>>However, the atmospheric density only becomes really substantial below roughly about 40,000 feet (approximately one diameter for a K/T type impactor), so that travel time becomes about 8.75/30=0.3 seconds, loosely. << Oops. I was too quick with my calculator finger. 40,000 feet is 7.57 miles, so 0.25 seconds, loosely, rather than 0.3.