[jamesmaniscalco]

There is a nice calculator for terminal velocity of a sphere in air here:

http://hyperphysics.phy-astr.gsu.edu/hbase/airfri2.html

That being said, I'm not sure how relevant terminal velocity is for anything but the smallest/slowest ends of the parameters of this app. For an asteroid going 38,000 mph (default speed on linked page), drag just doesn't play a big part - the asteroid passes through the atmosphere in ~5 seconds, not enough time for it to slow down significantly.