Within a near-future setting, where ships can move between planets using existing or currently experimental technologies (e.g. fusion), it seems that rail-guns are an extremely effective weapon.

Are there any plausible (that fit within the existing understanding of physics) defenses that can be employed against rail guns?

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    $\begingroup$ The only defence is to be somwhere else. Armour is too heavy for spaceships flying on existing tech. Basically, pack bigger engines and rely on long shell flight time (and lack of maneuverability) to make any turn a dodge. $\endgroup$
    – M i ech
    Apr 15 '17 at 4:40
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    $\begingroup$ @AptAnole Armor requirement will go up faster. For a rail gun round if double the speed it is 4 times the energy. $\endgroup$ Apr 15 '17 at 4:49
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    $\begingroup$ At existing and near future tech, any armour is too much. Weight is a real buzzkill. $\endgroup$
    – M i ech
    Apr 15 '17 at 5:03
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    $\begingroup$ It's fine so long as the ship is built in space and not expected to land. We can push massive ship through the ocean, we can move massive ships through nothing. $\endgroup$
    – rclev
    Apr 15 '17 at 6:21
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    $\begingroup$ @rclev: A ship in space is entirely different than a ship on water. The water ship only needs to overcome the friction of the water (plus a little bit for initial acceleration); the space ship must deal with changing gravitational potential. The energy needed for that is directly proportional to its mass - and until you invent a reactionless drive, that also includes the mass of propellant/reaction mass. See the rocket equation. $\endgroup$
    – jamesqf
    Apr 16 '17 at 18:24

10 Answers 10


In the realm of passive defense, there's been a lot of talk so far about thick armor, but you'd be better served to think about angled armor. If there's a small projectile coming at you at an absurdly high speed, don't try to stop it cold by absorbing its even-more-absurdly-large kinetic energy. It's likely going to be easier to deflect it by redirecting its momentum and sending it off into space while still traveling absurdly fast.

Which ties into the already-mentioned option of stealth, since (modern) anti-radar stealth designs also incorporate flat surfaces and hard angles, just like what you'd want for deflecting projectiles.

And then there are active defense options, which have gone largely unexplored in previous answers aside from making evasive maneuvers. But there are other options:

  • If you have high-powered lasers available (and, if you're using a fusion reactor, you probably do), you can zap the incoming slug with them. If the slug is small enough and your laser is powerful enough, you might be able to completely vaporize it. (This wouldn't make it completely disappear, but its mass would spread out over a much larger area, making it easier to absorb the impact. If you can vaporize it far enough away, much of the pass will miss you entirely.) If you don't have enough power for that, you can target the side of the projectile and the material boiling off the side will impart a slight lateral thrust, which may cause it to miss if you can target it accurately at a long enough range.

  • If you have railguns of your own and they can fire quickly and accurately enough, they can also be used to intercept incoming projectiles. Whether lasers or railguns are better for point-defense fire is largely a matter of how the rest of your setting works and the flavor you want to give it.

  • Use a cloud of robotic drone "plates" which can position and angle themselves to deflect the projectile while still at a distance from the ship, rather than waiting for it to reach you before attempting a deflection. In addition to the benefits of keeping the slug further away from you, such drones would also be able to adjust position and angle more quickly than the ship itself, as they would be smaller, lighter, and not constrained by the limits of human G-tolerance.

  • Another option with drones would be to use them as reactive armor, self-destructing and using the power of their own detonation to slow, damage, and/or redirect the incoming projectile.

