The return of this ongoing series on space warfare is inspired by recent comments in the Running on Rails thread, and also on Part V of this series, regarding laser weapons. Commenters Byron and Turbo10k independently raised the question of the threshold between deep space and local space, in maritime analogy between 'blue water' and 'brown water.'
Thus the image of the Spanish landing on Terciera in the Azores in 1583, showing the advanced state of amphibious doctrine at that time. Though the remainder of this discussion considers mainly 'blue water' action, in deep space at a higher techlevel. And here is an arbitrary but always appropriate link to Atomic Rockets.
Suppose an expeditionary force is approaching from deep space. For our purpose it makes no matter whether the attackers are in transfer orbit from Mars, Europa, the rings of Saturn, or from a wormhole.
The defenders may choose to defend from low orbit, or even from the surface. I argued in the first of this series, Space Warfare I: The Gravity Well, that the disadvantage of 'low ground' may be countered, or more than countered, by the advantage of concealment ranging from submarines to missile-launching trucks. In particular, surface launched kinetics are convenient and deadly against ships in low orbit. For the wider defense of local space, kinetics launched from low orbit can take advantage of an efficient Oberth effect boot to kick them into the teeth of an incoming attacker.
A defense based on the ground, or orbital forces, can also dispense with costly deep space ships with their specialized propulsion and long duration life support. Ground troops need only plain old barracks, while orbital forces can obtain provisions and support from the civil orbital infrastructure.
On the other hand, a close in defense means fighting amid that same orbital infrastructure, implicitly leaving it vulnerable to the attacker. Urban warfare is hard and costly for an attacker, but it is also pretty damn hard and costly for the city. There is good reason to fight the wolf at the threshold and not by the hearth.
Fighting at the threshold instead of the hearth may make the battle less interesting, because if we are fighting away from the clutter we don't need to worry about hitting civilians, can pretty well distinguish good guys from bad guys, and therefore can give free play to automated systems firing at Stupendous Range.
This battle is likely to be boring, but shame on us for complaining. These people aren't fighting for our entertainment - well, on a meta level they are, but to them it is sheer bloody murder. (Even if the battle is robotic, fortunes in hardware are being thrown away.) The battle is also likely to be Lanchesterian, which is of more consequence.
What constitutes the threshold and not the hearth will be determined first and foremost by weapon ranges, as the range of smoothbores set the traditional three-mile limit.
Even under the modest, midfuture assumptions I typically use, these ranges qualify as Stupendous Range. Consider a 1 gigawatt UV laser, zapping at 100 nanometers through a 10 meter main mirror - a main armament worthy of a laser star, but nothing magical. It can concentrate its beam onto a 1 meter spot at 40,000 km range, and burn its way through Super Carbon Nano Stuff armor at about 1 centimeter per second, a meter in less than two minutes of steady zapping.
At 400,000 km - lunar distance - our battle star can burn through a millimeter in less than 15 seconds (spot size 10 meters), scorching surfaces and destroying any sensitive surface equipment, such as sensors.
Kinetics, if launched at more than escape velocity, have nominally unlimited range. Their practical range is determined by flight time and target performance. If the target can veer farther in the time before intercept than the kinetic's deflect motors can reach, then it is home free. Otherwise it is at risk of a hit and must engage the kinetic with point defenses.
Kinetic target seekers, we will assume, have chemfuel deflect motors, with high acceleration but limited delta v, perhaps 3 km/s. (This is distinct from the initial boot that sends the bus toward the target.) If the target is a deep space ship with a high ISP drive, its acceleration is limited to the milligee range. At 3.5 milligees the target can put on 3 km/s in about 87,500 seconds, just over a day.
So if flight time is more than a day the ship can 'outrun' the target seekers. Less than a day, and it will have to engage them. And at a closing rate of 10 km/s the target seekers' intercept envelope extends to 875,000 km - twice lunar distance.
These are nominal figures, and highly sensitive to input assumptions, but they give us a sense of the scale of things. Given no ambiguities about who the players are, a range of 100,000 km is knife fighting distance in space. Relatively unsophisticated kinetics can reach out and touch you at distances approaching a million kilometers, and plausible midfuture lasers can scorch you at a similar distance.
You can defend a zone this large from low orbit, but your orbital infrastructure is vulnerable to an attacker at a similar distance. Thus the frontier zone, on order of a million km, within which your close-in defenses are effective, but beyond which you must mount any forward defense.
For the attacker it is the same picture, viewed from the other side. Within the defender's frontier zone you are exposed to direct attack by home defenses; beyond it you can only be engaged by 'forward' defense forces.
And for some acts of war - notably deep space blockade - you as attacker need never enter the defender's frontier zone. You need only engage departing ships after they leave the defended zone, or arriving ships before they enter it. Which could involve some interesting deep space chases, especially if the requirement is to board targets, not simply blast them.
A deep space blockade is in some ways easier to sustain than a traditional sea blockade. No storms will drive you off station, and while on station you consume only stationkeeping propellant.
The blockade constellation might be headed by a base station, a jumbo tender with most of the support functions, kept well back, and supporting half a dozen 'cruiser' laser stars fitted to carry boarding crews, gunships, etc.
In response to this threat, a defender who does not wish to pay for a full deep space force might build 'monitor' laser stars, not configured for true deep space missions but with sufficient delta v to force a blockader to engage or fall back onto an escape abort orbit that lifts the blockade.
Now for a little opera. The Hegemony need not rule planets, nor even dominate their local space; it is enough to blockade them. You can be independent, but you will be isolated. When people can't get their Rigelian green fuming brandy, governments tend to bend. The Hegemony's loose grip can be broken only by someone willing to build deep space forces and force it to actively defend multiple blockades ....
Related posts: Here's the Space Warfare series, plus some closely related ones. Note that my views are also a moving target - the form of killer bus I proposed in Part VII turned out to be hugely inefficient given even present-day guidance technology.
I: The Gravity Well
II: Stealth Reconsidered
III: 'Warships' in Space
V: Laser Weapons
VI: Kinetics, Part 1
VII: Kinetics, Part 2 - The Killer Bus
VIII: Orbital Combat
IX: Could Everything We Know Be Wrong?
X: Moving Targets
Battle of the Spherical War Cows: Purple v Green
Further Battles of the Spherical War Cows
And last but not least
Space Fighters, Not
Space Fighters, Reconsidered?