Propagation of impact-induced seismic waves into the interior of a rubble-pile asteroid

We present experimental results and numerical simulations on the study of propagation of impact-induced seismic waves in granular media.

The influence of static compression on the propagation of waves is studied to mimic the pressure variations induced by self-gravity in the asteroid interior. Pressure in the interior of a 200m asteroid are in the range 1-10 Pa; while for 2km asteroids, the pressure grows up 100-1000 Pa.

In the laboratory experiments, granular material such as sand, gravel and glass spheres are placed into a 50-cm acrylic cubic box. Pressure inside the box is controlled by a movable floor with a hydraulic jack and monitored with several pressure sensors. Impacts are generated on the upper face of the box through a 10-cm diameter circular hole. Projectile velocity ranges from a ~60 m/s up to ~300 m/s, attainable with a crossbow, CO₂ gun and a nitro-piston rifle. The projectiles have diameters from 4.5 to 5.5 mm, and they are made of steel or lead. An array of accelerometers is placed at several depths inside the granular material to detect the mechanical wave.

Impact generated waves are studied by estimating velocity and attenuation of the wave. A parameter study is then performed changing impact energy, properties of the granular material and static pressure.

We observe a strong correlation of the velocity of wave in the media with the confining pressure. For confining pressure of pressures a few tens kPa, the velocity is ~150 m/s for the different media, and it increases up to 500-700 m/s for confining pressures of ~200 kPa. Extrapolating these results to the very low internal pressure typical of km size asteroids would imply very low wave velocities.

We compare these results with numerical simulations using the DEM package ESyS-Particle. The mechanical properties of the granular material and the projectile as well as the impact velocity of the projectile are varied in a wide range of values. Numerical results regarding velocity, attenuation and energy transmission are compared with the experimental ones. With the numerical experiments we expand the results of the laboratory ones to the low-gravity environments of agglomerated asteroids as well as to very high pressures, covering a pressure range of 7 orders of magnitude.

These results are relevant to understand the outcomes of impacts in rubble/gravel pile asteroids. They are useful to discuss the feasibility of the kinematic impactor alternative to deflect an incoming hazardous asteroid, as it will be tested in the NASA/DART mission.