The U.S. Department of Defense have released information obtained by U.S. satellites on the fireball event that occurred over Siberia on Sept 24. Eye-witness accounts of the event reported a large luminous object falling to Earth near Bodiabo in Siberia. Hunters in the region have also reported the existence of a crater surrounded by burnt forest suggesting that an impact event had occurred. The event was detected by near-by geophones as a moderate-earthquake.
U.S. satellites initially detected the fireball at 57.91N and 112.90 E at an altitude of 62 km and it was tracked to 58.21 N and 113.46 E at an altitude of 30 km. The satellite measurements indicate that the total energy radiated from the fireball was roughly the equivalent of 0.2 kilotonnes of TNT.
Although the amount of energy of a meteoroid that is converted into heat and light as it falls through the atmosphere is not simply related to the size of the meteoroid it can be used to estimate the size of the event. The fireball seen over Alaska on January 18, 2000, for example, was likewise observed by U.S. satellites and released an energy of around 0.26 kilotonnes. This event, like the Siberian fireball, was detected by earthquake geophones and resulted in the fall of the Tagish lake meteorite. Only around 1 kg of Tagish lake was recovered from the frozen surface of a lake, most of this fragile meteoroid was probably destroyed during atmospheric entry. If the Siberian meteoroid, however, consisted of tougher materials such as iron then more mass may have survived to collide with the ground. Weak meteoroids also lose more of their energy during flight through the atmosphere because of their break-up.
The Sikhote Alin meteorite fell in Russia in 1947 and more than around 70 tons of this meteorite were recovered, many from steep sided impact pits in soft soil. Estimates of the energy released by Sikhote Alin vary, however, its is likely to have been up to 10-20 kt with most of this energy lost during fall through the atmosphere. The event in Siberia on Sept 24 is unlikely to have been as significant as Tunguska or the event that generated meteor crater in Arizona since both of these liberated tens of megatonnes of energy. It may, however, prove to have resulted in the fall of an iron meteorite although the fact that the fireball was tracked to an altitude of 30 km, where most fireballs associated with meteorites end, suggests it is perhaps not likely to be as large as the Sikhote Alin fall. Only once scientists have located and examined the impact site, however, will the size of the event be known for certain.
