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?