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First off habitable planets play a major role in this setting. Furthermore this is a setting whose elements somewhat require that the surface of planets are not soft targets.

In particular planets are defended not just by laser satellites and surface to orbit missiles, but also by umbrella shield generators that protect cities and fortifications in particular.

However I run into a major issue where the importance of planets becomes difficult to justify because the shields that somehow can defend entire planet bound cities could similarly be used on space stations or larger space ships.

The exact nature of the fields I'm still toying around with as well.

The closest handwavium candidate I have is something that interacts with a planet's gravity well. Which makes the explanation something to the effect of 'Something something, gravity well, something to do with gravitational gradients, and something to do with flat space and curved space

However, while I can't be sure, I get the feeling it's a bit of a sloppy justification, in particular I can't even articulate what the shield is even doing in the first place and therefore leaves the potential for holes I might not be prepared for.

So what pseudo-plausible but consistent handwavium could I use to explain why shields that planets use in defense, are strangely more practical in operating on the surface of planets than other environments?

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  • $\begingroup$ Do you HAVE to explain? Not explaining something that seems reasonable/obvious could be better because you can later invent other properties that wouldn't work with your preferred explanation. $\endgroup$ Commented Apr 26, 2023 at 13:02
  • $\begingroup$ @ZizyArcher It depends. Do I need a scientific/detailed explanation? No. In general I want to avoid the problem a lot of soft sci fi has where the writers kind of forget their technology is essentially a magic system that needs to be handled as such. $\endgroup$ Commented Apr 26, 2023 at 18:09
  • $\begingroup$ Planetbound shields have access to more power... bigger reactors, with decades of fuel in a siege situation. They can just be bigger in general, because shield generators are mass, and spaceships are limited in the mass they can take and still be useful. And if that wasn't enough... the atmosphere itself acts as a secondary shield making particle beams and lasers weaker, making missiles heat up and require more thermal shielding, etc. About the only weapon the atmosphere makes worse is nukes. The fireball is largely caused because of the air. $\endgroup$
    – John O
    Commented May 11, 2023 at 16:24

18 Answers 18

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The shield offloads the energy in the atmosphere.

Energy is a strange thing. We think of a nuclear bomb as a lot of energy, but a breeze across a prairie holds similar energy. Weapons try to damage something by having a lot of concentrated energy, your shields mostly function by distributing this energy across an immense space, possibly even the ground.

Shields in space don’t have that atmosphere to offload the energy and will need to tank everything with purely the shield and some stored materials instead.

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    $\begingroup$ Perhaps the shields could use a ground-source heat-pump to push the damage energy into the ground water, raising the overall temperature of the ground around and underneath the city by some amount. This could also provide a plausible explanation for limitations to how much energy the shields can handle - it takes a while for the ground area used as the heat-sink to recover back to the point where the heat pumps can operate efficiently. $\endgroup$
    – brhans
    Commented Apr 25, 2023 at 11:01
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    $\begingroup$ this works with kinetics too, I remember a discussion of the problem with shields on spacecraft is sooner or later the shield is limited by how well attached to the ship it is. sooner or later kinetic energy has to go somewhere, the ground can absorb a tremendous amount of kinetic energy. $\endgroup$
    – John
    Commented Apr 25, 2023 at 23:27
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    $\begingroup$ This is a great answer. You could put the same shield on a ship or space station. But dissipating that much energy into it's atmosphere (or structure), will just cook everyone one inside. $\endgroup$ Commented Apr 26, 2023 at 5:48
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    $\begingroup$ +1. Play Children of a Dead Earth and notice all those radiators? Space is a good insulator and weapons/shields/etc generate a lot of heat. So it's like running full blast in a parka, except that "getting to hot" means molten metal and other not-so FUN stuff. $\endgroup$ Commented Apr 26, 2023 at 6:31
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    $\begingroup$ Thank you very much for this. You know when I read this solution, my first thought was 'how did I not think of this' especially since the energy distribution bit fits with another piece of technology I have. $\endgroup$ Commented Apr 26, 2023 at 18:12
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Space stations are too small.

Planet shields have the full power grid of a planet to draw on, and an immense amount of mass. As such, they have hundreds of times more power to draw on than most space stations or space ships. This means they can generate immensely powerful shields that are far stronger than anything a ship can generate.

The largest capitol ship is about the size of a single city with a million people, and has a similar power draw. There are 500 cities today with over a million people. Each of them are committing their power to a shared power grid, and around 80 of them are super massive cities with closer to 10 million people which produce even more power. How can a ship or station compete with that much power?

