I want to talk about a concept a lot of people seem to struggle with, how range would work in real space combat. I want to talk about this, because I feel there are a lot of misconceptions and maybe having a conversation would clear some of these up.
Lets break it down by catagory
1) What is the range of a kinetic round in space?
If you are aiming at something in a on the surface of an object with a fixed orbit, like a planet, its infinite. The limit here is how long are you willing to wait for your round to hit it's target, and secondly how accurate you are, but for planetary bombardments, close enough is probably within a few kilometers. Just calculate where your target will be given flight time, shoot, and forget about it, target will probably die.
But what about a moving target? It then matters how fast your round is and how fast your target is. Lets say you shoot something at 0.1c, or 10% the speed at light. It would hit with immense force, if you manage to hit. But say your target is about as far as the moon, which is 1.25 light seconds from earth. That means your target has 12.5 seconds to dodge your incoming shot. If your ship can accelerate at 1gee, 10m/s, it will be moving at 125m/s after 12.5 seconds, or average speed of 61.25m/s. that means if it accelerates in a strait line, it can dodge 765 meters from where it was originally.
So at the distance from the earth to the moon, a moving target that can pull a leisurely 1 gee of thrust, can still dodge your shots by 765 meters, making hitting a target with kinetic rounds moving at a blistering 30,000 km/s very inaccurate even at the relatively short distance from here to the moon. You could fire a spread of shots, but whatever the cross section of your target is, against a possible target area that is a circle of 765 meters radius, (1.84 square kilometers) only a tiny number of shots would hit.
If your target can move 10 times faster, (10 gees like a good jet) or your shots are 10x slower, a "mere" 3000km/s muzzle velocity, ("only" 50,000x faster than a 50 caliber round), then your effective range will be 10x less. If you tried to shoot the target near the moon, you would expect your target to be able to move 7.65 km away from your shot. Trying to hit something somewhere in an circle with a radius of 7.65 km is nearly impossible (184 sq. kilometers or about 1/4 the size of New York City).
What about orbital defenses, they don't move right? Well, the ISS does. In fact all our satellites move. They have small thrusters that push them out of the way of all the junk in space. Those guys orbiting our planet already have the tech to dodge incoming shots, called space debris, and also need to occasionally adjust their orbit so drag doesn't pull them down (in orbit there is a tiny amount of air slowing your satelite down). So assume that in the future, this will continue to be true and orbital defenses will have at least enough maneuverability to force their attacker to get reasonably close before shooting. If the satellite has even a measly 0.1 gees of thrust, it can avoid 0.1c shots from a distance of 10 light-seconds away reasonably well, since 100 seconds at 1m/s gives it an average speed of 50m/s over 100 seconds or a target circle with a radius of 5km, which is a pretty big area (78.5 square kilometers, or about 1/10 the size of New York City).
2) What about light speed weapons, like lasers?
Ok, lasers are going to be able to hit from further away. But they face an interesting problem: they get wider the further away you shoot them.
Our tech is only able to make fairly crude lasers, if you shoot a modern decent laser at the moon, they will end up kilometers wide when they hit the surface. The degree to which it can maintain its original size over distance is a measure of how focused the laser is, and we simply can't make lasers that stay focused in strait lines forever.
Even with amazing super tech, you still run into this problem, (edit) because of reasons I dont think I can properly explain, but you are free to research yourself, having to do with decoherence, divergence, and the effect of interstellar gas/dust.
In other words, physics itself makes it impossible for us to maintain a laser properly focused across interstellar distances. It in't just about how crude our tech is. You can't shoot a laser at mars and expect it to actually do anything, no matter how good your tech is, all the energy would be scattered across too wide of an area.
So what is the real range of a laser? It depends on your sci-fi tech, but in general, assume that you can't use lasers for long range bombardment like you can kinetic rounds. Kinetic rounds stay the same shape, they dont spread out and lose effectiveness over long distances. Lets go ahead and assume that future tech will allow us to shoot the moon though, since that doesn't violate the laws of physics. There would be some expansion, but at say, 1 light minute, you could still expect your laser to be able to do damage, maybe just over a wider area.
