One of the issues with creating a model of space warfare that is both realistic and interesting is the ease with which one can destroy things using high-velocity kinetic kill vehicles (KKVs). For a civilization with frequent space travel, crashing a large asteroid into a planet is a trivial task. Any space combat model that does not result in either mutual destruction or a cold-war type scenario (good for spy-type fiction, less good for military fiction) needs a way to solve this problem. What are ways in which high-velocity kinetic impactors can be stopped or deflected to prevent a Mutually Assured Destruction scenario in space combat?
The best way to stop a high tech planet busting KKV is a very low tech device, the white flag. The white flag is here a metaphor for signaling "We Surrender! We Surrender! Call off your doomsday weapon!"
Unless a fictional society has a number of independent but totally peaceful nations which have given up war forever, developing advanced interplanetary travel and the ability to create KKV weapons will mean that as soon as a totally space living nation exists that doesn't include any land on the original planet of the species, that space dwelling nation will have any nations that include a lot of land on the original planet at their mercy in any war that might be fought.
The totally space dwelling nation can threaten to use a KKV weapon on the home planet and any partially space dwelling nation that includes a lot of land and people on the home planet will have to agree to their terms. If they don't agree to those terms all their people on the home planet will be killed - plus the people of any other and neutral nations on the home planet.
Such threats will be effective, but to a lesser degree, in conflicts between totally space dwelling nations. If one nation lives only in domes on Ganymede and the other only on domes on Callisto, for example, they can be attacked with small asteroids aimed at each individual dome.
The destruction of individual domes will ruin the biosphere of Ganymede or Callisto, but since the environment there was already deadly dangerous and humans can only live inside the domes, the environmental wrecking will not harm humans in domes that aren't destroyed. Thus the goal would be to have the power to smash each and every dome with a targeted small asteroid to deter war and to use it when and if war breaks out.
An alternate strategy would be to hit Ganymede or Callisto with an asteroid big enough to make the entire surface molten hot to a depth of several miles. That will wipe out all citizens of the enemy nation on Ganymede or Callisto - as well as any citizens of neutral nations that might live on Ganymede or Callisto.
Such threats will be effective, but to a lesser degree, in conflicts between totally space dwelling nations who live only in artificially constructed space habitats. If Nation A consists of 1,000,000 citizens living inside one large space habitat, one single asteroid would be enough to destroy the habitat and the nation. If Nation B has 10,000,000,000 citizens living in 1,000,000 space habitats each containing 10,000 people, it will take 1,000,000 KKV to destroy Nation B.
Thus it is theoretically possible that the threat of KKV will lead to mutually assured destruction keeping the peace in space forever.
It is also theoretically possible that the threat of KKV will make space governments much less likely to go to war, it will greatly reduce the probability of war in any particular moment of time, but it will not reduce the probability of war to zero. So wars will still happen from time to time and result in the elimination of one or both nations or alliances of nations.
Thus there will be a gradual elimination of space nations until there is only one government in the solar system. Either all the nations will be exterminated except one, or one nation will conquer other space nations to form a space empire of many nations, or all surviving space nations will agree to mutually surrender to each other and unite to form a space empire of many nations.
Another possibility is that all nations on the home planet will reform and give up war forever before they begin space colonization. Thus everyone who colonizes outer space will be educated to believe that war is evil and nonviolence is the only good way. And so there will be peace in outer space forever and no space wars using KKV. I find that rather hard to believe.
Another possibility is that all nations on the home planet will be united by conquest and/or negotiation into a single empire of many nations before the colonization of outer space begins. Thus everyone who colonizes outer space will be educated to believe that war is evil and the imperial government is the only way to avoid the horrors of war and have peace forever.
Therefore it seems to me that the end result of colonization of the solar system and all of the space wars that might hypothetically happen over many thousands of years would probably be waving the white flag and surrender of remaining nations to form an interplanetary empire ruling the entire solar system based on the ideology that any independent government anywhere would make war, death, and destruction inevitable.
Except that if interstellar colonization is possible in voyages lasting decades, centuries, or millennia it may be impossible to unite colonies in other solar systems and there might not be enough contact with those colonies to make any war with them possible. So the ideology might be that no independent government with reachable distance can be tolerated, but independent governments too far away to make war with are tolerable. And presumably, each and every colonized solar system would have its own empire with the same ideology.