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    $\begingroup$ Counterpoint: Look at modern Aircraft carriers. They already carry the electricity generation of small cities and they could easily afford more if it made sense for them to do so. And "being able to block incoming rounds reliably" would be a really good reason $\endgroup$
    – Hobbamok
    Commented Apr 27, 2023 at 15:20
  • $\begingroup$ New York city uses around 500 times more power than an aircraft carrier. A small city of 10-20k might use as much as an aircraft carrier, but there are thousands of them them to 20 aircraft carriers. $\endgroup$
    – Nepene Nep
    Commented May 18, 2023 at 23:00
  • $\begingroup$ yes, but only because there is not much reason for Aircraft carriers to produce/use more power since the aircraft and weaponry doesn'tt (yet?) run on electricity/heat (directly) and a ships top speed isn't really affected by the energy available. So 0 benefits to go with bigger reactors. But if there were a reason the situation would change really quickly $\endgroup$
    – Hobbamok
    Commented May 22, 2023 at 16:10
  • $\begingroup$ The reactors that power a Gerald Ford Aircraft carrier cost about a billion dollars to the 10 billion dollar price tag, and they're not very efficient at power generation since they're built to function on a ship which rocks and has limited space. It's hard to make more. And they actually are built with power to spare- they have half of their power capacity spare, to help handle future upgrades. $\endgroup$
    – Nepene Nep
    Commented May 22, 2023 at 21:17
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Big shield energy

The amount of power required is so ridiculous that it's only viable on a planet. You need massive generators to generate a shield of any size, and those generators need a lot of fuel and a lot of cooling. Now a planet will have access to things like natural resources to provide massive amounts of fuel, like solid ground to provide massive amounts of building space, and like oceans to provide massive amounts of cooling.

And who says it has to be scalable? This whole shield physics only works at absurd levels of energy, and the area or volume of the shield barely matters if at all.

Try to put that on a starship, it'll sooner melt it because you can't dissipate the heat.

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Tangential to Demigan's answer (but along the same lines):

The Shields need an Atmosphere to reflect off to form the edge of the shield. In space, the energy that is projected to make the shield work has no atmosphere, therefore the energy projected never gets to form that edge.

It's the intraction between the handwavium shield tech and the ionizing effects of the Atmosphere that makes a protective layer.

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    $\begingroup$ So the shield operates like an esoteric plasma window that uses the atmosphere as a medium essentially? That works too. $\endgroup$ Commented Apr 26, 2023 at 18:17
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Ozone bonding technobabble

One of my favorite physics effects is that microwaves work because they're at the right frequency to push water dipoles around. Let's say that your shields work on a similar effect that locks ozone molecules in a quasi-solid quantum grid. Obviously, no ozone means no shields.

This could have other fun effects. You'd have a ten-mile-thick barrier above most cities, but you wouldn't be able to fly your own jets through it. An explosion that dissociated the ozone would reduce the thickness of the barrier because the ozone wouldn't be there to harden.

Also, you'd lose your ozone layer, so long-term warfare would result in increased UV radiation in the effected areas.

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  • $\begingroup$ This also works very well too. Thank you $\endgroup$ Commented Apr 26, 2023 at 18:14
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A couple ideas come to mind:

  1. Your shields work on the same principle as a plasma window (see here) which is the closest thing we have in real life to a force field (They can actually hold back 9 atmospheres worth of air pressure). On a planet the shield ionizes a shell of air and uses it to make the shield. In space, any plasma would quickly disperse, and either needs to be replaced (meaning that your shield can only be run for a finite time) or needs to be contained with magnetic fields, meaning that it has high power draw and can't be turned off without losing all that plasma.

  2. One major issue for all space based equipment is cooling. Simple heat sinks in an atmosphere allow you to dissipate the waste heat of the shield into the atmosphere. This effectively allows you to use the entire planet as your radiator. In space, that waste heat has to be radiated away, which is extremely inefficient and requires massive radiator arrays, which are themselves vulnerable to enemy fire, and increase the size of the object you're trying to defend with shields. Pound for pound, or kilogram for kilogram, ground based stations would simply be able to generate more powerful shields without melting.

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  • Planetary shield generators do not produce domes or spheres, they produce flat disks. Something like this was implied by a certain franchise far, far away, but later contradicted.
    Planetary defense and attack planners are very much concerned with terrain contours and lines of sight. How far in would an attacker have to come to get a clear line of sight on an infrastructure target or headquarters in a valley? Which air defense installations can bear on that firing position? Do these locations have line of sight to outside the rim, or can they be shielded by hills? If all goes well for the defenders, a cruise missile to destroy the shield generator would have to run the gauntlet of multiple SAM sites and missile defense lasers.
    For a station, such a flat shield leaves the entire other hemisphere open to attack. The enemy just has to send two squadrons, not one. A mobile defense is a better investment than a half-helpful shield.

