Improved first strike capability is worthless íf it isn't crippling, and "devastating enough" first strike capability from orbit is completely unaffordable, and impossible to build up unobserved.
Being in orbit is a hindrance more than anything, really, because maintenance becomes ruinously expensive, everything is trivially observable for all your adversaries and you have to align the orbit with your target beforehand, too (which, again, everyone can observe).
> It doesn't even have to be rational though, combinations of graft and brinkmanship would be enough.
Enough for what? Threatening to nuke some satellites? Because anything else you can do easier, cheaper and on a larger scale from the ground. Why would you bother with nuclear warheads in space when you can just build/maintain like 10 ICBM silos for the same cost?
From GEO, no. From LEO, still probably no.
There may be a bird positioned just right so a small deörbit burn pots Moscow quicker than an ICBM could. But the moment you start burning, you’re caught. (Same as an ICBM.) And unless you have a really obvious orbital configuration that bunches a bunch of birds in a way useful for practically nothing but such a strike, you only get one or two such “early” shots before a wall of ICBMs would have landed.
Nukes in space aren’t about nuking the ground from space. It’s about space area denial through EMP.
"Elements within the Soviet space industry convinced Leonid Brezhnev that the Shuttle was a single-orbit weapon that would be launched from Vandenberg Air Force Base, manoeuvre to avoid existing anti-ballistic missile sites, bomb Moscow in a first strike, and then land. Although the Soviet military was aware these claims were false, Brezhnev believed them and ordered a resumption of [satellite destroyer] testing along with a Shuttle of their own."
Nukes from LEO aren't impossible. They're just impossible to do better than the current triad. Any breakthrough in propulsion that would make a plane change easier or less visible confers the same advantages to an ICBM. There is a narrow window in which orbital nuking can outperform, and that's almost entirely taken care of--and exceeded, in stealth--by SLBMs.
Space-based missile defence is not what’s implied by weapons that are “already up” and thus “give a lot less warning in a first strike situation?”
No. Your missiles have further to fly from geostationary orbit than a missile silo on the ground, not to mention the additional complexity of designing your ICBM for reentry
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