Or is it like Indians trying to form a navy with their canoes?  There aren’t any significant space trade lanes to block or keep open (…someday…), but a space force could focus on establishing “space supremacy” in a manner analogous to air supremacy.

Why Separate Service Branches Exist

The army is the oldest and most basic military service (indeed, “military” used to refer only to the army).  I’m not familiar with the history of navies as a distinct organization from the army, but humans have been plying the oceans since well before recorded history.  It’s clear that the crew and officers of military vessels were at least somewhat distinct from soldiers even in Antiquity.  What’s important is that a navy does something that doesn’t and can’t involve the army: enforcing or denying freedom of navigation.  This requires establishing command of the sea.

Sometimes an army may need to move things across the water in support of their operations on land, and they might have some vessels to enable this.  Likewise, navies use marines in support of their own operations.  But “the navy” doesn’t invade Donovia, and “the army” doesn’t blockade ports.

Which brings us to air forces.  The fundamental reason that independent air forces exist is strategic bombing.  Armies and navies only operate on the two-dimensional surface of the earth or ocean.  If an army wants to burn down the enemy’s capital, they have to move there, and if anyone else is in the way they have to fight them and win first.  The navy has to get off their boats if they want to take anything over, especially if the objective isn’t near the coastline.  With the airplane, you just fly over the enemy, drop your payload, and the target dies.

In theory, at least.  Air power advocates had and continue to have a serious problem overestimating the effects of strategic bombing.  That being said, it ain’t worthless, it isn’t going away, and it empirically exists outside the purview of the army and navy — that’s what the “strategic” adjective is really doing there.

In an air power-centric view, establishing air supremacy (which is air superiority, but more of it) is a derivative mission of strategic bombing.  Air forces must establish some control of the air in order to attack targets on the ground and bomb the enemy into submission.  Destroying enemy aircraft accomplishes very little, materially, on its own.  Of course the army and navy don’t want to be attacked by enemy aircraft either, and in reality this is part of the reason you want air supremacy, not just because it enables you to conduct strategic bombing.  At any rate, we can see that air supremacy is basically a “shaping” or enabling rather than decisive mission, not something that achieves strategic objectives on its own.  You’ll note that “air operations in direct support of ground forces” or close air support doesn’t really fit into this picture; this is the source of endemic squabbles over whether the USAF should have any responsibility for it at all.

The air force isn’t the only novel service branch.  The Soviet Union maintained a separate Air Defense Force.  I’m not directly familiar with the logic for doing so, but I would guess it is because their view of air supremacy aligned with the above: it’s to enable strategic bombing and maybe protect expeditionary ground and naval forces, not to defend against enemy bombing. Russia continues to have a separate “Strategic Missile Force” service for the operation of nuclear missiles, viewing this as sufficiently different from strategic bombing by aircraft to justify a separate service.

So What

Right now, the individual services put things into space in support of their operations on the ground.  Joint coordination must occur to make sure no one crashes into each other, and to mitigate the formation of information silos, but that’s about it.  Claiming that this justifies a new service branch, on its own, is a bit like deciding that naval aviation or army mariners should form their own service.

The lame answer is that the Space Force just serves as a coordination center for defense-related space operations, sort of like the Office of the Director of National Intelligence.  This might fix some hypothetical coordination problems between the uniformed services and civilian agencies like the National Reconnaissance Office, but while the current state of such coordination is far from public knowledge there aren’t any obvious signs of problems.

The most obvious role for a Space Force is “space supremacy”: deny the use of space to the enemy and enable it for the United States.  Since there are no strategically significant activities in space outside of Earth’s orbit, the space force’s purview would be limited to near Earth, as a practical matter.

The space force could also supplement or supplant civilian agencies like NASA in conducting space exploration, in a Faustian bargain exchanging militarization for funding.  However, while space pilots usually come from the military, the majority of work is done by civilians doing identifiably civilian jobs in science, engineering, and administration.

Notice that space supremacy doesn’t derive from the need to live or trade in space.  Even space-based weapons would only be incremental upgrades from currently existing strategic bombing methods. So far norms against space weapons have held up pretty well, if only because any “improvements” over ICBMs are likely to be self-defeating for game theory reasons.

Perhaps someone in the White House or DoD thinks they can skirt the relevant norms and treaties by putting only “space-to-space” weapons in orbit — the real fear being space nukes — by avoiding nuclear or even kinetic weapons (orbiting jammers).  I don’t know if anyone has ever recovered an enemy satellite from orbit for investigation — it’s possible.  If so, sensitive satellites might have anti-handling devices.  EOD…in space.

The need to police and control access to off-world or orbital installations, which would pretty strongly justify a space service, is completely irrelevant for the foreseeable future, as those things don’t currently exist.

Space Supremacy

So, at least for the foreseeable future, “space supremacy” is probably going to be the basic mission of the space force.  Separation from the air force would be justified on the basis of differing technical requirements and — maybe — even to avoid excessive militarization by keeping the brimstone scent of strategic bombing away.  What space supremacy is, is obvious by analogy to air supremacy: allow friendly use of space and deny it to the enemy.  How to go about it, and then actually doing it, might be something for a new service to work on.