Momentum enhancement due to hypervelocity impacts into pumice

Momentum enhancement refers to the effect of more momentum being transferred to a target than resides in the impactor. This increased momentum transfer is due to crater ejecta leaving the impacted body in the direction opposite to the impactor travel direction. An application for this effect is using hypervelocity impactors to deflect potentially hazardous asteroids and comet nuclei. A concern with historical work with consolidated material is the effect of porosity in the impacted body on momentum enhancement. Hypervelocity impact tests were performed of aluminum spheres striking pumice boulders. The pumice boulders ranged in mass from 70 to 486 kg and had an estimated porosity of 70 - 75%. Two different projectiles diameters were used (2.54 cm and 4.45 cm) to study the effect of size scale. Impacts occurred at roughly 2.1 km/s. Depth of penetration was measured, an estimated crater radius was measured (though the crater geometries were irregular), and momentum enhancement was measured. Significant scatter was observed in the results even though the tests were carefully controlled, with measured momentum enhancements ranging from 1 to 1.69 and the average being β = 1.3.