Impacts play a fundamental role in shaping the physical and chemical properties of the objects in our Solar System. Given the challenges in replicating such collisions through laboratory experiments, computer simulations are an important tool to investigate their outcomes. Accurately modelling material properties such as shear strength, porosity, and the formation of cracks is crucial for understanding impacts on small bodies like asteroids and comets. Very large and massive objects are dominated by self-gravity and can be approximated as a fluid. In this regime the equation of state used to model the behaviour of the constituent materials plays a key role. However, for bodies of several hundred kilometres, which are already spheroidal due to self-gravity, shear strength must still be considered. This impact regime is most challenging to model and therefore often overlooked in publications. In this review we present different impact regimes and the relevant physics that must be included. We then discuss their application to a variety of Solar System objects and assess how recent observations and numerical simulations, focussing on the Smoothed Particle Hydrodynamics method, can be used to inform our understanding of impact processes and solar system formation.

The NASA Double Asteroid Redirection Test (DART) mission successfully changed the trajectory of asteroid Dimorphos by hypervelocity kinetic impact. The spacecraft imaged a boulder-strewn surface, yet the asteroid's internal structure remains unknown. To understand how boulders surrounding the impact point influence ejecta and momentum transfer, 3D impact simulations were performed using the isale shock physics code, varying the number, size, burial depth, and radial distance of proximal boulders. Results show that boulders positioned off-centre, adjacent to the impact site, can locally suppress or redirect ejecta, producing peripheral effects on the momentum enhancement factor, beta, with net variations within 8 per cent compared to the no-boulder case. These effects are confined to a radial zone extending up to around 10 meters (around 16 impactor radii) from the impact point, within which proximal boulders can efficiently govern ejecta and momentum transfer. Outside this zone, the influence of boulders on deflection efficiency becomes negligible. This underscores the limited influence of boulders surrounding the impact site and points to the dominant role of those within the projectile-target coupling zone, offering insights for planetary defense applications. To probe impact ejecta over extended time-scales, axisymmetric 2D simulations were conducted with a mass-strength rescaling that conserves boulder mass density and strength per boulder 'ring'. This reproduces beta trends in 3D while enabling longer 2D simulations of boulder-rich rubble-pile targets in spheroidal volumes. Simulated beta values remain close to the observed range, suggesting plausible interior solutions for Dimorphos and supporting efforts to probe its interior ahead of ESA's Hera mission.

The successful impact of the Double Asteroid Redirection Test (DART) spacecraft on Dimorphos enabled the first-ever extensive observation of a postimpact ejecta tail from a binary asteroid system. Studying the ejecta can provide insights into impact physics and asteroid composition and inform future asteroid missions. In this research, the orbital capture of the impact ejecta around the Didymos binary asteroid system is investigated. The ejecta dynamics are described using an augmented bicircular restricted four-body model, which incorporates the binary’s irregular gravity field and solar radiation pressure (SRP) acceleration. Typical periodic orbit (PO) families, including planar Lyapunov and terminator orbits, are selected as the candidates for capture analysis. The candidate POs are perturbed and backward-propagated using the invariant manifold theory, and eligible trajectories intersecting the asteroids’ surfaces are recorded. The ejecta characteristics of different POs are summarized from three principal aspects: ejection location, ejection velocity, and ejection angle. The influence of two critical factors is qualitatively assessed, including the geometry of the asteroids in the binary system and the strength of SRP acceleration. Lastly, the likelihood for the ejecta from the DART impact being transferred to candidate POs is assessed.

Kinetic deflection is a planetary defense technique delivering spacecraft momentum to a small body to deviate its course from Earth. The deflection efficiency depends on the impactor and target. Among them, the contribution of global curvature was poorly understood. The ejecta plume created by NASA's Double Asteroid Redirection Test impact on its target asteroid, Dimorphos, exhibited an elliptical shape almost aligned along its north-south direction. Here, we identify that this elliptical ejecta plume resulted from the target's curvature, reducing the momentum transfer to 44 ± 10% along the orbit track compared to an equivalent impact on a flat target. We also find lower kinetic deflection of impacts on smaller near-Earth objects due to higher curvature. A solution to mitigate low deflection efficiency is to apply multiple low-energy impactors rather than a single high-energy impactor. Rapid reconnaissance to acquire a target's properties before deflection enables determining the proper locations and timing of impacts.

