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OK, so you've got an empire in space. And that empire has a frontier. Now on Earth, we have many ways to defend a frontier. We can build border fences or even walls, we can build watch towers, and of course we can let people patrol; given that the border is just a line, patrolling is quite efficient.

However, the space frontier is a whole 3-D surface, lacks the ground on which you could build fences or walls, and indeed would simply be too vast to completely cover it with a fence-like structure. Patrolling is of course possible, but should require very many ships simply because the border now is a whole surface. This doesn't seem too efficient, however, so one would hope that there are more efficient means.

So what would efficient border control and defence look like in space?

To set the scale, assume that the empire's space is approximately a sphere with a radius of 20 light years. Only sub-light speed is possible (signalling of course with light speed), but technology allows one to go arbitrarily close to light speed. No worm holes, hyperspace or other technology which would allow to "jump" across the border.

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    $\begingroup$ Other than the frontier depicted in "The Fifth Element"? I'd argue that border defence would strongly rely on the early detection of threats (long range sensors of some kind, intelligence) and the capability to launch an interception force that reaches the intruder in time (speed and operational range of the ships). $\endgroup$ – Ghanima Feb 15 '15 at 20:16
  • $\begingroup$ Related: worldbuilding.stackexchange.com/q/9717/2072 $\endgroup$ – Shokhet Feb 16 '15 at 3:05
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    $\begingroup$ This question would be a good one if some technological assumptions and scale were specified. Without those, possible answers are endless. $\endgroup$ – Keith Feb 16 '15 at 3:54
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Mines, exploding space-mines everywhere!

Plagiarizing one of my earlier answers. Note that this answer assumes any and all alien/foreign forces are hostile and also a relatively high tech level for your empire.

There's No Invisibility in Space

While nobody can hear you scream in space, everybody can see you. To the hyperadvanced spacefaring civilizations, even a black-painted intertially-driven scout stands out as a sore thumb, since the defending civ has all the little objects in the system and their trajectories for the next million years calculated and recalculated over and over, so anything NEW, even a few stray photons bouncing off your invading ship from long-range radar sweeps would be ringing alarm bells. Moreover, if you're planning on doing anything in particular in the system you need to slow down. That takes reaction mass (assuming you don't have reactionless drives), so you might as well be announcing your arrival with giant flares. If you don't slow down, well, massive relativistic-speed objects also tend to stick out. If you've got space-warping technology, the warping you induce into space is also going to be easily detectable, since alcubierre warps don't just happen on their own.

Where to defend

You don't want to wait until the enemy gets into your star system. If they're within light-hours of Home, it's already game over. They can blast it with TerraWatt gamma lasers that go through your planet's crust like a bullet through warm butter, or perhaps launch dozens of relativistic kinetic accelerators at it, and believe me, those are a hassle to stop, and your bunkers aren't any good in a molten slag state. No. Your defenses for Earth should probably be far out in the Oort cloud at the very least, preferably a few light-years away from any world you're defending if you have the technology.

How to defend

You will defend in depth. You will have rings and layers, upon rings and layer. The first layer should be your centimeter-sized Remote Telemetry observation satellites. They will serve as an early warning system. Spaceships' heat signatures in space are very hard to disguise. Of course, a 1-meter probe will have an easier time slipping by than some massive colony ship or a warship armored to resist nuclear impacts. Besides optics, your detectors would use gravitational distortions, if far enough out from massive bodies. Since an object's propulsion jet is visible as well as its speed, you can easily tell the mass, so (accelerated) decoys would not work unless identical in mass to the spaceship you're trying to disguise. This confers an advantage to defense, since they don't need to break or accelerate. Your microsatellite sensor grid info could be sent narrowbeam at lightspeed (or darkspeed if you accept FTL and the mess it makes of causality) back to a defense outpost or used to activate some local defenses.

While the micro-satellites are passive detectors, as your next detection layer every cubic AU or so you should have an active scanning station, a radar beacon blasting away into the enemy darkness. This will quickly paint a signature of any approaching target. This information will allow your sectoral defense AIs to order active denial actions, activating your first-line defenses.

