yes, perhaps in SoaSE it is better to build in space, but if you consider a hard SF scenario (as in as real as you can make it), most materials will have to be produced on planets at first, because that is where the most people will be.
In a hard-sci setting of the scenario that we're talking about (interplanetary combat), it is likely you'd get some sort of orbital factory and orbital cities too. I would hardly think that a planet wouldn't invest in some sort of orbital infrastructure, if they have the ability to start fighting across interplanetary (or interstellar) distances.
my basic point is that a giant orbital cannon doesn't have anything for its recoil force to be distributed/dissipated against. A planetary cannon does.
I provided a recoil solution to your above statement... not that I endorse orbital, recoilless, relativistic projectile guns. I think they're dumb, aside from the 2x power needed issue.
However, I should have been more clear; a space-based platform of that is nonemitting is invisible. Non-emitting includes IR emissions; that is, heat. I shall take your example: a missile pod. Functionally, a chemical-powered orbital missile pod is a far, far better platform, since they are essentially inert, can function close to background temperature, and most importantly, do not need to deal with a gravity well. And you can deploy thousands of them in many different orbits.
The only way to detect these things reliably at any sort of range is active sensors. Passives couldn't pick these things up; there's simply nothing to sense. Could a laser be run at zero emissions through low-power operations? I wouldn't be surprised. How about a mass driver? Probably.
The search technique you speak of is not really sensible.
1. It takes place from the surface of a planet; hardly the place to be if you're an invader trying to figure out the orbital defense satellites.
2. Valid only to the 12th magnitude, presumably from the surface of the planet. I'm going out on a limb here, but 12th magnitude is not very bright, and I would think that something a few hundred cubic metres in size, at a distance of, say, a million kilometres, and is made of stealth materials, emits far lower than a 12th magnitude star.
3. Differentiation between an orbital defense platform and one not meant for defense is non-existant, even if you detect them.
4. It takes *hours* to do it. By that time, they will have fired on you and you've already seen them, so no point in trying to find them that way.
But here's how I see it, as far as surface vs orbital defenses:
Disadvantages of surface defenses:
1. Immobile if hardened. They can't move if they're been hardened against attack (not that hardened defenses would work against relativistic kinetic energy strikes anyways)
2. Gravity well. Missiles will need to be many times larger to get out of the atmosphere, and would be correspondingly less capable. They will be have to be much bigger initially, will travel much slower terminally, and carry a smaller payload for given size of rocket.
3. Atmosphere. Interferes with efficiency of energy weapons. Weather may be an issue.
4. Extremely vulnerable to kinetic energy strikes. A planet's a big target to lob stuff at, and there's little you can do to stop kinetic energy strikes.
5. Limited space to put weapons on. You're limited to the surface area of your planet if you want to mount weapons there. Space... is a lot bigger. In practice, if an enemy wants to blindly by randomly shooting stuff until your defenses are dead, there's no question that space-based defense are much hardier.
6. Just as vulnerable to detection after they go into action. Launching missiles, RPs, lasers, plasma beams, whatever. Once you start doing it, they know where you are.
Advantages of surface defenses:
1. Easier to hide. Bury them, and they're pretty hard to find.
2. Easier to network defenses together, they have a common reference point.
3. Logistics is much easier. Transportation network makes it easy to service ground-based platforms.
4. Size limitations of single installations are not subject to exponential material increases (that is, it takes a lot more material to construct a similar sized orbital platform than on the surface of a planet)
5. Cover. The earth can serve as a partial shield against attack.
Disadvantages of orbital defenses:
1. Much easier to detect if not a zero-emission platform. If it's got a significant emission above background emissions, then careful searching will find them easily.
2. Mass drivers take twice as much energy to use.
3. Size limitations. You need to engineer everything into a single platform that it needs. That means that the size and material requirements of orbital grow rapidly relative to planetary defenses as you grow in size.
4. Stand alone systems. Every platform needs every physical system needed to operate it (power, launch tube, reactors, etc. whatever)
(got anything else?)
Advantages of orbital defenses:
1. Gravity not really an issue. Blah blah blah, orbit doesn't mean no gravity, whatever. Suffice to say, a structure in space takes less energy and material to put together in weightlessness (no gravity loads). And missiles/RPs lose less energy to climb out of a gravity well.
2. You've got a LOT of space. Many, many orders of magnitude more than a planetary surface. This also serves as a way to hide.
3. If you're zero-emission, background temperature, covered by stealth materials... you're effectively invisible. Combined with #2, there's very little an attacker can do to find them before they are fired upon by defenses.
4. You can intercept an invader at longer range than is possible with ground based weapons. Given equal capabilities, a weapon in space will stop an enemy sooner (and further away) from the planet.
5. Far more mobile. Every installation is technically mobile.
6. The biggest weapons must be orbital defenses. Building a space gun (it's stupid, but as an example) that is a few hundred kilometres long is impossible and impractical on a planetary surface.
The biggest issue I see with space-based platforms is that of stealth; with proper design, you can fix that. It's not a fundamental flaw, like launching missiles from deep within a gravity well compared to launching missiles from a geosynchronous orbit is. Thus, why I feel that orbital platforms are the way to go. Especially for something like missiles or energy weapons.