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    $\begingroup$ Minor nit-pick: there is no stealth in space. Radiating waste heat and energetic thruster exhaust, when observed through IR telescopes, might as well be a screaming horde of Mongolian raiders at a death metal concert. They're easy to spot on account of the lack of anything else at all being around it, and can be spotted from absurd distances. $\endgroup$
    – Sean Boddy
    Apr 16 '17 at 4:48
  • $\begingroup$ Comment after the comment: with good interstellar engines and plentiful energy, stealth may not be possible, but sneaky might be. An attacker could suddenly change orbits to pop out from behind a planet or moon with little warning, and likewise a defender on the run could enter orbit and burn hot all the way through a slingshot maneuver to attempt escape and put an undesirable backstop on the shot, assuming planetary bombardment was not on the bad guy's to-do list. $\endgroup$
    – Sean Boddy
    Apr 16 '17 at 5:11
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    $\begingroup$ @Sean: True, true. I really only mentioned stealth tech as an aside, "the same kind of design that deflects shots also has a side benefit of making you harder to spot with radar". Avoiding passive thermal detection in space is, as you said, a lot harder, and likely impossible during a thruster burn. $\endgroup$ Apr 16 '17 at 8:12
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    $\begingroup$ I've read that with hypervelocity projectiles, sloped armor stops being effective, since the timescale of impact is so short that deflection is impossible, but I can't remember where I saw it. Do you have any research that might support sloped armor being useful against 4+km/s projectiles? $\endgroup$
    – Catgut
    Apr 17 '17 at 13:37
  • $\begingroup$ @SeanBoddy I am not necessarily disagreeing with you, but you did seem to contradict yourself. "No stealth in space," then you proceed to describe a stealth in space situation. What you describe is stealth: sneaky is stealthy. The screaming horde of Mongolian raiders at the death metal concert that you describe is a great stealth tool if used properly; we did it to the Germans during WW2 by dropping a crapton of bits of foil into the air when we bombed them... their radar saw a screaming horde of Mongolian raiders at a death metal concert, and our bombers were invisible. That is stealth $\endgroup$
    – Loduwijk
    May 1 '17 at 20:43

Your best bet is to simply not get hit. That answer sounds a lot dumber than it actually is, so just bear with me. At interplanetary/interstellar cruising speeds (usually anywhere between a few dozen to a few thousand km/s), making minor (and most importantly random) course adjustments using small bursts of thrust at a fraction of a g every few seconds consumes very little fuel comparatively speaking and makes you a very hard-to-hit target. The speed of most spacecraft makes it so that even a small amount of thrust for a fraction of a second in any direction can result in you being dozens of kilometers away from your initial starting position by the time your enemy's railgun slug has intercepted your ship's original trajectory.

Alternatively, if you don't want to rely on Douglas Adams and the RNG to save you, turn your anti-collision lasers into point-defense systems and vaporize the incoming slug before it reaches you. Bonus points for shooting the slug with your own slug to knock it off-course. This all changes if your enemy has a railgun that fires slugs with their own built-in guidance systems, but smart bullets don't really work that well when combined with railguns for a variety of reasons, so you probably wouldn't encounter this.

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    $\begingroup$ It should be noted that random walk effectiveness depends on engagement range, randomness and not letting the attacker close distance. If the attacker can accelerate faster than you and has the fuel to do it, you are going to get shot. With near future tech, top thrust will be limited to human g- force tolerance, so the attacker gains advantage just by starting their burn earlier, but the defender can burn hard in a direction the attacker will have a hard time turning towards. Everyone will have killer radar and rear mounted anti ship weapons in case they end up running. $\endgroup$
    – Sean Boddy
    Apr 15 '17 at 9:32
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    $\begingroup$ In the real world, the definition of "close range" in space is a lot further than we normally think of. 15km is "kissing distance" in space -- it's a range where the ISS crew has to scramble to their life boats... and has happened twice in the ISS's career. If you're actually fighting at that range, I expect to see all hands on deck and the plank out and ready for boarding! $\endgroup$
    – Cort Ammon
    Apr 15 '17 at 23:57

Railguns aren't extremely effective weapon


I have to make some assumptions, as they aren't provided by OP. And those assumptions will cut out some possible use cases and technologies, which we may call railguns too. I have to do so to narrow the modeling and shorten the answer.

  • ships engines use thermonuclear power, with variable ISP. The same thermonuclear power is used to launch projectiles.

  • projectiles are dump chunks of metal, let's say Iron with 10GPa tensile strength, 2000 GPa Young's modulus, no maneuvers. (basically a chunk of steel 10 times stronger than usual)

  • speed of projectiles is arbitrary from 0.9c to 4 km/s

  • efficiencies are 100%, both for engines and for launching projectiles.

  • ships are aware of that they are in a combat situation, and they understand from which direction to expect a hit. (no invisibility in space for ships) Ships are equipped with detectors of IR signatures not worse than Spitzer with ran out of liquid helium coolant. More about what this assumption practically may mean in the answer

  • form of projectiles parallelepiped, proportions (1,1,3), mass 1 ton or less.

