Characterization of DART Ejecta Using 3D Radiative Transfer Modeling and LICIACube Observations

DART (Double Asteroid Redirection Test) was the first mission dedicated to demonstrating an asteroid deflection technique by altering an asteroid’s trajectory through kinetic impact. On September 26, 2022, the DART spacecraft impacted Dimorphos, the secondary component of the Didymos binary asteroid system, generating a substantial ejecta plume. This ejecta affected the momentum transfer and led to a larger-than-expected change in Dimorphos' orbital period, exceeding 33 minutes [1]. The Italian Space Agency’s LICIACube, deployed as a follow-up observer, captured this event with its LEIA and LUKE cameras.

A key parameter of the DART mission is the estimation of the momentum enhancement factor (beta), which quantifies the momentum transfer resulting from the impact. Accurate estimation of beta requires characterization of the DART ejecta and an estimate of the total ejecta mass. To characterize the physical properties of the ejecta and their temporal variations as observed in post-impact LICIACube images,  we have simulated the ejecta plume using  the 3D radiative transfer code Hyperion (https://www.hyperion-rt.org/). Hyperion employs the Monte Carlo technique to simulate photon propagation through a scattering medium, accounting for multiple scattering effects.

A detailed analysis of the ejecta requires consideration of the three-dimensional geometry of the impact event, including the relative positions of Didymos, Dimorphos, LICIACube, and the Sun. The geometric parameters of the ejecta plume were derived for the LUKE image taken 175 seconds post-impact using DART SPICE kernels [2]. In our modeling, we use the recently calibrated images reported in [3]. Dust modeling parameters include the single-scattering albedo, brightness and polarization phase functions, and the cross-sectional properties of dust particles, defined by their size and number density. These were modeled assuming a typical S-type asteroid composition for Dimorphos. We adopted a single-scattering albedo of 0.55 and used brightness and polarization phase functions based on laboratory measurements of various (up to millimeter-sized) silicate grains [4, 5]. The ejecta plume was modeled as a hollow cone, characterized by impact location, tilt, and opening angle. As a starting point for the dust number density distribution within the cone, we used the distribution derived from the mass distribution in numerical impact simulations [6]

Our current best-fit model reproduces the LUKE image taken 175 seconds after impact, with a focus on the near-surface region of Dimorphos. Assuming an azimuthally uniform number density, we successfully simulated the structure of the ejecta plume, including the dark band adjacent to Dimorphos. This feature is likely a shadow cast by one of the optically thick cone walls onto the other. To address inhomogeneity in the ejecta, we used azimuthally averaged brightness values at specific distances from the asteroid. Comparison between observed and modeled brightness profiles provides constraints on the dust particle size distribution and number density, as well as their spatial variation. We show that the observed and modeled brightness profiles follow similar trends, including the dark band near the surface. Fitting the observed radiance values across the plume allows us to refine the number density estimates and derive the mass distribution within the plume.

The developed numerical approach and its results can serve as a template for characterizing dust in active asteroids, providing their dust size distribution, number density, and mass loss.

[1] Cheng, A.F., et al., 2023, Nature, 616(7957), 457-460.

[2] Nair, H. and Costa Sitja, M., 2023. DART SPICE Kernel Archive Bundle. NASA PDS, NAIF p.104.

[3] Farnham, T.L., 2025, LICIACube Calibrated and Merged LUKE Images V1.0. NASA Planetary Data System, https://10.26007/5vhp-pe80

[4] Munoz, O., et al., 2020, ApJ Suppl. Series, 247,19.

[5] Lolachi, R., et al., 2023, PSJ, 4, 24.

[6] Raducan, S. D., et al., 2024, Nature Astronomy, 8(4), 445 -455.