Think of something about the size of a small beer fridge being stuffed
    with steel balls and explosives and you get the idea. Contrary to what
    you think you really don't want to fire it in the face of an oncoming
    target. The timing isn't that big a problem its more to do with the
    further contamination of neo with high kinetic particles.

To contaminate NEO, you have to be IN NEO, which is "hard."  The
simplification was to kill something in NEO with a merely sub-orbital
ballistic carrier, which only moves the "hardness" from "achieving NEO"
to "guidance", since your payload is no longer going to stay in the NEO
satellite's path for very long...

BillW

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