According to the Air Force, an kinetic orbital bombardment system could strike a target anywhere on Earth within 12-15 minutes. Replace the proposed kinetic weapons with nukes, and there’s your answer for how fast they could hit. So faster than an ICBM, which take about 30 minutes.
Much more expensive and vulnerable, though.
It’s really not true though. This is an error I see time and time again in sci-fi, the idea that if you’re in orbit, you can just casually drop things on Earth like Zeus hurling thunderbolts. Orbit isn’t just an extremely high piece of terrain, it’s a high-speed circular trajectory around the earth, and if you want to send a warhead back to earth, you need a significant deceleration burn. It’ll be far west of the target, in order to make that curved trajectory intersect the target, it’ll leave a massive heat signature from the burn and the re-entry heating, and it won’t surprise anyone at all.
You can’t just drop things from orbit!
This isn’t exactly true though. The Davy Crockett only had a yield of 10-20 tons TNT, which would’ve been called a fizzle if produced by a higher-yield test, but that’s what Davy was designed for. It definitely produced a true fission detonation with a significant blast wave and incendiary effects. It would’ve been extremely effective for wedging an armored column traversing a narrow pass.
But to your point, unlike in larger weapons, the neutron flux would’ve outrun the blast radius, like how a neutron bomb works. So in the above scenario, all the above armor would be too hot to mount or even approach and move, so the enemy would simply have to abandon the pass on foot (if they were able), and the friendlies could counterattack through it after a few days.
That is correct; satellites are traveling so fast that the are continually falling above the horizon, and to send a weapon or interceptor out of orbit the user would certainly need to apply enough momentum to cause the reentry vehicle to fall ‘below the horizon’; if they wanted to do so promptly (i.e. in the same timespan as a ballistic missile) then significant impulse is required. The only real advantage advantage––aside from providing only a minimal ground track to follow and project a target––is that the system already has substantial kinetic energy and even after it is reduced to get it to fall out of orbit the residual is still enormous. The ‘Rods from God’ scheme of using telephone pole-sized tungsten rods based in orbit to strike hardened ground targets would have provide as much energy as a small tactical nuclear weapon except with much greater ground penetration than an RV with a nuclear weapon would be able to withstand.
The undesirability of vulnerability, strategic instability, and invitation to preemptive kinetic warfare in orbital space should be obvious, and just the implementation and mainteance costs, even compared to a ground-based strategic deterrent, should make it evidence how impractical this notion is even if it could be implemented.
Stranger