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Imagine you have a big honking space gun mounted on your spaceship that launches slugs at a few percents of the speed of light. We'll call that fairly destructive.

However if you missed and the slug kept going, it could hit a planet directly behind, or any other object somewhere, sometime. If that doesn't concern you in the least, it does concern the Space Warfare Conventions Treaty which says you should feel concerned.

I feel that simply blowing the slug up would create a swarm of shrapnel which I don't feel is a better problem to have.

Is there a way to destroy such stray slugs? I'm looking for a solution that can be integrated to the slug, sort of a self-destruct mechanism.

Edit: Space may be big, but that's not the question. Authorities do care. The treaties that prevent you from getting glassed from orbit are the same that dictates you can't just fire blindly into space and hope for the best. It's not up for debate.

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    $\begingroup$ I'm interested to learn what you've done about Newton's Third Law, tbh. $\endgroup$
    – AakashM
    Commented Jun 10, 2016 at 14:52
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    $\begingroup$ Depending on where your battle takes place, you might not have to be very concerned about a stray slug. Space is very, very empty... $\endgroup$
    – user35915
    Commented Jun 10, 2016 at 14:55
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    $\begingroup$ This certainly doesn't invalidate your question (it's a cool concept) but do take into account just how empty space really is. Check out this article for a description of the probabilities of hitting anything even in our own galaxy, let alone another: what-if.xkcd.com/109 $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 15:07
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    $\begingroup$ @seeds Mass effect 2 the first time you enter the Citadel, "Once you fire this hunk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you are ruining someone's day, somewhere and sometime." Unfortunatly, they do not include a solution for this problem. $\endgroup$
    – Ryan
    Commented Jun 10, 2016 at 16:09
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    $\begingroup$ A single slug at 0.1c is a single slug. Break it apart and now you have a buck-shot at 0.1c. Are you sure that is what you want to do if you are worried about collateral damage? :D $\endgroup$
    – MichaelK
    Commented Jun 10, 2016 at 17:22

16 Answers 16

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Antimatter as added HE component or counterimpulse

The tip of your projectile includes an antimatter portion. If the projectile hits the target, this basically doubles the destructive power (because it needs to contain energy equal-ish to the kinetic energy of the projectile). If the projectile misses, the antimatter is rigged to annihilate some of the projectile's matter in a very directed way so the released energy decelerates the (pulverized or atomized at this point) remains of the projectile to non-relativistic speeds.

Directing the energy release of a matter/antimatter annihilation this way would probably be very hard if not impossible with real science, but if we're shooting projectiles at >1% C around we can probably handwave a bit ;)

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    $\begingroup$ I agree, antimatter is a good way to go about it. I would add however, that the slug be 'charged' before firing. If the slug hits it's intended target, the antimatter gives the added punch. If not, eventually the containing fields' charge wears off, and the antimatter consumes the slug in a rather brilliant display. $\endgroup$
    – owi2000
    Commented Jun 10, 2016 at 17:24
  • $\begingroup$ I'm not convinced that this actually helps - if you shot off all the resulting photons in the same direction, then they have the same four-momentum as the original bullet. This is very bad for anything that gets in the way of the photons. (And shooting them off in varying directions couldn't be any better than just blowing up the bullet in some conventional, but analogous way) $\endgroup$
    – Milo Brandt
    Commented Jun 10, 2016 at 17:48
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    $\begingroup$ Well, the question was if there was a way to destroy the slug so it didn't continue on to kill something else; this accomplishes that. Any resulting high energy particles that may remain wouldn't be much of a threat to a shielded space vehicle, much less a person down on a planet beneath a protective planetary magnetic field and atmosphere. $\endgroup$
    – owi2000
    Commented Jun 10, 2016 at 19:33
  • $\begingroup$ @owi2000 Photons are uncharged; they don't care about magnetic fields (at least not magnetic fields of the strength of planets'). They might not penetrate the atmosphere, but neither would a small enough projectile - and for larger projectiles carrying more energy, getting to the ground might not even be necessary to cause serious problems. $\endgroup$
    – Milo Brandt
    Commented Jun 10, 2016 at 21:50
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    $\begingroup$ @MiloBrandt - but the photons would go off in all directions, not one (building large lasers is quite hard), very quickly dissipating the energy about as effectively as at all possible $\endgroup$
    – Jonathan Dursi
    Commented Jun 10, 2016 at 22:21
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The best way to dispose of the slug would be to aim it at something that doesn't care, most likely a star. However, given the momentum of the slug, changing the direction it flies in would take something the size of the firing railgun, so that solution is close to impossible unless your enemies are considerate enough to fly between you and the closest star.

