1. Asteroids fly according to Fudd's law --
they'll "be on the same trajectory since the last time they changed trajectory." The vice president of asteroidpunishment must not send them toward Earth at all, because no Earthling ever lives long enough to be here when they arrive.

2. Just do the math. Forget intuition when calculating damage. Intuition is for building, not destroying. Of course, do the math when building, too.

You can send me email to beg for more
of this level of knowledge
at earthinspace@mail.com