Optical Characterization of the DART Impact Plume: Importance of Realistic Ejecta Scattering Properties

Abstract

The Double Asteroid Redirection Test (DART) mission will impact the moon Dimorphos of the (65803) Didymos binary and demonstrate the kinetic impactor technique for planetary defense. Observations from the ride-along CubeSat companion, LICIACube, of the spatial structure and temporal evolution of the resulting ejecta plume will help determine the vector momentum transfer and constrain physical properties such as strength and porosity. The optical scattering properties of the ejecta particles used in plume simulations will dictate the critical relation between observed brightness and inferred ejecta mass (and momentum). Here we examine the scattering behavior of plausible analogs for Dimorphos ejecta particles, including laboratory-measured “millimeter grains” selected based on a comparison with Bennu particle photometry. Over the range of phase angles observable by LICIACube (120°), brightness simulations for optical depths ranging from 0.01 to 10 show that scattering from these analogs is considerably dimmer than from grains scattering isotropically. Therefore, adopting realistic scattering properties for ejecta particles is critical for accurately interpreting and understanding observations of the DART impact.