According to Entering Space by Zubrin, pg 131, we get a 16 kt (4 m diameter) every year (but most of them hit the ocean or the desert) 120 kt every 10 yr, and 20 megaton every 400 years. Gulag prisoners and/or guards in Stalinist Russia probably killed by feb 1947 Vladivostok 128 kt impact.
Sometimes I have wondered if the failure of the government could be solved by making two entities: a highly profitable corporation, making anything whatsoever that makes money, giving all of it to a private space agency, and a private space agency using said money with the same ability to do the right thing as NASA but with ambition intact.
Also, I think we really need to consider Orion and NTR technology. For the size of things we could launch... and the posibility of lofting an Orion or using pure fusion bomblets.
Imagine the scientific progress if space lift was decreased to 1 dollar a kilogram. Even with no other advancements.