Good Old Kinetics

Now what would these defenses be? There's an extended discussion of classical kinetics/rockets/beams elsewhere, and you can read that at your leisure. All good ideas. Probably the best in the class is a straight up directed multiple warhead relativistic impactor, essentially just accelerated mass coming your way really really fast, and spreading out into slivers as it does to cover a vast area. Even with randomized defensive maneuvers, these puppies make it hard to miss -- and even 2 grams travelling a cool .99 c will leave you with $10^{15}J$ to deal with, or about 1 megaton of TNT equivalent. I'm guessing that will leave you with more than just a bad hairday.

But you can do even better.

Rift Weaponry

My other suggestion is rifting. Imagine you have devices with the capacity to bend spacetime itself. Done carefully on a starship and with insane levels of precision, this might effectively shorten long journeys, and maybe provide effective FTL spacetime translation. But if all you care about is damage, than a set chaotic, highly unstructured rifts will play havoc with the delicate innards of a ship (such as humanoids or computers and their minds).

Geometries bend at impossible angles, straight lines twist and intersect with each other, the ship's Combat Information Center and the million Kelvin engine room are suddenly superposed, leaving charred organic residue in the superpurified magnetic convection coils, past and present comingled as you see your mates both alive and and torn to shreds by the rift.

Your defenisve minded civilization has littered space with billions upon billions of such "mines" that quietly store all this energy, like a set of corks in a dam. When activated by the early warning sensor grid, they unplug their carefully designed set of cone-covering spacetime rifts and let it rip the intruding ships apart over a wide area.

Classical AI fleet

Anybody that survives that flashy welcome that the first few dozen layers of kinetics and rift-mines have extended will then be greeted by your local spherical-octant mobile defenses: AI-directed firing platforms who were set on an intercept course as soon as the lightcone (darkcone?) of information reached them.

These massive ships will have all the weaponry of your initial mines, dialed up to 11. Without squishy humans on board, these are capable of pulling 10,000 g accelerations with ease, so they will be on scene toute-suite. Without needing to be stealthy or discreete, these monsters will probably have an utterly obscene power output ...

And just in case they don't suffice, somewhere a few light-days or weeks behind them is the actual Capital Fleet, which is the stuff that mothers in alien civilizations use to scare their disobedient little reptilian brats who won't go to bed.

Inner System

In order to be able to field even such basic defenses as mentioned above, (i.e. nothing that would stop a determined Type II civ with a grudge), your civilization would need to be a high Type I, near Type II at least. Your system's industrial capacity must then be about $10^{26}$ watts, so generating $10^{11}$ (100 billion) kinetic impactors would take about 1 second of the civ's power output, while the rift devices might require more power, (but you'd need fewer of them). That said, a type II civ should be able to reach this basic level of militarization from scratch in about a year without any discernible impact on the inner-system standard of living.

Your innermost defenses, if the visitors actually came for a real fight, will be powered by your Mercurian Solar Grid. While this is normally reserved for the purposes of research and investment banking AIs, in times of emergency the output can be directed into a beamed weapon of near unimaginable strength, with a continuous power output in the range of $10^{25}W$, that would be hitting with the energy of 100 dinosaur-killer asteroids each second. It would take a really determined enemy to get past this one.

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    $\begingroup$ If each of your 1-centimeter oort cloud observation satellites can cover 1 billion square miles, then you only need 2,717,163,486,089,818 of them if you're just concerned about the innermost sphere of 5k AU. (that's 2.7 quintillion). To cover with mines at a distance of 1 light hour (also assuming about 1 billion square mile coverage), you only need 16,982,271,788 - or about 17 trillion mines. $\endgroup$ – Dan Smolinske Feb 15 '15 at 22:08
  • $\begingroup$ Yup. I think you're underestimating things a bit. I was thinking of placing them at 100,000 AU out. I'm assuming a solid near Type II should consider these nothing but basic precautiounary measures by a peace-loving civilization. $\endgroup$ – Serban Tanasa Feb 15 '15 at 22:10
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    $\begingroup$ Yeah, unfortunately this question is really open-ended as to the tech level, it's hard to tell if we're talking about monkeys with radar or monkeys with 1 kg/light-day rated gravity sensors. $\endgroup$ – Dan Smolinske Feb 15 '15 at 22:19
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This largely depends on your technology. Do we have warp drives? Wormholes? Or are we restricted to sub-FTL ships? Because your technology is going to drive your detection/mitigation strategies.

