Premise

Even at present day, space assets are very valuable:

Our space assets, day by day, are growing by leaps and bounds. It's
the value of that satellite to the economy. - Neil deGrasse Tyson

Historically, the US Space Command has been around since 1985, and has played several important roles and undergone restructuring over the years leading up to the 2018 reestablishment. Now, it's especially easy to conjecture that conflict in space will be a distinct possibility. Here is Neil deGrasse Tyson again on a 1967 space treaty:

Now that I'm old and tired, it's just: 'Why should we promise to not
kill each other in space when we are not successful at doing that here
on Earth.'

As Tyson articulates later, space weaponization is unlikely to start with weapons aimed at earth, for it would be far to difficult too implement. He argues, that conflict will likely start by targeting enemy space assets.

With the motivation mapped out, what then are the hardships of the endeavor of safeguarding space assets? Here is another Earthling's take:

If you give an Audi to the wrong person, and he/she uses it to hurt
people, you've just made that Audi a weapon. So, what does it take to
destroy something in space? It takes a satellite, a sensor and a
maneuver capability. I've just described 90% of the US, Russian and
Chinese on-orbit constellations. - General John Hyten USAF Commander

Question

In the emerging theatre of operations that is space, what kind of space force would a state in the world I'm building strive for to protect its space assets, given Hyten's cautionary words that there are many points of attack?

Quality Metric: Solutions that are cost-efficient are preferable to budget-intense solutions. Also solutions that rely on highly speculative technology are weighted-down, and solutions that use better-documented emerging technologies are weighted-up.

Assumptions:

• conflict hasn't started yet but will be inevitable


• lead time to build the space force is unknown, at most 10 years


• assume present to near-future technology (near as in a few decades from now)


• Budget is flexible, but quality metric prefers efficiency


• assume we advise a hegemony-esque state that is faces rising powers and rivals having an increasing presence in space but not quite as powerful as the hegemony (maybe that position doesn't sound too bad, but factoring in assymetric warfare threats, the hegemony is also assumed to be alarmed)

It starts with surveillance, numbers and stealth, then increasing capability, but not in the way you would think

Any object in space is defenceless in a frontal attack, as demonstrated recently a missile aimed at a satellite will destroy it with little trouble. Armour would not work, active defences can be countermeasured easily.

• Surveillance is a likely (and already used) aspect of space satellites to defend space assets. By constantly monitoring the ground, and other satellites, you would be able to build a picture of what is happening, and indeed what is likely to happen, and take pre-emptive action - perhaps even just using conventional ground forces. This is easily your first line of defence - prevention is always better than cure.




• Numbers is next - if you saturate space with a network of spy satellites with high levels of redundancy it would make it more difficult to dismantle. You can even place inordinate amounts of low-grade 'dummy' satellites as diversionary ways to expend enemy resources without much effort.




• Stealth - As in most scenarios, if you know something your opponent does not, it gives you an advantage. Stealth, or even the ability to have it, may give you a de-facto 'fleet in being', without requiring too much resources to maintain, and simply maintain peace because it is too risky due to unknown satellites. Nano-satellites, radar-absorbing material, data encoding in civilian satellites, are examples of this.




• Capability - so not a large fleet of space warships, which could be destroyed easily as they become only large targets, but simply large amounts of research into space technology, such that other countries find it difficult to 'catch up', and the strict non-disclosure of these. Examples of this is new drives, power generation, data encryption and increasing data bandwidth. Eventually bases placed 'out of reach' on the moon or far afield in space are the next step, but they will be redundant once detected or your competitor catches up with capability. (Hence a Space Race).

All of these are not a 'Space Force' in a traditional sense but are the likely initial steps of your scenario.

However, having said that, it is easy to see that the reason it is better to be Cooperative, instead of Confrontational, being that space conflict is expensive, difficult, and the outcome is not certain.

Cooperation yields a synergy between international civilian / commercial imperatives and national ones such that peace can be achieved cheaper and more assured, which has been the preference recently.

