All of the analyses which you mention have indeed been carried out, to the extent that resources were availalble to do them.

The error in your logic is in adopting the erroneous view that only asteroids hit the Earth. This is in fact incorrect, and has been demonstrated to be incorrect, by studies of impacts with the Earth by specialists in the relevant fields.

The engineering/cost-benefit analysis which you speak of has also already been carried out by a group of engineers at NASA's Langley Center. The funding of the development of some engineering test articles necessary to answer certain remaining engineering questions is likely to be approved in the next year.

Further, the Congress of the United States, working in a bi-partisan manner, has changed the NASA charter to establish dealing with this hazard as a priority for NASA. Even more specific instructions are likely to be passed in the next session.