Modeling the possible situations

High-velocity projectiles.

Destruction limits for the projectiles

For a chunk of metal with tensile strength of 10GPa energy needed for destruction of 1 cubic meter of it into 0.001m size chunks is something like 10/200 * 0.001 * 1e10 * 3 * (1/0.001) = 1'500'000'000 J (it is under assumption that we exfoliate it layer by layer, 1mm thick layers in each of 3D dimensions, and we do a work which is defined by elongation before it breaks(which is determined by Young's modulus and strength of the material) until we get bunch of small 1mm cubes from it and it seems that the energy is invariant in those assumptions which seem to be correct actually)

certainly, it represents the needed energy at the order of magnitude and depends only on the volume of the projectile. For our 1 ton projectile, it will be about 7.5 times less, so for the destruction the projectile as a solid object, we need 200'000'000J or less.

Collision of projectile with a thin flat shield

Same material used as a thin flat shield against a projectile and the question is How thick it has to be to destroy the projectile and the energy of the collision.

Assume collision is an inelastic collision, the thickness of the flat shield is "d".

Energy which will be spent on heating and destruction (ignoring relativistic effects) should be

$$0.5 \cdot \frac{m_p \cdot m_s}{m_p + m_s}(0.9c - 0)^2 > 200'000'000J \qquad (1)$$ $$m_s = \rho \cdot d \cdot \left(\frac{m_p}{3\cdot\rho}\right)^{2/3}$$

$$\frac{m_p + m_s}{0.5 \cdot m_p \cdot m_s \cdot 0.81c^2} < 1/200'000'000$$

$$m_s << m_p \Rightarrow \frac{200000000}{0.5 \cdot 0.81c^2} < m_s$$

For 1 tonne projectile we need a section of shield with mass 2×10^8÷(0.5×0.81×9×10^16) = 0.000000005 kg < ms to destroy the projectile as solid object. The destruction does not protect the ship yet because those remains are more than capable of destruction of the ship itself. We need to spend more energy to disperse those remains, to reduce the amount of debris which are potentially capable of hitting the ship.

Cone of destruction

Now comes in play the distance between the ship and destroyed projectile and energy we spend on the destruction.

The same energy which destroyed the projectile has to be used to give that debris the velocity to fly off the projectile. More energy is spent in collision faster those particles will disperse and higher velocities they will have perpendicular to the original velocity vector. This motion will form some sort of cone(probably with some complex density distribution, but for simplicity I assume it is even distribution)

To for a cone with angle 1 grad, those particles have to get in average 0.007853882c radial velocity, or for our 1 tonne projectile it means that energy of collision have to be more than 2.775755811×1015J thus mass of the shield(which collides with projectile) has to be 0.076152423kg, which is much more than it was needed just to destroy the projectile(so basically material strength does not matter), but still not that much.

  • Energy of collision can be greatly improved for high-velocity collisions by using thermonuclear fuel as a shield, thus requiring even less mass for the shield. This will work at about 0.1c relative velocities.

The thickness of the shield will be about
0.076152423÷7500×(3×7500÷1000)^(2÷3) = 0.000080924m $\approx$ 0.1mm

A square kilometer of it will weight 606930 kg or about 600 ton.

The distance between the shield and thus the place where the projectile will be dispersed has to be pretty distant if we would like to reduce the amount of debris which may potentially hit the ship. Let's assume an even distribution of the debris and that the ship can survive the collision with one gram of it and projection of the ship in the direction of attack is 10000 square meters, the angle of the cone is one grad.

To satisfy the requirements distance has to be
sqrt(1000000×10000÷3.14)÷tan(0.5) = 6'466'611 meters or about 6500 km

  • considering the speed of the potencial projectile (0.9c) and its mass (1'000'000 gram) it is a surprisingly short distance.

0.1c velocity projectile

Destruction energy the same - 200'000'000J
Mass of section of the shield for the destruction energy has to be 0.000000444 kg (which is 88.28 times more than with 0.9c projectile)
Radial velocity for the cone has to be 0.000872654c (which 9 times less than for 0.9c, obviously)
Energy of collision has to be 3.426859026×1013 (81 times less than with 0.9c)
Mass of section of the shield has to be 0.076152423 (the same as for 0.9c case)
Distance can be 81 times less than in 0.9c, as 1 gram at 0.1c carries 81 times less energy (ignoring relativistic effects for 0.9c, which is about 2.3 times difference in kinetic energy compared to newton physics, I'm too lazy to deal with it for 2.3 times difference in results)

Shield thickness about the same 0.1mm.