Without changing the momentum of the slug, the best you can do is to break it down into the smallest pieces you can manage. I imagine the slug being made of a material that starts breaking down the moment it's fired (or even the moment it's made if it's pressed/cast/3D-printed just before being needed).

If you hit your intended target, it will be long before any degrading happens. If you miss, the slug turns into powder in seconds. Then a tiny explosive blows the lump of powder into a cloud to spread it out, since a lump will still ruin someone's day at that speed. Hopefully, interstellar matter, solar winds etc will further slow down and disperse the particles.

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  • $\begingroup$ They have bullets that do this. They're called frangibles. The idea is that the first thing they hit has to soak up all of the kinetic energy of all of the parts. However, if they hit something hard, instead of ricocheting, they burst into a bunch of smaller pieces. The idea is that each of the smaller pieces has a much lower lethality, so acceptable to increase the number of pieces that ricochet, since none of them are lethal. $\endgroup$
    – Cort Ammon
    Commented Jun 10, 2016 at 15:35
  • $\begingroup$ breaking down into smaller pieces is not a real solution, the pieces still carry the energy of the original slug which is the real threat. $\endgroup$
    – RBarryYoung
    Commented Jun 10, 2016 at 19:33
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    $\begingroup$ @RBarryYoung That's true for a short distance, but not for distances on the scale of AUs or even light years. A secondary target a thousand kilometers away is going to get vaporized by a cloud of particles just the same, but if you spread the particles, that cloud is going be hundreds or thousands of kilometers wide at greater distances, so any target will only receive a small fraction of the energy. $\endgroup$
    – Cyrus
    Commented Jun 10, 2016 at 19:44
  • $\begingroup$ Would it do any good if instead of a solid slug it was molten metal? $\endgroup$
    – AmiralPatate
    Commented Jun 10, 2016 at 20:07
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    $\begingroup$ @Azor-Ahai Sure it would. In space, essentially the only way to lose heat is through radiation: there's nothing to conduct to. The temperature management challenge when designing a space suit is keeping the astronaut cool, not keeping them warm. $\endgroup$
    – David Richerby
    Commented Jun 12, 2016 at 12:44
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The slug has a lot of kinetic energy, and your goal is to reduce this to safe levels if it misses it's target. $KE=\frac12mv^2$, so your two options are to reduce it's mass or to reduce it's velocity. It's velocity can not change unless it hits something, which you don't want, so your only option is to reduce its mass. reducing it's mass in the absolute is impossible, so the only way to reduce it's mass would be to break it into smaller pieces (shrapnel) which you also don't want to do (even gas moving at the speed you mentioned would be pretty destructive). Space is really big and really empty, so if there were a Space Warfare Regulations Committee, it probably wouldn't care about bullets. Consider this: when you look at the sky at night, the % of the sky that is white from starlight is greater than the % chance a stray bullet would ever hit anything in space. If this is not satisfactory, your Space Warfare Regulations Committee could ban bullet-firing guns completely. Powerful enough lasers with even a slight cone would be just as devastating as short range, but harmless at greater distances.

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  • $\begingroup$ The only good answer on this question so far. Why it is way down here is beyond me. $\endgroup$
    – Dan
    Commented Jun 10, 2016 at 17:56
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Sadly, most of the answers ignore the fact that the energy and momentum of the projectile remains even if the thing is disintegrated into clouds of molecules. Instead of having a big slug of metal or metal/ceramic composites striking you all at once, you have a cloud of gas with the same total mass and energy striking the target. While maybe not as dramatic as that single point of contact, it is still going to hurt....a lot.

