Planetary defense in an interstellar society

Let's say that you are some sort of tyrannical dictator of an inhabited world with futuristic technology (ex. railguns, a fully automated labor force, efficient space travel.) You know that within your system exist several other space-capable tyrants, who may attempt to attack you and your people.

Your advisors inform you that your enemies have little care for the wellbeing of your planetary infrastructure, and as such will attempt to use the most devastating methods possible to render life on your planet miserable. These would likely either be

A. A barrage of nuclear warheads, or

B. Directly ramming one of their ships into the planet.

The second option is more troubling to you, as it presents a myriad of possible difficulties. You can't simply indiscriminately blast any ship that looks to be on an approach vector with your world, as that would devastate trade. You can't rely purely on conventional weapons, either, as the shrapnel from a ruined ship could do as much damage as the craft itself.

With these considerations in mind, how would you, humble space tyrant, go about rendering your world protected from your savage and barbaric neighbors in the most efficient method possible?

10 Answers
10

Don't allow traders to approach the planet. Let them approach a space-station in safe distance and do the trading there.

Or you could require that ships be escorted by your own escort ship within your territory and maybe also have one of your pilots on board to fly the ship. (similar to the escort practices in many harbors on earth)

Now you can shoot down anything that comes closer and is not your own or not controlled by you.

Preemptive strike

There is a fancy term for it, the whatever whatever I forget gambit, but basically it falls down to this. You have an enemy you don't know anything about, except for the fact that they have the capability to destroy you and you have the capability to destroy them. If they fire first your death is guaranteed and you might not even have a chance to fire back, if you both fire at once both of you will die, if you fire first their death is guaranteed and they might not have a chance to fire back.

Obviously using pure logic and zero morality the most beneficial reaction here is to fire first. Not only are you prepared for the potential counter-barrage guaranteeing that even if they fire back you will be able to at-least survive (albeit with heavy casualties). Surviving with heavy casualties is better than dying due to a 100% casualty rate if attacked and caught unawares.

Naturally you decide to hit them when they don't expect it and to ensure maximum likelihood of total annihilation of the target you use both a barrage of nuclear weapons in the gigaton yield and an automated kamikaze ship timed to coincide with each other's arrival. You also packed the ship completely full of highly radioactive long half-life isotopes because you're an asshole.

As you launch this barrage you of course have prepared your point defenses and moved any minions and commoners you would prefer not to lose to shelter just in case the counter barrage arrives. Spreading your keepers around in asteroid stations or orbital habitats and such is an excellent tactic to protect whomever matters enough to be protected as well. Once the enemies have been drowned in a deluge of nuclear warheads and get sucker punched from the massive kilometer long ship turned dirty bombs your defense network is scanning like crazy for any incoming return fire. Maybe they get a few shots off and you lose a few cities, but hey, you're the space dictator, you can fire up the ol propaganda machine and spin the story to say you were attacked first and were unfortunately forced to oblitorate them. If you are a particularly sociopathic space dictator maybe you intentionally allow a few missiles through to hit cities full of people you don't like just to turn tragedy into profit.

Congrats, the system is now yours, you are the hero of the war of enemy aggression, avenger of your people and bringer of peace. The newly created radioactive hellholes will make excellent penal colonies for any dissenters who might not have been taken out in the "accidental" lapses in your security grid. We'll repackage it as recovering from disaster. Could yield some pretty lucrative resource collection and it only has to be at the expense of people you didn't like or need very much anyways.

Bad News.

The bad news is that if you are already space-faring, you probably already have the means to destroy a planet - infallibly. All you need is to get one of a million asteroids already existent throughout any star system (or if you want to sacrifice one of your ships), and slowly accelerate it on a trajectory that will intersect your planet. It doesn't have to be particularly large.

Over interstellar distances this could reach relativistic speeds, really fast and hard to detect. Once impact with your planet has occurred, your planet is obliterated with little chance to support life. Nuclear warheads are useless in this regard - your relativistic asteroid/ship is your warhead.

It is easy to do, and difficult to stop. There is no defence once it enters your solar system.

But maybe some Good News.

It should be easy to see them coming if you have mega-telescopes and detection devices far afield of your solar system - the earlier you can spot one coming the more actions you could take.

For instance, the solutions could be:

you could try to accelerate your own asteroid/ship on a collision path with theirs, and knock it off course, or fire a powerful laser to try to influence it's course. An early lateral nudge is what you need to disable it, you would need an array of these at the edge (preferably beyond) of your solar system to make any difference. Chances are unlikely that you would have enough time given the energy and accuracy that you would require, but it depends on how early you detect it - therefore I predict that early detection and reconnaissance is a major priority in your galaxy. From spies on the opposing planet, to finely tuned devices that can detect movement and intent.