On 2022 September 26, the Double Asteroid Redirection Test (DART) spacecraft intentionally collided with Dimorphos, the moon of the binary asteroid system 65803 Didymos. This collision provided the first full-scale test of a kinetic impactor for planetary defense. Images from DART’s DRACO camera revealed Dimorphos to be an oblate spheroid covered in boulders of varying sizes and shapes. Very little was known about Dimorphos prior to DART’s impact, including its shape, structure, and material properties. Approach observations and those following the DART impact have provided crucial knowledge that narrows the parameter space relevant to modeling the impact into Dimorphos. Here we present the results of a suite of hydrocode simulations of the DART impact on Dimorphos. Despite remaining uncertainties, initial models of DART’s kinetic impact provide important information about the results of DART (e.g., potential crater size and morphology, ejecta mass) and the properties of Dimorphos. Simulations here suggest that Dimorphos has near-surface strength ranging from a few Pascals to tens of kPa, which corresponds to crater sizes of ∼40–60 m. Simulated crater sizes provide a crucial comparison metric for the European Space Agency Hera mission when it arrives at the Didymos system. Hera’s measurement of crater size in combination with measurement of Dimorphos’s mass will allow us to assess our simulations and provide the information needed to make the DART impact experiment both the first test of a planetary defense mitigation mission and the first full-scale planetary defense simulation validation exercise.

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.

Abstract Hypervelocity impacts play a significant role in the evolution of asteroids, causing material to be ejected and partially reaccreted. However, the dynamics and evolution of ejected material in a binary asteroid system have never been observed directly. Observations of Double Asteroid Redirection Test (DART) impact on asteroid Dimorphos have revealed features on a scale of thousands of kilometers, including curved ejecta streams and a tail bifurcation originating from the Didymos system. Here we show that these features result naturally from the dynamical interaction of the ejecta with the binary system and solar radiation pressure. These mechanisms may be used to constrain the orbit of a secondary body, or to investigate the binary nature of an asteroid. Also, they may reveal breakup or fission events in active asteroids, and help determine the asteroid’s properties following an impact event. In the case of DART, our findings suggest that Dimorphos is a very weak, rubble-pile asteroid, with an ejecta mass estimated to be in the range of (1.1-5.5)×107 kg.

We present a series of numerical simulations using a shock physics smoothed particle hydrodynamics code, investigating energetic impacts on small celestial bodies characterized by diverse internal structures, ranging from weak and homogeneous compositions to rubble-pile structures with varying boulder volume packing. Our findings reveal that the internal structure of these rubble-pile bodies significantly influences the impact outcomes. Specifically, we observe that the same impact energy can either catastrophically disrupt a target with a low boulder packing (≲30 vol%), or result in the ejection of only a small fraction of material from a target with the same mass but high boulder packing (≳40 vol%). This finding highlights the pivotal role played by the rubble-pile structure, effectively acting as a bulk shear strength, which governs the size and behavior of the resulting impact. Consequently, understanding and characterizing the internal structure of asteroids will be of paramount importance for any future efforts to deflect or disrupt an asteroid on a collision course with Earth.

Kinetic deflection is a planetary defense technique that delivers spacecraft momentum to a small body to deviate its course from Earth. The deflection efficiency depends strongly on the impactor and target. Among them, the contribution of global curvature was poorly understood. The ejecta plume created by NASA's DART impact on its target asteroid, Dimorphos, exhibited an elliptical shape almost aligned along its north-south direction. Here, we identify that this elliptical ejecta plume resulted from the target’s curvature, reducing the momentum transfer to 44±10% along the orbit track compared to an equivalent impact on a flat target. We also find lower kinetic deflection of impacts on smaller Near-Earth objects (NEOs) due to higher curvature. A solution to mitigate low deflection efficiency is to apply multiple low-energy impactors rather than a single high-energy impactor. Rapid reconnaissance to acquire a target's properties before deflection enables determining the proper locations and timing of impacts.

