HR: 1330h
AN: P52A-0478    [PDF]
TI: A Theory of Impact Cratering in Extremely Low Density Solids and Application to Track Shape Formation in Aerogels
AU: * Dominguez, G
EM: domi@socrates.berkeley.edu
AF: Physics Department, University of California at Berkeley, Department of Physics University of California, Berkeley, CA 94720-7300 United States
AU: * Dominguez, G
EM: domi@socrates.berkeley.edu
AF: Space Science Laboratory, University of California, Space Sciences Laboratory @ Grizzly Peak, Berkeley, CA 94720 United States
AB: Aerogels are superior in their ability to capture partially (if not completely) intact hypervelocity projectiles. The capture of hypervelocity projectiles of modest velocities ($v\sim1-7$ km s$^{-1}$) in ultra low-density solids such as aerogels typically results in the formation of carrot-shaped impact craters. Several sample return missions currently in transit (e.g. Stardust) or in planning use aerogel as a capture medium of hypervelocity dust particles. In addition, several aerogel collector arrays have previously been deployed in Low Earth Orbit (LEO) since the early 1990s. These collectors, in addition to recording carrot shaped tracks, have recorded impact events with crater morphologies that do not have laboratory analogs. The origins of these anomalous tracks (and the micrometeorites that created them) are unknown because of the absence of a theoretical understading of impact cratering in aerogels. In this paper, therefore, I propose and develop a general model for impact cratering of a compactile type in a extremely porous media such as aerogels; my model adopts general arguments that derive shock wave attenuation properties in porous solids and apply these to aerogel. The model proposed here details the relationship between the energy loss of a projectile and impact cavity formation. I empirically test this model by self consistently accounting for the energy loss of projectiles in aerogel using a simple drag model together with a component that accounts for the mechanical strength of the aerogel. I show that this model suitably accounts for the slowing of spherical glass beads shot into aerogels of various densities and at various velocities. I find that the range of 20 $\mu$m sized glass beads fired into 14 mg cm$^{-3}$ and 50 mg cm$^{-3}$ aerogels at hypervelocities is substantially shorter than what one would expect based on previous work with 106 $\mu$m glass beads. An examination of captured projectiles reveals that aerogel aggregation by the projectile is a significant contributor to the anomalous slowing and is responsible for the observation that the range of projectiles captured into aerogel is not a single valued function of the velocity. Together with a simple energy loss model I generated theoretical track shapes and compared these with actual track shapes in 14 mg cm$^{-3}$ and 50 mg cm$^{-3}$ aerogels. The agreement between actual impact craters in aerogel and my model is remarkable given the simplicity of the model. I conclude by discussing implications that these results may have for the Stardust mission and impact cratering on porous asteroids such as Mathilde.
DE: 2100 INTERPLANETARY PHYSICS
DE: 3944 Shock wave experiments
DE: 6015 Dust
DE: 6022 Impact phenomena
DE: 6094 Instruments and techniques
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting