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In the book The Killing Star, Relativistic Kill Vehicles (RKV) are described as the ultimate weapon.

They are described as impossible to stop, so that any civilization that develops them better start using them against everyone or else risk total destruction itself!

I'm brainstorming for ways to defend against RKV. I was thinking that making the orbit of my planets more complicated would make them harder to hit, since it takes some time to accelerate an object to 0.9 c from outside a star system; for this reason if your trajectory is hard to track it gets harder to "aim" the RKV. At the beginning, I thought of making my world a Trojan moon - to add "cycles" to the orbit. Then I thought of replacing my planet with a Bank's Orbital to reduce its profile.

Will adding bodies in a tight configuration (was thinking the hexagonal rosette) help with making the system even more unpredictable, while still providing basic stability?

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    $\begingroup$ Dust. Lots and lots of it in orbit around your planet. When they are coming in at that speed, even microscopic particles will have a very severe effect on those projectiles. And you do not even need to cloud the sun for that. Even a ratio of 1 to 1000 area coverage will still provide quite good an obstacle for these things. The big problem becomes getting in and out of there yourself. You will need to create dust-free openings. $\endgroup$ – MichaelK May 13 '16 at 15:36
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    $\begingroup$ I've thought about space war in such environment for a very long time. The best defense for such a weapon is to put your valuables into motion (e.g. under acceleration). Anything that could be in a predictable place at a designated time, could be destroyed. Putting your valuables into motion makes a preemptive strike much less likely to succeed. $\endgroup$ – Jim2B May 13 '16 at 16:19
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    $\begingroup$ @MichaelKarnerfors Not so sure a dust shield would work. But even if it does it only works once, the killers would just launch two waves of RKV, the first to clear the area $\endgroup$ – SilverCookies May 13 '16 at 17:06
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    $\begingroup$ I wouldn't expect a kinetic weapon to clear the dust. When it hit the cloud and broke up, it would contribute to the cloud. $\endgroup$ – T.J.L. May 13 '16 at 17:41
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    $\begingroup$ I agree with Michael, that dust is the answer, but the dust needs to be smart dust. The dust needs to be able to sense and maneuver and communicate just enough to be able to skew the trajectory of the RKV to miss. Remember, even at relativistic speeds in one direction, the laws of physics still work in the other two. The real question is time. How long do you have from the moment the weapon is detectable to the moment of impact. That lets you know how much sideways acceleration you need to impart to the RKV $\endgroup$ – Cort Ammon May 13 '16 at 17:50
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Making it more "complicated" doesn't make it less predictable. What you need is to make it chaotic. It needs to move in a way that can't be predicted far in advance, either because it is so chaotic that no measurement is good enough, or because you actively mess it up at random.

An example of this are the outer three moons of Pluto Because they orbit a changing off-center primary (a binary object) they orbits don't repeat exactly, although they are held in formation through complex resonances so they don't fly apart or crash.

Interestingly, the rotation of these moons is even worse. (Though I knew about Hyperion long before these were discovered, and its chaotic rotation has been featured in science fiction stories.)

Nix can flip its entire pole. It could actually be possible to spend a day on Nix in which the sun rises in the east and sets in the north. It is almost random-looking in the way it rotates.

You want the body's position to be this random, though the tumbling is a more severe form of the effect. So here's an idea to turn the rotation into a position: imagine two rocks on opposite sides of the moon. Their locations in space, if the rest of the bulk was ignored, is random.

So make a second-level binary object. The two bodies rotate around each other, but the axis of rotation is subject to this chaotic gravitational torque and precesses all over the place, on a rapid time scale. If they are artificial habitats you could use tethers to directly have a rotating body with a mostly-empty middle.

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    $\begingroup$ And a recipe for chaos would be to have a substantial number of planets with similar masses and nearby/intersecting orbits. However this is also likely (in geologic time at least) to cause planets to smash into each other and kill everyone. $\endgroup$ – hobbs May 13 '16 at 19:54
  • $\begingroup$ Right, if civilization declined so they could no longer maintain active control, their days would be numbered. $\endgroup$ – JDługosz May 16 '16 at 15:47
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Active defence

The key to this is early warning, you have to know as soon as possible and as far out as possible that these things are coming. The VDA (Very Dangerous Array) is a good place to start with this as it's self defending and the array nature means it can't be easily knocked out.

