2. Current models
Several different models have been developed in order to fit all the available data on the Tunguska event (e.g. Chyba et al. 1993, Grigorian 1998, Hills & Goda 1993, Lyne et al. 1996). All these models have contributed significantly to a general improvement in our understanding of the atmospheric disruption of meteoroids. They usually assume that the fragmentation process starts when the aerodynamic pressure is equal to the mechanical strength S of the cosmic body. Relating air density to airburst height, this allows one to derive the meteoroid speed (V):
where is the atmospheric density at sea level, h is the height of first fragmentation and H is the atmospheric scale height (about 8 km). From Ben-Menahem's analysis we infer that there was a single fragmentation event; there is no evidence of multiple explosions, as it should occur during multiple fragmentation events (Ben-Menahem 1975). Thus Eq. (1) can be used to derive V, provided one assumes that the first fragmentation coincided with the airburst occurred at km.
Table 1. Speed of the Tunguska cosmic body vs. strength according to Eq. (1)
Now, since before exploding large meteoroids undergo a limited mass loss during their atmospheric path, the pre-explosion speed must be close to the (geocentric) orbital speed, and thus must be greater than the Earth's escape velocity (11.2 km/s). Therefore, according to the results derived from Eq. (1), the most plausible solution would be that of an iron body. However, the iron body hypothesis is not consistent with the recent on-site recovery of microremnants from a stony object (Longo et al. 1994, Serra et al. 1994).
Actually, taking into account the uncertainty in the value of S and the different measurement errors (both of which are difficult to quantify), the stony object solution could not be entirely ruled out using this argument (typical geocentric speeds for near-Earth asteroids are km/s). However, it is known that the interaction of large meteoroids (or small asteroids) with the Earth's atmosphere is characterized by a great variety of behaviors, and any quantitative theory should take into account a large number of variables: size, shape, rotation, composition, internal structure, orbital speed, flight path angle. Thus for the time being each bolide must be seen as a case study, which can provide useful insights for a future comprehensive theory. As a further consequence, Eq. (1) cannot be trusted to provide quantitatively reliable results in every case.
For instance, we know that sometimes meteoroids explode at dynamic pressures much lower than their mechanical strength (Ceplecha 1995). In the case of the Lugo bolide, an interesting possibility is that this behavior may have been related to a porous structure of the meteoroid (Foschini 1998). However, Table 1 shows that in the case of Tunguska we have the opposite problem, and that we should assume an anomalously high mechanical strength. Therefore, I will look into another direction for a possible solution of the conundrum.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998