You cannot protect your space assets.

Not against a determined, near-peer competitor. Putting a ball bearing or a small guided KE interceptor on a collision course will be cheaper than launching the intelligence or navigation sat, and you cannot afford to dodge all the time. The delta-V of your sats is too precious for that.

Sats will move on fairly predictable orbits, and they will likely overfly the territory of the competitor.

That being said, make them easily replaceable.

You cannot win a war of attrition, but be prepared for some losses.

• Make your space assets small and networked. Instead of a few big, multi-purpose assets, launch many small ones.


• Be prepared to launch replacements on short notice if a few of your sats are damaged.

Deter attacks against your space assets.

Make it clear that they are not just another unmanned drone. They're valuable to you.

• Tell people that you will take an attack on your space assets at least as seriously as an attack on your homeland. Say so consistently, over a long time. React to small provocations to drive the doctrine home.


• Tie some of them into your nuclear command-and-control system. That makes it credible that you would see an attack on your space assets as a first strike against your strategic forces, and that you would respond with a nuclear strike.


• Make your space assets valuable to other global powers. Right now, an attack on the GPS net would have vast global consequences. Anyone who did it would anger third countries.
  
  
  
  
  • Route your sat communications through contracted commercial sats which are used by the civilian economy.
  
  
  • Place your military assets where the debris of an attack would interfere with civilian assets (a "civilian shield" strategy might not be illegal if the shielding assets are unmanned).
  
  
  
  

Hide a few space assets.

That malfunctioned commercial sat, that payload shroud from a space launch, are they really orbital junk or are they passive listeners (or in-orbit spares)?

There are several ways to build a space force, but most are counterintuitive, essentially because space is a different environment.

The first and easiest way to build a space force is to start building capacity on the ground. Your force will need massive numbers of satellite vehicles and launchers to replace the ones being disabled or destroyed, but the safest place for space hardware is in sheltered hangers on the ground. The vehicles can be given routine maintenance and upgrades in their hangers as time passes, and when technology changes, new build vehicles can be simply driven by truck to the hanger bays, and old vehicles removed for disposal.

When the balloon goes up, you can relatively quickly send masses of small satellites in constellations in orbit from ground launches from the territory you control. You can vary the pace of launches and adjust the orbits to meet whatever needs your force has both tactically and strategically. By building large numbers of satellite vehicles, you also have a steady supply chain and can continue to make up the losses of satellite vehicles as the war progresses. Instead of X-wings or TIE fighters, you are sending up fleets of cubesats.

The major element of your space armada

Since you do need to keep some elements on orbit in order to see what the other side is up to and have an immediate response if needed, you will need to have your satellites in orbits which are difficult to intercept. The Russians developed the Molniya orbit in order to have communications spacecraft have long "dwell times" over Soviet territory, but you can utilize this for any reason, including have satellites able to "dive" towards areas of interest. A series of overlapping orbits can be used to provide a wide range of coverage, and a confusing set of target zones for enemy ASATS to reach

Molniya orbit

Lastly, once enough space assets and infrastructure are in place, you need to get strategic depth. Sending spacecraft to the Moon and Near Earth Objects (NEOs) provides that strategic depth in that assets are now weeks or months removed from Earth (so any enemy will have to take a much longer time frame into account). Even if there is no resource extraction to extend the range and scope of your Space Force (you simply "garage" space vehicles in insulated bubble shelters), there is still going to be a huge change in the strategic calculus for any opponent, since you can bring forces to bear across a much greater expanse of space and time, complicating any sort of planning they do.