The sooner you spot them incoming the more accurately you can plot their trajectory and get into position to fire something equivalently energetic back along its path to intercept. If you're really good you could hit it with a much lower energy object across it's path and knock it far enough off course to no longer be a danger.

It's a whole lot cheaper than accelerating a planet. If you have multiple VDAs up and they all get knocked out simultaneously, just accept your fate.

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    $\begingroup$ "Something equivalently energetic" What exactly would that be and how would you accelerate it so quickly? We are talking about something moving at .9c, which is a heck of a lot of energy and not something you could easily intercept in the small window of time you have $\endgroup$ – Kevin Wells May 13 '16 at 21:06
  • $\begingroup$ @KevinWells I left that deliberately vague as the technology level is unknown to us $\endgroup$ – Separatrix May 14 '16 at 7:52
  • $\begingroup$ Vague here isn't really a good thing, I think the OP is looking for concrete solutions $\endgroup$ – Kevin Wells May 15 '16 at 15:55
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    $\begingroup$ He'll have to tell us what technology is available for me to design it for him. At the moment he seems to have the ability to move planets around casually so he should be able to get something up to a reasonable percentage of C without too much trouble. $\endgroup$ – Separatrix May 15 '16 at 16:32
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The problem here is two fold:

  1. An RKKV is not going to be detectable very far in advance, since it is essentially coming in just behind its light cone. Any sort of active or passive defense will not have much time to react to the arrival of an RKKV. Indeed, if a defense is deployed, it must be totally autonomous, since there will be no time to report to a higher authority.

  2. Planets have immense amounts of inertia. Moving a planet is a long and involved process (for example slinging asteroids past the planet to exchange momentum). XKCD has a useful example A Kempler rosette simply has bodies moving in a regular orbit around their common centre of mass, and individual planets, or even the entire rosette are not going to move anywhere very quickly.

Defending against an RKKV would probably involve many of the different ideas expressed in these answers being used in conjunction. Clouds of gas and dust would certainly degrade a wave of RKKVs, and the incoming weapons would be rapidly converted to plasma by the intense energy release (see Relativistic Baseball and Diamond for a more detailed description of what happens at these speeds). Any object that is being targeted will need the ability to move quickly, so while it sucks to be on the planet, if you are in a spaceship in orbit around the potential target you blast away at maximum thrust the moment the plasma clouds appear in the outer Solar System. Moving in a random or chaotic path is the only sure way to avoid being targeted, but as noted, with planets you run the risk of colliding with other bodies, which is not going to make your day much better.

So the real issue is the mismatch between the time you have to react and the amount of energy needed to actually move a planet. You might also consider that suddenly applying petajoules of energy to a planet in a matter of minutes to hours to get out of the way will also result in massive earthquakes and other geological damage to the world.

The most "realistic" answer is the civilization in question may quietly evacuate their homeworld and scatter to low energy enclaves throughout the Oort cloud to deny a lucrative target for the RKKV's, and have a MAD like defense system ready to backtrack incoming RKKVs and launch a counterstrike when they are detected.

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  • $\begingroup$ This, this, this. Once you start assuming access to unreasonable amounts of energy for setting such weapons in motion, the apparent velocity can easily exceed $c$. In extreme cases if could be tens of $c$: so getting a signal from your tracking array light months out gives you a week or so to try to divert or disperse an object with $10^{14} \,\mathrm{kg\,m/s} or more momentum that is only meters across. And keep in mind the level of kinematic focusing in collisions at that speed; just vaporizing the RKKV isn't going to save you because the gas column still has that momentum and energy. $\endgroup$ – dmckee May 14 '16 at 4:06
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The orbits of your planets will always be predictable from observations if you are not constantly adding delta-V. A rosette will shrink the window of opportunity for some angles, but have very little effect overall.

Since we're talking about planets here, any noticeable delta-V is going to be hideously expensive. If you have this energy available, you might as well just add giant thrusters to your planet and zoom around randomly, though at a fixed distance from the star.

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  • $\begingroup$ interesting. Any idea of how to move a planet without losing mass and keeping it close to orbit? $\endgroup$ – SilverCookies May 13 '16 at 17:04
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    $\begingroup$ Hmmm, planet-sized wormholes? $\endgroup$ – Cyrus May 13 '16 at 17:52
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Defense is Easy

Anything coming for you at such speeds will hit dust or gas and produce gamma rays. It will be highly visible. Move small spacecraft out the way or large asteroids in the way. Or hit it with a high power laser. Containing the projectile is hard but atomizing it and spreading out the energy could be done by putting something in the way, even thin solar sails would turn it into a cloud of fast atoms.

