Gigantic wall of text coming up...
I've seen some mentions of planetary anti-ship weapons mentioned here and I'd just like to say how much of a mistake such weapons are, if you'd like to not be glassed that is. If we operate under the idea that glassing a planet is discouraged and can have severe diplomatic repercussions (aka, it's the nuclear option) then having any form of planetary weapon is tantamount to suicide.
Completely wrong - it depends
entirely on the assumptions you make about the setting. Principally:
1. Do FTL scanners exist?
2. Can I scale up shield generators to continental or planetary scale?
3. Is waste heat an issue?
If FTL scanners exist (and for the record, if FTL
communications exist then FTL
sensors exist by default... think about the relationship between
receiving radiowaves for a radio, and
reflecting radio waves for a radar installation), I can shoot down any relativistic missiles you lob my way. And if it's an asteroid, all I need to do is heat up one side of it, which will likely cause it to veer off course (NASA want to do similar for asteroid defence purposes).
If planetary or continental shield generators exist, I can spam them all over said planet.
If waste heat is an issue (see IRL), the planet wins again because it's got a bajillion tonnes of planet to act as a gigantic heat sink, whereas your invading fleet has... well a few radiators and portable heat sinks. Good luck with that.
And finally, for every capital ship with its multi-megaton lasers you attack me with... why exactly can't I counter with massed batteries of bigger, meaner planet-based lasers? I've got the whole resources of a planet to work with - you've just got whatever your invasion fleet can bring and keep hidden from my planetary gamma-ray laser defence grid. Because a 2.9e-11m wavelength gamma ray laser with a 20m diameter lens will have a spot size of about 26 metres...
on a target sitting Pluto. In other words, everything in line of sight of my defended planet is a valid target, light-speed lag permitting.
Now, taking real life as our model here, we have two basic means of attacking a planet:
1. Lasers: spot size on target depends on wavelength. A UV laser (1e-8m wavelength) with a 20m diameter lens will have a spot size of almost
ten kilometres at the 100 AU range I used in the above example... "oh noes, not the sunburn!"
. You won't even
notice visible light lasers at that distance.
2. Missiles: assuming the defenders cannot interfere with them via jamming etc, they can be shot down (see above re massed batteries) and are not easily replaced. The defender on the other hand has basically unlimited power for their banks of lasers etc.
Particle beams are too short-ranged (that is, their optimum range is better for things like ship to ship combat), and I'm discounting technobabble solutions because we don't know what's available in Stellaris yet.
So, the best option for the attackers is gamma ray lasers (or x-ray ones etc) or missiles. They hide behind an outer-system planet, pop up, shoot, and pop back into hiding. The defenders meanwhile can be building their own fleet, reinforcing the static defences... all that kind of thing.
Oh yeah... what happens if the defenders can keep up constant fire on the planet the attackers are hiding behind? Because if you can (roughly) predict where the enemy fleet is, you can spam so much firepower towards the planet (or rather,
around the planet) that anyone even thinking of poking their head above the parapet will get it shot off more or less immediately.
Regardless of weaponry a planet should be allowed to surrender before being subjected to bombardment. Otherwise it's going to be a feeble excuse diplomatically.
What if you can't spare the occupation / police forces and it's a total war? By all means if you're re-enacting the Hundred Years' War in space it might be okay, but if you're fighting Space Nazis instead... well there's a reason most historians consider dropping nukes on Japan to be the
least bloody way to force their surrender.
It might be fun to use a Kessler defense. Just fill up local space with so much junk it's hazardous to go near the planet. Sure it blocks interplanetary trade and whatever, but we didn't really want their stuff anyway. A thick enough layer of junk might even provide protection from long range (non-relativistic) kinetic kill weapons.
All that energy and momentum has to go somewhere. If your relativistic kill vehicle smacks into, say, the International Space Station... it'll keep going. Sure, it might resemble a blob of plasma more than a rock now, but big deal - it'll still hit the atmosphere... and then dump all its energy into said atmosphere:
In the forests below, lakes caught the first rays of the rising Sun and threw them back into space. Abandoning the two-dimensional sprawl of twentieth-century cities, Sri Lanka Tower, and others like it, had been erected in the world's rain forests and farmlands, leaving the countryside virtually uninhabited. Even in Africa, where more than a hundred city arcologies had risen, nature was beginning to renew itself. It was a good day to be alive, she told herself, taking in the peace of the garden. Then, looking east, she saw it coming -- at least her eyes began to register it -- but her optic nerves did not last long enough to transmit what the eyes had seen.
