As part of a series of questions about a hard sci-fi universe (for more answered questions about it, please check this and this)

The setting is the same:

I'm making a world where sub-FTL interstellar travel is a thing, and so is interstellar war, but for the purpose of this question we'll assume the scale to be of a single system.

Spaceships have evolved quite a bit since our current era, and they range in size from several meters (something like 2 times larger than your average air superiority fighter, having a space-worthy manned vessel can't get any smaller than this in my universe) to up to 20 kilometers in the shape of a scaled-up space submarine.

Weapons comes in varying shape and size (proportional to the ship that bear them) and can be split in 3 categories:

  • Missiles : Just your everyday payload carrying self propelled firestick, you could also call them torpedoes at this point, it doesn't really makes a difference as far a I know. They won't get bigger in size than an ICBM and the tech didn't evolve much aside from targeting and space-worthiness.

  • Lasers: They are mainly used as a countermeasure and/or to mess with enemy targeting systems at close range (and occasionally to blind the enemy commander trough the window as a prank).

  • Railguns: This is where R&D was the most successful, since those guns can get pretty big (remember the 20km ships?) and the biggest projectile to date is a whopping 100m long and can travel at 30Km/s. Picture throwing Saturn V at the enemy vessel). Please note that for smaller projectiles the speed can get up to 150km/s, but that is a very specialized gun and the average is less that 100km/s for a decent gun.

Concerning energy requirements, capacitors went a long way and are now extremely efficient, as well as cooling, even in space (firing a railgun more than once won't melt it for the first couple consecutive shots) and fusion is the go-to mean of energy production across the civilization. Dyson sphere project started in some systems, and the question of the access to the host star sparked the wars in the systems.


Ships in my universe are armored, but since I want to maintain a decent amount of consistency, I can't just strap a 5m layer of whatever armor on the hull and call it a day. Even worse, Unobtainium seem to be... unobtainable. I then went for some whipple shields and reactive armor and the likes, to get decent protections with limited wheight. The thing is, all of this works wonderfully against space rocks, but when some weapons (say, a missile) have counter measures to your counter measures, things start to get tricky. So what I need is a shield, separate from the hull, and far enough to make any missile useless, or at the very least less deadly.


So far, the shield I came up with is based on generating local magnetic fields to have some iron dust orbit the ship at high speed. This would give me a hollow sphere of iron rotating a some speed (at least 3km/s at the equator of the thing) with a density of matter of at least 1kg/m3. the weak point at the poles of the sphere would be fixed with a second sphere perpendicular to the first (and of slightly different radius of course).

Such a shield should be able to destroy incomming missiles even when undetected, and prevent any boarding of the ship by small assault vessels should they survive the fixed defences of the ship. Whith some luck it may even deflect a fast moving inert projectile (fired from, say, a railgun) As of the size of the ship, i'll be going with the biggest (20km submarine shaped) in my universe, and energy requirement are moot (exept if this system requires the power input of a star).

Tech level is several centuries ahead of current tech, so extrapolating to the limit of physics is acceptable a long as the tech is plausible.

Now for the fun part: Would this shield system be realistically possible?

Bonus: If it's nonsense or simply impossible, what kind of current tech would make a good remplacement for this system that isn't an insult to reality?

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    $\begingroup$ Countermeasures to your countermeasures is the history of warfare and related technologies. If nobody had invented armour there's a reasonable chance we'd still be hitting each other with sticks, but we needed a better stick to penetrate the armour and a better armour to stop the new sticks. The next thing you know you're dropping rocks from space and all because someone made some better armour. $\endgroup$
    – Separatrix
    May 24, 2018 at 8:46
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    $\begingroup$ I don't have the expertise to help you on this one, but I have another consideration to point out. If this works, how will your shielded ship return fire? Are you thinking they would have to turn off the shields to fire? It seems like it would take a while to get the iron particles up to speed and stopped between shots. (But like I said, I'm lacking the expertise) $\endgroup$
    – Josh
    May 24, 2018 at 13:04
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    $\begingroup$ Interference between the two fields would cancel out your shield $\endgroup$
    – nzaman
    May 24, 2018 at 13:27
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    $\begingroup$ @Josh Firing anything that is not a fast moving chunk of metal (did you say railgun?) would require some preparation, such as the creation of an artificial launch window by fine tuning the magnetic fields, and it would be far from instantaneous and probably easily detectable, making it a launch window for the enemy as well. $\endgroup$
    – Alexcommil
    May 24, 2018 at 13:29
  • $\begingroup$ related worldbuilding.stackexchange.com/questions/12520/… $\endgroup$
    – Aify
    May 24, 2018 at 18:21