So, in general, there are not that many changes compared to the situation of the 0.9c projectile, and no changes in efficiency for the same shield, but it is because the energy we should have to make the cone is orders of magnitude higher than the energy needed to destroy(make loose) the projectile.

low-velocity projectile

At some point energy needed to destroy the projectile will be close to the energy needed to form the cone and begins to play a more significant role.

For the 1-grad cone, it will be at velocities about 72 km/s , so, let's see the same for the projectile at 70 km/s

Destruction energy the same - 200'000'000J
Mass of section of the shield for the destruction energy has to be 0.081632653 kg
Radial velocity for the cone has to be 610 m/s
Energy of collision has to be 186587642 J
Mass of section of the shield has to be (for energy of collision 200'000'000J + 186'587'642J) - 0.157790874 kg
Shield thickness about as twice as much as for 0.1c and 0.9c projectiles - 0.2mm

with lower speeds, the needed thickness begins to grow as crazy, because the energy of the inelastic collision proportional to the square of relative velocity differences - so for 35 km/s projectile it will be 4 times thicker, for 7 km/s it will be 100 times thicker. Still, it is better than solid armor, and that is Whipple shield velocities. 1km/s projectiles and we are in the field of usual armor.

Efficiencies of (whipple style) shield at different speeds

0.9c - very efficient
0.1c - very efficient
70 km/s - efficient
7 km/s - kinda efficient
1 km/s - not efficient
0.99999c - it depends.

Notes about tactic in the situation.

First of all - the distance between ships is the friend in the situation, against high velocity and for low-velocity projectiles, but for different reasons. For low velocity, because of they just too slo-o-o-o-w. For high-velocity projectiles - it needs noticeable distance between countermeasures and the ship itself, for the base of destruction cone to be big enough, compared to ship projection in direction of attack.

Lasers as point defense are just inefficient, just forget about them for high-velocity projectiles they are just useless.

Unmanned drones which deploy the shield are useful things. They may deploy and keep with acceleration speed of the main ship. (there are different ways to do so, and they are technology dependant)

Great distance is good for missiles, the higher velocity at the target, especially those missiles which are with thermonuclear engines.

High-velocity projectiles will emit IR signature, no matter how efficient was their launch, just because of interstellar medium

Shield of death.

The Recent invention of space combat industry presents you a shield of death. Mobile, relatively light weight, almost tested in "Children of a Dead Earth" A grid of guided interception missiles.

A volume which filled with interception missiles, with low delta-v, small, at distance of about 5km from each other, with some kinda weak guide system.

Or modified version of it, with tethers between nearby nodes.

Will be good for stationary bases and to protect volumes. Might be good against missiles, and low and medium velocity projectiles. 400 km (80 layers) will guarantee the safety of your gold-pressed latinum and your life.

Buy now - satisfaction guaranteed, call 666-777-42, do not wait, the enemy does not sleep, call now 666-777-42.


Railguns aren't extremely effective weapon by any means. Missiles are a better choice, but they aren't perfect either.

Rail guns are close combat weapon, and to allow the enemy on the distance which is effective for them, it needs to be very simple minded. Especially in space where you see the possible enemy at a.u. distances.

I haven't used all my assumptions because it would make the answer needlessly too long, but the problem is pretty rich on details, and small changes in those details may change the picture drastically. Implementing a bit more sophisticated technologies to those projectiles may make them significantly more effective, but the same will be true for defense.

At the end better prepared will win the battle and it will be not those who have only rail guns or have them at all.

I kinda recommend the game "children of death earth" - it's not ideal in the available technologies, but it might help to choose the characteristics of weapon you may need. Definetly it's better than nothing, and at the moment there are no other easy available options for simulating the kinda stuff.

