low

  • @JBH mention big space, and it was okay in one aspect, but the other consequences were slipping from him. As an eaxample 100 trillion people work for some corp which produces space ships - if they produce less that trillion units a day I would say they waste everything - like high tech, automation, etc.

Resources are limited in space, that is true, but even a fraction of those resources is bigger than everything we have here or can have, bigger by order of magnitudes, 1000x and more.

So there are objective problems, so as subjective problems, so as just plain demand(level of current good enough) which leads to space battles, space civilisations being a poortly researched topic, which leads to - when they are used creators have to stick to accepted tropes, to a certain set of rails, and can't/won't depict what other possiblies can be.

In a sense a set of rails, any, may depict some possibility, because of diversity of potencial strategies which can be based on actual technologies sides are equipped.

We need a refreshment of the tropes, because when those tropes were formed, drones were not a thing, hacking was not a thing, different ECM were not a big deal.

So the answer is - for the same reason as other flaws do exist - it simple enough to handle, and it is outdated

Is it possible to invent a strategy which protects a planet and makes it hard to blow by a camicadze ship or an asteroid - yes.

Can it all be rendered useless by some change in capabilities of attacker force - yes.

In general in some moderatly okayish setting, when ships can't flash too close to a star at whim - defence of a planet will start with a entire sphere of a radius 60 a.u. that is if when not a lot of efforts are put in it, and few times of that with a little bit of more efforts, too much more not neccessarly makes any positive change.

A grid of missiles, 5 million km apart, may fill that volume and it just 24 billion missiles, realy is nothing much but it can be an aspect of such planet9or star system) defence.

Planets may have denser grids of all kinds of measures in their orbital proximities.

  • that notion that a planet may deliver a bigger firepower than space means is 100% bs, once they lost their space part they are toast. One more reason to have space habs I guess, and do not live on planets.

The stuff goes on and on, there is no end. Missing the scale of space, and its resources and opportunities which it may provide and ways required to have those opportunities be used - missing all those leads to poor depictions. And everyone has custom set of capabilities, right and wrong for their space civs, it all also does not help. So as regularm shield and sword problem which also has no end to it.

In a sense a trope when they do not destroy a planet is not necessarly wrong, it can be imagined as acronym for some actual situation, when a whole system is filled with space habs, and destroying a biggest one (by size, not population) does not change anything in the situation, and in a sense a planet is a collective image of supressing and invading each and everyone space habitat in a system, and same way as a planet is a substitution for actual situation, same way actions to take it down are substitutions for the actions required to take down all those space habs (a billion space habs, with defence systems, and few millions ppl in each, all over that star system)

So yeah, the pit is way much deeper that the q

PS

All this complexity is not required, lol, importance of simplicity can't be underestimated and on a full display with @Mary answer. People do not need real space battles, it in space, blinks with fire - good good.

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Beuracrats.

Whatever Mr Vader may think, the power to destroy a planet is very much significant compared to a lone space magician. It requires a massive industrial process to fit and supply a city scale engine on an asteroid, even at the low power end which would take years of continuous thrust to redirect into the target. In the beuracratic overhead alone, there will be hundreds of spies or defectors willing to alert the opposite faction and arrange a preemptive strike to avoid global genocide. Building such a superstructure under false pretence also gets thwarted by beuracrats, simply because their regulations prohibit anyone (who again might be a spy) from commanding the energy scales that might threaten their own planet.

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    $\begingroup$ I find your lack of faith disturbing. $\endgroup$
    – user86462
    Sep 25, 2022 at 18:16
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Planets have become highly mobile

The question implicitly assumes that planets are immobile compared to spaceships. Relax this assumption. Some future motor has power proportional to surface area to the 4th power; a planet ends up hugely more mobile than a comparatively slow ship, let alone some asteroid that a ship accelerates.

Homeworlds nimbly jump aside if anyone tries to launch an asteroid. In fact, not only do they nimbly jump aside, but the slow, ponderous ship that tried it must now face an immeasurably more powerful opponent that is also faster than it.

The engines take a long time to build and are extremely costly; this tends to favour their use on populated planets, rather than ships/asteroids/etc.

An alternative is that a planet can phase out to some other loosely linked dimensions and become ethereal. The same bigger = better physics and economics applies.

The EE Doc Smith Lensman series features a planet called Medon which was mobile, as well as several planet destroying weapons against which planetary defences prove effective/ineffective, if you feel like trawling through some space opera for ideas.

