This process is like an explosion. For this reason, the atmospheric fragmentation is often called "explosion" or "airburst" in the scientific literature, even though they are not related to TNT or nuclear explosions.
Airburst of a small cosmic body. When and how
does it begin?
Turning asteroids into stardust
by Luigi Foschini
| Not always a cosmic body
heading toward a planet does actually hit the ground. It
must be of certain size or composition, to get throughout
the atmosphere and collide. How does this work? How big
must an asteroid or a comet be to pass the atmosphere
layer by layer and hit? Some put the limit of strength
dominated regime at 150 m. Below this limit, the presence
of the atmosphere plays an important role in defining the
impact hazard. Indeed, in this case, the fragmentation begins
before the impact and, after the break up, the small
asteroid/comet begins to expand, increasing its cross section to the
air flow, which in turn results in increasing the drag
and deceleration. This process is like an explosion. For this reason, the atmospheric fragmentation is often called "explosion" or "airburst" in the scientific literature, even though they are not related to TNT or nuclear explosions. |
|
 Fallen trees in the Tunguska area (1928). Tunguska Page of Bologna University |
The best known event of this
type is the Tunguska event of 30 June 1908, when a 10-20
Mton explosion at 5-10 kilometres height, destroyed 2150
square kilometres of siberian taigà and more than 80
million of trees. The nature of the cosmic body which
caused this event is still under discussion and on July
1999 (Tunguska99)
an expedition went to Siberia to collect data and
samples, hoping to find the decisive argument. The key problem in the study of airburst is to understand when the cosmic body begins to break up. First models put this condition when the dynamical pressure in the front of the asteroid/comet is equal to the mechanical strength of the material. However, during nineties Zdenek Ceplecha showed that this is not true. He studied airburst from tiny asteroids , that is, meters-sized bodies: for some of these bodies there are available photographic and video records, that made it possible to calculate with great precision the height of fragmentation and the corresponding dynamical pressures. |
It resulted that the pressure is
systematically lower (more than one order of magnitude) than the
mechanical strength. Some authors invoked special features of the
incoming body: non-homogeneity, additional flaws and cracks,
internal voids. But all of them could give explanations for some
episodes, not a general theory.
One possible solution to this
conundrum -the one I have proposed to the discussion - is based
on the study of unsteady flow around the small asteroid/comet. In
this case, the distortion of the schock wave created during the
passage in the atmosphere, interacts with the turbulence, which
results in sudden outburst of the dynamical pressure, up to
twelve times its nominal value. Therefore, even though the
dynamical pressure cannot reach the mechanical strength, the
amplification of turbulence can produce the required effect. This
could also explain why meteorites reaching the ground are cold:
the break up occurs after a simple mechanical process and not
because of thermo-mechanical stress.
It is worth noting that the condition for
the existence of this mechanism is that the Mach number (i.e.,
the ratio between the speed of the cosmic body and the local
sound speed) is changing. If it is constant, the interaction
between shock wave and turbulence does not occurs, and the
fragmentation occurs because of thermo-mechanical stress.
This solution is developed only from a
conceptual point of view and numerical modelling is necessary to
fully develop and test the theory. However, for the moment, it is
the only available.
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