Characterization of the DART Impact Ejecta Plume on Dimorphos from LICIACube Observations

Abstract

We modeled the geometry and the three-dimensional orientation of the ejecta cone triggered by the impact of the DART spacecraft on the asteroid Dimorphos. We used eight LUKE images of the impact acquired by the CubeSat LICIACube that flew by the Didymos system shortly after the impact. These images, which show the ejecta cone in both face-on and side-on profiles, enabled us to reconstruct the ejecta cone in inertial space. We started our model as a simple cone with a circular base and developed it to a rotated cone with an elliptical base that best fit the data. The cone axis points to R.A., decl. (in J2000): 147 − 10 ° + 1 ° , + 16 − 6 ° + 4 ° . The cone is characterized by two perpendicular half-angles of η = 69 − 3 ° + 1 ° , γ = 51 − 11 ° + 1 ° and a rotation of ω = 12° around its axis. The apex of the cone is located near the center of Dimorphos within 15 m. The intersection of the cone and the surface of Dimorphos (surface enclosed by the cone) would correspond to a crater with a maximum radius of about 65 m. The characterization of the cone axis is directly related to the computation of the momentum enhancement factor (β) of the impact, and it hence proves the crucial need of studying impacts in the context of planetary defence scenarios. The results of this work could potentially be used to constrain whether the impact took place in a strength-dominated or a gravity-dominated regime. This work shows the important scientific return of the LICIACube CubeSat in the context of planetary defence.

Achievement of the Planetary Defense Investigations of the Double Asteroid Redirection Test (DART) Mission

“…The wide opening angle of the ejecta cone viewed by LICIACube in the minutes following impact was confirmed in these telescopic observations. An analysis that combined the LICIACube and HST observations and accounted for the different viewing conditions of each also showed an elliptical ejecta plume (Hirabayashi et al 2024), with cone geometry results similar to those obtained by analyzing only LICIACube images (Deshapriya et al 2023). Ground-based telescopes and HST also obtained views of the evolution of the ejecta over its first few hours, evolving from a cone to the initial indications of a tail of material leaving the Didymos system within a few hours of DARTʼs impact (Li et al 2023;Murphy et al 2023;Opitom et al 2023;Rożek et al 2023;Lister et al 2024).…”

“…Analysis of the LICIACube images shows a wide cone of ejecta, with an opening angle of ∼140° (Dotto et al 2024). Further analysis of LICIACube images have shown that the geometry of the ejecta plume is an elliptical cone that can be described by two angles, a wide opening angle of about 138°and a narrow opening angle of about 102° (Deshapriya et al 2023). Additionally, the intersection of the derived ejecta cone with the surface of Dimorphos corresponds to a region roughly 65 m in radius, which is a substantial portion of the impact hemisphere of Dimorphos (Deshapriya et al 2023).…”

“…The Dimorphos rotational lightcurve is also suggestive of Dimorphos having different post-impact axial ratios (Pravec et al 2023), outside of the uncertainties associated with the axial ratios that were determined using the pre-impact DRACO images (Daly et al 2024). That the intersection of the derived ejecta cone with the surface of Dimorphos corresponds to a region roughly 65 m in radius also suggests large-scale modification of Dimorphos' surface (Deshapriya et al 2023). Different post-impact axial ratios for Dimorphos are evidence of the creation of a large crater or reshaping of the body due to DARTʼs impact.…”

“…A Monte Carlo approach was used to produce a distribution of velocity changes that were consistent with the period change determined by the ground-based observations (Thomas et al 2023a) and that accounted for uncertainties in the Didymos-system parameters (Cheng et al 2023). The net ejecta direction was constrained using observations from Hubble Space Telescope (Li et al 2023) and LICIACube images (Dotto et al 2024), which showed the net ejecta direction to be opposite of DARTʼs incoming trajectory to within roughly 20° (Cheng et al 2023;Deshapriya et al 2023;Hirabayashi et al 2024). The net ejecta direction and Dimorphos' orbital velocity direction were thus roughly the same, and hence, β was calculated in the along-track direction (Cheng et al 2023).…”

“…Further analysis of LICIACube images have shown that the geometry of the ejecta plume is an elliptical cone that can be described by two angles, a wide opening angle of about 138°and a narrow opening angle of about 102° (Deshapriya et al 2023). Additionally, the intersection of the derived ejecta cone with the surface of Dimorphos corresponds to a region roughly 65 m in radius, which is a substantial portion of the impact hemisphere of Dimorphos (Deshapriya et al 2023). Examination of the RGB images returned by LUKE suggests that the ejecta plume became spectrally redder with increased distance from Dimorphos, which is attributed to either the smaller size of the dust grains in the outer ejecta or less altered material excavated from Dimorphos' subsurface (Dotto et al 2024).…”