Hypervelocity impacts on asteroids and momentum transfer I. Numerical simulations using porous targets

In this paper, we investigate numerically the momentum transferred by impacts of small (artificial) projectiles on asteroids. The study of the momentum transfer efficiency as a function of impact conditions and of the internal structure of an asteroid is crucial for performance assessment of the kinetic impactor concept of deflecting an asteroid from its trajectory. The momentum transfer is characterized by the so-called momentum multiplication factor beta, which has been introduced to define the momentum imparted to an asteroid in terms of the momentum of the impactor. Here we present results of code calculations of the beta factor for porous targets, in which porosity takes the form of microporosity and/or macroporosity. The results of our study using a large range of impact conditions indicate that the momentum multiplication factor beta is small for porous targets even for very high impact velocities (beta < 2 for nu(rmp) <= 15 km/s), which is consistent with published scaling laws and results of laboratory experiments (Holsapple, K.A., Housen, KR. [2012]. Icarus 221, 875-887: Holsapple, K.A., Housen, KR. [2013]. Proceedings of the IAA Planetary Defense Conference 2013, Flagstaff, USA). It is found that both porosity and strength can have a large effect on the amount of transferred momentum and on the scaling of beta with impact velocity. On the other hand, the macroporous inhomogeneities considered here do not have a significant effect on beta. (C) 2013 Elsevier Inc. All rights reserved.