9 Answers 9


Point Defenses and Manuevering are better than "Force Fields"

Lets assume 3 things:

1: Computers are faster and able to process larger amounts of data in that smaller time allotment.

2: A corresponding advancement in sensory technology has made sensors both very sensitive, very fast, and very long range.

3: Lasers have become much more powerful (in the megawatt ranges at-least)

A suitably powerful laser could burn through meters of steel per second, making such an emitter is actually within current tech ability, its just powering the damned thing isn't economical enough to try. So a rail-gun projectile or missile is coming at your ship, the sensors detect it, the computer calculates its vector and determines that it will impact the ship, then it fires one of the point defense pulse-lasers at it and vaporizes part of it, which either causes the missile to explode, or vaporizes enough of the rail-gun projectile to nudge it juuuuuust a wee bit off course. Maybe a firing Solution isn't needed. Maybe the ship just needs to alter its acceleration by .5% to avoid the projectile. This is space and the distances involved are so immense dodging a rail gun projectile is simply a matter of not moving at the same velocity when it arrives as when it was fired. Even just a tiny nudge can equate to kilometer wide margins of error for the gunner trying to hit the target. In space combat we are talking about battles taking place at thousands or even millions of kilometers. Ships may even be firing from one orbit into another. This means that no matter how fast projectiles are there will always be ample time to detect them and either evade or open fire. This is why lasers were practically blessed by our universe's physics to be used in space combat since they move at the speed of light and no matter what your enemy is throwing at you unless its another laser you will always be faster.

Another great way to use lasers is to blind enemy sensors, even at ranges where the beam is too diffused to cause outright damage to the enemy it can still damage sensitive electronics and fine equipment, which sensors tend to mostly be comprised of. At extreme ranges you could force the enemy to shutter or shelter their sensor arrays until they are within your engagement distance.

The Problem With Your Shield

The issue your shield is going to have is that your own cloud of iron dust is going to mess with your sensors. The more sensitive detection equipment is the easier it is to make a mistake. You will essentially be blinding your long range sensors (radar, ladar, thermal, IR, UV, etc etc). Like I mentioned earlier the biggest advantage in space combat you can have is long range detection. In space combat the defender will always have the advantage over the attacker since they can be detected from millions of kilometers out and anything they are doing cannot really be hidden. Giving up that advantage just to maybe stop a few missiles that your long range lasers could have handled half a million klicks out just doesn't make sense. Accurate powerful weapons with standoff distance, more powerful and sensitive sensors, and targeting computers with faster and more accurate reaction time are the closest thing to a shield that a vessel is going to have in our current understanding of physics.

  • 2
    $\begingroup$ If the shield is made of well defined projectiles moving in controlled orbits, sensors can compensate for those as they know exactly where the shield particles are. The sensors will receive less light/radiation as some of it is blocked. But if that is a problem you can turn the shield into a sensor advantage by having each projectile be a small sensor platform, all sending their data to the main ship to be combined. In that way such a shield would become a sensor advantage. It needs a lot of computer power but that shouldn't be a problem in the future. $\endgroup$
    – JanKanis
    May 25, 2018 at 12:21
  • $\begingroup$ I don't think that there's any plausible extrapolation of current technology that can "burn through meters of steel per second" but other than that I think you make good points. It's also worth remembering that lasers generate a lot of waste heat which is hard enough to get rid of in atmosphere with effectively unlimited mass to throw away heat into. In a spaceship, this is likely to be a very big problem. $\endgroup$
    – Mark Olson
    May 26, 2018 at 15:26
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    $\begingroup$ popsci.com/technology/article/2011-02/… A 1 MW laser can burn through something like 2000 feet of steel per second. The US Navy is presently trying to build one. $\endgroup$
    – TCAT117
    May 26, 2018 at 17:45

Would this shield system be realistically possible?