And then, after many thousands or millions of years, a faster than light (FTL) drive might be invented making contact between the different system empires fast and cheap and easy, thus making it possible for them to have reasons to go to war. And so there might be wars between different system empires involving KKV weapons. Perhaps there might be a sort of Lensman arms race and invasion fleets might be accompanied by numbers of asteroids, planets, and stars with faster than light drives to smash into targets.
And after a longer or shorter time, there might be a union of all the system empires into one interstellar empire. And all new colonies would be colonized by people loyal to the interstellar empire or by refugees from the interstellar empire, refugees which the interstellar empire might seek to conquer and annex when it discovers them.
And possibly an expanding interstellar empire might encounter other expanding interstellar empires. And possibly there might be a shorter or longer period of wars between expanding interstellar empires. And eventually, all the interstellar empires might unite to form a galactic empire.
Depending on the speed or acceleration attained by the FTL interstellar drive it may or may not be possible to rule an entire large galaxy like the Milky War Galaxy.
Depending on the speed or acceleration attained by the FTL interstellar drive it may or may not be possible to travel to and colonize star systems in other galaxies in voyages taking days, weeks, months or years. It is quite possible that the speed or acceleration of the FTL drive make voyages to even nearby other galaxies take decades, centuries, or millennia.
There is a question about whether a galactic government would be possible without instant teleportation.
My quite long answer describes how such a galactic government might be able to work. So if a writer is convinced by that discussion that a galactic government with slower than light travel was possible, they could select a speed or acceleration for their FTL drive that made ruling a galaxy practical, perhaps with difficulty, but made ruling other galaxies impractical and colonizing other galaxies just barely possible with generation ships.
Therefore other nearby galaxies could be colonized, but only by voyages taking decades, centuries, or millennia, too long for any sort of regular contact between galaxies or for galaxies to have any reason to go to war.
So possibly a number of daughter galactic empires will be formed by colonists from the home galaxy. And possibly alien civilizations will form their own galactic empires in various galaxies.
And maybe after many thousands or millions of years a new FTL drive might be invented which makes travel between galaxies in mere hours, days, weeks, months, or years possible, instead of in decades, centuries, or millennia. It suddenly becomes easy, cheap, and fast to travel between galaxies, meaning that it is now possible for galactic empires to fight wars against each other.
Thus there may be wars between galactic empires. And eventually, an entire supercluster of galaxies might be united in a supercluster empire.
And maybe after many thousands or millions of years, an even faster FTL drive might be invented, making it possible to reach any place in the universe in hours, days, weeks, months, or years instead of the decades, centuries, or millennia it might previously have taken.
This will make contact between supercluster empires easy, fast, and cheap, and so it will be possible for supercluster empires to find reasons to go to war against each other. Thus there could be a shorter or longer period of wars between supercluster empires.
And eventually, supercluster empires might unite to form a universal empire.
Thus in the history of that universe, there would be five general stages when there would be space wars.
1) Wars within a solar system eventually resulting in either extermination or a system empire.
2) Interstellar wars between system empires eventually resulting in either extermination or an interstellar empire.
3) Wars between interstellar empires eventually resulting in either extermination or a galactic empire.
4) Wars between galactic empires eventually resulting in either extermination or a supercluster empire.
5) Wars between supercluster empires eventually resulting in either extermination or a universal empire.
And it is possible for different regions to be at different stages at the same time. A person might fight in interplanetary wars resulting in the formation of a system empire, and their child might see their system empire contacted by an expanding interstellar empire, and their grandchild might see their interstellar empire contacted by an expanding galactic empire, and so on.
And any of those stages might see the use of KKV weapons, whether missiles, ships, meteors, asteroids, comets, moons, planets, stars, neutron stars, black holes, or whatever, possibly involving a Lensman arms race.
And any one of those stages of on and off space wars might last for decades, centuries, millennia, etc. But if the civilization involved in any stage of a space war is going to last for a long time, the period of space wars will be a relatively short and minor period in the history of that civilization. The age of space wars cannot last forever.
And IMHO the best defense against KKV weapons is the white flag of surrender, preferably a negotiated mutual surrender to form an empire before the fighting starts.
Very good, ubiquitous surveillance systems.
If a KKV is coming at you at 0.99c from a couple light years away, it will take a couple years and a few days to hit you. That's orders of magnitude more time than you need to:
Calculate the trajectory for an interceptor KKV of your own with an app running on a 2010's smartphone.
Pick a proper, prebuilt counter KKV of your own or make a new one;
Launch your KKV at the oncoming KKV.