In general though, it's just not possible.

Most stories that involve effective space defense fall into the 2D-space trope. And it does seem to make sense from that perspective. But the reality is that your border is 3-dimensional, which increases the difficulty of defense immensely.

Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

Given the sheer size of space, it's not really practical to defend your frontier. You might be able to defend a planet or other fixed installation, but realistically if someone throws enough rocks at you at high enough velocities, you're not going to be able to stop it, and then your day is ruined. Depending on your relationship to enemies, you might be ok if everyone is avoiding those kind of attacks due to a MAD scenario, or because they want to take your planets and not just destroy them.

You also need to decide what it is you're trying to detect. A death star approaching your border is orders of magnitude easier to spot than a smuggler who runs silent through your border region and isn't going toward anything tactically significant. Unfortunately in a realistic space war scenario that smuggler has a built-in WMD, and once they're inside your border they can go wherever they want.

So, here's what you can do:

  1. Defend your important planets and installations against everything but MAD attacks. Make sure your enemies know that you have your own defenses in place, and if they take out your planets you'll respond in kind. Forget about smuggling entirely, it's not something you can stop.
  2. Make sure your ship movements are randomized to some extent. A least-time optimal course is efficient, but predictable. In military terms, predictable is synonymous with dead. Space is big enough that you can aim for, say, 95% efficient and still have tons of room - that lets you almost entirely eliminate piracy or privateers.
  3. Given the above restrictions, wars will likely be fought primarily with intelligence and special forces-type units. Concentrate on those.
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    $\begingroup$ +1 for the Hitchhiker's Guide to the Galaxy Reference. If I could give you more, I would. $\endgroup$ – PipperChip Feb 15 '15 at 20:43
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    $\begingroup$ +1 for the superb quality and thoughtfulness of your answer, not because of a brief pop culture reference $\endgroup$ – Lightness Races in Orbit Feb 16 '15 at 10:53
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    $\begingroup$ What is a MAD and WMD? $\endgroup$ – user5005 Feb 16 '15 at 11:05
  • $\begingroup$ @user5005 "MAD = Mutually Assured Destruction" - the concept that if you've got nukes and I've got nukes, we both die if either of us uses them. "WMD = Weapons of Mass Destruction" - Nuclear/Atomic weapons, or major chemical/biological weapon. Usually a weapon that would kill tens or hundreds of thousands of people at once, rather than 1-1000 people like a 'conventional' weapon $\endgroup$ – Jon Story Feb 16 '15 at 11:25
  • $\begingroup$ @user5005: I added links to the relevant wikipedia articles for those concepts, thanks for pointing that out. $\endgroup$ – Dan Smolinske Feb 16 '15 at 18:34
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The best defense is a good offense!

You're likely looking at a system of early warning sensor arrays, some drone satellites that can launch interceptor missiles and the capability to make sure that anyone who fires at you won't enjoy the retaliation.

I'd like to question the assumption that fences, walls, towers, patrols etc... are an adequate defense. Historically they've rarely worked for anything larger than a small town area. Even then very many walled castles have fallen to enemy invaders.

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In short: you can't.

Dan Smolinske's answer sums it up pretty well, but I did want to add a few things.

First, there is no concept of a "border" in space. Your borders are the planets you control (also possibly things like asteroid fields or space stations as well). You can defend your planets and space stations, but trying to control a "border" of 3-dimensional space is impossible.

In space, most military campaigns are going to center around planets. Furthermore, you can make the assumption that any life form must have evolved on a planet, and therefore will want to USE your planets as their own, so they aren't going to completely destroy them or risk rendering them unusable for themselves (unless they just want to kill all of your citizens). Unlike terrestrial battles, where every inch of ground (almost) can be useful in some way (farming, factories, etc.), in space, there are NO resources between planets. Therefore, planets, especially resource rich planets, will be highly valued and fought over.

