100 Years Since Tunguska

Monday, June 30 marks the 100th anniversary of the Tunguska incident in 1908, in which a meteor or comet fragment entered the atmosphere over Tunguska in Siberia producing an enormous explosion.

“We know that a rather massive body flew into the atmosphere of our planet,” said Boris Shustov of the Russian Academy of Sciences.

“It measured 40 to 60 meters in diameter. Clearly, it did not consist of iron, otherwise it would have certainly reached the earth. The body decelerated in the atmosphere, the deceleration being very abrupt, so the whole energy of this body flying with a velocity of more than 20 meters per second [probably should be: kilometers per second] was released, which resulted in a mid-air explosion, very similar to a thermonuclear blast,” he told Tass news agency yesterday.

“The yield of the explosion totaled 10 to 15 megatons, which matches the yields of the largest hydrogen bomb ever tested on the planet [actually, the largest reported test in October 1961 had a yield in excess of 50 megatons]. The explosion felled some 80 million trees [but] it is generally assumed that the blast did not kill any people,” he added.

“The Tunguska phenomenon showed that the asteroid-comet danger is quite real. It happened not in the era of dinosaurs, but in our recent history. Russia was definitely lucky; had the body flown up to the Earth several hours later, it would have hit St.Petersburg. The consequences would have been horrendous,” he said.

“Impacts such as the Tunguska incident are thought to occur about once in one hundred years based on the density of impact craters on the Moon,” according to a White Paper on Planetary Defense attached to the 1994 U.S. Air Force report Spacecast 2020.

A 2007 NASA summary report to Congress on planetary defense is here (pdf). A longer account is here (pdf).