A number of folks have offered accounts of what might happen if that 30m rock that narrowly missed Earth had, in fact, hit. Clayton Cramer posted the following (via this site):
Fifty thousand years ago a huge boulder crashed into the desert flatlands in what is now Arizona leaving behind a bowl-shaped hole 4,000 feet wide and 570 feet deep. A study published in the journal Science concludes the stone that came in from space that day was a nickel iron meteor 100 feet in diameter and weighing 60,000 tons, traveling at speed of almost 45,000 miles an hour.He goes on to cite the 30km destruction radius that is represented by the Arizona crater. This is fine as far as it goes, but the current rock was moving substantially slower, and that makes a big difference. Calculations follow:
A 30m (round) nickle-iron asteroid has a volume of (4πr3/3) or 14,000 cubic meters. A cubic meter of water masses a metric ton. Iron is 7 times heavier than water, so such an asteroid would mass something like 100,000 tonnes.
Next, the current object was moving at about 16,000 miles/hour in open space. Once it hit the atmosphere, it would be slowed. Assume in the 30 seconds it spent driving through the atmostphere it is slowed by a factor of 4 (heating itself and the air something terrible, and producing a god-awful burst of light). This gives an impact of about 1800 m/second.
Energy released is mv2/2 or about 1.6x1014 joules. According to this a kiloton of TNT = 4.2x 1012 joules. So the impact of this object at that speed would yeild about 40 kilotons of TNT. Pretty bad, but not earth-shattering by any means.
Note: One big uncertainty in this is the degree to which the object slows in the atmostphere. The energy removed by slowing is mostly imparted to the air (the rest producing bright light), in proportion to the square of the velocity change. It is quite possible therefore that the superheated air and other atmospheric effects would produce outward winds larger than the immediate blast effect.
Note that the Arizona impact object was moving almost 3 times faster, with 8 times the original energy, assuming that the cited velocity was in free space, not at impact (which would be far worse).