  The NASA Double Asteroid Redirection Test (DART) spacecraft will impact the
secondary member of the [65803] Didymos binary in order to perform the first
demonstration of asteroid deflection by kinetic impact. Determination of the
momentum transfer to the target body from the kinetic impact is a primary
planetary defense objective, using ground-based telescopic observations of the
orbital period change of Didymos and imaging of the DART impact ejecta plume by
the LICIACube cubesat, along with modeling and simulation of the DART impact.
LICIACube, contributed by the Italian Space Agency, will perform a flyby of
Didymos a few minutes after the DART impact, to resolve the ejecta plume
spatial structure and to study the temporal evolution. LICIACube ejecta plume
images will help determine the vector momentum transfer from the DART impact,
by determining or constraining the direction and the magnitude of the momentum
carried by ejecta. A model is developed for the impact ejecta plume optical
depth, using a point source scaling model of the DART impact. The model is
applied to expected LICIACube plume images and shows how plume images enable
characterization of the ejecta mass versus velocity distribution. The ejecta
plume structure, as it evolves over time, is determined by the amount of ejecta
that has reached a given altitude at a given time. The evolution of the plume
optical depth profiles determined from LICIACube images can distinguish between
strength-controlled and gravity-controlled impacts, by distinguishing the
respective mass versus velocity distributions. LICIACube plume images
discriminate the differences in plume structure and evolution that result from
different target physical properties, mainly strength and porosity, thereby
allowing inference of these properties to improve the determination of momentum
transfer.
