As predicted, David Morrison has commented on the new results about the Tunguska impactor:
The main story in this edition of NEO News concerns a proposed downsizing of the energy of the 1908 Tunguska airburst, with associated increase in the expected frequency of such impacts. Mark Boslough of Sandia has generated supercomputer simulations of the Tunguska atmospheric explosion. In part his models require less energy in the explosion because he includes the substantial downward momentum of the rocky impactor, rather then modeling it as a stationary explosion. If this revision (down to an estimated energy of 3-5 megatons, and a corresponding diameter of about 50 meters) is correct, the expected frequency of such impacts changes, from once in a couple of millennia to once in a few hundred years. If smaller impactors can do the damage previously associated with larger ones, of course, the total hazard from such impacts is increased.
Remember, this is a statistical frequency. We might not see another 5 megaton blast for a thousand years, or we may get three next month. But over time, the numbers average out. What this all really means is that we’d better figure out a way to push even little rocks aside, which means finding them first. That’s no small task, since they are extremely dim. Most of the time, we don’t see even bigger objects until they have already passed us.
This is a real issue, and one that needs much thought, and much attention.
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