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  • $\begingroup$ Now what prevents them from moving a gas giant on top of your planet? It would be even nimbler than the planet. $\endgroup$
    – toolforger
    Sep 27, 2022 at 12:12
  • $\begingroup$ @toolforger You'd need to build engines all over the gas giant. Not economically possible. $\endgroup$
    – user86462
    Sep 30, 2022 at 1:07
  • $\begingroup$ If the power is proportional to the (surface area)^4, i.e. radius^8, you need just one motor. And a tenth of a motor to move a star. $\endgroup$
    – toolforger
    Oct 1, 2022 at 20:51
  • $\begingroup$ @toolforger The surface area covered by the engine, not the surface area of the body that the engine is on. $\endgroup$
    – user86462
    Oct 2, 2022 at 9:17
  • $\begingroup$ Ah ok, I overlooked that bit. So motor size is proportional to m^(3/4), which gives the installations on larger bodies a slightly better effort-to-agility ratio. Still, I'm wondering... why wouldn't an attacker just use another, same-order-of-magnitude-size planetary body to crush the defender's planet? He'd have to spend time to build it and/or bring it to the attacked system, but planets would just jump around, you wouldn't get a space opera setting that way. You'd have to add something like "larger motors cannot run for that long" or something, which starts to complicate explanations. $\endgroup$
    – toolforger
    Oct 2, 2022 at 12:53
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Your maths is off.

The Chicxulub asteroid was 10-15 km diameter, and it "only" killed all life between Mexico and New Jersey.

https://en.wikipedia.org/wiki/Chicxulub_crater

The thing that killed the dinosaurs was not an asteroid, it was a lack of sunlight (caused by an asteroid). Your scifi race can ride through a decade long cloud, caused by a 10-15 km diameter asteroid as long as the majority of their food is lab grown and fortified with vitamin D - a technology we have already worked out, but not yet bothered to scale up.

If 10-15 km asteroids can destroy 2 countries, we might be talking a solid object of 20+ km diameter to destroy a planet.

In Warhammer 40k, famous for having big ships and disposable planets, 12 km was the most readily available max size of a typical battleship. Some larger vessels exist, but these are more analogous to a Death Star than a Star Destroyer, probably couldn't be propelled at a planet, and could easily destroy a planet single handed without collision.

But these 12 km long ships aren't actually that big compared to a 20 km diamater asteroid. A 20 km asteroid has volume of 4,200 km^3. A 12 km long battleship might be 2 km tall by 3 km wide, with 50% of the volume actually just being air, equals around 36 km^3 of metal - around 1% the mass required.

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Orbital bombardment only works in a vacuum (heh), but if you consider anything else it immediately becomes clear its a technique no one would want to use.

  • Real world example: nukes.

If Russia would nuke Ukraine or anyone else now, all its credibility is gone. No one would trust Russia and isolationism would quickly occur, as well as a high risk of rebellion of both the common people and the higher ups. That is assuming retaliatory strikes dont happen in a "oh shit nuke them before they nuke us" deal.

  • it takes a buttload of time.

Most people say "just grab an asteroid and throw it". But unless you can FTL it to the planet any useable rock will be months if not years away, and the acceleration would be easily detectable by the defenders. The defenders would only need to train a laser on it, shoot at it or place things in its path to divert its path, which means the attackers are suddenly at the backfoot as they need to defend the entire process for all the time they bring it.

  • its a political no-no

If the entire universe is just two factions and one of them lives on a single planet you might get away with it, despite you just cutting the liveable area in half. But in any multi-planet faction war it just means everyone will start using it against you. Any faction not participating in your war will NEED to join against you as they could be subject to total annihilation next.

  • its an economic no-no

Many people say "at intergalactic times, waiting for a planet to become liveable again is tiny". But that does not excuse using orbital strikes as the expenditure to capture a planet relatively intact is earned back by the speedy recovery of the planet and its population working for you in the time you would normally wait for the planet to settle. Because the planet earns you stuff faster you can exploit the system faster and expand from there. This gives you an exponential increase in return compared to waiting for the planet to become liveable again and rebuilding a society regardless of looking at a short or galactic timescale.

Additionally the cost for rebuilding can be many times greater than the cost of an invasion.

  • MAD becomes the norm

Lets imagine that orbital strikes are easy and cheap. That means that every planet has a bunch of ships parked in dead space ready to perform strikes whenever they notice their parent planet is destroyed (several beacons in the Oort cloud could detect who and what attacked the planet, as could stations in the system). So any orbital strike would simply be answered by orbital strikes against you.

