Abstract
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].