- the size of Russia
- The range of surface to air missiles
- the chance of missing an object that does not give off
a heat signature until it is about to hit.
Any kind of suggestion that we have the tech or the means (financially) to do this for a large chunk of the planet is nonsense, and for a smaller area the odds of hitting something are probably so bad that it isn't worth it.
Some combination of mass and speed detected by radar (or satellite?) triggering an S2A response.