Perhaps the best analogy is to think of the projectile as a hypervelocity sabot round from a tank, and the destroyed version of the projectile as a stream of gas from a plasma torch you get the idea.

Add in extra devices to the slug to disintegrate it and extra energetics to widely disperse the cloud would be quite expensive and make the slug more massive. The various devices will also have to be military grade so they are fall safe in the ship's magazine and don't explode while under high acceleration in the railgun itself, but still are 99.9% reliable in doing the job after the slug passes the target.

At this point, I would suggest that instead of a slug, use the railgun as the first stage of a missile launcher and fire a missile or torpedo instead. The missile can use its fuel to make last minute corrections to strike the target, and if it misses it can be programmed to fire the rest of the propellant to bring its relative velocity down to mere interplanetary speeds for recovery or destruction.

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  • $\begingroup$ Excellent points and a good suggestion about the missile version of the projectile. That would make for a "smarter" weapon anyways. Perhaps combining that fuel usage to slow down and then detonating itself would reduce the threat to manageable levels even more. $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 16:40
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    $\begingroup$ Slowing it down is hard. get it to aim at the nearest star. $\endgroup$
    – Donald Hobson
    Commented Jun 10, 2016 at 16:56
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    $\begingroup$ I don't thnk it's reasonable to fire a rocket with enough delta V to slow itself significantly from a railgun that launches at several percent of lightspeed. New Horizons is the fasters rocket we have launched, at 16000 m/s, which is 0.00005337026 of lightspeed (5/10000 of a percent of lightspeed). And of course, New Horizons launched on a huge, multi-ton rocket, not a railgun firable projectile. And disintigrating the projectile CAN be helpful, because it helps the atmosphere to absorb the energy as heat, rather than the kinetic energy of a projectile being transfered into the ground. $\endgroup$
    – Dan
    Commented Jun 10, 2016 at 17:50
  • $\begingroup$ The reason most of the answers ignore it is that if you manage to break it up in a cloud (which you point out is not the easiest thing to do), it will continue to disperse, causing anyone that's hit by the remnants of this projectile to only intersect with a minuscule fraction of its original mass. This of course assuming that there will be some distance between the initial dispersal event and the accidental target, which I think is safe to assume considering that you don't want to stay too close to an intergalactic war unless you're directly affected. $\endgroup$
    – pipe
    Commented Jun 11, 2016 at 10:45
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Chemical dissolution

  1. Design your slugs with a central hollow portion.

  2. Fill this hollow cylinder with a glass tube filled with a liquid or gaseous chemical that is specifically designed to eat the material the slug is made from

  3. The firing process ruptures the glass so that the slug starts being eaten away from the inside as soon as its fired.

The chemical can be designed to eat the slug at an appropriate rate so that the slug is mostly intact when it strikes (or passes by if you miss) your intended target but is disintegrated shortly thereafter.

This has the benefit of not dictating your combat maneuvers and tactics as if you miss it doesn't matter, the slug eats itself, you don't need to aim with a secondary backup target in mind. It should be pointed out that if you miss your primary target you aren't likely to hit a secondary one anyway.

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    $\begingroup$ Law of conservation of matter applies. Even if it dissolves into a different state or different compounds, it still has mass and velocity, and can still destroy stuff... except now there's acid in the mix. $\endgroup$
    – Devsman
    Commented Jun 10, 2016 at 16:43
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    $\begingroup$ At that speed chemical composition is irrelevant. $\endgroup$
    – Donald Hobson
    Commented Jun 10, 2016 at 16:52
  • $\begingroup$ @DonaldHobson I don't disagree but at least it spreads the impact out and over interstellar distances the resulting cloud would be so scattered it wouldn't much matter. $\endgroup$
    – James
    Commented Jun 10, 2016 at 19:58
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    $\begingroup$ Do you have an idea of how a cloud of acid would behave in vacuum? Or if that even matters in any capacity? $\endgroup$
    – AmiralPatate
    Commented Jun 10, 2016 at 20:14
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    $\begingroup$ Really like that one. Especially at it would not be a cloud, but a veeeeery long stream of gas – such bullets would not be eaten in the blink of an eye, but possibly bleed material over a longer period. $\endgroup$
    – kaiser
    Commented Jun 10, 2016 at 20:31
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I accept and reinforce your question. Earth is already experiencing a big problem with space debris. And I'm not just talking about derelict satellites; the bigger problem is from bits of flaking paint (seriously and literally). A paint chip from a deteriorating satellite or spent booster traveling at orbital speeds packs a very dangerous punch. The big viewing window on the ISS has discoloration due to a debris strike, most likely from a paint flake; experts are debating the risk implications. So I can postulate that any civilization advanced enough to be engaging in space warfare has also survived a space debris crisis. Most likely, lives have been lost and/or their entire orbital infrastructure was threatened. If they put Geneva Convention-style rules of warfare in place, limiting collateral debris would be high on the list.