  • There is the opposite to the answer by Demigan: Operating the shield generators creates significant waste heat. A planet can act as a heat sink, so can a moon. But a station cannot.

  • And yet another option: Shield generators suffer from something even harsher than the square-cube law. Basically, you can build a shield generator for a station, but it will not be much smaller than the generator for an entire planet. And a single planetary shield generator will take a significant bite out of the defense budget -- only the richest system can afford to shield their most important stations on top of that.

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  • $\begingroup$ yeah, just realized it after reading it again. No idea why my brain inverted its meaning when I read it yesterday $\endgroup$
    – Hobbamok
    Commented Apr 28, 2023 at 7:05
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Shields only work in the direction opposite the weight vector of the shield emitter, and their effectiveness is proportional to the magnitude of the weight (for a given machine - you can't just weld some weights to your emitter to make it more effective). Big space ships can accelerate towards the threat and spin up the emitter array centrifuge to project a patchy dome that's better than nothing, but they're always vulnerable from the rear, and spinning up a big heavy centrifuge puts a lot of strain on the ship and prevents it from turning quickly without ripping itself apart, compounding the vulnerability. Planets don't need to maneuver, they can hold up unlimited weight, and they don't have a rear to attack.

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Orbiting shields are easily bypassable

I'm assuming you cannot create a shield bigger than some square km.

Protecting a static target on the surface from space is not as straightforward as it could seem (unless you have so many satellites you could basically shield all of the planet upper orbits).

In particular, there is no reason why the enemy wouldn't create a missile intelligent enough to pass through an area not covered by satellites; then it would correct its path and direct itself toward its designated target.

Some other points af attention of using orbiting shields to protect targets on the surface:

  • Since the surface of a sphere increases quadratically with the radiud, on orbit, the % of surface of the sphere you can cover with N shields is lower than the area the same N shields could cover on the surface.
  • The satellites, unless in a geostationaty orbit, don't cover the same area on the surface, but move with respect to the surface of the planet
  • Moreover (at least for Earth) the geostationary orbit is so high (>35000 km) that it would be impossible to cover a significant area, and a missile could easily bypass it and have enough time to aim itself toward a suitable target.
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  • $\begingroup$ I think the question is more why do the space stations don't have shields protecting themselves. $\endgroup$ Commented Apr 27, 2023 at 23:47
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Concentration of Force

We think of shields as contiguous domes or spheres of energy that somehow destroy of deflect an inbound projectile, but this is a horrible and pointless waste of power when a typical inbound weapon only has a cross section less than a meter across.

Instead of thinking of a shield as a solid wall, think of it as an array of point-defense beams that converge on an inbound target to stop it. This is in fact how Boeing's "plasma shield" system works. A ship or space station has a relatively small total surface area (and as others have pointed out, power output), but a planet has literally millions of square kilometers of surface area to mount point defenses on, so if only a tiny fraction of that surface is point-defense, then a planet could easily have way more defensive firepower than an entire fleet could ever muster.

So, if an enemy kamikaze ship tried to slam into Earth, it could be met by a massive salvo of simultaneous high powered beams fired from England, France, Germany, Italy, Greece, Russia, etc. all as part of a single massive blast capable of turning even the largest of capitol ships into an instant cloud of plasma, and if a whole fleet tried firing 1000s of nukes at a planet from a safe distance, it has enough individual point defense systems to shoot them all down independently.

enter image description here

Plasma Opacity

Another part of what makes plasma shields work well only on a planet is Plasma Opacity. When you superheat the air, it becomes opaque which can block a laser. The phenomenon has been one of the biggest obstacles in designing offensive lasers in the real world because once you make something hot enough, the plasma will block additional laser energy from reaching the target. So, if you converge a bunch of lasers at one point in the air and optimize thier frequencies to maximize this effect, it will superheat the air into a ball of plasma that will block an enemy laser weapon. So space shields using the same tech could only shoot down missiles, but planetary shields could block missiles, beam weapons, and even explosive shockwaves. Again this is how actual plasma shields currently being developed for real military use work.

enter image description here

How to keep the shield from also being a weapon?

The other possible concern is not about what makes a planetary shield so strong, but what keeps it from being a totally OP planetary weapon system. Lasers scatter a bit when fired through an atmosphere; so, while they would still have enough energy density to take out a target a few hundred km above the Earth's surface, the farther out you get, the more those lasers scatter until thier energy is no long able to be concentrated enough to do any meaningful damage.