For a KKV to be effective as a terrorist weapon it would have to be fired from up close. But as long as governments can know where every sufficiently-sized launcher is and destroy or capture them beforehand, everybody should be safe.
Slow-moving asteroid must originate in target system.
Moving any object at a speed much slower than the speed of light from another system would take thousands of years and make war as we know it meaningless.
So, an enemy must scout a proper object, likely in target system's Kuiper belt, and direct it towards the inner planet. This process should take years, even if object's orbit is perturbed enough to make direct hit without several rotations. Target civilization should be advanced enough to detect this kind of activity in its own backyard and take measures before the asteroid is set on the dangerous course.
And, at any rate, with similar tech level, it would take defending civilization less time to push the asteroid off-course than for the offending civilization to put in on collision course.
The above covers the "slow asteroid" scenario. For the impactors moving at relativistic speeds, situation would be different.
You assume that if space travel is trivial, then throwing rocks will be trivial too. But what you are missing is that living in space instead of on planets would be just as trivial! Why bother living on a planet if it takes so much more effort to get things off of that planet? Most people would be living in space-based cities, nice and controlled (with the technology they would have it would be) with the option to actually redirect the entire thing off the course of a KKV, which you can't do with a planet.
Throwing KKV's would be a weapon of terror to kill off population, but the lion's share of materials, construction and living will be done off-planet.
Now that the homes of your people are much safer, war can be around the KKV's. Even with early detection, no one is going to sit around until those KKV's come flying. So you set up scouting parties that go out and find KKV's that are still speeding up, giving you a chance to easily send a warning signal to potential targets to get out of the way and a place to wage war: Hunt down teams that set up KKV's and the KKV's themselves, while you are trying to find suitable KKV's and protect them while you swing them at your enemy.
In a wider perspective, it is easy to place the defender in a position of advantage in an interstellar war (In the hypothesis that only one side has a long-time established colony on the planet).
Since the attacker must make a hyperspace jump to reach the defending system, every ship of the attacking fleet must be equipped with a bulky jump engine (in addition to the slower-than-light engines necessary to navigate in the system of the star).
So, the ships of the attacker will be inferior as weaponry with respet to the ships of the defender, since the latter ones don't need to use jump engine (having been built inside the same system).
In order to attack a planet with an asteroid, the attacker will need:
- time to overview the system to find a suitable impactor (if no intelligence about the objects in the system was gathered before)
- time to reach the asteroid (which, by the way, must be near enough to the planet to destroy/attack)
- time to build the facilities to modify its trajectory (I don't think that pushing the asteroid with the starships themselves would do the trick)
In the meanwhile the defender will have plenty of time to detect and attack the enemy fleet (being in a position of advantage, as explained above). So probably the attacker should find different ways to conquer a planet, maybe outnumbering the defender or trying guerrilla-like techniques.
Three thoughts, all taken from my experiences with the Traveller RPG.
My first thought is to make technology unable to push big dangerous masses in a manner convenient for warfare. The Problem with that is that starships typically move at dangerous speeds and have good armor, making them potentially devastating "bullets". If they also rely on big power plants too, they could be potentially devastating bombs.
So my second thought is to field a sufficient orbital (and even system-level) defense, including early warning systems, automated systems, what-have-you, all with the ultimate intention of deflecting incoming destructive masses as early as possible. The Problem with that is that low-tech worlds are at the mercy of high-tech worlds.
As an illustration of the problem with #2, consider the very common Traveller scenario where a world at about the level of Earth in the 1970s is attacked by a nearby star system that is, say, a couple hundred years' its superior, with antigrav, interstellar drives, and cheap fusion power. I can't see any outcome of the above scenario where the "Earth" above does not become a vassal state, unless there are external protective forces at work -- a galactic government, or a "Big Brother" system.
- A contrived solution might suggest that interstellar wars are never "to the death" but rather are economic -- all about controlling resources -- and therefore big rocks thrown at near-C velocities are for the realm of the insane genius madmen bent on annihilation. The problem with this is that sometimes in order to secure a resource over here, you have to stop the technological industry of a system over there. How are you going to do that, if not by hurling a bunch of kinetic masses at it until you've stone-aged them?
A few ideas:
- If moving asteroids towards planets is trivial, and I suspect that these asteroids would have to travel quite far, then moving asteroids into intercept-paths with other asteroids would only be slightly less trivial, as long as you could detect the KKV well in advance. In a cold-war scenario, it might be incumbent on a defending planet to put several large bodies into safe orbit around itself, with propulsion attached to them, in preparation for just such an event.
