I would imagine a nuke having such a large explosion that missing the target by a kilometre won't matter too much. But with a kinetic projectile that kind of deviation would not be acceptable (unless we are talking about seriously big projectiles, like in Tunguska etc).
Obviously you don't want to do a powered de-orbit for reasons mentioned (you burn fuel & lose kinetic energy). However, if you're flying low enough, couldn't you aerobrake your projectiles after release and have them de-orbit themselves?
For comparison, the X-41/51 scramjet programs appear to be aiming at the mach 5-9 region. So less than 1/2 as fast.
As complicated as the materials science and guidance has to be for any X-71-based projectiles, slowing down and terminal guidance (ablative coatings and sacrificial control surfaces) seem like easier problems than boosting up to ridiculous velocities.
As for tracking, you get the heat bloom as it aerobrakes, but if you manage to keep it coherent through re-entry then the ridiculous speed largely moots that.
At 5 km/s, with a prograde orbit, you're from Istanbul to Beijing about 24 minutes (by my sleepy calculations?). The exercise seems more of a question of "How steep can you dive (aka how much heat can you handle)?" than anything else.
PS: Well, and "How the hell do you communicate-with / sense-from a platform surrounded by air that hot?"
Of course that's all pointless because a ballistic missile launched from Earth could accomplish the same mission at a far lower cost with greater reliability and survivability.
Regardless of what orbit you start from, you dip into the atmosphere, deploy your payload from there, voila.
Now anything you dropped has to deal with a furnace of superheated air, and the question of whether it's possible to have control in those conditions, but it's definitely going to de-orbit.
And the atmosphere is going to supply most of the energy, rather than direct retro burns from the delivery vehicle.
That's not how orbiting works. If you want your projectile to travel straight down, you need to cancel all it's orbital velocity. For geostationary orbit, that's 3km/s of delta V needed.
Obviously, you can de-orbit with less delta-V, but then guidance starts becoming necessary.
> The slug is simply released with no indication of ingress. From a geostationary orbit, it just starts getting closer very fast. Cover it with EM absorbant, non-reflective material, and there's even less hint of activity, until the re-entry burn at approximately 60 miles altitude.
There is going to be some kind of burn to start de-orbit, you could simply monitor for that.