Missiles and cannons will also be a problem for planets to defend themselves with because ships can shoot down missiles and dodge cannons with relative ease.

In this way, planetary technology makes for much better shields, but also much worse weapon systems than they do on ships. So it would be easy to harden a planet against attack, but also not very good at returning fire against an invasion fleet making sure that planets still need fleets to defend and retake thier orbital space.

If it must be a "Force Field" these same principles can still be applied

Ever notice how in a lot of Sci-Fi, the shields are transparent until hit. One possible explanation of this is that at passive load, the shield is evenly distributed across a large surface, but when struck, the shield does one of 2 things. It either dissipates that attack across a large area of the shield or it converges the power of the shield into a small point to "brace" for the impact. Both of these interpretations also rely Concentration of Force force. It's just a matter of semantics regarding how you apply your available force field energy to stop the incoming threat.

There are many ways to explain how available power can be converged from a large defense surface to a small attack surface, but what ever way you go with, the principle of concentration of force still holds true.

enter image description here

Plasma opacity could also be applies to a solid shield. A beam weapon could have only a marginal interaction with a shield dome, but if it heats up the air, then you still get plasma opacity kick in.

As for mitigating offensive value, contiguous shield tech basically addresses this in its fundamental description and design.

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  • $\begingroup$ Thank you for the idea. For a number of reasons I'll go with force fields. But I'll include this as well since it does give me another element to add in the defense of planets. $\endgroup$ Commented Apr 26, 2023 at 18:11
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    $\begingroup$ @AllSeeingEye33 Even if it is a "forcefield", the same principle can be applied. If power converges in one part of the shield in anticipation of an attack, or if the energy from the attack can be dissipated across the whole surface, then a shield is exactly as good as its power input instead of being reduced by some Square or Cubic relationship. $\endgroup$
    – Nosajimiki
    Commented Apr 26, 2023 at 18:36
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    $\begingroup$ @AllSeeingEye33 Also, I don't know why I did not think of this before, but plasma opacity is a real thing that should answer your question exactly. It's a phenomenon that is used by Boeing's experimental Plasma Shield system which involves using converging lasers to superheat the air into a wall of plasma that can block lasers, missiles, and explosions, but does not work in a vacuum. I've added it to my answer. $\endgroup$
    – Nosajimiki
    Commented Apr 27, 2023 at 16:17
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As per answer above about heat but with some more 'Handwavium' details added.

The shields use a 'quantum' level effect to 'smear' the energy contained in photons that strike it across the entire surface of the field. Those same effects means that half of the energy then radiates outwards from the surface of the field while half radiates inwards.

So take a scenario where a capital ship fires say 200 megawatts of laser energy at a shield in a tiny faction of a second. All of that energy is concentrated in a beam the width of a couple of centimeters but on impact that energy is instantaneously distributed across the entire surface area of the field. It then radiates outwards and inwards (since they're the only two directs it can go) and the result is that 200 megawatts of energy are spread across the entire surface area of the shield. And the larger the surface area the more heat that can safely be radiated away!

A very large shield would (on the first few impacts at least result) result in a light warming effect close to the surface of the field itself. Which of course means that the beauty of this tech is that that is that the larger the shield (say one over a large city) the stronger & more long lasting the shield will be. So large shields covering entire cities or districts are better than ones covering single households.

Of course nothing is perfect. Dump enough energy into a shield and eventually it collapses because it can't radiate heat away quickly enough to maintain integrity. The result is an instantaneous release of whatever energy is in the shield at the moment of collapse. (Warning: Do not be standing next to it when this happens unless you want to roast your marshmallows). But again of course while there will be damage inflicted on anything close to a shield that collapses large shields that arch thousands of meters into the air over a city will see a relatively lower impact at ground level.

And here you come to the trouble with using them on ships. Yes you can do so but ships don't have any easy way to radiate heat away in the vacuum of space as there's no atmosphere or bodies of water to carry it away. Thus multiple strikes on a ship using one would rapidly result in the crew cooking themselves alive! Plus the ship is blind since it won't be able to receive information about what is happening outside the shield.

Frame challenge: Perhaps ships do have shields but they're limited in duration and effectiveness because since they can't easily radiate heat away like ground based shields would do (unless your talking about ships so large they rival or are greater that cities in terms of surface area. So when shields do collapse in space not only do the shield generators go off line and take time to be spun up again but half of the heat contained in that shield radiates back into the ship itself.

This makes it critical for ship captains to carefully consider when and where to use them while under fire as they will only be good for a a short time and a ship will always be effected by the thermal effects of a shield collapse if/when one happens. Hence careful judgement is required.