- If you have FTL travel, then depending on how you do it you may incorporate the same kind of technology into your planet shields.
- For example, if FTL is accomplished by warping space, and a small ship is only capable of generating enough power to warp small space nearby itself, then a large power generator on a moon may be capable of warping a large space far away from itself. As soon as the KKV is detected, the moon activates and warps space in front of the KKV, effectively transporting the KKV some distance in any direction without changing its velocity. If carefully done, the space could be warped into a sort of toroidal shape and then released, so that the KKV is sent in another direction -- perhaps back on the enemy.
- If FTL is accomplished using "antimatter fuel", then access to antimatter in large quantities may imply the ability to generate antimatter bombs, set to detonate immediately after coming into contact with a physical object. The bomb would pass right through the KKV and implode immediately behind it, creating a small temporary black hole (perhaps). The intended effect would be to simultaneously destroy the propulsion device on the KKV while pulling the KKV backwards -- slowing it down or stopping it altogether.
- If FTL is accomplished by entering "hyperspace", presumably via a "hyperspace gate", then (if scifi has taught me anything), since things in hyperspace can't interact with regular matter, the KKV could be rendered harmless by forcing it into hyperspace. Maybe this can be done by throwing a hyperspace "entrance" gate in front of it and then destroying the "exit" gate once inside. It's an expensive solution, and the cost is increased because, since hyperspace things move so quickly, you'd have to place the exit gate very far away in order to be able to destroy it in time. The way I see this being accomplished is by saying that the gates are entangled somehow -- if you destroy one, then the other destroys itself. Furthermore, since you need to be able to quickly generate lots of these, it will be important to keep one gate open at all times to pass parts through it to make more exit gates at the "endpoint" location, only to have them destroyed when another KKV comes in range.
- If advanced space travel implies advanced radio capabilities, then strong beams of radio waves (microwaves), much more powerful than what we're capable of producing today, could be used to cook the inside of the KKV, causing it to melt and burst into smaller, more manageable pieces. If done right, the smaller pieces may harmlessly disintegrate on contact with their target atmosphere.
Hope these ideas help!
For relatively slow impactors, such as typical asteroids, an advanced telescope survey system, along with deflection technology — gravity tractors, kinetic impactors, nuclear devices (possibly even adapted nuclear shaped charges), and the like — would probably catch almost anything an enemy could throw. (Other answers have already covered that case better than I have.) If the enemy is sending relativistic impactors, though, deflecting them is largely futile — even nukes won't do anything more than turn the projectile into a still-relativistic cloud of exploding gas hurtling at the planet the same as before. The sheer amount of momentum involved with relativistic KKVs means that deflecting them is virtually impossible without (1) ridiculous amounts of warning (i.e. possibly longer than the light-travel time from their launch point) or (2) gravity control technology (which is certainly possible but well beyond technology foreseeable in the near-to-medium range).
Move the planet.
See, the problem with this is that it takes ridiculous amounts of energy to move a planet — and even more ridiculous amounts of power to do it quickly. So that's out of the picture. End of sto... Wait. You say it's possible. Umm... But there's no possible way to move an entire planet that fast without gravity contro... Wait... Ok, I see what you're saying.
Move the planet, for a given value of "move the planet".
Not the entire planet, for obvious reasons. The most important thing is that the planet not be in the way of the KKV when it arrives — but the planet is far to heavy to move. This seems like a conundrum, but a surprising solution presents itself: the impactor will have to pass through the planet. Not by "phase shifting" or some other pseudo-scifi magic tech, but with a good old-fashioned tunnel — move only the part of the planet that's in the impactor's way.
This presents problems in of itself. Although quickly building a tunnel through a planet is incomparably easier than deflecting a relativistic projectile or moving an entire planet to the side, it is still incredibly difficult by near-future technological standards. But I wouldn't say it's impossible.
Here are the components of my imagined version of a planetary defense system using this idea:
- Extensive, frequent, and powerful telescopic surveys to spot incoming projectiles from up to light-years away. (This could be combined with surveillance focused on known or suspected enemy systems to spot launches immediately.)
- Extreme-precision tracking, to pinpoint exact impact locations to accuracies measured in meters.
- Some way of disabling KKVs' maneuvering systems. Something like a tiny, well-aimed laser mounted on a fast interceptor craft or a vulnerability in the KKVs' guidance software, maybe.