  • $\begingroup$ At .9 c relativistic effects become important something like 25% additional kinetic energy. Additionally, I disagree with your inelastic collision assumption for the projectile/shield impact. At these speeds, the projectile would hardly notice a shield 2mm thick. You may get a flash fusion reaction on contact but given the mass and velocity of the projectile, I expect a significant chunk to make it through. That is in addition to the spreading cone of relativistic metal vapor. $\endgroup$ Apr 16 '17 at 0:18
  • $\begingroup$ @JoeKissling actually the difference of kinetic energies due relativistic effects at 0.9c is way much more that +25% compared to newton physics. It is 3.195450219 times difference. at 0.1c it is 1.007563052 times difference(less than 1 percent). Was not my point to be too precise, the point was to show that faster not necessary means better for that type of weapon. The thickness of the foil is not 2mm but 0.2m (actually 0.16mm). As for fusion/fission happening - if you can calculate the percentage then you are welcome, but even 10% energy loss is not big deal here $\endgroup$
    – MolbOrg
    Apr 16 '17 at 1:11
  • $\begingroup$ (if it more than that then just great, a new way to make materials cheap). There will be a lot of stuff happening during the collision, but I have no way to calculate them all and it is not necessary for the precision of the answer as it is already a very rough aproximate of the situation intended to show the tendencies. $\endgroup$
    – MolbOrg
    Apr 16 '17 at 1:12
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    $\begingroup$ As for not noticing the barrier - it will notice and exactly that is the mind-bending truth which I would like to show. Usually, people think the right opposite of it - how small dust particles are a great danger for an interstellar ship at fraction of light speeds. Making the projectile more elongated will probably help with penetration, there are solutions for that too. $\endgroup$
    – MolbOrg
    Apr 16 '17 at 1:12
  • $\begingroup$ I suppose I should double check my relativistic gamma calculation then. When I say not notice I mean that the shield will do little to reduce the danger of the round. If it hits a shield .2mm thick you are going to have an explosion akin to a nuke, but you are still going to have the majority of the mass of the round coming at you at .9c. It's moving too fast to be affected by the expanding plasma that its contact with the shield will produce. $\endgroup$ Apr 16 '17 at 1:21

Get hit but not hurt.

Consider Al-Queda. Some dude is captured. He is out of commission. He is of no help against the larger organization because he is clueless and has had little or no contact with them.

Now consider the good ship Al-Queda. It is freaking enormous, a half-mile across, modular and chaotic, with components rearranging themselves, joining, drifting etc. This is strictly a space ship. It is not going to land. You have a railgun which launches a little piece of metal which will put a hole right through whatever you shoot. You can shoot many of these. At what? You are not sure what part of the ship is important. You can be pretty sure than any given part of the ship is multiply redundant, including crew and AIs. How do you hurt something like that with a little hole?

Apollo 13 was the opposite of the Al-Queda: extremely dense with nothing nonessential. Shooting Apollo 13 with a railgun would be very effective. Shooting the Al-Queda just wastes time you could be using to steer in a nuclear missile.

I should add that nuclear missiles have more in common with Apollo 13 than with the Al Queda. Railguns are unsurpassed for disabling incoming nuclear missiles.

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    $\begingroup$ Put the nuke in the rail gun round? $\endgroup$ Apr 15 '17 at 23:18
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    $\begingroup$ @MolbOrg Not with the correct MIL-SPEC call out. Besides there have already put a nuke in an artillery shell. $\endgroup$ Apr 16 '17 at 0:10
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    $\begingroup$ @MolbOrg Acceleration is a killer, no doubt there. Right now there exists tech that can tolerate >150000 G's I don't see it being too terribly unreasonable that this can be improved upon. $\endgroup$ Apr 16 '17 at 0:34
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    $\begingroup$ @JoeKissling Yes, it can be improved and I would say very very significantly. It just a trope of those types of weapon to assume them dumb. There is another moment there, at some point the projectile is better than a nuke just by itself(which is one of the selling points of it) and spreading of the projectile can be achieved by simpler means. But considering the improvements, makes the considering of the situation more complex and most people are not capable of doing so, and they have no interesting ideas on improving the technology. Lack of fantasy, knowledge, demand. $\endgroup$
    – MolbOrg
    Apr 16 '17 at 1:23
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    $\begingroup$ @Joe Kissling: that enormous energy in the relativistic projectile only hurts if it is delivered to the target. I envision these projectiles, like cosmic rays, travelling completely through the Al Queda and off to into space with 97% of their initial energy. $\endgroup$
    – Willk
    Apr 16 '17 at 3:07

There's four that I can think of that may fit into your setting.