Physically, no. You have two problems to begin with:

  • you cannot accelerate iron particles using a central magnetic field. You would need charged particles and, even then, you can't control them with a static or variable field.

  • you cannot have two hollow spheres one inside the other with different planes of rotation, because the field that controls the farther one would inevitably disrupt the other.

Then again, you have a further problem - the ship would need to move. This would require a much greater degree of control, as any collision might send particles against the ship.

And, of course, you could never shoot from inside the shells - your missiles or laser beams would collide with your own shield.

Finally, the mass of the shield would make things awkward - one kg per m3 may seem little, but the volume of a spherical shell surrounding a starship has a lot of m3.

But in space, without air resistance, you could easily prepare a "shield" with very thin, reflective film at some distance from the ship, kept in place with very thin whiskers. Inside that volume you could also be able to manipulate electric fields and transmissions, and (e.g.) make it seem that the ship is in a slightly different position or orientation. Of course, as soon as you fire the engines, the ruse will be uncovered. The film can reflect lasers and perhaps disrupt some kind of projectiles ("Whipple shield").



Other answers already list some problems with your proposed shield of iron dust. An IMO better design would be to have the shield consist of small active projectiles. The projectiles can generate their own magnetic field to interact with the one the ship is generating and those of other projectiles. With this active control each projectile can have its own unique orbit and change that orbit, you're not stuck to a sphere of dust rotating all in the same direction.

All this would require a lot of computer power by todays standards, but nothing unrealistic for a space faring future.

The magnetic fields would change in strength and orientation semi-randomly, but as each projectile knows how the field is going to change it can adjust its own field in accordance. This also ensures enemy weapons cannot disturb the shield too much, as they don't know how the field is changing and so they cannot counter it in a predictable way, though they could still try to disturb the shield with their own random magnetic field.

Such an active shield also gives better protection against (detected) weapons, as a large number of shield projectiles can arrange to intercept the incoming weapon. Of course the ship has an automated factory to produce replacement shield projectiles, and you might have a mix of different kinds of projectiles that could vary depending on the expected environment.

There is still the question of how effective this shield is. An incoming weapon will collide with the shield projectiles, and given the speeds all the participants will break apart and partially vaporise. You will then have a cloud of vaporised metal and scrap hitting the ship at many km/s. If the shield is far enough out from the main ship the cloud will have expanded a bit, so it will be less damaging than a single mass of an intact missile hitting your ship. Enough whipple shield armor might then stop the debris, but I wouldn't be able to say how much you need to realistically stop it. But at least the debris cloud won't be deploying countermeasures, so the ship would have a chance.

projectiles and propulsion

The part I don't buy in the description of your world is that missiles aren't much better than what we have today. If these people are traveling interstellar distances they probably have ships that can reach much higher speeds than what we have today, and therefore much more effective drive systems. The speeds of several dozen km/s you mention for big railgun projectiles is probably already within reach of current rocket tech. Just launching into orbit already requires a delta-V of ~10 km/s. If whatever drive technology they have is applied to missiles, those would also be reaching speeds similar to the railgun projectiles, and be equally effective, except you don't need a railgun.

Of course, you may want to combine both systems. A railgun-like magnetic accelerator launches a missile to a high speed, and then the missiles drives fire to speed it up even more and provide some maneuverability etc. Making the missile survive the high-g acceleration doesn't sound like a big problem, the US was already using radar-rangefinder equipped artillery shells in WWII that had to withstand 20000 g.


Use your magnetic fields by themselves.

So far, the shield I came up with is based on generating localmake magnetic fields this strong, set them up and let them run with nothing in magnetic fields to have some iron dust orbit the ship at high speed.