So, solutions. Outlawing space weaponry is the best, but makes for a boring story. So we address relativistic projectiles. Seeds is closest to the mark: Vaporize your slugs. If you can convert your slug to an expanding ball of diffuse gas or neutral plasma, then eventually it becomes indistinguishable from the solar wind. Blowing it apart into shrapnel won't do it. You need total conversion into a diffuse gas or plasma. Vaporization energy of metals is high; a metal shell with a non-nuclear explosive core is unlikely to vaporize completely. You need something either that will sublimate or evaporate on its own, like water or a frozen volatile, or else a plastic or light metal shell filled with enough explosive that you can get 100% vaporization when it detonates. Perhaps your most expedient solution is to make the whole shell/slug out of a self-decomposing material like nitrocellulose, with a timing mechanism small and light enough that you are guaranteed complete vaporization. You could see a complex regime of regulations and matching technologies: Restrictions on shell materials and even firing arcs based on the location of the battle. Treaty conventions could limit engagements to deep space, or a given distance from the ecliptic plane. I can imagine the earlier, "civilized" part of an interstellar war when incoming ships have to rush through a gantlet of hostile fire in deep space until they reach the legal safe zone of planetary orbit, where all battle is banned due to debris non-proliferation conventions. Your canon also serves as propulsion. Again, treaty conventions could insist on self-deflagrating shells, or else limiting "arcs of fire" (in this case exhaust vectors) to specific "safe" orbital elements. A mass-driven ship might have to "tack" its way into or out of orbit, firing mass only along prescribed trajectories. A space DEQ would monitor mass driver exhausts for excessive particulates. If your mass accelerator (canon) goes out of tolerance, your shells (military or propulsive) start eroding and shedding particles as they are launched and your ship gets grounded until its drive can pass an inspection.

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    $\begingroup$ FYI: The Geneva convention controls the legal treatment of human beings, e.g. if you don't fight according to the rules of the convention, you're not protected by them, if has nothing to do with weapons. It's the Hague convention that controls what is and is not a "legal" weapon. It is of course, a spectacular failure. Just has the Geneva convention is enforced by retaliation, so is the Hague convention. That means everyone has to have banned weapons on hand in order to enforce the convention. Something of a catch-22. In any case, only liberal democracies are held responsible. $\endgroup$
    – TechZen
    Commented Jun 12, 2016 at 23:34
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    $\begingroup$ The space debris issue something out of the 60s when sensors and computers were just getting concern. It's only a theoretical concern if the space craft have no defensive armament, which some now do and in this scenario all ships will have an anti-collison grid if nothing if. Since space is a perfect fish bowl and even flecks of a millimeter or so are resolvable on infrared and radar, any debris will be zapped out to charged plasma and then deflected by charged plasma, magnetic fields or absorbed by sames. $\endgroup$
    – TechZen
    Commented Jun 12, 2016 at 23:41
  • $\begingroup$ Excellent answer and comments.Would allow for some "non-compliant" parties to act like terrorists, shooting within the "wake zone" at military vessels that are banned from responding and just have to high-tail it out to a safe orbit. $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 1:48
  • $\begingroup$ The key focus would be on the "expanding and diffuse gas" part of it. If you can redirect the momentum to be more of a wide cone it could very quickly lose the focused energy required to do damage. $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 1:50
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One could make the slugs from ice, or something else that will sublimate with time. Doesn't help with hitting a different space target, but if it gets too close to a star or planet it will melt/burn up. If you need to use it in a railgun, a ferrous discarding sabot of some kind could be used to propel the round and fall away after leaving the barrel. This means debris is created, but not immediately dangerous to folks downrange.