Armor. Lot and lots of armor. Assuming these rail guns are capable of getting their bullets to at least a small fraction of the speed of light, then the resulting destruction is comparable to our nuclear devices. Luckily for us, we already know of a way to protect against blast of such intensity and already employ it in our super special bunkers.

Armor. Lots and lots of armor. You can go the Battlestar Galactica route and slap so many thick chunks of armor onto your space ships that the railguns just end up shaking you around a bit.

EDIT: As Joe pointed out, no current bunker can survive a direct hit. You'd need to make your armor out of something stronger than your average steel and concrete. Would also need to be able to absorb much of the kinetic energy to protect the people inside. Thick layers of carbon nanotubes might suffice, but not sure on that.


Second is simple speed. Maybe your group heavily invests in speed when it comes to building your ships. If your fast enough, any enemy ships are going to have a hard time getting a decent shot at you. Though the downside of this is that your ship will most likely be more vulnerable if it does end up getting hit.


Electronic Warfare and Stealth technologies. If they can't see you they can't shoot you. Or perhaps you manage to interfere with their targeting system and your actually miles away from where they think you are. Space is big, and they'd have a hard time looking out a window and finding your ship, so they'll be completely reliable on what their scanners tell them.


Probably the most Sci-fi answer, but plausible. A super strong magnetic may be able to deflect incoming projectiles away or stop them completely. Similar to the kinetic barriers of the Mass Effect games. And while they would take a massive amount of energy, if you have a room temperature super conductor in your story, you'd pretty much only have to charge the barriers up once.

Hope I helped

  • $\begingroup$ No bunker can take a direct hit from a Nuke $\endgroup$ Apr 15 '17 at 4:39
  • $\begingroup$ Boo, you're right. Maybe his ships armor can be made from thick artificial diamond or maybe carbon nanotubes would be strong enough. $\endgroup$
    – rclev
    Apr 15 '17 at 4:41
  • $\begingroup$ Neither would tolerate a Nuke at all. Now, for a hypervelocity projectile, those could be useful. You would need both, diamond for its hardness and carbon nanotube composite to catch the fragments. Diamond will shatter completely. $\endgroup$ Apr 15 '17 at 4:45
  • $\begingroup$ Good answer, wish I could upvote. I don't think it's plausible to outmaneuver the targeting systems, simply because a small change in angle would cover a larger distance far away. $\endgroup$
    – AptAnole
    Apr 15 '17 at 4:45
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    $\begingroup$ @JoeKissling That's a very good point. It could depend on the rate of fire and the speed. For particular engagement distance you would have to calculate what the maximum distance a ship could travel in the time it takes the projectiles to reach and how any projectiles you would need to fire to cover that area. $\endgroup$
    – AptAnole
    Apr 15 '17 at 5:00

Man-made fusion isn't an energy source, experimental or otherwise. Perhaps it will be, but I wouldn't bet on it. Even with fusion power, you still need reaction mass.

The more reaction mass you have on board, the more reaction mass you need to use to change the velocity vector of your craft. The faster you change velocity or direction vector, the higher the acceleration you expose the crew to.

Machines are both quicker to change direction and more able to withstand acceleration. They are smaller and cheaper than manned spacecraft. If the manned ship is within a couple of seconds of the attacker (say, the rail gun can accelerate its projectile to 3,000 or even 10,000 km/sec) then the target will be toast. If they are further apart, dodging around might be possible - if the projectile is incapable of course correction.

The problem is, there's no reason why is would be unable to correct course, if such "smart" weapons were needed. The only real defense against such weapons is distance. With distance, you can deploy countermeasures, you can send back noise to disguise your exact location, and you can hide (possibly - if something to hide behind is available).

The real problem with fighting in space is explaining what the benefit is and why country A would tolerate country B's attack on its spaceships. Space is too big to defend. As the recent influx of refugees into Europe has demonstrated, even in two dimensions borders are economically impossible to defend.

The only scenario where A attacks B is one in which A and B agree to engage. This only works as long as neither has much to lose. As soon as one side starts losing, they'd drop a rock on the other's capitol. Wouldn't you?