Spacecraft observations revealed that rocks on carbonaceous asteroids, which constitute the most numerous class by composition, can develop millimeter-to-meter-scale fractures due to thermal stresses. However, signatures of this process on the second-most populous group of asteroids, the S-complex, have been poorly constrained. Here, we report observations of boulders' fractures on Dimorphos, which is the moonlet of the S-complex asteroid (65803) Didymos, the target of NASA's Double Asteroid Redirection Test (DART) planetary defense mission. We show that the size-frequency distribution and orientation of the mapped fractures are consistent with formation through thermal fatigue. The fractures' preferential orientation supports that these have originated in situ on Dimorphos boulders and not on Didymos boulders later transferred to Dimorphos. Based on our model of the fracture propagation, we propose that thermal fatigue on rocks exposed on the surface of S-type asteroids can form shallow, horizontally propagating fractures in much shorter timescales (100 kyr) than in the direction normal to the boulder surface (order of Myrs). The presence of boulder fields affected by thermal fracturing on near-Earth asteroid surfaces may contribute to an enhancement in the ejected mass and momentum from kinetic impactors when deflecting asteroids.

The detailed observations of the boulder populations on rubble-pile asteroid surfaces such as Ryugu, Bennu or Dimorphos provide insights into their physical properties and the evolution of their host asteroids [1]. Asteroid surfaces evolve through impacts and thermal processing – both mechanisms that may lead to boulder fragmentation (Figure 1). Figure 1: a) - c) Images acquired by the DART spacecraft approaching the surface of Dimorphos. Boulders which may have originated as a result of an impact event on the asteroid surface are highlighted in red (NASA/Johns Hopkins APL). Here, we present numerical simulations using the Bern SPH code [2] to study the impact fragmentation of boulders on asteroid surfaces. Before applying our models to asteroid scales, we successfully validated them by comparing the results with boulder disruptions observed in laboratory impact experiments [3].In scenarios of impacts on rubble-pile asteroids, our Bern SPH simulations include the displacement and late-stage evolution of the boulder fragments, using a recently developed approach [4,5]. This allows us to determine their final position on the asteroid surface or if they are ejected from the asteroid. Our initial results suggest that only for boulders with a relatively low bulk tensile strength (

The NASA Double Asteroid Redirection Test (DART) spacecraft impacted the moon Dimorphos of the [65803] Didymos binary system and changed the binary orbit period, demonstrating asteroid deflection by a kinetic impact and indicating that more momentum was transferred to Dimorphos by escaping impact ejecta than was incident with DART. Images of the DART impact ejecta plume were obtained by the Light Italian cubesat for Imaging of Asteroids (LICIACube) in the first few minutes after the DART impact. The ejecta plume imaged by LICIACube 158 s after the DART impact prior to closest approach shows no evidence for plume clearing at low altitude. The ejecta plume imaged 175 s after the DART impact is optically thick up to projected altitudes of 200 m above the surface of Dimorphos. These observations are compared with models of the impact ejecta plume optical depth, structure, and evolution, which are developed from point-source scaling models fitted to numerical simulations of the DART impact into a rubble pile Dimorphos with different material strengths. The observations of the impact plume optical depth and the high momentum transfer from the DART impact are not consistent with impact and ejecta plume models assuming the Dimorphos cohesive strength to be as high as 5000 Pa. Models with 5 and 50 Pa Dimorphos cohesive strength provide the overall best consistency with plume opacity observations and high momentum transfer.