It was quite small for what it could do -- small enough to fit into an average-sized living room -- but it was moving at 92 percent of light speed when it touched Earth's atmosphere. A spear point of light appeared, so intense that the air below snapped away from it, creating a low-density tunnel through which the object descended. The walls of the tunnel were a plasma boundary layer, six and a half kilometers wide and more than 160 deep -- the flaming spear that Virginia's eyes began to register -- with every square foot of its surface radiating a trillion watts, and still its destructive potential was but fractionally spent.
Thirty-three kilometers above the Indian Ocean, the point began to encounter too much air. It tunneled down only eight kilometers more, then stalled and detonated, less than two-thousandths of a second after crossing the orbits of Earth's nearest artificial satellites.
Virginia was more than three hundred kilometers away when the light burst toward her. Every nerve ending in her body began to record a strange, prickling sensation -- the sheer pressure of photons trying to push her backward. No shadows were cast anywhere in the tower, so bright was the glare. It pierced walls, ceramic beams, notepads, and people -- four hundred thousand people. The maglev terminal connecting Sri Lanka Tower to London and Sydney, the waste treatment centers that sustained the lakes and farms, all the shops, theaters, and apartments liquefied instantly. The structure began to slip and crash like a giant waterfall, but gravity could not yank it down fast enough. The Tower became vapor before it could fall half a meter. At the vanished city's feet, the trees of the forest were no longer able to cast shadows; they had themselves become long shadows of carbonized dust on the ground.
In Kandy and Columbo, where sidewalks steamed, the relativistic onslaught was unfinished. The electromagnetic pulse alone killed every living thing as far away as Bombay and the Maldives. All of India south of the Godavari River became an instant microwave oven. Nearer the epicenter, Demon Rock glowed with a fierce red heat, then fractured down its center, as if to herald the second coming of the tyrant it memorialized. The air blast followed, surging out of the Indian Ocean -- faster than sound -- flattening whatever still stood. As it slashed north through Jaffna and Madurai, the wave front was met and overpowered by shocks rushing out from strikes in central and southern India.
From "The Killing Star", referenced on the always-awesome Atomic Rockets website.
The real problem is the lack of habitable planets, it simply does not make sense to destroy and make inhabitable, or very hard to recolonize, a perfectly healthy planet. It costs you more to use more conventional means in the short run but in the long run you got a healthy planet ready for exploitation and colonization, if you just carpet bomb everything and nuke from orbit you get a bare rock.
Define "destroy"... if you mean, say, nuclear strikes on all the enemy cities etc, then the radiation threat's been overstated. If nothing else, nuking cities = best done with airbursts = minimal fallout. Factor in cleaner warheads and the likelihood of being able to find other places to build your own cities (never mind hypothetical radiation-cleaning tech), and it won't be too bad at all.
Meanwhile options like glassing a city with direct energy weapons like lasers will largely avoid the fallout / radiation problem as well.
You can not laser guide a relativistic projectile.
You can not wire-guide a rail-gun round across multiple AU.
In fact, I highly doubt wire-guiding a rail-gun round would even be possible to begin with.
If you think, as you said, that 'There is nothing particularly out-there about anything just listed', then you have absolutely no idea what you are talking about.
Well... a wire-guided one will not work (transmission lag + sheer mass of wire + tensile strength etc), but a laser-guided one is fine. You heat up part of the shell, generating a small amount of plasma. Said plasma will act like a tiny thruster, adjusting the shell's velocity.
Nope. This is a major misconception I keep seeing.
Whatever your source of energy is, you can build it more easily and better in space. More importantly you can build it denser in space, because in space you can build in three dimensions better than you can on a planet surface.
You're forgetting that building a giant solar panel in space is fine
until it needs defending. Conversely, you can spam fission / fusion / antimatter reactors on the surface (and to an extent, within) of a planet, cover them with solar panels, and then throw in geothermal power as well, because why not.
To be sure, it's probably easier
when at peace to simply rely on power beamed directly from your orbiting solar arrays... but this is obviously a war scenario.
Again, only solar power is viable for space mounted defenses.
Why? An orbital station doesn't need to manoeuvre much, so it can be really (really) big compared to a spaceship, which has to accelerate regularly (and size matters A LOT for mechanical stresses). In other words, you can shovel huge amounts of reactor fuel into it, and have it run off that - or better yet, have it run off that
when its solar arrays are destroyed (and they will be).
but it also needs to be a lot more powerful from the ground because you lose so much power shooting through the atmosphere
You do know that not all wavelengths are absorbed equally by the atmosphere, right?
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Of course... it's likely that Paradox will use game mechanics to stop players building Invincible Planet-Forts of Doom (TM), because that makes for boring gameplay. However, given that game mechanics wasn't part of the argument here, I hope you can see my point that the assumptions you build into your sci-fi setting will determine whether planetary defences are a practical thing or not.
