Re: Tunguska
Tunguska class objects represent only 1 % of the threat from a casualty standpoint. Even these small objects are estmated to hit every 100 to 300 years, with 70 % of these hitting the ocean. Less than 100 years later most of the effects of the impact have virtually disappeared. It is really not surprising that there are few known examples of this class of impact.
While I doubt there is a better explanation than impact for Tunguska, loosing it as an example would not substantially change the NEO impact odds which are based on Lunar Impact data and NEO population estimates based on observed rates of discovery.
Regards,
Mark