- The actual tunnel-construction technology. This could be a more typical tunnel boring machine, adapted using novel forms of waste heat removal for the incredibly harsh conditions of the planet's hot (probably, but depends on the planet) interior, or something more outlandish like a rocket-propelled line fired through the planet and then inflated to form a tunnel. Whatever it is, it needs to be fast — the worse the telescopic surveys, the faster.
- Some means to ensure a vacuum within the tunnel. A "tunnel" will likely have to be created through the atmosphere as well to keep air from venturing in from around the edges. The tunnel material will also have to be well beyond current technologies by being able to withstand 6000K temperatures (in the case of Earth; extremely advanced active cooling may also reduce this) while maintaining structural integrity and having a negligible vapor pressure. Although a few molecules here and there won't cause much of a problem (aside from maybe destroying the tunnel behind the passing projectile, which is the least of your worries at this point), any significant amounts of matter in the would-be impactor's way will either devastate surrounding regions due to extreme energy release or, worse, explode the impactor, causing a bona fide collision.
In all, I think this is a possible solution, although of course I haven't run any numbers to double-check (in any case its speculation about technologies that don't exist yet, so I wouldn't be able to check those). Even though there are tons of incredibly difficult engineering, physics, and materials science problems to be solved, though, there isn't really any other good way to keep a relativistic weapon pointed at a planet from devastating said planet. Creating a tunnel through the planet for the impactor to pass through safely is the only way I can think of to do this without manipulating gravity itself or handwaving the problem away.
This solution works just fine for most classical types of relativistic weapons. However, none of this applies if the enemy is smart enough to vaporize the projectile themselves before it hits the planet.
Science fiction workaround: E=mc2 brake bomb.
I made this up as a workaround to prohibit the sort of war you want to avoid while allowing other types.
Usually when something fast hits something else, the kinetic energy of the impactor turns to heat and also kinetic energy of the masses impacted. Energy cannot be created or destroyed. But energy is mass, and one could dispose of unwanted energy by converting it to mass.
When the brake bomb hits a moving object (of specified energy or greater), the kinetic energy difference between the bomb and the object is turned into matter. This small amount of matter is added to the matter of bomb and object, which continues on its prior trajectory at whatever (low) velocity it has remaining.
The bombs are small and cheap, and are set to orbit occupied planets. They also have peacetime uses as they could slow then stop a runaway train or act as a cushion for falling objects.
It is difficult to use them offensively although an offensive use of these bombs would be a fine thing to have occur in the course of the fiction.
My favorite way to turn energy into matter is by fusion of iron or heavier elements: an endothermic "reaction". Perhaps the brake bombs have kinetic energy catalyzed fusions. If that is how they worked, these bombs would produce heavier elements on being triggered.
First thing to consider is how things burn up in an atmosphere, the faster you are moving, the more likely you are to burn up; so, a ship that can survive re-entry 20,000 kph will just spectacularly explode in the upper atmosphere moving at 100x that speed. The faster you move, the higher you vaporize, meaning the less opportunity your energetic explosion has to propagate to the denser lower atmosphere to do meaningful damage; so, super fast, smaller things like ships are not that dangerous.
I'm sure once you reach truly absurd speeds, you eventually make atmosphere a non-issue, but most sci-fi assumes that FTL technology does not actually involve accelerating to relativistic speeds, but rather warping of reality so that a "slow moving" thing can travel as though it were moving very fast. If you follow this convention then gravity may disrupt your warp bubble spitting out your FTL ship at its actual speed which may be no faster than modern spacecraft.
So, to survive reentry and result in meaningful damage, you'd need a bigger slower thing like a giant asteroid, but a space aged civilization could see that coming so far ahead of time that they could deflect it using the same technology their enemies used to put it on course to begin with making that a non-tactic as well.
This leaves carpet nuking, but a good array of ground based lasers could just destroy those moments after they are launched. So, these nukes would need really good shielding to survive these defensive weapons; meaning it would not be cheap at all. The question is then why spend that much money destroying a world just to irradiate it too much to use for your own resources.
In Dune, Frank Herbert envisioned a kind of personal "shield" (force field) that would stop fast-moving object but not slow-moving objects. That was the explanation for why you couldn't shoot someone with a gun, and everyone was fighting hand-to-hand with knives. No reason you couldn't steal this idea and scale it up to the level of a planetary defense.
