About six to ten
The center of the field had been the nation 's capital , the third of the great American cities to be destroyed in the holocaust of 2048 . `` How big was the asteroid they modeled ? '' asked Joe . `` About six to ten miles across , '' said Mark . `` And the one Zimbardo has aimed at Earth is forty miles long ? '' `` Bigger . A little bigger than that , '' answered Zip quietly .