When people consider a war between planets, they immediately think of throwing big rocks at it. What my question is, when you know another planet is going to attack, what is the first thing that should be done ? Do you immediately start building a giant death star ? Do you ramp up your production of anti-matter ?

Assume for this question, that people can travel at .9c but can only send messages instantly. We(Earth) are fighting a planet that is 1 light-month away, and that planet is really determined to destroy Earth (We don't know why). Also, everyone on Earth knows this and is working together (except for like 10% who protest and slow things down and 5% who help, but in a way that benefits themselves, which also slows things down). Earth and the enemy planet are both Type 2 civilizations .There is a world government, and then country governments. (For Earth)

Question: What are the first things that should be done -

  • Political-wise : You have to act as if you can win this war, and start mobilising the scientists, but what do you tell ?
  • Military-wise : What weapons to create first ?
  • Defense-wise : How will you defend yourself ? What steps do you consider ?

(Just answer these 3, and this is the first question in a series of questions)

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    $\begingroup$ @Euphoric Your answer is good, but the enemy is one light-MONTH away from Earth, and it is hard to be stealthy in space. (Especially from a type 2 civ, even if the enemy is also a type 2 civ). $\endgroup$ Commented Jun 29, 2016 at 9:17
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    $\begingroup$ Obligatory XKCD reference. $\endgroup$
    – PTwr
    Commented Jun 29, 2016 at 11:17
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    $\begingroup$ Isn't one light month still ridiculously close to the Sun, though far beyond Pluto? Given Alpha Centauri is 52 times as far away.. $\endgroup$
    – Chieron
    Commented Jun 29, 2016 at 12:37
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    $\begingroup$ One wonders how they can have instant messaging and how much more fun would everything be if they hadn't, taking 1 month for every message to arrive its destination. :D $\endgroup$
    – xDaizu
    Commented Jun 29, 2016 at 13:38
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    $\begingroup$ @KingofSnakes For solar systems near the galactic core, having two solar systems only a couple of light-months away is feasible (the average distance would be somewhat below half a light-year according to some guy on reddit) but the closest solar system to us is light-years away (and the average is also really high, though I don't know the number) so if one of the two planets is earth, there are some problems with that scale of distances. $\endgroup$
    – Jasper
    Commented Jun 29, 2016 at 17:09

5 Answers 5


Everyone is going to die.

An observed reality of warfare is for offensive technology to ever outstrip defensive technology. One obvious example is guns-vs.-knights (guns are still effective centuries later). As your Kardashev level increases, the stakes increase exponentially (literally; it's an exponential scale). As a direct result, the only way to truly win a war in the far future is to either strike unilaterally, or to have a vastly superior tech level.

However, your situation is balanced technologically, and both sides are in agreement that a state of war exists between them. So both sides kill each other. I'll unpack for you exactly why, also explicitly answering your questions:

  1. Political:
    There is massive denialism abut the above facts, and nationalist (or Earth-ist, as the case may be) pride assures the people that they'll win. It isn't a problem to motivate people against existential threats so long as they're near-term.

  2. Offense:
    Offensively, the answer is obvious; from as many sources as you can, you send out relativistic projectiles toward everything the enemy owns. Back them by laser bombardment.

  3. Defense:
    Defensively it won't really matter. But you can try to shoot lasers at the projectiles that they will launch before they reach final speed (throwing off the intercept). If you're ambitious, you can try to move something like a planet in-between you.

These actions are predictable by both sides, and both sides do them. Ultimately, though, a single relativistic cloud of sand trumps any laser defense system you could possibly build. You can counter any shielding by shooting from the sides (or just concentrating missile after missile on a single point, since it can't be reinforced).

Since both sides are technologically equal, the only way to win is to strike unilaterally (or, equivalently, first). Given the delay involved in interstellar travel, there are no second chances. So, both sides must strike as aggressively and quickly and thoroughly as possible.

Both sides run through these same conclusions and attack each other in every way they can possibly conceive, countering every defense they anticipate of their enemies. Since offense outstrips defense, every target is destroyed, and the war ends in a grim tie of mutual annihilation.

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    $\begingroup$ Decent, but assumes solid matter bound civilizations. A planet bound type 2 civilization isn't reasonable, they have too much energy. The civilization is processing a dissipating a star's worth of energy every year. The sand-attack is one civilizaiton concentrating their energy budget and trying to drop it on a specific target. This requires intelligence on the attack vector, or an easy to hit target from interstellar distances (non-mobile), or vastly more energy than the target is already capable of processing. Hosting a type-2 civilization on earth turns it to plasma without weapons. $\endgroup$
    – Yakk
    Commented Jun 30, 2016 at 13:44
  • $\begingroup$ @Yakk fair enough; I was assuming Earth is the primary target, but certainly basic thermodynamics mandates extensive space infrastructure too. The larger narrative still applies, though: offense is easier than defense. Relativistic warfare is an obvious good choice because there is literally no limit to the amount of (relativistic kinetic) energy you can give a rock, and you can make enough of them to do massive damage to a huge area. But maybe you build a star-pumped x-ray laser or do magic instead/also. $\endgroup$
    – geometrian
    Commented Jun 30, 2016 at 23:19
  • $\begingroup$ but the problem is, the civilization you are attacking digests energy on the same scale as your weapon. Using the sun's power to create a beam of energy? They are already eating a sun. Your attack could overwealm their ability to handle it via concentration only. Sending a fast rock runs into the problem that it plows through the interstellar medium, destroying itself. If turned into expanding plasma, its energy flux falls until it becomes harvestable like the sun. $\endgroup$
    – Yakk
    Commented Jul 1, 2016 at 0:08
  • $\begingroup$ @Yakk I don't see any problem. Warfare is all about concentration on weak points--whether that's weak links in social engineering or abutments in a tank's armor or the output of a star on Earth. Is the issue that you're suggesting that relativistic missiles don't work as an attack vector? $\endgroup$
    – geometrian
    Commented Jul 1, 2016 at 12:53

