Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p32b0559o&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P32B-0559
Physics
6022 Impact Phenomena, 6205 Asteroids And Meteoroids, 6210 Comets
Scientific paper
Instabilities play an important role in impact phenomena. We found that the impact penetration modes can be characterized by two dimensionless parameters: 1) Projectile/target uncompacted density ratio - ({ρ p } / {ρ t} ), and 2) Vaporization number - U2/Ev, where U is the impact velocity and Ev is the target energy of vaporization. The magnitudes of these dimensionless parameters were found to define the following penetration modes: 1) Spreading, projectile spreading, and smooth transient cavity wall formation; 2) Breakup (target vaporization)- projectile breakup due to target vaporization induced Rayleigh-Taylor (R-T) instabilities and 3) Breakup ({ρ p /{ρ t } > 3)}, projectile breakup due to R-T and Kelvin-Helmholtz (K-H) interface instabilities. We developed analytical models for the above penetration modes and confirmed these with detailed computer code calculations. For the impact of projectiles with targets of similar density (1.0 < {ρ p} / {ρ t} <3), the impactor expands to line the transient cavity and neither R-T or K-H instabilities at the impactor/target interface have time to grow large during penetration. The transient cavity is smooth and hemispherical at the time of maximum penetration for density ratios near unity. For greater magnitudes of the density ratio, the transient cavity becomes more elongated during the penetration phase. When the impact velocity is high or the vaporization energy is low (U2/Ev > ~ 10 ), then instabilities can occur for density ratios greater than 3. In this case, the material in front of the projectile is vaporized and the vaporized material decelerates the forward motion of the projectile and drives R-T instabilities at the interface at the front of the projectile. This eventually fragments the projectile. The ejecta in this case is not just propelled above the surface at the edges of the transient crater (ejecta curtain), but is propelled throughout the transient cavity because of the vapor pushing upward on the projectile fragments. For density ratios less than 3, which is characteristic of impacts of dense projectiles into comets, porous asteroids and meteoroid capturing media, the projectile initially flattens and expands normally to impact velocity vector. This radial expansion of the projectile is arrested by K-H instabilities that strip mass from the edges because of the velocity difference between the projectile and target material. The projectile penetration is terminated by R-T instabilities that form on the front of the projectile and eventually shatter it. We found that the magnitude of the target porosity did not affect the requirement for the development of instabilities and that the uncompacted density ratio independent of porosity was the determining factor. Our calculations support the results of Housen et al., 1999 for impacts into porous refractory targets. We found that for impacts into refractory targets, porosity produces a compacted zone in the crater cavity of near normal density material and that much of the impact energy goes irreversibly into compaction. In addition, porosity reduces the velocity and the mass of material that is ejected from the transient cavity.
Ahrens Thomas J.
Okeefe John D.
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