The Double Asteroid Redirection Test (DART) mission is the world’s first planetary defense mission. Reaching the binary (65803) Didymos-Dimorphos asteroid system in late September or early October 2022, it aims to change the orbit of the secondary member, Dimorphos, through kinetic impact deflection. The spacecraft will hit the 160 m diameter Dimorphos at a speed of approximately 6 km/s with the objective of changing its orbital period about Didymos by at least 73 s and creating an impact ejecta plume in the process. These events will be observed both from Earth and by its ride-along companion SmallSat, LICIACube. These observations will be used to determine and understand the momentum transfer efficiency of the impact [1]. The resulting plume properties, including ejecta momentum and consequently momentum transfer efficiency are controlled by several global factors related to the asteroid material: strength, porosit, cohesiveness, and internal structure (e.g., is it a “rubble pile”? is there a regolith layer present?) [2,3]. However, factors local to the impact site can also play a major role. For instance, the value for transfer efficiency can change dramatically depending on whether DART impacts into a boulder or regolith [4]. One method of characterizing the impact ejecta is via optical observations of the evolving impact plume brightness coupled with radiative transfer reconstructions of sunlight scattering by ejecta particles. This approach can give information about composition, and the developing spatial and mass distributions of ejecta material. Using radiative transfer models to analyze and reconstruct an impact plume has a precedent. Previously, simulations were conducted using results from the Deep Impact mission in order reconstruct the plume 1 s after impact in order to analyze its composition [5]. For DART, an initial radiative transfer prediction study of the LICIACube flyby observations was carried out by the mission team [1]. Estimates for geometric optical depth of the impact plume, as well as order-of-magnitude approximations for plume surface brightness were made, consistent with the measured Didymos geometric albedo of 0.15 [6]. These estimates were made assuming large, isolated plume particles, i.e., extinction coefficient of ∼2, an assumed isotropic phase function and single scattering. Unfortunately, if the same methodology is applied to reconstructions of the actual plume observations it is likely to result in large radiance differences and misinterpretation of ejecta properties. This is because it is vital to any such modelling effort to have a realistic treatment of the plume particle scattering properties, as well as the effects of large optical depth. Using the flyby geometry of the study [1] we have performed our own reconstructions of the DART impact ejecta plume observations combining a 3D plume geometry, realistic phase function and the multiple-scattering radiative transfer software DISORT [7].
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