Randomly zig-zag your planet - an out there solution, but you could establish an irregular orbit from pushing or pulling your planet in and out to stop prediction of where you planet will be. By the time the light reaches your adversaries star system, it would already be outdated information and difficult for them to plan a trajectory.

As Sun Zhu said - prevention is better than cure, deception is better than battle.

Assumptions: hard science, no special technology

M.A.D. Mutually Assured Destruction

It doesn't really matter what you do in terms of system-defence, this is one case where the weapons have FAR outstripped any likely defence. So your chief approach should be psychology and overwhelming firepower.

Not an especially difficult one. Maintain a fleet of relativistic ram-ships, one or two for every planet your enemy has. If your civilisation is destroyed, the ramship fleet will accelerate to a significant fraction of the speed of light using Hydrogen Ramjets and each hit a planet with enough force to smash it like an egg.

Whether this is a suicide run or a computer-guided drone after launch is up to you. You'd probably be better off with the second one, simply maintain a number of isolated outposts in deep space far from any space-lanes, manned by a skeleton crew and regularly checking in with civilisation to make sure it's still there.

By making sure you have the means to unstoppably obliterate your enemy even if they first-strike and destroy your entire civilisation, nobody would be insane enough to try. Just make sure you can do it and that your enemy knows it.

Downside: People are crazy and technology is fallible, expect that things may go wrong. Also, by placing your biggest weapons at arms reach, you do leave them open to attack if your enemy ever finds out where they are. Worth maintaining a series of bases all around your territory to prevent any single solution to destroy them.
Reference: The Cold War

If you have the necessary technology to travel between stars, you have the necessary technology to screw around with the orbits of comets and asteroids, and THAT is the threat you should be concerned about. Not nukes. Not ramming spacecraft. Dinosaur Extinction Rocks is what you have to be able to defend against.

So.

In order to defend against Big Freaking Rocks, you need two things.

First: You need to be able to detect them coming, which means a REALLY well developed sensor network. You probably want radar or some other kind of active sensor because you can't rely on telescopes to find these things if their albedo is low enough.

Second: You need to be able to deal with the rocks once you find them. Ideally you want to be able to detect them far enough in advance that you can just counter whatever orbital weapon your opponents used to throw the rock at you and just redirect it into the sun or something. The viability of this option depends entirely on what kind of technologies your setting is relying on. If you can't apply enough force to the rock to make it miss your planet entirely, then you want to apply enough force to break it into chunks small enough to NOT represent an existential threat to your biosphere. Said chunks would need to be ~100m or smaller to avoid catastrophic damage (for comparison, the earth gets hit about once a year with an asteroid this size. They create an explosion equivalent to the nuclear weapons deployed in WWII, but because these explosions occur very high in the atmosphere, they don't usually cause damage. The Tunguska Event is an unusual example to the contrary.)

For reference: The impactor that caused the K-T extinction is estimated at between 10-15km in diameter, so that's the scale that your defensive weaponry needs to be able to operate at.

Stalemate.

So you're in a cold war with a weapons-of-mass-destruction-and-overwhelming-force situation? Build your own, and let the other guy know yours is bigger than his.

With the science-fiction tag, we can do a bunch of thing.

Capture as many asteroids as you can, build colonies on them: You now have second/multiple strike capabilities, so the enemy knows that if he strikes your planet, he's not done with you. This is really the only option in a sustained cold war.

Protect your asteroids with force fields, or your planet if possible: This will, again, only delay the inevitable overwhelming power of the other guy's weapons, but this delay can keep the stalemate.

Automated semi-intelligent defenses: Drone swarms in space (NASA's on it, already), changing position all the time, equipped for defense/offence without having to get a command from a human. The other guy may get all of your colonies and main planet/s, but they know it won't save them from retaliation.

Espionage network based on the latest tech: I can write a few pages here...

you name it.

Anything to keep ahead of the other side. Of course, they have already thought of the same things... So don't trust your wife of the past 10 years, she's obviously a Fembot.

What you'll end up with is the only logical option: Peace. But you didn't get through six years of Tyrant School by making friends, now did you? :-)

Assuming you only want to go for defense, what you would do is setup a ring of protection starting at your core planet and expanding out, a fundamental web of surveillance that expands across your entire region and look into the enemy territory. Now space is large and it would be nearly impossible to cover every single path.

Firstly your going to want to figure out the most effective way for the enemy to travel to you. You throw a bunch of probes and sensors along those paths. You keep expanding your scope as you go. The quicker, cheaper and safer paths to your core planets are easily filed with thousands of probes which scan every single ship that enters the area. Basically you setup access points into your region along the easiest to travel routes.

