To intercept a missile you need to intercept it - that's it. You don't need to be traveling the same speed, or have the same mass, you just need to be physically where it is at the time it's there.
ICBMs aren't armored - and they can't be. They're spacecraft of a sort, and doing almost any amount of intentional damage will prevent them from functioning. Nuclear warheads are hard to detonate - the behavior if disrupted is the weapon likely goes inert because you've ruined the implosion mirror or timing circuitry.
The problem is that interception is hard because your engagement times are very short, and you're off-trajectory. Initial deviations in your aiming create larger and larger errors at the target, and any sort of maneuverability on the warhead's part means you're unlikely to be able to correct in time - hence the focus on trying to hit missiles in the boost phase, when they're by necessity more predictable.
There's no ICBM by the time any realistic interception scenario takes place. There's a slightly diverging cloud of warheads, chaff, jammers, light (inflatable metallized film) and heavy (small thingies that produce plasma in the higher atmosphere) decoys. The interception of a single target is hard because of what you said, the interception of any realistic salvo is impossible due to the sheer volume of incoming stuff.
Nope, the MIRVing/decoy deployment is done right after the boost phase, that way you need the least fuel to achieve the same target spread. That's some 5 minutes into the flight and mere hundreds of miles from the launch site. If it's Russia you're talking about, the missiles (or rather what's left of them) are still over their territory, some 200km up.
To intercept a missile you need to intercept it - that's it. You don't need to be traveling the same speed, or have the same mass, you just need to be physically where it is at the time it's there.
ICBMs aren't armored - and they can't be. They're spacecraft of a sort, and doing almost any amount of intentional damage will prevent them from functioning. Nuclear warheads are hard to detonate - the behavior if disrupted is the weapon likely goes inert because you've ruined the implosion mirror or timing circuitry.
The problem is that interception is hard because your engagement times are very short, and you're off-trajectory. Initial deviations in your aiming create larger and larger errors at the target, and any sort of maneuverability on the warhead's part means you're unlikely to be able to correct in time - hence the focus on trying to hit missiles in the boost phase, when they're by necessity more predictable.