As M i ech said, don't be there.

The use of non light speed (lasers) or tracking (missiles) weapons at anything other than knife fighting distances is like playing the lottery.

Each target is surrounded by a halo or blob (there's probably a technical term for it) of where it could be when the payload arrives. This halo gets bigger as the distance to the target and the thrust available to the target increase and gets smaller with the speed of the payload.

If the halo is big enough (eg. the target only occupies 1% of the halo), then you are left with a "spray and pray" strategy.

All of this only really matters if the target is taking "evasive maneuvers." A target that doesn't know the attack is incoming is much more predictable.

So, big thrusters around the ship will add to its defense. G-force protection for any crew will increase the amount of thrust you can safely use.

However, for near tech, things are much simpler. High thrust is expensive. If they run out of reaction mass before you run out of rounds for your rail gun, you've got them.

  • $\begingroup$ Concerning the technical term: Where the other ship is would have an area of uncertainty which would account for position error. The same term could probably be used in the case you describe, the ellipse within which target will be located at time-0. Total area of uncertainty would be a function of both initial fire-time position uncertainty and the added future-position uncertainty. That could be very large and is essentially the haystack you are "hiding" within. Both initial position error and movement-over-time error will change with distance, so distance is key. $\endgroup$
    – Loduwijk
    May 1 '17 at 21:13
  • $\begingroup$ @Aaron, yep. Thus, knife fighting distances. Note that these same issues pose problems for lasers if you can make the area the laser hits big enough through distance or movement, you decrease the energy per square inch/centimeter. Tracking weapons are the only reasonable long range weapon in my opinion. $\endgroup$
    – ShadoCat
    May 5 '17 at 21:18

There is no real defense, except to dodge or brace for impact. You could use a nanotech foam that hardens on impact, which would make it lightweight for space. It might be 2-5 meters in thickness around the ship and very bulky. I don't know how think it would have to be. When the projectile hits, it would distribute the force along the entire ship's hull instead of a single spot, hopefully absorbing the impact. There is currently a similar material call D3O for snow/ski/bike/motorcycle/military helmets and elbow/knee pads.


Another strategy would be to make a very large, very open, very spaced out ship with thin walls that are self-healing. When hit, the projectile just makes a small hole and the walls are pressurized with fast curing foam/gel that seals the holes. It would have backup systems for major components like engine, life support, navigation, etc. People inside would just hope they themselves were not hit. It would be like trying to pop a taped balloon with a needle, which is impossible.


  • $\begingroup$ At these speeds everything splatters. The energy not the momentum is the problem, the energy released is that of a nuke. $\endgroup$ Apr 15 '17 at 22:47
  • $\begingroup$ the second link is neat, and such solution may be a valid way to deal with the threat. It can be improved - making a multihull ship which moves its parts away from the trajectory of the projectile, making the projectile fly between those parts. No fuel required, low energy demands, low-tech solution. With a better technology, the same can be done on a greater scale and better, so changes for humans to suffer from collisions will be about to zero. $\endgroup$
    – MolbOrg
    Apr 15 '17 at 22:58

There already exists armour that protects against this type of 'attack'. It is in use right now on many satellites currently in orbit. The armour is called a Whipple Shield.

Whipple Shields are used to protect satellites from collision with small debris objects traveling at extremely high relative speeds (of the order of 15km/s!). They work by having many thin (i.e. not heavy) layers of material. Each layer is hit and upon impact the hyper velocity impactor disintegrates into much smaller peices and loses a fair amount of its momentum. After usually two layers of Whipple shielding the impactor is effectively a collection of particles similar to a gas, which the inside most layer of the Whipple Shield defends against with ease (basically elastic deformation). If you want to know more about Whipple Shields you can ask a question over at the space exploration stack exchange! :)

As I mentioned at the start of the answer, these are used today in orbit. They're flight proven, meet general spacecraft system requirements (size/weight etc.) and well understood


You might consider as a last-ditch, desperate act of defense against a railgun projectile "some sort of" specially targeted detonation with the aim of creating a high-order explosive event which would allow for a focused wavefront of ejecta to meet the projectile and which may lessen the impact.

Even less convincingly, if you were capable of some ginormous magnetic field control you might be able to bend the projectile around you, or slow down its approach so that your explosive armor can provide some protection.


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