But there is a lot of space in space. So your going to need a ton of telescopes pointed along less plausible paths that constantly monitor them for incoming ships. Throw in an occasional patrol and now only small ships or bands will be able to get through. Nothing that you shouldn't be able to contain.

Finally your going to want some super powered telescopes pointed directly that the planets of your enemies. Considering that Hubble can see several billions of light years away, you a space dictator are going to have no problem assembling a fleet of even more powerful telescopes that will focus in on your enemies year round. This way you can see fleet and resource moments of your enemies and plan ahead. As long as you don't have faster than light travel you will always know if your enemies are moving a massive amount of resources off planet and hence be able to prepare. Heck, you would track any large ship or resource movements to try find out where their ship yards are, so you can determine the condition of their fleet and how to handle it.

You have to remember that we currently, split into multiple nations with many internal and external conflicts are still able to almost track most objects within our galaxy. NASA has done an amazing job already with what limited funding they had and limited technology they used back in the day. You have a space faring dictatorship. There is nothing stopping you from expanding your surveillance powers to monitor for potential threats over several light years if you want to put the money and resources into it.

Of course, the problem is if you have faster than light travel. The only way in these situations would be to threaten them with a similar destruction to you. Think of the cold war but on a space level. No one wants to completely lose so its just a massive arms race and then nothing happens. You could try your best to spy on them, but if you can move faster than light, there is no way you can reasonable expect a spy to get a message to you before a fleet of ships is knocking on your door.

You're a tyrant. Act like one, and with brutal elegance.

But before that, rebuild all your bleeding edge infrastructures in space. Ideally, hollow out asteroids and create O'Neil cylinders, Bernal Spheres, Bishop Rings, and other gravitated space habitats inside them. Artificial habitats in the care of minds superior to humans, while harder to create, are far safer and easier to sustain than flashy planets. They are more movable, self-sustainable, and unnoticeable. Reliance on stealth mechanisms instead of ironclad armor are rarely ineffective against unknowable threats.

The best way to avoid being victim of a planetary destruction, is to avoid being on a planet.

By blasting your own world, you can demonstrate an unimaginable power that all of your barbaric neighbors will come to know. Note that I didn't specify that it is your power that was demonstrated. Let them think whoever did that to your world should be feared. Most importantly, let them think you're out of the game. Then, slowly begin your new interstellar reign from the shadow of an imaginary monster that you have created. Reliance on stealth mechanisms for defense are just as effective for offense.

Some advantages of hollowed asteroids as both offense and defense are:

They are highly mobile. Unlike planets, space habitats inside hollowed asteroids are more akin to spaceships than habitats. As long as there is enough space and enough fuel, they can move around quite well. Equip them with far-reaching, fast-acting sensors and they can even dodge RKV's .

They can cannibalize/reproduce. What happens when your home asteroid runs out of resources? You just find another one and tear it down, atom by atom if you have to, then feed those atoms to your own home. Or, just create a new home entirely if the asteroid seems to be pure space rock.

They can function as Relativistic Kill Vehicles. Enough said. They can go kamikaze, accelerating at just the fraction of c towards a target planet to trigger extinction events one after another.

You'd think a dead enemy is harmless, until its ghost is haunting you in your dreams while strangling you in your sleep.

Expand and fortify. If you can't stop the enemy from shooting, give them more to shoot at. Your homeworld should be your last line of defense, not your first. It's less a question of protecting a planet, and more a question of fortifying a solar system.

As other answers have pointed out, if you've got efficient space travel, you can effectively annihilate a planet comparatively easily. One good asteroid nudged on a collision course, one big enough kamikaze ship (or fleet of kamikaze ships), and wham bam, no more planet (to speak of). It doesn't take much to render a planet uninhabitable. So how do you protect against your comparatively probable doom? You don't let them get close to your planet. You expand, you fortify.

First things first, you do the obvious things to trick out your homeworld. Slap an artificial orbital ring around it covered in guns and starship hangars, give it a network of a couple thousand defense laser satellites, get yourself a couple mile-or-so-long ground-based railguns. Make yourself a network of next-gen fallout shelters miles underground, big enough to house most of your population (or at least those of your population you like—you are are a dictator). If anyone gets all the way to your homeworld, you're already in deep doody, so this should be your last, most drastic line of defense. Also a good place to keep your most complex megatelescopes—this should be the safest place in your empire, and you need as many eyes on the skies as you possibly can. You loose your sensors, and you're blind to any incoming attacks. Space is big and mostly empty, so if you've got telescopes, you'll be able to see pretty much any attack coming.

