High-Speed Boulders and the Debris Field in DART Ejecta (2506.16694v1)

Abstract: On 26 September 2022 the Double Asteroid Redirection Test (DART) spacecraft collided with Dimorphos, the moon of the near-Earth asteroid 65803 Didymos, in a full-scale demonstration of a kinetic impactor concept. The companion LICIACube spacecraft documented the aftermath, capturing images of the expansion and evolution of the ejecta from 29 to 243 s after the impact. We present results from our analyses of these observations, including an improved reduction of the data and new absolute calibration, an updated LICIACube trajectory, and a detailed description of the events and phenomena that were recorded throughout the flyby. One notable aspect of the ejecta was the existence of clusters of boulders, up to 3.6 m in radius, that were ejected at speeds up to 52 m/s. Our analysis of the spatial distribution of 104 of these boulders suggests that they are likely the remnants of larger boulders shattered by the DART spacecraft in the first stages of the impact. The amount of momentum contained in these boulders is more than 3 times that of the DART spacecraft, and it is directed primarily to the south, almost perpendicular to the DART trajectory. Recoil of Dimorphos from the ejection of these boulders has the potential to change its orbital plane by up to a degree and to impart a non-principal axis component to its rotation state. Damping timescales for these phenomena are such that the Hera spacecraft, arriving at the system in 2026, should be able to measure these effects.

• The paper presents a detailed analysis of high-speed boulders using LICIACube imaging, revealing their velocities, spatial distribution, and momentum contributions from the DART impact.
• Refined data reduction and calibration techniques, including bias/dark removal and signal enhancement via triplet exposure merging, improved image fidelity.
• 3-parameter parallax measurements provided accurate boulder positions and velocities, offering critical insights into impact physics and potential orbital changes of Dimorphos.

Analysis of High-Speed Boulders in DART Ejecta

This paper (2506.16694) presents an analysis of the ejecta resulting from the Double Asteroid Redirection Test (DART) impact on Dimorphos, focusing on the high-speed boulders observed by the LICIACube spacecraft. The paper details the data reduction and calibration techniques applied to the LICIACube images and provides a comprehensive overview of the flyby, describing the observed events and features. By measuring the positions, velocities, and brightnesses of individual boulders, the research investigates their spatial distribution, momentum contribution, and implications for understanding the impact physics and the overall effectiveness of the kinetic impactor concept.

Data Reduction and Calibration Improvements

The authors reprocessed the LICIACube/LUKE images, employing a refined bias/dark removal process that uses a short exposure image of blank space acquired shortly after the Didymos encounter. This approach mitigated residual vertical striping and background truncation issues present in the calibrated images available from the Planetary Data System (PDS). They also derived independent absolute calibration coefficients using LUKE observations of the spectrophotometric standard star Xi2 Ceti, ensuring consistency with radiance levels obtained from independent observations and DART spacecraft measurements. To optimize image utility, the team merged data from triplets of exposures, maximizing the signal-to-noise ratio (S/N) while minimizing saturated regions.

Flyby Overview and Ejecta Morphology

The LICIACube flyby is divided into three phases: approach, flyby, and departure. During the approach phase (29-140 s post-impact), LICIACube's trajectory was primarily radial, allowing direct measurements of the projected velocities of the early ejecta. Clumps were observed with speeds of 340 m/s and 240 m/s. The flyby phase (140-195 s post-impact) offered the highest resolution images and the best parallax for 3-D analyses. The spacecraft's translational motion revealed the dimensionality of the ejecta as streamers moved in front of Didymos. Post-C/A images revealed a classic ejecta cone with a dark ring at its base, attributed to a dense cloud of ejecta casting a shadow on the material beneath. The departure phase (195-243 s post-impact) provided additional perspectives for evaluating the debris field's structure and evolution.

Boulder Measurement and Analysis

The paper identified and measured 104 individual boulders in multiple images. Parallax measurements were used to determine their absolute positions and velocities relative to Dimorphos. Two approaches were used to derive solutions: a 3-parameter fit, which solves for the position vector and assumes a linear trajectory, and a 6-parameter fit, which solves for both position and velocity vectors. The authors adopted the 3-parameter fit solutions due to the large positional errors associated with the 6-parameter fit. Photometric measurements were used to estimate boulder sizes, with radii ranging from 0.2 to 3.6 meters.

Boulder Distribution and Origins

The spatial distribution of the boulders is non-uniform, with most concentrated in two primary clusters. The first, containing approximately 70% of the measured objects, is a dense cluster located to the south with high ejection speeds (30-50 m/s) and low ejection angles relative to the surface. The second cluster, located to the northeast, is less dense with lower velocities (<20 m/s) and higher ejection angles. Trajectory analysis suggests that the southern cluster may be remnants of the boulder Atabaque, which was struck by a DART solar panel, while the northeastern cluster may be related to the destruction of the boulder Bodhran.

Momentum and Energy Budget Implications

The combined kinetic energy of the 104 measured boulders is estimated to be 1.6 x 10^8 J, representing approximately 1.4% of the DART spacecraft's kinetic energy. The boulders' total momentum is estimated to be 1.1 x 10^7 kg m/s. The momentum component aligned with Dimorphos' velocity vector suggests that these boulders contributed approximately 9% of the momentum that changed the orbital period. The momentum in the X and Z directions could have altered Dimorphos' orbital plane inclination by approximately 0.85 degrees and induced a tumbling state.

Comparison with HST Data

The paper compares its findings with Hubble Space Telescope (HST) observations of boulders surrounding Didymos. The HST-observed boulders exhibit lower velocities (~0.5 m/s) and are likely a different population ejected later in the impact event. The momentum contribution from the HST boulders is estimated to be around 0.2 of the β=3.6 experiment result. The paper finds no evidence to support seismic shaking as a primary ejection mechanism.

Conclusions

This analysis of LICIACube data provides insights into the dynamics of the DART impact ejecta, particularly the high-speed boulders. The boulder measurements reveal unique information about the early impact processes, suggesting that they are remnants of larger boulders shattered by the impact. The momentum contribution of these boulders highlights the importance of considering ejecta in all directions when evaluating the effectiveness of kinetic impactors. The findings suggest that the Hera mission may encounter a Didymos system with Dimorphos in a tumbling state and an orbit inclined to Didymos' equator.