Introduction: NASA’s Double Asteroid Redirection Test (DART) aims to test a controlled deflection of a Near-Earth asteroid, by impacting the smaller component of the 65803 Didymos asteroid system, Dimorphos. ESA’s Hera mission [1] will arrive at Dimorphos several years after the DART impact and will perform detailed characterisation of Dimorphos and of the impact outcome. Past studies of the DART impact outcome [e.g., 2–4] have shown that the amount by which Dimorphos can be deflected is strongly dependent on its surface, subsurface and internal properties. Moreover, [4] showed that in order to validate the predictive capabilities of our numerical models and to understand and ultimately reproduce the asteroid deflection technique, we need a measure of both the impact deflection efficiency (often referred to in terms of β, where β = change in momentum of the asteroid/impactor momentum) and the crater size and morphology. Small asteroids, of less than ~10 km in diameter, are believed to be rubble-pile objects, aggregates held together only by self-gravity or small cohesive forces [5]. Moreover, recent results of the SCI impact on Ryugu [6] inferred that at least the near-surface of the asteroid may not be dominated by strength and impact events are controlled to a large extent by gravity, despite its very low value. These findings might also be applicable to Dimorphos. Studies by [7] suggest that for a low cohesion target, an impact on the same magnitude as DART might have a large enough specific impact energy to cause global deformation of the target. Here we numerically simulate DART-like impacts on weak homogeneous and rubble-pile asteroids that use realistic material models and aim to evaluate the resulting target morphology and the global shape change caused by such impacts. Additionally, we aim to quantify the deflection efficiency, given different target scenarios.Numerical Model: Here we use Bern’s parallel Smooth Particle Hydrodynamics (SPH) impact code [8, 9] to model DART-like impacts (500 kg projectiles at 6 km/s) on spherical asteroid targets, and track the evolution of the target for up to 2 hours after the impact. Bern’s SPH code has been previously validated against laboratory experiments and benchmarked against other codes [e.g., 10, 11]. Recently, the code has been validated against laboratory experiments of impacts into heterogeneous targets [12]. To quantify the effects of the target properties and structures on the post-impact morphology, degree of shape change and on the momentum transfer efficiency, we considered the following target scenarios: 1) Homogeneous spherical targets with low cohesion (Y0 = 0 to 50 Pa) and varying coefficient of internal friction (f = 0.4 to 1.0); To describe the shear response, the target material was modelled using a simple pressure-dependent strength model typical of pre-damaged rock materials [13]. The initial target porosity was kept constant at 40%. 2) Rubble-pile spherical targets with three different distributions of boulders, embedded into a cohesionless matrix material: 2a) grid distribution of 2.5 m in diameter boulders, with one boulder diameter spacing between each boulder in the x, y, and z directions; 2b) random distribution of 2.5 m boulders; 2c) Random distribution of boulders with random sizes between 2 and 10 m. In all rubble-pile target scenarios, the boulders had a tensile strength of YT = 1 MPa and the matrix material was modelled the same as in (1), with Y0 = 0 Pa and f = 0.6.Results and discussion:Homogeneous asteroids: The size and morphology of the DART crater is of paramount importance for determining the asteroid’s near-surface properties and structure. In the strength dominated impact scenarios, the cohesion is the dominant force that stops the crater cavity from growing. Therefore, with decreasing target cohesion, more material is displaced or gets ejected above escape speed. We found that for homogeneous asteroids, impacts into targets stronger than Y0~10 Pa create well defined bowl-shaped craters, while impacts into weaker targets create morphologies that do not resemble an impact crater anymore (e.g., Fig. 2, [14]). For these scenarios the curvature of the target also plays a major role. Fig. 1 shows the momentum enhancement factor, β, as a function of target cohesion, for four different coefficients of internal friction. Our results suggest that for a low friction, cohesionless target, the momentum enhancement can be as high as 6. Rubble-pile asteroids: Fig. 2 shows the final target morphologies from DART-like impacts into a (1) cohesionless homogeneous target and (2a, b, c) rubble-pile targets with different boulder size-frequency distributions. In all four impact scenarios, the DART impact produced target morphologies that are dissimilar to cratering and changed the global morphology of the asteroid. Initial simulation results revealed that the presence of boulders in the target can decrease the deflection efficiency by up to 40% compared to an impact into a homogeneous target. At the same time, the initial boulder size-frequency distribution on the surface of the asteroid seems to also influence β. Conclusions: The DART impact on cohesionless spherical bodies is likely to produce morphologies that are dissimilar to cratering and change the global morphology of the asteroid. Our modelling results together with future Hera mission observations will provide constraints regarding the evolution of the shapes and structures of small asteroids. Future work will investigate impact events into low-cohesion asteroids with more diverse asteroid shapes and boulder distributions. Acknowledgements: This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 870377.References: [1]Michel, P. et al. (2018) Adv Space Res. 62:2261–2272. [2]Jutzi, M. & Michel, P. (2014) Icarus, 229:247–253. [3]Raducan, S.D. et al. (2019) Icarus, 329:282–295. [4]Raducan, S.D. et al. (2020) Planet. and Space Sci. 180:104756. [5]Richardson, D.C. et al. (2002) Asteroids III, 501–515. [6]Arakawa, M. et al. (2020) Science, 368:67–71. [7]Jutzi, M. et al. (2017) A&A, 597:A61. [8]Jutzi, M. et al. (2008) Icarus, 198:242–255. [9]Jutzi, M. (2015) Planet. Space Sci, 107:3–9. [10]Jutzi, M. et al. (2009) Icarus, 201:802–813. [11]Raducan, S.D. et al. (2021) LPSC,1908. [12]Ormö, J. et al. (2021) LPSC,1965. [13]Collins, G.S. et al. (2004) Meteorit. Planet. Sci, 39:217–231. [14]Raducan, S.D. & Jutzi, M. (2021) LPSC,1900.

