Momentum transfer in hypervelocity cratering of meteorites and meteorite analogs: Implications for orbital evolution and kinetic impact deflection of asteroids

Kinetic impact is regarded as an effective way to divert an asteroid on a collision course with the Earth. However, asteroids show a wide diversity in their mineralogy, porosity, and water content. Asteroids range in composition from primitive undifferentiated objects to differentiated objects. Among the undifferentiated asteroids they range from more reflective, carbon-poor bodies to darker, carbonaceous bodies. Porosities of asteroids range from 0 to > 50%, with most > 20%, and some asteroids exhibit a 0.7 mu m water feature in their reflection spectra. Mineralogy, porosity and hydration are each expected to influence the momentum transferred in hypervelocity collisions. We conducted a series of measurements of the post-impact momentum, which is characterized by a factor beta, the ratio of the total linear momentum acquired by the target to the momentum of the impactor. We measured beta for anhydrous meteorites, which sample their asteroidal parent bodies, spanning a wide range of porosities: including 7 samples of the CV3 carbonaceous chondrite Northwest Africa (NWA) 4502 (2.1% porosity), 7 samples of the ordinary chondrite NWA 869 (6.4% porosity), 4 samples of the ordinary chondrite Saratov (15.6% porosity), and, to extend our measurements to higher porosity than is found among meteorites, 2 samples of terrestrial pumice (80% porosity). We also measured beta for hydrous meteorite analog targets: including 2 samples of terrestrial serpentine (17.9% porosity) and 4 samples of terrestrial montmorillonite (51.5% porosity), the two clay minerals that dominate the composition of the hydrous CI carbonaceous chondrite meteorites, as well as 4 samples of hydrous meteorite analog material prepared by powdering and hydrating an anhydrous carbonaceous chondrite. We found that for both the anhydrous and the hydrous samples beta decreased with increasing porosity, consistent with hydrocode modeling. However, the beta values we measured for the similar to 5 cm/s impacts onto these anhydrous samples, with beta = 3.55 for NWA 4502, 2.69 for NWA 869, 2.10 for Saratov, and 2.15 for pumice, are larger than results from hydrocode modeling for 10 km/s impacts into relatively strong, porous rock targets. The beta values for the moderate porosity (17.9%) hydrous serpentine targets (beta = 4.70), the highly porous (51.55%) hydrous montmorillonite targets (beta = 2.79), and the moderately porous CI-analog targets (beta = 2.99) are each significantly larger than the beta value for anhydrous targets of comparable porosities. This is likely due to jetting of water vapor, which could significantly affect the deflection of hydrous asteroids and icy comets in natural or human-induced collisions.