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Fate of the projectile in oblique impacts

Hydrocode modeling of oblique impacts: The fate of the projectile
E. Pierazzo, H.J. Melosh

Meteoritics and Planetary Science 35(1), 117-130, 2000.

ABSTRACT

All impacts are oblique to some degree. Only rarely do projectiles strike a planetary surface (near) vertically. The effects of an oblique impact event on the target are well known, producing craters that appear circular even for low impact angles (>15° with respect to the surface). However, we still have much to learn about the fate of the projectile, especially in oblique impact events. This work investigates the effect of angle of impact on the projectile.
Sandia National Laboratories' three-dimensional hydrocode CTH was used for a series of high-resolution simulations (50 cells per projectile radius) with varying angle of impact. Simulations were carried out for impacts at 90, 60, 45, 30, and 15° from the horizontal, while keeping projectile size (5 km in radius), type (dunite), and impact velocity (20 km/s) constant.
The three-dimensional hydrocode simulations presented here show that in oblique impacts the distribution of shock pressure inside the projectile (and in the target as well) is highly complex, possessing only bilateral symmetry, even for a spherical projectile. Available experimental data suggest that only the vertical component of the impact velocity plays a role in an impact. If this were correct, simple theoretical considerations indicate that shock pressure, temperature, and energy would depend on sin2(theta), where (theta) is the angle of impact (measured from the horizontal). However, our numerical simulations show that the the mean shock pressure in the projectile is better fit by a sin(theta) dependence, whereas shock temperature and energy depend on sin3/2(theta). This demonstrates that in impact events the shock wave is the result of complex processes that cannot be described by simple empirical rules. The mass of shock melt or vapor in the projectile decreases drastically for low impact angles as a result of the weakening of the shock for decreasing impact angles. In particular, for asteroidal impacts the amount of projectile vaporized as always limited to a small fraction of the projectile mass. In cometary impacts, however, most of the projectile is vaporized even at low impact angles.
In the oblique impact simulations a large fraction of the projectile material retains a net downrange motion. In agreement with experimental work, the simulations show that for low impact angles (30° and 15°), a downrange focusing of projectile material occurs, and a significant amount of it travels at velocities larger than the escape velocity of Earth.

 

COLOR FIGURES (To download GIF files click on the figures)

Distribution of shock melting and vaporization inside the projectile for the various hydrocode simulations. To show the shock state inside the projectile for the 15° simulation, regions with shock pressures lower than 70 and 50 GPa are also shown. Vectors represent the direction of impact in the various cases.

 

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