Edit: if the sabot is spring loaded to separate from the main slug, a nearby EM field can capture the debris.

Edit2: Build the sabot capture magnet into the last part of the barrel, so it is pulled directly away/back from the slug.

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  • $\begingroup$ Good ideas, but regarding the sabot, remember that space doesn't have air resistance so a shell like that wouldn't simply slow down and fall away like it does on earth. It would keep traveling at the same velocity until it hit something. Probably not as damaging as the slug itself but still a concern if it's over $.01C$ $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 16:35
  • $\begingroup$ A small charge to neutralize the momentum of the carrier, or an electromagnet to capture it for reuse/recycle. $\endgroup$
    – Seeds
    Commented Jun 10, 2016 at 16:39
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    $\begingroup$ Interesting concept. I think you just invented the first eco-friendly weapon of mass destruction! "Here at Kill Them All labs we take pollution seriously. Our weapons are designed with Mother Nature in mind, so you can kill your enemies without killing the environment. Try our new Certified-Organic Free-Range All-Vegan Relativistic Railgun today!" $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 20:25
  • $\begingroup$ it will evaporate by it selfs over time, because of Interstellar medium. How long it will take through. $\endgroup$
    – MolbOrg
    Commented Jun 12, 2016 at 1:00
  • $\begingroup$ @MolbOrg Heat death will get everything... eventually $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 1:42
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Make it out of a semi-stable element whose nucleus will disintegrate over the desired time. So if you haven't hit your target in N milliseconds it will just begin showering betas and gammas isotropically outward.

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  • $\begingroup$ How do you suggest that they prevent the projectiles from disintegrating while in storage, before they're fired? $\endgroup$
    – ApproachingDarknessFish
    Commented Jun 11, 2016 at 21:11
  • $\begingroup$ @ApproachingDarknessFish, great question! Maybe The Quantum Zeno effect? Put them in special containers that always measure some observable of the bullet. The Zeno containers serve as a fancy quantum bullet shell. $\endgroup$
    – user1717828
    Commented Jun 12, 2016 at 11:29
  • $\begingroup$ @ApproachingDarknessFish Could be a magnetic field holding them together. Is that a thing that can happen to atoms? $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 2:29
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    $\begingroup$ @thanby No, radioactive decay involves the weak nuclear force. The electromagnetic force won't have any effect. $\endgroup$
    – ApproachingDarknessFish
    Commented Jun 13, 2016 at 5:33
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Since your setting is in the future, why not go into fantasy a little bit [1]. Make the bullet out of exotic matter which is pulled out from gluon field temporarily and returns back to its non-existent state after a while. Obviously this state transition should require energy to bring it to reality and energy release (say in form of x-rays) after it expires. But this energy will dissipate to all directions. Anything near will get heated, but after a distance, it will not have much effect.

Second idea would be to create matter/antimatter orbiting around each other. It would take sometime before their collapse, which will emit relatively-harmless-at-long-distance x-ray. But if they hit into a hull of a spacecraft it will either cause a blunt (at 0.01C) damage (matter hit) or annihilation (antimatter hit) plus most of the damage of emitted x-ray. If you send enough slugs with this way, alternating hits will probably decimate your target.

[1] If you question the science behind this: there are virtual particles can pop into existence. Even it seems that there is a way to make them stable for longer periods.