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  • $\begingroup$ Your point about duration is interesting. Most sci fi has shields that are perpetually turned on and static. Having it so that ships can't keep them on perpetually does make space battles more dynamic. $\endgroup$ Commented Apr 26, 2023 at 18:19
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3 shield generators are necessary to create a shield covering the area between them. So, these generators form something like cell tower system on a planet surface to provide coverage. Optimal distance between the generators is about 20 km, at 15 or 25 the shield becomes much weaker.

There is about 20m spot around a generator where shield is relatively weak, so they are generally buried deep underground and static defense impenetrable for space based weapons is built on top of them. Alternatively, they are hidden and very hard to detect.

It is possible to use this tech in space, 4 generators may form a pyramid with 20km side but they have to be aligned exactly right at all times and there is no realistic way to protect them in space.

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We turn to our good friend, the Friis transmission equation. It's not exactly what we need, but it's a close enough analog.

Pt =Pr * Ar * At / (d^2 * λ^2)

Where: Pr = Power of our radiator, Pt = Power recieved at our target, Ar = Aperture of our radiator, At = Aperture of our target, d = distance, λ = wavelength of our electromagnetic radiation

What does this tell us? It tells us what some posters have already told us, we need a lot of power available to put a lot of power into the target. It tells us we want to have large targets, but we can't really control that. It tells us we want the target to be close, but we can't control that very well either. Finally, it tells us we want our antenna to be large. A planetary shield's antenna can be easily distributed across the entire surface of the planet, and even underground. This enormous antenna will allow the shield to direct energy in a much more precise manner than otherwise possible, in addition to doing so with a far greater supply of power. The only way for a space-based system to be so precise would be to have an equally large planetary shield in space, which might as well be thought of as a planetary shield without a planet.

(Yes this also applies to weapons.)

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  • $\begingroup$ This antenna/aperture size is immediately related to Nosajimiki's "Concentration of force" idea. It is precisely this increase in aperture size which increases one's ability to concentrate force. This is also related to why telescopes, particularly interferometers, use massive aperture-sizes: they want information on a very "concentrated" part of the sky. $\endgroup$ Commented Apr 27, 2023 at 15:49
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One aspect I don't see anyone mentioning: inverse square law. In short, the further away from a shield generator, the intensity of your shield reduced by distance square. So either you need a lot of shield generators in orbit, or much more powerful shield generators. Or just put the shield projection on the ground, project a short distance and have a much better shield. Even if the orbit is relatively low compare to the diameter of the planet, the increase still rise fast with the quadratic function.

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Gravity Inversion

The shield does nothing more than "simply" invert gravitational fields.

Incoming projectiles with significant mass will bounce a space-station off like a billiard ball. This is undesirable.

Try that on a planet though and thanks to the target's relatively huge mass it's the incoming projectiles that do all the bouncing away.

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Magnets!

Well, sorta. See, most planets that would be worthwhile for big colonies would have a magnetic field to protect them, like the Earth. Otherwise they'd be vulnerable to radiation and solar wind blasting away their atmosphere.

Your shield generators rely on this magnetic field to shape/power/handwave themselves into existence. Without such a hugely powerful magnetic field as one provided by a planet, the shields wouldn't be able to exist.

Could such a magnetic field be produced outside of a planet with sufficient power? Potentially... But the power draw would be unsustainable.

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Kinetic energy. (This is related to Demigan's answer)

He's right that the energy doesn't just vanish, but cooking isn't the problem. If it were just heat you could have a big mass of very cold material to work as a heat sink, it wouldn't be perfect but it would be far better than nothing.

Instead, the problem is shield generators just transfer the energy to themselves and thus push against whatever they are mounted to. To deal with the power of that h-bomb the shields just stopped you need to spread it amongst a lot of shield generators that are supported by bedrock. You need enough generators that no one generator has to deal with too much energy and mobile craft are simply too small to do this. Try to put a shield on a ship and the shield mounts fail with the first hit. Note that this does permit lesser versions of shields to deal with things like grains of sand floating around in space, they're just too weak to be meaningful in combat.

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The shield is a secondary defense. The atmosphere of our Earth protects us pretty well. Iron based asteroids and space junk burn up while falling into the atmosphere, and deadly solar radiation is rendered fairly harmless. Lazer weapons have difficulty staying coherent in the atmosphere, and the Earth's magnetic field reduces electromagnetic interference from space. The only thing a planetary shield would need to defend against is something that can penetrate the atmosphere (and, on Earth, the magnetic field). Satellites have none of these protections and are vulnerable to a high-speed piece of metal. A satellite shield would take the brunt of any attack, instead of being a backup to the natural protection of an atmosphere.

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