that people can travel at .9c

This is basically a MAD situation. If you can accelerate substantial mass to .9c, then that becomes a planet-shattering un-interceptable super weapons. And if both sides have the same level of technology and have some of those super weapons prepared, then if one side attempts to use it, the other will immediately use theirs.

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    $\begingroup$ That assumes that the other side has time to retaliate before it is extinguished. If the weapon is actually a repurposed spaceship that would be expected to approach at that speed, at the time they realize it is used as a weapon, it's probably milliseconds before the planet's life is wiped out. $\endgroup$
    – celtschk
    Commented Jun 29, 2016 at 6:37
  • $\begingroup$ @celtschk There was question here exactly about that, but I can't find it. Simply said, trust is required for approach of such a spaceship. If even speck of doubt exists, the planet would assume any such approaching spaceship to be a threat. Only special circumstances would allow such spaceship to approach. $\endgroup$
    – Euphoric
    Commented Jun 29, 2016 at 6:44
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    $\begingroup$ @celtschk: during the cold war, a B52 was always on air with, basically, the order to retaliate in case of attack. This can be done also in space: you have a couple of this weapons ready in orbit with the order to be launched if the planet is wiped out $\endgroup$ Commented Jun 29, 2016 at 7:06
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    $\begingroup$ They know that it's moving in your direction, they don't necessarily know it won't stop. $\endgroup$
    – Erik
    Commented Jun 29, 2016 at 10:20
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    $\begingroup$ @Euphoric then do not park it, but accelerate your rock, too. Even a minor change in course will cause it to miss Earth. Heck, you could even install engines on their rock to change its course. $\endgroup$
    – Chieron
    Commented Jun 30, 2016 at 7:56

Depends what you want to accomplish, how much destruction is acceptable, what your endgame is, and what technology you have at your disposal.

If the goal is elimination without regard to civilian casualities (aka massive war crime) and long time consequences, then throwing rocks is a fine solution. It's crude but effective. There's little reason to engage in a costly war, or building expensive weapons when you can just put thrusters on a rock. At .9c you'd have trouble finding a more destructive and cost effective weapon than rocks.

There are little defense against this besides prevent the other side from installing said thrusters on said rock, or hacking the Mainframe™ to take control of the rock's guidance systems to deflect it (if applicable).

You would be justified to retaliate with the same kind of tactics and disregard for collateral damage the moment you detect them trying to launch rocks at you. Though, you know, if you can detect that there's a good chance it's already too late.

Now what do you say? "These aliens want to destroy us, we'll show'em a good time, hooah!" I mean, if it's about survival, it's an easy sell. Humans like to survive.

It's a different story if your goal is conquest and you don't want to kill everyone on the planet. Then you have a full scale war on your hands, and there are a variety of ways to go about it, which can end in a various degrees of disaster for both sides.


Build a gigantic network of observation satellites.

Given sufficient time to prepare, anything that can be spotted can be stopped. A meteor lobbed at your planet can be intercepted with a meteor of your own, either deflecting it away from the planet or breaking it into small enough chunks that it will burn up in the atmosphere. A fleet of antimatter bombs can be met with a fleet of interceptors, with multiple redundant interceptors targeting each incoming missile. Once you see something, its trajectory will be relatively fixed, and you can deal with it.

Instantaneous communication means that you can react as soon as you see something, but if you wait until you can see it from your planet, it's already going to be $\frac{9}{10}$ths of the way there. Your first order of operations will be to extend your detection range as far as possible. Any reasonable approach trajectories from the other planet will be your first target to observe, moving progressively further from your planet. Your ultimate goal will be to get your satellites in orbit around your opponent's planet, so you can see all of their launches in close to real time.

Your second goal will be to stop them from doing the same. A successful attack on your enemy will be largely dependent on your being able to launch it undetected. If you can deny your opponent forward observation posts, the light from your launches won't get to them until just before your attack does, which will hopefully be too late.

