Ejecta Mass Estimates from the DART Impact Plume Inferred from LICIACube Images

We report ejecta mass estimates produced by the Double Asteroid Redirection Test impact with Dimorphos, the secondary of the Didymos asteroid binary system. This first demonstration of an asteroid deflection technique was witnessed by the Light Italian Cubesat for Imaging of Asteroids (LICIACube). The LICIACube Unit Key Explorer (LUKE) RGB camera images are used to estimate the ejecta mass, critical to understanding the impact conditions and physical properties of the asteroid. This requires realistic scattering properties for the ejecta particles, which were obtained from model and laboratory scattering analog particles (sizes <1 μm to ∼1 cm). The particle size distribution (PSD) of ejecta is constrained from a series of 18 images acquired at widely separated phase angles (50°.6 to 110°.2) using the size dependence of the analog scattering phase functions. Total plume radiance in the LUKE RGB channels is determined by spatially integrating over the field of view in each image. Plume radiances are then used to retrieve ejecta mass assuming an optically thin plume, giving lower limit estimates of 0.85 to 1.19 × 107 kg. Results indicate that the PSD of ejecta in the nascent plume follows a single power-law coefficient (k ≈ −2.5), rather than the broken power law inferred from later observations. Using a single, high signal-to-noise image, the observed areal mass-brightness relation is extrapolated inward, yielding an increase of ≈77% for ejecta mass in the optically thick inner region. Our ejecta mass estimates are consistent with Dimorphos having weak cohesive strength (<5000 Pa but more likely ∼50 Pa).