Late last year, for example, scientists at Sandia National Laboratories ran one of the most advanced supercomputer simulations yet on the Tunguska event. They concluded the object might actually have been much smaller than previously believed for the amount of damage caused. Upon exploding in midair, the simulation showed, the object set off a massive fireball that blasted downward toward the Earth's surface faster than the speed of sound. That effect, coupled with new data taking into account topography and ecology (the forest was apparently not as healthy as once thought), indicated the impact might have been equal to a three- to five-megaton blast, rather than the 10- to 20-megaton explosion earlier models suggested.
Just two weeks after the Tunguska event's 100th anniversary, an international team of scientists announced new findings to help dispel lingering theories the explosion was a purely terrestrial one (a naturally occurring nuclear reaction, for instance, or a "super-burp" of natural gas from deep underground). The team reported it had found high levels of carbon-13 and nitrogen-13 isotopes in the peat layer formed at the time of the impact, as well as evidence of a massive acid rain fallout: some 200,000 tons of nitrogen and nitrogen oxides.
"The levels of accumulation of the heavy carbon isotope 13C measured right on the 1908 permafrost boundary in several peat profiles from the disaster area cannot be explained by any terrestrial process," said Tatjana Böttger of the Helmholtz Center for Environmental Research in Germany.
The team concluded the object hitting Tunguska could have been a C-type (carbonaceous) asteroid or a comet similar to Borrelly, which has been extensively studied by Deep Space 1 and the Hubble Space Telescope. (Observations have found that Borrelly is a dark, hot and dry comet, rather than a "dirty snowball," with no detectable water ice at its surface.)