Two related concepts, the Electrostatic Defense Field and the Plasma Shield, will be discussed in their own articles. Though mentioned many times in passing in science fiction (often called "deflector shields",) a serious discussion of magnetic fields as a physical defense could not be found. Some of what follows is an extrapolation on current research.
The idea of electromagnetic fields as a defensive barrier originates from the golden age of science fiction, when mad scientists and aliens in pulp magazines used fantastic-seeming "fields of force" to create a barrier against attack. In nature, Earth’s magnetic field helps to shield us from both solar and cosmic radiation. In high-energy physics, powerful magnetic fields contain and shape super-hot plasmas and particles travelling at near-light speed.
NASA is also currently looking into integrating active electromagnetic fields into spacecraft hulls to shield them from radiation hazards. Because solar wind particles and cosmic radiation tend to be charged, deflection via active electromagnetic fields are thought to be an effective way to protect a crew during a long-duration space flight outside of Earth’s protective magnetic blanket.
Using such fields as barriers against attack are another matter, however. A defensive magnetic field in principle is fairly easy to generate; all one needs is high grade conductive coils in abundance and a power source. Higher tech levels may use superconducting materials instead, to generate more powerful fields using less energy and less material.
The more powerful you want the field, you either add more wires coils or more current, within limits. Too much current, and the electrical resistance in the wire will generate too much heat, degrading the field or perhaps even melting the conductor. Superconducting systems can handle much more current than normal conducting wires, but even they have limits.
These fields work best against metallic objects, but fields of sufficiently immense strength would be able to deflect almost any kind of normal matter. How far an object can penetrate the field will depend greatly how much mass and inertia is backing it up. Projectiles with sufficient velocity may be able to penetrate magnetic fields no matter how powerful such defenses can be.
This technology is usually associated with spacecraft and space stations, mainly because of its previously mentioned potential use in protecting such assets from radiation. It can be used in an atmosphere, but at diminished efficiency; the constant ionization of air molecules will sap away some of the energy of the field.
The principle for using magnetic fields for deflecting attacks is straightforward. Upon encountering field, an incoming projectile will become ionized (if its not already) as electrons on its outermost layers are repelled away by the field. Upon encountering the inner, stronger portions of the field, the now-charged object is pushed away, hopefully curving the trajectory of the incoming projectile away from the ship. Low-powered lasers directed at incoming threats could also help to ionize targets, increasing the potential effectiveness of the field.
Besides physical threats, such fields can also potentially deflect charged particle beams and plasma weapons. They would only have a negligible effect on lasers and neutral particle beams, however, no matter their strength.
One of the big drawbacks to using these fields defensively is their potentially enormous energy drain. Deflecting radiation such as in current NASA proposals does not require great field strength, but larger, speedier threats like micrometeoroids (which at orbital speeds can hit like bullets), missiles, shrapnel, particle beams, and so on require a magnetic field many thousands of times more potent than the one Earth generates naturally. Creating such a field around, say, a tank or a space station would require a constant drain on even potent energy resources.
Another big drawback is the effect the field will have on both operating personnel and nearby electronics. Powerful electromagnetic fields are known to have an effect on long-term health, leading to medical complications later in life. Fields of the strength discussed here could also cause immediate injury (including headaches, nausea, and loss of consciousness) or even outright death.
Electronics will suffer short-circuits, and glitches as their circuits pick up stray charges from the field. In both cases, heavy shielding of the crew and operating equipment may be necessary, adding to design complications and cost. Unfortunately, projectiles fired against the target can also be similarly shielded, nullifying the shield’s effectiveness.
Such defense fields may also play havoc with communications, but this can be at least partially bypassed by extending a shielded radio mast or antenna out past the field.
Other vulnerabilities exist. For ground-based vehicles especially, the presence of incidental conductive material in the field (spent shrapnel, casings, debris, etc) can help sap a field’s strength. A wily enemy could pepper a battlefield with such material with the express purpose of degrading any protective magnetic fields present.
Finally, there is the consideration that in combat, an active electromagnetic defense field would show up like a bright star on enemy sensors, pretty much drawing a large metaphoric bullseye around the target vehicle.
Magnetic Shielding for Spacecraft:http://www.islandone.org/Settlements/MagShield.html http://www.space.com/businesstechnology/technology/technovel_magnetic_041217.html http://thayer.dartmouth.edu/~Simon_G_Shepherd/research/Shielding/index.html
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