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.