Image from NASA's Neowise mission. 10,000 objects to choose from

Much of this can be done with existing hardware. Cubesats can be configured to act as communications relays, elements of orbital reconnaissance constellations or even offensive vehicles capable of manoeuvring against enemy satellites or de orbiting and striking ground targets with immense amounts of kinetic energy. SpaceX has demonstrated mass produced inexpensive launch vehicles are possible, and a Falcon 9 analogue can loft 20,000 kg to orbit, while a Falcon heavy analogue can bring either massive payloads to orbit or launch payloads to the Moon and beyond (Elon Musk's car is past the orbit of Mars at the time of writing)

https://www.youtube.com/watch?v=sB_nEtZxPog

Space vehicles cannot be practically armoured against the massive kinetic energy of other objects in space, but water or dust mined from the Moon or NEOs can be used to provide some shielding. Shaping the shield and utilizing various methods for cooling or minimizing surface reflections will also be useful.

"Misty" stealth satellite concept. Filling the shroud with water would provide some additional protection.

So much of your space force will be in shelters on the ground thoughout most of their service lives. The elements on orbit will be in strange elliptical orbits, until it becomes possible and practical to start basing them on the moon and in deep space. This may not be very romantic, but it makes both economic, tactical and strategic sense.

Strictly speaking, you only need two things for this, and they don't even need any 'new' technology:

• You need to know where everything (not just satellites, but debris too) is up there.


• You need to be able to have your satellites dodge incoming objects.

The second part is easy, most of the satellites that would be attractive targets already have some form of maneuvering thrusters for station keeping. The first part is actually the tricky one. The general public actually has access to information on where most of the active satellites are up there. The various space agencies have reasonably accurate maps of debris on top of that, and the government knows where their own classified stuff is. This means they just need to figure out where other people's classified satellites are, as those are the most obvious tool for attacking things up there.

Once you've got that information, you just watch for deviation from expected orbits, and dodge things coming your way. The mapping and tracking itself could be easily automated, and with a bit of work, the whole setup could be automated, which would reduce your long-term costs to whatever it takes to maintain the equipment (probably on the order of at most a few million a year, possibly as low as a few hundred thousand if we can avoid the typical governmental inefficiencies).

There are three limitations to this simplistic but inexpensive approach:

• Attacks will still cause some disruption. This is actually unavoidable unless you can deorbit the hostile objects, but allowing for that would exponentially increase the budget requirements.


• It doesn't get rid of the threat. Again, doing this safely would require forcibly deorbiting the hostile objects.


• It depends on the friendly satellites' maneuvering thrusters not being fuel-limited. IOW, they need to use something for station keeping that doesn't require refueling, otherwise this approach drastically reduces expected operational lifetimes for the satellites.

First you have to understand the threat. It makes no sense to destroy the enemy's satellites, the debris will continue in orbit, destroying other satellites, including your own. This is also something Neil deGrasse Tyson has noted.

If anybody takes to blowing up satellites, they are all going down in a chain reaction. So if that's what you want, include explosive charges on every satellite you send up, and when the time comes, destroy your own. The debris will destroy everybody else's.

Thus, presuming you want to keep the advantage of your OWN satellites,you need to disable their satellites without creating any debris. The likely way to take the offense here is using non-destructive weapons up close; like Electromagnetic pulses to fry electronics, or high-power lasers to fry their solar panels, or robots could even find the navigational ports and fill them with some kind of expanding foam or solid, so the satellite, though continuing in its orbit, can no longer navigate. A robot might also sever exposed wires or cause damage; you could have ruined the Hubble Telescope by spraying a few ounces of paint on the lens; likewise you can very cheaply attack (or sever) exposed antennae and disable communications with the satellite.

Your Space Force would most likely be Earth bound, communicating with intelligent mobile robots in space, that guard the important satellites from being attacked by other robots.

That would create an arms race in space, between defending and attacking robots, but still -- Nobody that wants to keep their satellites wants to create debris, every satellite (or robot) that gets shattered into debris increases the chances your own satellites will suffer, and that can reach a tipping point, where the satellites can't dodge because no matter where they move something will hit them. This is (as @Harper noted in a comment) called the Kessler Syndrome, the premise in the movie Gravity. It is not considered speculation, really, but something that can happen.