Low-altitude airbursts and the impact threat.

Abstract We present CTH simulations of airbursts in the Earth's lower atmosphere from hypervelocity asteroid impacts. The intent of these simulations was to explore the phenomenology associated with low-altitude airbursts, with the particular goal of determining whether the altitude of maximum energy deposition can be used as a reasonable estimate of the equivalent height of a point source explosion. Our simulations suggest that this is not a good approximation. The center of mass of an exploding projectile is transported downward in the form of a high-temperature jet of expanding gas. The jet descends by a significant fraction of the burst altitude before its velocity becomes subsonic. The time scale of this descent is similar to the time scale of the explosion itself, so the jet simultaneously couples its kinetic energy and its internal energy to the atmosphere. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are felt at the surface than would be predicted by a point source explosion at the height where the burst was initiated. For impacts with a kinetic energy above some threshold, the hot jet of vaporized projectile (the descending “fireball”) makes contact with the Earth's surface, where it expands radially. During the time of radial expansion, the fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. We suggest that the surface materials can ablate by radiative/convective melting under these conditions, and then quench rapidly to form glass after the fireball cools and recedes. Possible examples of such airburst glasses are the Muong-Nong Tektites of Southeast Asia and the Libyan Desert Glass of western Egypt. We suggest an enhancement of entry dynamics models to account for the downward advection of shocked and heated material, and the lowering of the apparent airburst altitude. The actual differences between the effects on the ground from a point source approximation versus a full flow field still need to be quantified by running more realistic high-resolution 3-D simulations with a variety of impact parameters. A re-evaluation of the impact hazard is necessary to properly include this enhanced damage potential of low-altitude airbursts.

[1]  S. P. Worden,et al.  The flux of small near-Earth objects colliding with the Earth , 2002, Nature.

[2]  H. J. Melosh,et al.  Planetary science: Meteor Crater formed by low-velocity impact , 2005, Nature.

[3]  I. Friedman,et al.  Libyan desert glass: Its viscosity and some comments on its origin , 1969 .

[4]  Christian Koeberl,et al.  Confirmation of a Meteoritic Component in Libyan Desert Glass from Osmium-Isotopic Data , 2000 .

[5]  Robert C. Whitten,et al.  An analysis of the physical, chemical, optical, and historical impacts of the 1908 Tunguska meteor fall , 1982 .

[6]  N. V. Vasilyev,et al.  The Tunguska Meteorite problem today , 1998 .

[7]  Eugene M. Shoemaker,et al.  ASTEROID AND COMET BOMBARDMENT OF THE EARTH , 1983 .

[8]  M. Boslough,et al.  Shoemaker‐Levy 9 and Plume‐forming Collisions on Earth a , 1995 .

[9]  Peter H. Schultz,et al.  Atmospheric effects on ejecta emplacement , 1992 .

[10]  Christian Koeberl,et al.  The record of Miocene impacts in the Argentine Pampas , 2006 .

[11]  Samuel Glasstone,et al.  The Effects of Nuclear Weapons , 1952 .

[12]  Ari Ben-Menahem,et al.  Source parameters of the siberian explosion of June 30, 1908, from analysis and synthesis of seismic signals at four stations , 1975 .

[13]  J. Wasson,et al.  Large aerial bursts: an important class of terrestrial accretionary events. , 2001, Astrobiology.

[14]  C. Chyba,et al.  The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid , 1993, Nature.

[15]  V. Svetsov,et al.  Total ablation of the debris from the 1908 Tunguska explosion , 1996, Nature.

[16]  A. F. C. Haldemann,et al.  Evidence for a ∼ 200–100 ka meteorite impact in the Western Desert of Egypt , 2007 .

[17]  V. V. Svetsov.  Thermal Radiation on the Ground from Large Aerial Bursts Caused by Tunguska-like Impacts , 2006 .

[18]  I. T. Zotkin,et al.  Simulation of the Explosion of the Tungus Meteorite , 1966 .

[19]  Mark B. Boslough,et al.  Postshock temperatures in silica , 1988 .

[20]  V. A. Bronshtėn,et al.  Nature and destruction of the Tunguska cosmical body , 2000 .

[21]  John T. Wasson,et al.  Iridium anomaly associated with the Australasian tektite-producing impact: Masses of the impactor and of the Australasian tektites , 1993 .

[22]  R. Jeffery Lawrence.  Enhanced momentum transfer from hypervelocity particle impacts , 1990 .

[23]  K. Zahnle,et al.  Airburst origin of dark shadows on Venus. , 1992, Journal of geophysical research.

[24]  David A. Crawford,et al.  Comet Shoemaker‐Levy 9 Fragment Size Estimates: How Big Was the Parent Body? a , 1995 .

[25]  Falko Langenhorst,et al.  Evidence for shock metamorphism in sandstones from the Libyan Desert Glass strewn field , 2001 .