  • $\begingroup$ +1ed for detailed description of scenario's information warfare. I think the problem is that blocking a k-projectile takes more effort than sending one. For example, if the attacker accelerates a rock and then explodes it, how is the defender going to stop every resulting grain of sand individually? With a dedicated missile? I calculate each grain could level a city block. If you're a K2 civ, you can throw a lot of rocks, and if each rock produces 10^28 sand grains . . . it doesn't really matter how far away you see them coming. $\endgroup$
    – geometrian
    Commented Jun 29, 2016 at 19:52
  • $\begingroup$ Wouldn't the grains all burn up in the atmosphere? Anything passing through the atmosphere at relativistic velocities would absorb a huge amount of thermal energy before hitting the ground, which should vaporize basically anything. Alternately, I could just place a cloud of fluid droplets in the path of the sand cloud. A collision with a fluid droplet at 0.9c should be more than sufficient. $\endgroup$
    – ckersch
    Commented Jun 29, 2016 at 20:03
  • $\begingroup$ Burning up in atmosphere refers to depositing energy. At .9c, the energy of sand grains is a lot of energy. The KE of Ceres at .9 c is 1.3 x E21 kg. (And yes, I measured energy in kg: the KE of something at .9c is 1.3x its total conversion energy). It takes about 1 J to heat up 1 g of air by 1 K: there isn't enough atmosphere. There isn't enough planet: that is enough energy in the rounding error of these equations to turn the Earth to plasma. (electron binding is 1 million times weaker than rest mass) $\endgroup$
    – Yakk
    Commented Jun 30, 2016 at 13:39
  • $\begingroup$ Launching Ceres at that velocity is out of the realm of possibility for a K2 civilization, though. Converting to Joules, that would take around $3.4 \times 10^{37} J$, which is over 100 times the energy budget of an an entire K2 civilization for a year. $\endgroup$
    – ckersch
    Commented Jun 30, 2016 at 14:48

Everyone has so far mentioned the use of "Fast Rocks" as an offensive weapon. If I were defending a planet from "Fast Rocks", I would do as follows, creating a set of defensive perimeters.

The farthest away would be the distance related to defending from obvious, undisguised attacks. This distance would be set such that, no matter the direction of approach, I could bring my own "Fast Rocks" in to position and speed for intercept. This would likely be very far from my own planet, but not anywhere near the enemy planet.

The second perimeter would be the distance away I could safely intercept an approaching object with a set of "Fast Rocks" already prepped and in position. This would be much closer to the planet.

Finally, I would have an approach corridor to my planet. This approach corridor would be shaped much like an hourglass, with the thinnest point at or around half-way between the far perimeter and near perimeter. Objects of any type entering the thin "Checkpoint" must start to decelerate at a rate that will make sure they reach a "reasonable" speed by the inner perimeter. Any object that is deemed suspicious is told to decelerate to a stop by the second perimeter. If an object exits the approach or acts in a dangerous manner is assigned a "Fast Rock" for intercept from the stockpile near the approach.

This plan should allow for the defense of the planet while allowing shipping to allow continue, maintaining the all-important industrial capacity of my civilization. This would allow for "Customs" to occur off-planet at the Checkpoint, and would be easy to adapt for peacetime use.

In terms of offense, it depends on many factors. If I knew that I had a stronger industrial capacity and more access to rocks than my enemy, I would simply stockpile and set them up before shelling the planet until it has no more rocks of its own, having used all of them on intercept.

Literal MS Paint Diagram enter image description here Edit: I know this is a fairly incomplete answer, but a lot of clarification would be required for further investigation. In addition, it is just an exploration of defense from large, weaponized meteors or ships as kinetic weapons.

Edit 2: This is assuming that the civilizations have highly advanced scanning equipment. I am not worried about the detection side of things because in order for common interplanetary and interstellar travel, a ship would need to be able to be very sure that there is nothing in it's path, both when moving at normal speeds and near the speed of light. The energy imparted by even the smallest of objects moving at 0.9c is truly impressive. In order for advanced trade between planets, there would need to be very sophisticated scanning equipment.

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    $\begingroup$ Think of the scales involved. None of this would reasonably work. $\endgroup$
    – Polygnome
    Commented Jun 29, 2016 at 15:02
  • $\begingroup$ @Polygnome Can you point to anything specific you think wouldn't be successful? $\endgroup$
    – Dent7777
    Commented Jun 29, 2016 at 17:34
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    $\begingroup$ I doubt any attacks would be obvious. Finding objects, even moving ones, in space is not easy. It took a long time for us to find Pluto, and an object smaller than that moving much faster than that (therefore much harder to detect in time) would be devastating to the Earth. Another issue with detection is for an object moving at .9 c, we would get the light (or any radar reflections, etc.) from the object very close to the time the object itself arrives. Covering surface area of a sector in the most likely direction of attack with deflection defenses would take way too much material. $\endgroup$ Commented Jun 29, 2016 at 18:26
  • $\begingroup$ To me... At that distance from your planet, the amount of ressources needed to maintain that perimeter is just too much to be a sensible idea:/ $\endgroup$
    – Patrice
    Commented Jun 29, 2016 at 18:30
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    $\begingroup$ good enough answer, with ftl communications will work well, specially in such dumb attacks op suggests. $\endgroup$
    – MolbOrg
    Commented Jul 3, 2016 at 15:23

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