On 26 September 2022, NASA's DART spacecraft successfully demonstrated the deflection of a near-Earth object by intentionally impacting Dimorphos, the secondary body of the Didymos binary asteroid system. This historic event marked the first extensive observation of the post-impact dynamics ejecta from a binary asteroid system [1, 2], offering invaluable insight into the behaviour of an impact event in “active” asteroids.Post-impact observations of the system were facilitated by LICIACube, the Hubble and James Webb Space Telescopes, and ground-based observatories, thus, revealing a wide particle size distribution, with some material experiencing temporary orbital capture within the binary system [3]. Additionally, smaller millimetre-sized particles, strongly influenced by solar radiation pressure, were quickly expelled from the system, whereas larger, meter-sized particles remained in orbit for longer periods [4]. These findings highlight the complexity of the ejecta dynamics, including the potential for temporary orbital capture of larger objects.Despite these observations, a systematic probabilistic mapping [5, 6, 7] of post-impact material transfer onto Didymos remains as of yet unexplored.In view of ESA’s Hera mission, launched on October 7, 2024 [8] and due to arrive at Didymos in late 2026, an ideal opportunity arises to perform a comprehensive investigation of particle dynamics within the binary system. Hera’s arrival offers a unique chance not only to survey Didymos and its composition but also to study the long-term orbital capture and evolutionary behaviour of particles within this environment [8]. Moreover, an opportunity to track reaccreted fresh material post-impact.In this work, the dynamics of a perturbed two-body problem is used to provide the first comprehensive probability maps of ejecta re-impacting onto Didymos. The model is formulated in a Didymos-centred inertial frame, incorporating the point-mass gravity of both Didymos and Dimorphos, solar tidal acceleration computed from SPICE kernel ephemerides, eclipse-modulated solar radiation pressure, and third-body solar gravity.In implementing this model, the surface of Dimorphos is discretised into nodes, each launching massless particles at a grid of ejection speeds and five distinct elevation angles (15, 30, 45, 60, and 75 degrees) [6]. Trajectories are integrated utilising a variable-step Adams-Bashforth-Moulton scheme. This study adapts and expands the probabilistic ejecta-transfer methodology previously developed by Chappaz et al. [6] for Mars and its moons, specifically tailoring it to the Didymos-Dimorphos system.From these simulations, the resulting transfer-efficiency maps quantify both the global mass-exchange fraction for the complete DART ejecta population and illustrate how variations in angle and ejection speed shape surface-contamination patterns on Didymos.Unlike Langner et al. [9], who investigated only specific subsets of boulder sizes and velocities using a shock-physics and N-body pipeline to characterize individual re-impact trajectories, this study adopts a fully parametrised probabilistic approach. By covering the entire ejecta parameter space, this research produces high-resolution probability and contamination maps and detailed quantitative estimates of mass transfer. Additionally, the current analysis considers a broader spectrum of perturbations, whereas Langner et al. [9] restrict their model to MacCullagh gravity with minimal solar and tidal effects. Future extensions of this work, if feasible, will incorporate modelling of higher-order gravitational effects, further improving the study’s depth and accuracy.In conclusion, this systematic probabilistic survey contributes to the understanding of ejecta dynamics, enhancing secondary-ejecta hazard assessments for the spacecraft operations, directly supporting ESA's Hera mission objectives, facilitating safer navigation and more effective scientific interpretation of remote-sensing data from the post-impact Didymos environment. AcknowledgementsDr Stefania Soldini was supported by the Medical Research Council UKRI-FLF grant number MR/W009498/1. References[1] J.-Y. Li, M. Hirabayashi, T. L. Farnham, et al, Ejecta from the dart-produced active asteroid dimorphos, Nature 616 (2023) 452–456.[2] A. Graykowski, R. A. Lambert, F. Marchis, et al, Light curves and colours of the ejecta from dimorphos after the dart impact, Nature 616 (2023) 461–464.[3] E. G. Fahnestock, A. F. Cheng, S. Ivanovski, et al, Pre-encounter predictions of dart impact ejecta behavior and observability, The Planetary Science Journal 3 (2022) 206.[4] F. Ferrari, S. D. Raducan, S. Soldini, M. Jutzi, Ejecta formation, early collisional processes, and dynamical evolution after the dart impact on dimorphos, The Planetary Science Journal 3 (2022) 177.[5] D. J. Kessler, Derivation of the collision probability between orbiting objects: The lifetimes of jupiter’s outer moons, Icarus 48 (1981) 39–48.[6] L. Chappaz, H. J. Melosh, M. Vaquero, K. C. Howell, Transfer of impact ejecta material from the surface of mars to phobos and deimos, Astrobiology 13 (2013) 963–980.[7] E. J. Opik, Collision probabilities with the planets and the distribution of interplanetary matter, Proceedings of the Royal Irish Academy. Section A : Mathematical and Phisical Sciences 54 (1951) 165–199.[8] X. Fu, S. Soldini, F. Saveriano, Orbital capture of the dart impact ejecta around the didymos binary asteroid system.[9] K. Langner, E. Martellato, R. Luther, F. Marzari, A. Rossi, Secondary impact debris in the didymos system: what could be observed by hera?, arXiv preprint arXiv:2503.08158 (2025).