Therefore, you can setup your defenses to defend the planets, not necessarily worrying about the empty space between planets. Sure, you can setup long-range scanners (and at the speeds you are talking about, you will see them LONG before they reach you), and have patrols that can move to intercept threats, but the vast majority of your military will be dedicated to planet defense and protection.

As an aside, your last paragraph makes it clear that an empire with no hyperspace/warp technology would be very at risk of raiders or similar, because even with signaling at light speed, it would take TWENTY YEARS for the signal to reach the other side of the territory... Most raids take a few HOURS at most. Without some kind of faster speed or heavy planet defense (shielding, etc.) I can't see how feasible it is to attempt to defend an empire that size, especially if you are relying on a centralized military/government.

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    $\begingroup$ This is why the only universes where a military makes sense without FTL communication are those where jumps happen at discrete jump points in space. These could then in theory be mined, defended with forts, and have fleets deployed. Otherwise anyone willing to go to the limit can take out your planet or orbital factories more or less at will. $\endgroup$ – Oldcat Feb 19 '15 at 18:10
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Classic defense strategy is defense in depth. You do not expect that outer layer of your defense will be able to fight off attacker. All you want is to give you information where attacker is attacking, what is structure of enemy's forces, and give you time to prepare more focused defense. You want to focus your defense capabilities on attacker, not to spread them too thin and wide. You want to have overwhelming power, and all advantages, when engaging your opponent.

Mines, AI probes, bases - all is just examples of defense in depth.

After attack is detected, you need to start one or more of prepared defense strategies.

And one of the defensive strategies could be a counter-attack, either hitting attacking forces from unexpected direction, cutting off supply/escape route, or attacking unexpected objective, like important base (or homeworld).

As says Sun Tzu: victorious strategist only seeks battle after the victory has been won, whereas he who is destined to defeat first fights and afterwards looks for victory.

Obviously this depends on weapons available, and difference in technology. Which might be huge, few thousands of years might make decisive difference, and then any question of fair fight is moot. One civilization either win decisively, or gets wiped out. Choose the planet you born on wisely!

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  • $\begingroup$ The problem with this is that it assumes you will be able to delay the attacker and get in front. If they are as fast as you, heading right for your home world will mean they arrive there when you do. $\endgroup$ – Oldcat Feb 19 '15 at 18:07
  • $\begingroup$ Unlikely - because I have already someone on my homeworld. Obviously this depends on weapons available, and difference in technology. Which might be huge, few thousands of years might make decisive difference, and then any question of fair fight is moot. $\endgroup$ – Peter M. Feb 19 '15 at 23:34
  • $\begingroup$ For equally sized fleets, the defender has to have some guard the perimeter which can be bypassed, giving a numerical advantage to the attacker. Ships in orbit can't keep the enemy from bombing the surface with missiles at high speed. If they aren't in orbit, they can be bypassed too. If you hold a huge fleet on your home planet in orbit, the peripheral fleet can be attacked and destroyed, and then the home fleet is outnumbered...and any installations in the outer system can be attacked and destroyed. $\endgroup$ – Oldcat Feb 20 '15 at 0:38
  • $\begingroup$ Not if I can setup invisible wormhole portal feeding incoming enemy to a blackhole. All this space war crap is ridiculous because by time civilization has interstellar travel it has other technologies - whole stack of them - which we cannot pretend to comprehend. $\endgroup$ – Peter M. Feb 20 '15 at 0:44
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I think the most likely would be to have a 'net' of sensor drones that report back. Both what it sees and what it doesn't. you could have outposts scattered around that the drones report to directly for faster response and it would allow a much smaller force to guard vast areas of space.

Keeping the drones fairly simple. They relay what they detect and any changes in their area, they report to all the surrounding drones on a regular basis, if one doesn't report, then the rest send an 'alarm' to the nearest outpost. The outposts of course will be automatically doing a similar thing through the drones to 'nearby' outposts so if anything causes a disruption, someone will be notified. Of course if things like worm holes are able to be used and can 'skip' a net then there better be some way to detect their creation or all bets are off.

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