Total ablation of the debris from the 1908 Tunguska explosion

THE origin of the explosion over Tunguska, central Siberia, in 1908 has long been an enigma. Models1–3 of the disruption of solid objects entering the atmosphere indicate that the Tunguska explosion occurred at an altitude of 6–10 km, and that the source object was probably a stony asteroid4. But important questions concerning the nature of the object remain5,6, particularly as no fragments have been identified in the area of the explosion. Unlike smaller objects (such as meteorites), which decelerate high in the atmosphere and can thus escape complete ablation and/or pulverization7, a Tunguska-sized object penetrates deeper into the atmosphere, where it will experience a greater aerodynamic load: the object should be disrupted into a vast number of fragments, each no larger than about 10 cm (ref. 2), which are then widely dispersed. Here I calculate the flux of radiation both inside and outside the fireball associated with the fragmenting object, and show that this is sufficient to totally ablate the dispersing fragments. The apparent absence of solid debris is therefore to be expected following the atmospheric fragmentation of a large stony asteroid.