Fundamentally, you need to do three things in order to protect against a KKV attack:
- Develop and stockpile an interceptor capable of destroying or deflecting an incoming KKV (this could be a KKV or something else entirely).
- Clean up your backyard. Remove anything in your general vicinity that would be dangerous if it impacted your planet.
- Deploy a network of satellites to monitor for incoming KKVs.
With these three things, you will be able to see any KKV attack incoming and destroy it at a safe distance. The specific distances here will depend on the level of technology used by you and your enemies. If it takes you $X$ hours worst-case to receive a signal, prepare and interceptor, and launch it, then you'll probably want your monitoring satellites at a distance of at least $0.00035*X$ light years to ensure you can intercept with plenty of room to spare. It's probably worth extending your "clean" zone to around $0.0005*X$ light-years to make sure you can easily detect anything crossing into your cleared zone.
You're only vulnerable to attacks that are launched so close to your planet that you don't have time to react and neutralize them. If you can ensure nothing hostile gets that close, then you don't have much to worry about.
- If it's multiple projectiles, have a defensive weapon that forces those projectiles to smash into each other.
- Watch the end of the movie "The Beyond" on Netflix and use the "alien tech" at the end of the movie as an example--it's basically portable shielding.
- If it's one large projectile, as others mention, an early warning system is best.
- If the projectiles have stealth capability, this could ruin early warning systems.
- A "thick/reinforced/shielded atmosphere" would probably destroy most small projectiles.
- A "planet mover" technology could just move the whole planet out of the line of fire.
- Consider the fact that a planet that has been hit by an asteroid would probably have very low value to the "captor".
- Shielding moons. That is, a movable moon that can intercept projectiles.
- If "warp bubble" technology exists, then bending spacetime around the asteroid could alter the course of a projectile... so much so that you could send it back to the civilization that threw it at you.
- Similarly, if you could make thousands of small warp bubbles, you could fragment the projectile into much smaller shards, capable of much less damage.
- Ionize it. At a few thousands degrees, it would turn into a lava-like substance and its structural capabilities would be greatly diminished. At a couple million degrees, it'd become plasma and hitting the atmosphere would make it look like northern lights.
- Contact with antimatter will create a total annihilation (and a big boom).
- Interfere with the opponent's guidance system.
- Create a "solar atmosphere", where the entire solar system acts like an atmosphere and tears apart incoming projectiles. Think of "fluidic space" from Star Trek Voyager. The idea is that most things can't stop a 50 caliber bullet, but a lot of anything can (so, 20 phone books can stop a sniper round, but a single steel plate cannot).
- Portable black holes or gravity control.
- Subspace barriers. If the projectile must move along a smooth patch of spacetime, any interruption in the fabric of spacetime would prevent that movement, like a speedboat hitting a sandy beach.
- Super advanced civilization. Restore your planet from a backup. Physical matter reforms to its last known stable state, including auto-resurrection. The asteroid would be little more than a pebble thrown into a pond.
- Friendly intervention. Friendly civilizations could help you monitor and mitigate asteroids. Using asteroids as a war tactical result in a multi-civilization counter-attack.
- Jamming. If they use teleportation, jam it. Subspace transport. Jam it. Hyperspace. Jam it. Peanut butter. Jam it.
Words of warning:
- Slow moving asteroid ideas are highly "played out".
- Don't try using "solar powered" object movers, since solar power is lost at the square of the distance, meaning solar power is useless at significant distances from a star.
- Ionic propulsion takes a very long time to get up to speed and eventually runs out of fuel.
- Throwing asteroids at an enemy seems like the equivalent of rock throwing in a third world country. There are probably much better ways to fight an opponent.
Assuming a universe where crashing large asteroids into planets is both possible and for whatever reason used for offensive purposes, we could also imagine that a defending planet with access to similar technology could counter the attack by setting into motion an equivalently sized asteroid on a collision course.
You don't. If something is going at .99c towards you throwing things at it is useless because the KKV's time frame is much slower then your time frame. The antibalistic things you throw at it won't have time to shatter and spread the KKV's matter. Also, if you throw something at the KkV with enough energy to change it's course you are actually doing what particle accelerators do, but instead of a few protons you are doing with things that have the mass of a car. I can't do the calculations, you should ask in the physics stackexchange, but it probably won't be healthy to be in the same solar system in which this collision is happening.
You survive by getting out of the way, living in small, mobile, fast space habitats: a civilization of space mongols riding their ships in the black steppe.