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  • $\begingroup$ Wonderful departure from the other answers so you definitely get a +1. But virtual particles are more of a "theoretical thing to explain something we observe" and the guys who "discovered" them (whoever the heck it was) don't necessarily think they're a real thing, it's just a placeholder until we can figure out what actually happens. $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 2:26
  • $\begingroup$ Well, I kept reading after posting this. It seems that they use virtual particles to create matter, anti-matter by stabilizing the virtual particles, at which point they become real particles. Quantum mechanics are out of this universe. $\endgroup$
    – Cem Kalyoncu
    Commented Jun 13, 2016 at 6:40
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Hitting a planet behind your target is a non-issue, unless your slugs are VERY big or have a LOT of momentum. It will just burn up in the atmosphere. Well, you did say a "few percent of the speed of light", that is a lot of momentum. Easy enough to make up numbers for mass and exact speed and calculate kinetic energy, but I don't know how to get from there to calculating what happens when it hits the atmosphere. Maybe Tunguska was a stray slug from an alien space battle. :-)

If your battle is in deep space, probably a non issue. Space is very big. The probability that someone will run into this slug in all the hundreds of cubic light years between any two stars is minuscule.

Where it could be an issue is if your battle is in a planetary orbit, and both sides are firing lots of slugs. Now you're adding a lot of space debris. But then if you're firing at a few percent of the speed of light, it's not going to stay in orbit, it's going to go flying off into deep space. Let's see, it takes light approximately 5 hours to travel from the Sun to Pluto. So at 1% of the speed of light, that would take 500 hours or about 3 weeks. So in three weeks, your slug has exited the Solar System and is in deep space. Back to non-issue.

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    $\begingroup$ This does not really answer the question, but questions its goals. $\endgroup$
    – kaiser
    Commented Jun 10, 2016 at 20:34
  • $\begingroup$ @Kaiser I'd say "questions its premise" rather than "goals", but yes, exactly. To make a deliberately silly example: Suppose someone asked, "How could I design a car that does not subject the hamsters to the scalding heat of the engine? I think it's cruel to subject those poor hamsters who run the hamster wheels to make the engine go to such inhumane conditions." Surely a discussion of heat shields or cooling systems would be beside the point. The only right answer would be to explain that car engines are not run by hamsters turning hamster wheels. :-) $\endgroup$
    – Jay
    Commented Jun 12, 2016 at 5:24
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    $\begingroup$ While I would agree if this would be a completely technical stack site, I think this site and question has one speciality to it: The fictional law dictates that. Like if a fictional law would tell you to put hamsters inside an engine bay to turn the sprockets – you can do nothing than help those furry fellows have a nice time there :) $\endgroup$
    – kaiser
    Commented Jun 12, 2016 at 16:49
  • $\begingroup$ A relativistic projectile the size of a baseball ball can create horrifying effects in an atmosphere. Not to mention a proper, weapons grade, projectile. $\endgroup$
    – Davidmh
    Commented Jun 13, 2016 at 14:52
  • $\begingroup$ @Kaiser If someone posted, "In my story, stray projectiles from my spaceships guns are a permanent hazard to astrogation because ...", and some reason why this is necessary for the story, then I'd agree that saying that the physics behind that reason doesn't work is an unsatisfactory answer. You have to either grant the fictional physics for the sake of the story, or propose some alternative justification or mechanism. But if someone says, "I'm stuck in this story because I don't know how to get around X", an explanation of why X isn't really a problem seems totally appropriate to me. $\endgroup$
    – Jay
    Commented Jun 13, 2016 at 21:26
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There are several good answers here, but some that are misinformed as well. I'd like to toss in one additional element that could work in tandem with some of the better answers.

IMHO the best answers have to do with creative ways to disintegrate the projectile into a gas or plasma. However care must be taken to also drastically redirect the subsequent cloud of former-projectile or else the same kinetic energy will be imparted to the thing it all eventually hits. For example, see real-world applications that use phase-state changes to actually enhance destructive effects, like the shotgun slug.

In situations like that, projectiles are (more-or-less) converted mid-air into a liquid, to actually do significantly more damage than they would if they stayed in their original solid phase.

What you could make use of is simple rifling. Rifling spins the projectile to increase its accuracy in air. Space needs no rifling because there's no air resistance, but rifling combines neatly with some of the aforementioned techniques to vaporize the projectile, because if its spinning already when the phase transition to a gas occurs, it will naturally widen out into a much-less-harmful conical cloud in a very short span of time, basically "flinging" itself in all directions at once.

