So Khrennikov and colleagues simulated the effect of meteorites made of rock, metal or ice, moving through the atmosphere at a speed of 12 miles per second (20 kilometers per second). (Meteorites enter the atmosphere with a minimum speed of 11 kilometers per second.)
Friction with the atmosphere immediately heats these objects. But while iron vaporizes at around 5,432 degrees Fahrenheit (3000 degrees Centigrade), water vaporizes at only 212 degrees F (100 degrees C). So icy meteorites do not last long.
Indeed, Khrennikov and colleagues calculate that an icy body large enough to cause such a large explosion would have traveled no more than 186 miles (300 kilometers) through the atmosphere before vaporizing completely. That suggests the Tunguska meteorite could not have been made of ice.
Instead, Khrennikov and colleagues say a different scenario fits the facts. They say the explosion must have been caused by an iron meteorite about the size of a football stadium. This must have passed through the upper atmosphere, heated rapidly, and then passed out into the Solar System again. The shock wave from this trajectory was what flattened trees.
The shock wave would have caused an explosion of about the right magnitude, and any vaporized iron would have condensed into dust that would be indistinguishable on the ground. Crucially, this scenario would not have left any visible asteroid remnants.
It could also explain reports of dust in the upper atmosphere over Europe after the impact.
Lucky miss
If Khrennikov and colleagues are correct, then Earth had a lucky near-miss that morning. A direct impact with a 656 foot-wide (200 meter-wide) asteroid would have devastated Siberia, leaving a crater 2 miles (3 kilometers) wide. It would also have had catastrophic effects on the biosphere, perhaps ending modern civilization.
In the event, the Tunguska impact is thought to have killed perhaps three people because the region is so remote. It could clearly have been much worse.
Ref: Krennikov et al. 2020. On the possibility of through passage of asteroid bodies across the Earth’s atmosphere. arxiv.org/abs/2009.14234