This is a great idea except for the iron dust part. If you have the ability to them. You will not have to worry about changing direction of the ship with the inertia of your iron cloud, or that the iron cloud is too thin, or that you have run out of iron. Your magnetic fields can stop incoming ordinance in the manner of an eddy current brake

A conductive surface moving past a stationary magnet will have circular electric currents called eddy currents induced in it by the magnetic field, as described by Faraday's law of induction. By Lenz's law, the circulating currents will create their own magnetic field which opposes the field of the magnet. Thus the moving conductor will experience a drag force from the magnet that opposes its motion, proportional to its velocity. The kinetic energy of the moving object is dissipated as heat generated by the current flowing through the electrical resistance of the conductor.

A high velocity conductor entering the magnetic field will generate enormous eddy currents within itself and convert its kinetic energy into heat (of itself). Incoming missiles will heat up to glowing hot, which will be hard on their components.

Also, your magnetic field will be a fine detector. Something entering one of your fields will be easily detected by the perturbation it causes, and all of your field generating power can be swung to bear to the relevant field - this would take a nanosecond.

Someone posted this proposition (stop bullets with eddy currents) to the physics stack but those fuddy duddies closed it. But not before there was a fine and math-rich answer posted. https://physics.stackexchange.com/questions/238332/can-we-stop-moving-bullets-by-eddy-currents

I am thinking how cool the CGI of this would look. As opposed to bouncing off an invisible barrier, the projectiles would slow and also glow with heat as they slowed. Consider smoke - on earth smoke trails behind a smoking object because it is slowed by air resistance. In space there is no air resistance, and the smoke would not be slowed by the magnetic field. Smoke, vapor and molten droplets (the smaller the object the weaker the eddy current) would retain the momentum they had and would shoot forwards off the slowing, glowing hot projectile.

  • $\begingroup$ I feel like this is a good idea but implausible. Its true you could slow the projectile with eddy currents, but the missile itself will be travelling way faster than a 9mm bullet and that is a lot more energy to slow down. It would require a much larger field to have any meaning full time impact on the missile. And secondly the field would impact your own missiles and anything metal that is moving on your own ship and there would be a ton of that in any sensors or weaponry that you have. $\endgroup$
    – Shadowzee
    May 25, 2018 at 0:16
  • $\begingroup$ @Shadowzee - the faster the projectile, the greater the eddy current it makes. The current produced is proportional to the velocity of the projectile. Re your own missiles - just turn it off and turn it back on once missiles away. $\endgroup$
    – Willk
    May 25, 2018 at 0:20
  • $\begingroup$ It is also exposed to the field for less time which will reduce the deceleration of the missile. I tried the same equation in the link you provided, with just the speed increased and that would require 15B (which I think is roughly the same as 15T?) applied to the entirety of your ship if you wanted a 1m stopping distance. If you wanted the shield to expand further or cover an uneven shape it would make the immediate field even stronger due to the inverse square law. 1/2 $\endgroup$
    – Shadowzee
    May 25, 2018 at 0:47
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    $\begingroup$ Turning the field on and off would generate those same eddy currents across your entire ship and it would have a much stronger affect on your ship since everything is closer to the field generators. I'm not sure of the impact this would have, but it isn't good in anyway. 2/2 $\endgroup$
    – Shadowzee
    May 25, 2018 at 0:50
  • $\begingroup$ This has already been used; it's the basic principle behind Dalek shields. $\endgroup$ Sep 19, 2020 at 23:42

$1 \ kg/m^3$ is the same order of magnitude of the density of a gas at sea level.

If a flimsy shell fired from a rifle can pass through it, imagine what the effect will be on anything launched by what you devised in your previous questions: almost null.

If you had a stopping power equivalent to the launching power of the enemy's rail gun (meaning you can deliver the same acceleration given by the rail gun), you would need about 18 km to stop the shell.

IMHO your best options are (even better if combined):

  • evasive maneuvers: as you pointed out, firing those shots won't be stealth. Take advantage of this and move out of the trajectory as fast as you can.