But that's all just for the worst-case scenario. Fundamentally, you don't want folks even getting to your homeworld. Short of magic hyperspace dimensional popping, people are going to have to get through your solar system to reach your (presumably inner-planet-equivalent) homeworld. The trick is to not let them get far enough. Build bases orbiting every planet, fortify every asteroid, every rock, every speck of space dust. Put guns on everything. It's cheaper and easier to make space habitats than to terraform planets or launch ships out of atmospheres, anyway. Slap outposts everywhere, let your population boom, and give them all guns. You don't just have megatelescopes and laser arrays around your planet—you have them all throughout your system. You have bases all the way out in your Oort Cloud, if you can get them. You want to be able to see everything coming from every direction, and aim a thousand guns in that direction to blast it to bits. There's no stealth in space, so if you have the infrastructure, it becomes easy to sight possible threats.

But as you said, how can you be sure if a ship approaching is a threat or not? Any reasonably fast ship is nearly an apocalypse weapon by itself—how can you protect against that? Sure, millions of defense arrays helps draw the fight away from your people, but what if some loser pretends to be friendly until he's in your atmosphere?
When it comes to this, all your space guns come in handy for a different reason: Intimidation. Creating the illusion of control and dominance is just about as good as actually having it. Technically speaking, anyone could crash their car into any random building and cause a great deal of damage, but no one does, because we're all certain the repercussions from the Powers that Be would be devastating. And, if you've got enough defensive infrastructure, that certainty wouldn't necessarily be misplaced.

As a final bonus, if you really want total military dominance over a solar system, and even its surrounding systems, you can build a Nicoll-Dyson Beam around your sun (effectively an array of super-gigantic mirrors). Such a beam would transform it into a Starkiller-Base-esque laser that could fry even the distant homeworlds of anyone who tries to pull terrorism on your turf.

Mobility

Leading up to World War 2, France built the Maginot Line, a line of turrets along the Franco-German border. This was a stationary form of defense. Germany built-up a rapidly-dispatchable military utilizing the Blitzkrieg rapid-attack strategy. Germany was able to go around the Maginot Line and seize Paris; and France did not have an equally-sized, equally-mobile military to counter the offensive. It turns out that a mobile military can provide appropriate offense and defense, and can be recalled to your capitol if need be. However, a stationary defense is very easy to circumvent unless you spend significantly more resources to build it literally everywhere. My point is, you should not use many stationary defensive turrets. You should use a dispatchable fleet of patrol ships. This offers the added benefit that if the planet does get destroyed, you will have a fleet of ships in space that can repopulate your species and/or launch a counterattack (related to the mutually assured destruction theory).

Regulated Boundaries

There could be a 3-D border around the planet in which ships must receive clearance before passing through it. Ships that violate the border might be detained with tractor beams or blown-up. It is possible that the space border might be tens of thousands of miles thick, or it could expand throughout the solar system.

Automated Pass Clearance

Pass clearance could be mostly automated for anyone who has previously visited the planet. You could use technology similar to E‑ZPass to receive clearance via electromagnetic radiation pass codes. There could even be space stations in the outer perimeter of the planetary region that assess security clearances and distribute passes before a ship gets close enough to violate the spatial region.

It should be noted than in various Star Trek series, especially Star Trek: Voyager and Star Trek: Enterprise, the namesake vessel has routinely been intercepted with authoritarian warnings of "you have violated our space. Turn around." From the viewers perspective, these kinds of hostile responses are extreme. But from the perspective of the question, these security measures would be within the tyrant's expected options.

Threat Detection

Even if a ship has a pass authorizing it to cross the border, that doesn't mean it hasn't been hijacked and filled with a payload of WMD's. If this is a serious concern, you should attempt to implement some sort of technology that detects massively destructive cargo (e.g. devices that detect plutonium cargo on ships).

Whack-a-mole

If you are a tyrant who is largely concerned with your own safety, and if your enemies are largely concerned with assassinating you personally, then your best defense might be having no fixed location. Your species should have numerous off-planet colonies located on moons, asteroids, and space stations. You, the tyrant, should be constantly transferred between locations so that the enemy has no idea where to attack. You should have numerous decoy look-alikes who are also shifting between locations. This has the added benefit that if one of your decoys gets attacked, you may not only survive, but you may be able to raise your threat condition level in anticipation of elevated risk.

Mutually Assured Destruction

Other answers have already mentioned this.

Diplomacy

There are numerous ways that present day nations discourage nuclear warfare, ordinary warfare, development of nuclear weapons, etc. These include embargos, trade agreements, humanitarian aid, nuclear treaties (with provisions to permit inspections), financial incentives for respecting treaties, and military alliances.

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