Keep in mind it would still be lethal soon after vaporizing, so you could even include story elements where a self-destruct wasn't issued in time so the full force of the kinetic cloud of particles was imparted to the victim anyways (and further plot elements about whether that was an actual accident or a planned "oops" moment to kill a target "by accident" could make things juicier.)

Any way you take it, I think you're on to something fun. Hope you don't mind if I use some version of it in a story sooner or later.

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Simple Have Slugs that break apart after go a certain distances that way if you miss your target you slug will break apart into smaller pieces that will not hurt the planets behind your battle. This would limit your range of course but it would prevent any collateral damage in a space battle. It also we make it harder for one of your ships to be destroyed by friendly fire.

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    $\begingroup$ The problem is, even the shrapnel will still contain the net energy that the slug originally had, so will be equally destructive. That may seem counter-intuitive because that's not how it works on earth, but that's because earth has air, which provides increased resistance when you increase your surface area (by breaking the slug apart). Space doesn't have that benefit $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 15:13
  • $\begingroup$ The second half of this article illustrates that point pretty well: what-if.xkcd.com/26 $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 15:19
  • $\begingroup$ @thanby I think we may be overestimating the damage any one piece of shrapnel will do. Consider that the uncountable micrometeorites in space, with their vast cumulative mass, are considered a nuisance, but not much more. Meanwhile a single Tunguska or a Chicxulub, with less mass than the sum of the micrometeorites is decidedly more than a nuisance $\endgroup$
    – Cort Ammon
    Commented Jun 10, 2016 at 16:13
  • $\begingroup$ But if the same mass of micrometeorites (as the Chicxulub meteorite) hit the earth at the same time the net effect would still be their total kinetic energy being transferred to earth, just primarily through the atmosphere instead of through the ground. The link I provided actually explains that point too, in the last couple paragraphs. $\endgroup$
    – thanby
    Commented Jun 10, 2016 at 16:31
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    $\begingroup$ @CortAmmon micrometeorites aren't moving at relativistic speeds. Even small projectiles can create huge effects: what-if.xkcd.com/1 $\endgroup$
    – Davidmh
    Commented Jun 13, 2016 at 15:15
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You don't.

Do you realize that that's about 10^13 J = 10'000 GWh (when using a 25 g slug), right? So that's the energy a nuclear fission reactor puts out over almost 3 hours. You don't just slow that thing down without bumping into something really massive, even when making a giant cloud out of it like others suggested (just ignoring the chemical reaction guys who apparently never heard of conservation of momentum).

Btw.: What do you mean by "if you missed and the slug kept going"? Like it didn't kept going did it hit. A very small object with the energy a freaking nuclear reactor puts out over several hours doesn't care whether it hit something. It just keeps going.

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  • $\begingroup$ You make a good point about the energies involved and I think you're onto something with it, but your answer isn't fully addressing the question. There are many possibilities for tackling the problem. Try editing it to include what would be required to spread that much energy out to make it less-harmful instead of simply saying "nope". Remember this is future tech, so cell phones probably have the power output of a small nuclear reactor at this point (which would make for some really epic versions of Candy Crush). $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 2:17
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tl;dr:

  1. Space is big. Unless you're near an inhabited region, you're probably not actually going to hit anything.

  2. Planets and spacefaring species already have ways to handle stray space debris. A stray shot would be covered by those protections.

  3. Intergalactic authorities won't care about stray bullets; they will care about weapons of mass destruction and lingering wartime hazards.

Space is Big

The quote from Mass Effect 2 is actually not true. Not every object flying through space will eventually hit something. While it's true that if it does hit something, a stray shot could ruin someone's day, most things floating through space won't hit anything.

Unless you are so close to an inhabited thing that you can see it with your naked eyes, you've got nothing to worry about. Your shots will spread out enough that any given inhabited planet or space installation more than a few light-years away will only ever be on the receiving end of between 0 and 1 shots.

In a combat situation, a total risk of all combined collateral damage under 0.1% is not only acceptable, it's fabulously, unbelievably good.

There are already protections in place

Your slug is going fast, true, but there are other fast-moving, massive objects flying around in space capable of causing destruction. A mass slug from a railgun is similar enough to natural space debris that anything capable of protecting against stray space rocks will also protect against stray bullets. Any sentient civilization will have ways to deal with stray space debris, either directly via technology, or passively such as their planet's atmosphere.

