Foschini Hypersonic Flow
In the image: A schematic representation of a possible process by means internal cavities can increase
the deceleration and the airburst efficiency. As the cosmic body enters the Earth atmosphere, the ablation removes
the surface, discovering the internal cavities, which act as something similar to a parachute,
thereby increasing the deceleration (see papers pdf, 304 kb
and pdf, 18.9 Mb).
Foschini (refs. [¹] and [²]) developed a model studying
the hypersonic flow around a small asteroid entering the Earth’s atmosphere. This model
is compatible with fragmentation data from superbolides. The model considers a bow
shock in the front of the cosmic body that envelops the body. As the air flows toward
the rear of the body, it is re-attracted to the axis. Therefore, there is a rotation of the
stream in the sense opposite to that of the motion and this creates an oblique shock
wave (wake shock). Since the pressure rise across the bow shock is huge when compared
to the pressure behind the body, it can be assumed that there is a vacuum behind
the cosmic body. According to the model, the condition for fragmentation depends
on two regimes: steady state, when the Mach number does not change, and
unsteady state, when the Mach number undergoes strong changes.
In the latter case, the distortion of shock waves causes the amplification of turbulent
kinetic energy. So, a sudden outburst of pressure that can overcome the mechanical
strength of the body, starting the fragmentation process is expected. On the other hand,
in the first case – the steady state – the effect of compressibility suppresses the turbulence,
and then the viscous heat transfer becomes negligible. The cosmic body is subjected
to a combined thermal and mechanical stress.
The key point in fragmentation is how the ablation changes the hypersonic flow. The
existence of asteroids with an extremely low density, such as Mathilde (~1300 kg m–³),
suggests that such a body could have an increased efficiency in deceleration. A possible
process by means internal cavities could increase the deceleration and airburst efficiency
is shown on the image.
[¹] Foschini L (1999) A solution for the Tunguska event. Astronomy and Astrophysics 342:L1–L4
[²] Foschini L (2001) On the atmospheric fragmentation of small asteroids. Astronomy and Astrophysics 365:612–621
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