The Dimorphos ejecta plume properties revealed by LICIACube

The Double Asteroid Redirection Test (DART) mission, a planetary defense experiment, successfully impacted the asteroid Dimorphos on September 26, 2022. This event marked a significant milestone in our ability to potentially deflect hazardous near-Earth objects. The primary objective of DART was to alter Dimorphos's orbital period around its parent asteroid, Didymos. However, the impact also generated a massive ejecta plume, offering a unique opportunity to study the physical properties of the asteroid and the impact process itself. Prior to the DART impact, various models predicted the ejecta characteristics based on theoretical simulations and observations of other impact events, but direct observational data from the impact itself was limited. Understanding the properties of the ejecta plume – its size, shape, composition, and velocity distribution – is crucial for refining our models of asteroid impacts and improving our ability to predict the outcomes of future deflection missions. This research leverages the unique perspective provided by the Light Italian Cubesat for Imaging of Asteroids (LICIACube), a small satellite deployed by DART to capture high-resolution images of the impact event and its immediate aftermath.

Previous studies using ground-based telescopes and the Hubble Space Telescope (HST) provided valuable information on the post-impact brightness of the Didymos system and the general evolution of the ejecta plume. These observations showed a significant increase in brightness following the impact due to the scattering of sunlight by the ejected material. Graykowski et al. (2023) characterized the light curves and colors of the ejecta, while Li et al. (2023) investigated the complex temporal evolution of the plume using HST data. Studies by Thomas et al. (2023) and Cheng et al. (2023) focused on determining the momentum transfer and the resulting change in Dimorphos's orbital period. However, these studies lacked the high-resolution spatial details achievable through LICIACube's close-range observation of the plume. The data from LICIACube provided a complementary perspective to these ground-based and HST observations, offering unprecedented insights into the detailed structure and dynamics of the ejecta plume.

The LICIACube mission deployed two imaging instruments: the LICIACube Explorer Imaging for Asteroid (LEIA) and the LICIACube Unit Key Explorer (LUKE). LEIA acquired images from approximately 1000km from Dimorphos 5 seconds before impact to 320 seconds after. LUKE started image acquisition 29 seconds after the impact. Both instruments captured a sequence of images, allowing for tracking of the plume's evolution. The study analyzed a subset of these images, focusing on both pre- and post-closest approach (CA) geometries. Geometric considerations were used to determine the axis and aperture angle of the ejecta cone, based on the assumption of axisymmetry. Color information was obtained using RGB filters, allowing the analysis of color variations within the plume. Image processing techniques, including computer vision algorithms, were used to measure velocities of distinct morphological features within the ejecta plume (filaments, clumps, boulders). The analysis involved measuring pixel displacements between consecutive images and converting them to real-world velocities. Corrections were applied to account for perspective effects, and uncertainties were quantified. Analysis of the flux ratios in different color channels (red/blue, green/blue) allowed the characterization of spectral variations within the ejecta plume.

LICIACube's observations revealed a cone-shaped ejecta plume with an aperture angle of 140 ± 4 degrees. The plume's inner region was distinctly blue, gradually transitioning to redder hues with increasing distance from Dimorphos. This color variation suggests potential differences in particle size distribution and/or composition within the plume, with the inner region possibly dominated by smaller, blue-colored dust grains and the outer region containing larger, redder particles. The plume exhibited a complex, inhomogeneous structure, characterized by numerous filaments, dust grains, and individual or clustered boulders. Measurements of ejected material velocities revealed a broad range, from a few tens of meters per second to as high as approximately 500 meters per second, with the fastest velocities observed in the most distant structures. Notably, these high-velocity structures were significantly faster than those observed by the Hubble Space Telescope (HST). The inner region of the ejecta, within 250 meters of Dimorphos, revealed 18 main filaments, and tracking several of these filaments showed radial velocities of 47-75 m/s. These findings suggest that the ejecta plume resulted from a combination of different ejection processes. Analysis also estimated the non-illuminated cross-sectional area of Dimorphos' non-impacted hemisphere to be around 5300 m² (with uncertainty of ~200m²).

The findings from LICIACube provide valuable constraints on models of asteroid impacts and ejecta dynamics. The wide aperture angle of the ejecta plume suggests a significant amount of material was ejected at high velocities, consistent with the momentum transfer observed in the orbital period change of Dimorphos. The color variation within the plume supports the hypothesis that different particle sizes and compositions were ejected at different velocities. The high-velocity structures observed by LICIACube corroborate findings from HST, but add crucial spatial resolution. The data provides crucial constraints on the physical properties of Dimorphos, and the findings will inform future impact modeling efforts, significantly improving the accuracy of predictions for future planetary defense missions.

LICIACube's close-range observations of the DART impact on Dimorphos provided detailed insights into the ejecta plume's morphology, color variations, and velocity distribution. The findings indicate a complex interplay between multiple ejection mechanisms, resulting in a wide range of particle sizes and velocities. The high-resolution data significantly enhance our understanding of asteroid impacts and their consequences. Further research should explore the detailed physical and chemical processes involved in the ejecta plume formation, and investigate the long-term evolution of the ejected material and its interaction with the surrounding environment. LICIACube's success highlights the potential of CubeSats as valuable tools for planetary science missions.

The analysis relies on the assumption of axisymmetry of the ejecta cone. This simplification might not fully capture the actual three-dimensional structure of the plume, especially given the presence of inhomogeneities and filaments. The velocity measurements are based on projected velocities onto the LICIACube's field of view, which introduces uncertainties that are accounted for in the estimates, but these uncertainties should be considered when interpreting the absolute velocity values.