  • tactical orbital strikes and a change of warfare

Ok so suppose you try to avoid all that by using tactical orbital strikes, for example in the form of Rods from God. That just means that the defenders will use stealth, faked signatures and placing their forces around the stuff the attacker wants intact to protect their troops. At the end of the day the attacker has to rely on either spotting from space, which is relatively easy to fool, or using troops on the ground to spot targets. And those troops can be engaged making tactical strikes hard.

  • high-tech protection

Many high-tech sci-fi makes the defenders passive entities which do nothing while the attacker starts bombing them. But if you have access to shields you would create many layers of weak shields high in the atmosphere that basically function as spaced armor to spread the strike, if not push the debris from the strike back out once its dissipated enough. Or if you can make entire kilometer long ships hover in place above the surface then the whole "bottom of the gravity well" isnt that big a disadvantage anymore as protections and countermeasures can be placed high in the atmosphere.

  • cost of travel

Most sci-fi ignore this and that is fine, but the cost of travel should be pretty big right? Moving an entire fleet along with enough personel to occupy a planet cant be a small operation, so you might be required to bring limited war materiel and basically manufacture your supplies, weapons and gear in the system you are attacking. Building orbital strikes may simply not be efficient.

  • actual layered defense.

Most sci-fi will first have a space battle, often in orbit of the planet, before an orbital strike or invasion happens. This makes no sense, as the defender would build a layered defense. Any attack on the space-based defenses would automatically be engaged by ground-based weaponry, especially if its a high-tech sci-fi where the energy thrown around by even small ships is ludicrous. A single powerplant on the planet would eclipse the energy of an entire multi-kilometer long starship even if said starship tries to make itself the kinetic impactor. That means any attack on a planet would need to simultaneously attack the space-based assets and assault the planetary defenses in order to succeed.

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Realistic physics

In fiction we often like to talk about kilometers-long spaceships capable of going substantial fractions of light speed like it's nothing.

It's often treated as if merely being in space makes everything bigger and more powerful. But why would that be the case? In real life, doing anything in space is far more difficult and expensive than doing the same thing on planet-side. The payload has to be lifted from the surface, requiring enormous, expensive rockets. As a result, spacecraft humans can currently build have quite small and light payloads. Really big and heavy equipment stays on Earth.

If you don't have some hand-waved MacGuffin technology that makes everything in space super-powerful, then everything in space is going to be fairly light, weak, and expensive. The most efficient realistic spaceship motors, such as ion drives, output far less thrust than a chemical rocket - they are simply capable of maintaining that output for a much longer time. If your ion drive has a mere 100 pounds of thrust, that's already so far beyond current technology it's not even funny.

There is the possibility of mining the asteroid belt so that you don't have to lift everything from the planet. Or building an orbital ring or launch loop to make it cheaper to lift payloads from the planet. But you still are confronted with the difficulty of actually getting around in space. Chemical rockets produce a lot of force and last for only a handful of minutes before they've burned up their vast amount of fuel. Ion drives and photon drives can burn for years but produce tiny amounts of force.

Sure, "just" drop a 10km asteroid on a planet, and you'll do some real ecological damage. But how long is that going to take, if your thrust is measured only in pounds? It would be far easier and more cost effective to aim a bunch of nuclear weapons at key locations on the planet, than to build a motor capable of towing a 10km asteroid to where you want it in a realistic time frame.

Realistic space combat - projecting current technology a few hundred years into the future - is likely to revolve around destroying enemy spacecraft and satellites so they can't intercept your nukes. And even once you've done that, there would be land-based nuclear defenses to contend with. So it's really not so easy to strike land-based targets from space, even if you have destroyed the enemy space fleet.

If you want to have realistic space battles between different stars, you may still want a MacGuffin of some sort to enable the ships to get from one star to another without spending decades or centuries. So, you can make the MacGuffin a warp drive that only works outside the gravity well of a star. (And once you drop out of warp you're back at realistic speeds, such as 20 km/s). That way, space battles once inside a system would have to be conducted with realistic technologies.

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In Weber's Honorverse, there is the Eridini Accord. Strikes from space on a world result in the owner's home world being destroyed.

In many ways you can make an analog to the cold war. Both Russia and the U.S. could destroy the other. Why didn't they?

Flip side of that, there fanatic governments in the world that figure that having a tiny fragment of a planet that was "Pure" would be better than the present status quo.


But if you have two interstellar powers, and both of them can launch fractional c weapons, how certain are you that ALL of the attacker's ships have been neutralized. If you attack unreasonably, they will retaliate. This was the base for the geneva convention for treatment of prisoners.

Whatever you do, your own home planet is hostage.

There then is a huge incentive to play out brush fire wars on societies that don't have planet busters yet.

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