  • kinetic hit: fire a counter shot or a series of shots aiming at or around the projectile. The impact energy will severely damage the device before it hits you.

  • $\begingroup$ But missiles would get destroyed, right? Or is this system completely worthless $\endgroup$
    – Alexcommil
    May 24, 2018 at 8:47
  • $\begingroup$ @Alexcommil, I am afraid not. Too much kinetic energy to be dissipated in too short space. $\endgroup$
    – L.Dutch
    May 24, 2018 at 8:52
  • $\begingroup$ You omitted the part that says the shield rotates at 3 km/s, also, projectiles travel at much higher speeds than in our atmosphere, which makes them spontanously heat while entering the shield. Coupled with the rotation, maybe it could lead to deflection. $\endgroup$ May 24, 2018 at 10:48
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    $\begingroup$ @Mranderson, the OP stated in other questions that the projectile is travelling at least at 150 km/s. The density of the shield and its velocity are not effective. Even if it was, it is something which could be easily corrected, kinda of wind correction when aiming. $\endgroup$
    – L.Dutch
    May 24, 2018 at 10:55
  • $\begingroup$ I am aware that inert railgun projectiles won't even notice the field, but what about a missile? As they are slower and way more fragile, they should be influenced in some way, if not destroyed $\endgroup$
    – Alexcommil
    May 24, 2018 at 12:36

The shield idea is interesting, a bit like ablative armor, but its greatest limitation is the resource: iron. Unless you're hauling around a bazillion metric tons of it, you'll quickly run out of it. In fact, you could depend on your enemies developing weapons designed to vaporaize vast amounts of it just to clear it out of the way.

As for current-tech solutions, there aren't any. We don't have shielding today, nor is any technology remotely close to providing one.

However, you're not on the wrong track. Let's use some handwaving and suggest that you have a nuclear force shield, one that has the strength of the nuclear bonds between atomic particles, and that shield is rotating around the ship. It's purpose is to capture and redirect objects away from the ship (not unlike a street sweeper). I don't think this has been done before, and no mass-oriented object would be capable of penetrating it unless the energy of that object exceeded the energy applied to the shield.


This shield system sounds possible (so the answer to your question, strictly speaking, is "yes",) but not very effective at all for its intended purpose. It would be relatively easy for a missile to counter - either by generating it's own local field to move the iron out of the way, or simply match velocity with it and bump its way through the relatively static pieces of iron.

You're much better off with a proverbial "shield," one that works basically the same way as our current missile shields: actively detect incoming threats and intercept them with either anti-missile missiles or point defense fire. This is a lot easier to do in space than on Earth too, because there is no atmosphere to attenuate lasers (or other beam weapons,) no horizon to limit your detection range, and no unrelated activity in your surroundings to potentially cause false positives.


Sounds like it would work to me. Get the dust moving fast enough and it should act like a shield.

Keep in mind, though, that it would act like a gyroscope, increasing the ship's inertia. If you are accelerating the dust to relativistic speeds, it might make it almost impossible to accelerate or change direction.

The dilemma for ship captains would be whether they try and Dodge, or turn up the shield and hope for the best.


Kinetic Water shields perhaps?

For some reason I can't find a reference to them anymore except a few vague ones here.

But water shields, besides wonderful against things like plasma or radiation, could protect against kinetic impact. The faster a bullet enters water the more pressure it creates for itself causing it to crush itself and scatter kind of like a Whipple shield. I'm not sure how it will react to high-velocity impacts, but I can imagine that the amount of energy required to vaporize the water will absorb much of the energy and the additional "explosion" of sudden vaporization might work against shaped-charges that might be inside the missiles. As a bonus, if you blow a hole in a water shield the remaining water will move in to fill it up, forcing the enemy to chew through lots of your "armor" across you entire ship before they start dealing the real damage.

I still can't find a good reference, but I believed they wanted to use some kind of electrically powered hydroloving field that made the water stick together more and remain on top of a surface, even if you are under gravity (I originally read something about America looking into those supportive-frame Mechs and using a water-shield to allow the soldier to walk through bullethails with barely a scratch).


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