If your shots break apart into small fragments after missing their target, you further reduce the damage from any collateral hit to that of a micro-meteor. A planet might have to absorb more than 1 fragment, but a ship, even a huge one, would only have to deal with at most 1 fragment.

Intergalactic authorities won't care

Any intergalactic authorities won't care about stray shots from a railgun. They'll care about something because it is actually a problem. They would care far more about:

  • minefields → mines tend to be left behind long after a war is over. They cause a large area to be needlessly dangerous far longer than was ever intended.
  • area denial weapons/tools → Same basic idea as minefields. Anything that blocks safe travel in a wide area, and lasts long after the war is over, becomes at least a travel hazard.
  • weapons of mass destruction (WMD) → A single stray bullet could kill a person. A single stray nuke could destroy a city and poison the land for centuries. A stray WMD has the same small chance of hitting something important as a stray mass slug. Unlike a stray mass slug, however, a WMD will ruin more than just a handful of peoples' days if it does hit.

Conclusion

You're worrying needlessly. But if you want to reduce the extremely tiny risk further, just blow missed shots up into a cloud of tiny fragments.

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    $\begingroup$ I can't believe you wrote Space is big without following up with 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, but that's just peanuts to space. $\endgroup$
    – pipe
    Commented Jun 11, 2016 at 10:39
  • $\begingroup$ good answer about the general view on the problem. Add easy solution to OP Q, like dispersion of slug by antimatter and it will be sure the best answer of all at the moment. $\endgroup$
    – MolbOrg
    Commented Jun 12, 2016 at 1:16
  • $\begingroup$ re: ME quote. A 50kg slug traveling at relativistic speeds is going to have the same level of energy as a WMD. So anything it hits is going to know it, and so is everything around. It will likely hit something... eventually. Gravitational perturbations of the course, if nothing else. Maybe not for a million years, but eventually. $\endgroup$
    – Seeds
    Commented Jun 13, 2016 at 15:57
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§ Heisenberg Ammo

Ammunition under Heisenberg law is defined as shape-shifting ammunition, which must be liquid and of low viscosity and density in stationary state, but when loaded gets set under pressure in the catapulting device and is allowed to shift to solid state. Loaded with energy during the pressurization, the material must harden. As soon as the projectile gets fired and the initial pressure is lost, it must keep it's energy for a maximum of thirty seconds earth time. When the end of the time span – defined by Space Warfare Conventions Treaty about mass weapons – is reached, the material must shift back into liquid state to avoid hitting non-participating parties.

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  • $\begingroup$ I like where you're going with this but it still stands that the liquid would have as much kinetic energy as the solid, so something would have to spread it out to minimize potential damage. In fact many modern bullets are designed to sort-of work opposite of this, turning into a liquid in mid-flight so as to do more wide-spread damage to the target. $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 1:57
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    $\begingroup$ At relativistic speeds, the state of the matter is irrelevant because the forces involving impacts between the atoms of your slug and the atoms of your target are way bigger than the interatomic interactions inside the slug. $\endgroup$
    – Davidmh
    Commented Jun 13, 2016 at 15:16
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If you have the technology to fire a projectile at a significant fraction of the speed of light it is probably also safe to assume that you are not going to 'miss' as such. Your target might evade the projectile but you should be able to tell where it is going to end up.

With this in mind you could certainly have a targeting system which prohibits firing on any trajectory which is considered dangerous or illegal. This may mean that space battle involve a certain amount of maneuvering to stay in the shadow of a 'no target' planet but then again this sort of military/diplomatic cat and mouse happens even now, especially in low intensity warfare where large (and notionally law-abiding) nations are involved.

The other consideration is that even if you do hit your target your projectile will either pass straight through it and/or transfer some or all of its energy to it so you still end up with a load of hot space debris which may well eventually fall into any nearby gravity wells.

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    $\begingroup$ Excellent point about the impact causing more relativistic debris. That would definitely need to be addressed somehow. $\endgroup$
    – thanby
    Commented Jun 13, 2016 at 2:20

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