If they are impossible to stop wouldn't Mutually Assured Destruction stop anyone using them?

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  • $\begingroup$ The question is about using orbits to defend, not how to defend in general. $\endgroup$ – MichaelHouse May 13 '16 at 18:17
  • $\begingroup$ Turning into a cloud of fast atoms doesn't help if they all still hit you. The amount of transferred kinetic energy stays the same either way. This is like the suggestion to nuke comets that are going to hit earth. If all you do is spread it out, you are just spreading out the destruction, not preventing it $\endgroup$ – Kevin Wells May 13 '16 at 21:04
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    $\begingroup$ @KevinWells why wouldn't spreading it out help? Sunlight doesn't hurt, but if you concentrate the sunlight even from 2m^2 area (really not that big), you can melt pretty much anything. It's the same idea (in the opposite direction). $\endgroup$ – Rob Watts May 13 '16 at 21:51
  • $\begingroup$ @RobWatts While there are definitely situations where spreading out the impact helps, spreading out the impact in the situation we are talking about is essentially a question of, would you rather get shot by a shotgun slug or bird-shot? Either way its going to kill you even though the bird-shot will be a lot more spread out $\endgroup$ – Kevin Wells May 13 '16 at 22:04
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Your best bet might be to redirect the projectile. Create a micro-black hole that could alter the course of the RKV enough to miss the planet. Have it designed so the black hole collapses quickly enough to not be a problem.

If you want to avoid black holes, perhaps some sort of strong magnetic force could be used to form a sort of ramp out of smaller particles that can cause enough friction to change the direction the RKV is traveling. This answer suggests that even the smallest particles could cause enormous drag.

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  • $\begingroup$ The trouble with a tiny black hole is that it has both tiny gravity and tiny momentum. If it's not very close to the RKV it will have negligible effect on it. If it's very close to the RKV it may absorb the RKV, resulting in a larger black hole moving at almost 0.9c in roughly the same direction (still likely to collide with your planet). $\endgroup$ – trichoplax May 14 '16 at 1:16
  • $\begingroup$ @trichoplax true. The idea would be to perfect some way to create one accurately enough to avoid those issues. $\endgroup$ – David Starkey May 14 '16 at 1:25
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Dust

You turn the entire concept around on the attackers. If a tiny little RKV can kill an entire city, then a tiny little speck of dust can kill an RKV. Any RKV that tries to go through this will explode much earlier than intended.

An objection anticipated: but if an RKV his the dust shield, will that not leave a "hole" to shoot through again? Not really no. First the hole will be relatively tiny. So you have a really small target to try to shoot through again. And you cannot send a train of RKV through because the destruction of the leader will create even more dust and debris which will knock out the followers.

Second: the dust is not hanging there static. It is moving at several thousand meters per second in order to stay in orbit. And the attacker can never know which direction the dust — and thereby the hole — is moving. You can even have several layers of dust, at different orbits, moving in different directions relative to each other. This means the shield becomes self-sealing because any "hole" punched in one layer will instantly be covered by the other layers.

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Dodge.

No need to complicate things with more bodies with the planet.

If the RKV is already under way, and it cannot be redirected, just accelerate the planet to a slightly different orbit, or further along on the same orbit; the RKV will miss.

If the RKV can be redirected, same solution, but don't stop moving: monitor continously the RKV's trajectory, estimate where it will collide, and don't be there when it will pass. Since the RKV is much faster than a planet, it will overshoot if the planet makes a last-minute (er, last-week) impulse.

If the RKV isn't underway, just wait.

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  • $\begingroup$ The idea behind an RKV is that you don't have enough time to dodge. We are talking about changing the orbit of a planet, which is no easy feat in any situation, but if we add the extra constraint of not having much time between detection and impact it gets very much harder $\endgroup$ – Kevin Wells May 13 '16 at 22:05
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When life hands you a lemon, it's time to learn how to make lemonade.

Your society has the wherewithal to create and stabilize rosettes. The obvious next step is to create a Dyson Sphere outside the orbit of the rosette. This will serve both to hide the planet locations and to provide power for your civilization.

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