[Justa]

Space Command (or WrTF they're calling NORAD now) would pick it up on radar before impact. Probably not enough time to stop it but at least they'd know what caused the impact blast.

[My2Cents]

plus, the lack of radiation would show it wasn't a nuclear strike. In addition, it is highly unlikely that a rock of that size would endanger a city or population center. Most likely, it would hit the ocean, or the Australian outback, like SkyLab and Mir.