But what happens when you shoot someone who is 1 light minute away? (18 million kilometers, or 1/8th distance from earth to sun), well at that range you face two problems, you are actually shooting at where that target was 1 minute ago, and your laser will take 1 minute to reach target. Against a stationary target (orbiting on a fixed path) thats not a problem, you should be able to blast ground targets from a light minute away with very high tech focusing devices. But a moving target now has 2 minutes to dodge. If you remember from our talk about kinetic rounds, 2 minutes is a very long time. So how far could fast ship (10 gees) move in 120 seconds? 600 kilometers, which means that you could be shooting at something that will be 600 kilometers away from where you were aiming for at a distance of 1 light minute. That's a huge target circle, 600 kilometers radius is an area of 1.13 million sq. kilometers (more than 1/3rd the size of Rhode island), good luck hitting anything at that range. Although, if they didn't start moving till you shoot, they would only get 60 seconds to dodge-
So, to narrow it down to about 100 meters target radius, you are going to need to be a lot closer or be shooting at something a lot slower. Keep in mind though, these are ships that can cross entire solar systems in days that means they have to be blistering fast. To get from outer solar system to inner would take monthss at a leasurely 1g, so dont expect ships in stellaris to be that slow. So realistically, against a ship that can pull 10g's of thrust, you would want to be within 2.5 light seconds or so they "only" have 5 seconds to move away from where you were aiming, giving them a 250 meter target radius (196,250 square meters target area), which against very large ships, might score a reasonable chance to hit.
(edit) Of course, these target circles all assume that the target is able to accelerate randomly at any direction and might do so. I imagine many ships would not be able to thrust in any direction to juke any shots, so the actual target area might be smaller, and more cone shaped.
3) What is the range of a guided missile in space?
In theory, a missile could effectively have infinite range in space combat. In reality, it sort of matters how long you want to wait before your missile ever gets to its target. If you are launching your missile at a target that is just orbiting a star, then you could very well launch your missile from anywhere, and expect it to hit, eventually. We already do that when launching stuff at mars, it takes years for our rockets to reach mars, and we still have pretty good accuracy, landing our stuff where we want them to go within a few hundred meters. This means that we throw our missile at a target and it saves some fuel to slow down when it gets near Mars, and then carefully drops a package. If our missiles were more advanced than crude chemical rockets, then we could launch them at any orbiting object in our solar system, not bother to tell them to slow down, and still expect them to hit. This would be long range bombardment with missiles. Could take a while, but no range limit.
What if we are shooting at something that can move and try to shoot the missile? Well, then its more complicated, because you need to save some fuel for final maneuvers, so a missile would accelerate to get to its target quickly, then as its about to reach, it would do some evasive maneuvers to try to avoid point defenses, and importantly, it needs to be able to adjust its aim if the target does evasive maneuvers of its own. So in a situation like that, a missile's effective range is determined by how fast it is relative to its target. Being able to get there quickly means the target has less time to try to avoid the missile. Shoot from too far away, and the target can adjust its own path to such a degree that the incoming missile can't turn fast enough to compensate. You have seen this sort of thing in the movies with jets. In space it would be similar, if the missile has much faster acceleration (not final speed) than its target then its target can not evade the missile, but if the range is too great, then even a slower ship has enough time to evade the missile because the missile has to fight its own inertia to change directions if the target moves a lot.
Trade off between speed and range is complicated by the fact that the missile might not have infinite fuel, so it needs to spend some fuel to move towards target fast, then spend more fuel to course correct if the target moves. This means, that the range of a missile against a moving target is determined by the engines of both its target and the missile, and how good one is compared to the other. Once a missile has missed, there is no realistic way for it to turn around and try again, given the distances and speeds involved. So range and accuracy are a trade off, the further away you shoot a missile, the less likely it is to hit, because it gives the target more time to outmaneuver the missile.
But general, the range of a missile could well be a many times larger than even laser, because they can track their targets, and correct for any attempt to dodge, the problem of the target circle doesn't apply. If the missile tech is good, you could well still expect to hit a moving target several light minutes away, though you could be waiting hours or even days before your missiles reach.
This is why, most likely, for any foreseeable future, space combat will probably revolve around missile duels, with lasers for point defense, and no kinetic weapons except for orbital bombardment.
Thank you if you actually read this far, feel free to comment or disagree
(edit: actually I screwed up, confused acceleration with speed, someone else could check my math, but I went back and fixed the numbers as best i could...)
(edit 2: checked my numbers again, fixed some, most egregious error was confusing the speed of a 50 cal bullet, its meters/second to kilometers/second, so 1% light speed is about 50,000x faster, not 50, lol)