Asteroids are the building blocks of the Solar System and gaining insights into their mechanical characteristics and internal structures is not only key to understanding the Solar System formation, but also essential in safeguarding Earth from potential asteroid-related hazards. Recent space missions to small asteroids, including JAXA’s Hayabusa2, NASA’s OSIRIS-Rex and DART [1] have revealed that cratering events on these bodies occur in a regime that is not yet fully understood, where the interplay of low gravity and material strength (cohesion) influences the outcomes. The craters on asteroid surfaces are key to understanding their surface properties and evolutionary history. Impact simulations using so-called shock physics codes have been previously used to determine the target properties on asteroids based on observed craters [2]. When validated against laboratory experiments, these models become invaluable for interpreting the history of asteroids. However, the low-gravity, low-strength conditions on rubble-pile asteroids pose significant challenges for both experimental investigations and numerical modelling. In this regime, the craters formed can grow to approximately a hundred times the size of the impacting projectile over extended periods, needing considerable computational resources to accurately simulate the impact physics and replicate the final crater. Our recent work introduces a novel approach that directly employs shock physics code calculations to model the entire process of impacts in this challenging regime. This method has shown promise, notably in replicating the SCI artificial impact experiment conducted by Hayabusa2 on asteroid Ryugu [3], which helped determine the asteroid's surface mechanical properties2. However, to enhance the credibility and accuracy of our simulations, further validation through detailed laboratory impact experiments is essential.This research focuses on the effect of gravity on the size and shape of impact craters on rubble-pile asteroids. We used the Bern SPH [4] shock physics codes to model the outcomes of recent laboratory cratering experiments conducted under simulated low-gravity environments and performed at the Institute of Space and Astronautical Science (ISAS) in Japan [5]. By validating our numerical simulations with laboratory experiments, we aim to deepen our understanding of the physical processes involved in low-gravity impacts, including the interaction between particles and the material's crushing behaviour.The findings of this study will aid in interpreting the cratering history on asteroids Didymos and Dimorphos, targets of the upcoming ESA Hera mission in late 2026 [6]. This work is part of the Hera Impact Physics Working Group (IWG) and aims to develop new modelling strategies by integrating different numerical codes for simulating cratering on small, rubble-pile asteroids. The results from this research, and other benchmark and validation studies carried out within the Hera IWG [e.g., 7], will provide quantitative and reliable predictions about impact outcomes, which can be measured using spaceborne and in-situ instruments by the Hera mission. Acknowledgements: S.D.R. and M.J. acknowledge support from the Swiss National Science Foundation (project number 200021_207359). S.D.R. gratefully acknowledges the support received from the Swiss Society for Astrophysics and Astronomy (SSAA) MERAC Travel Award. M. K. and A. M. N. acknowledge support by JSPS KAKENHI (grant number, JP21H01148 and JP24K17116).References: [1] Daly, R. T. et al. Successful Kinetic Impact into an Asteroid for Planetary Defense. Nature 1–3 (2023) doi:10.1038/s41586-023-05810-5[2] Jutzi, M., Raducan, S. D., Zhang, Y., Michel, P. & Arakawa, M. Constraining surface properties of asteroid (162173) Ryugu from numerical simulations of Hayabusa2 mission impact experiment. Nat. Commun. 13, 7134 (2022).[3] Arakawa, M. et al. An artificial impact on the asteroid 162173 Ryugu formed a crater in the gravity-dominated regime. Science (2020) doi:10.1126/science.aaz1701.[4] Jutzi, M., Benz, W. & Michel, P. Numerical simulations of impacts involving porous bodies: I. Implementing sub-resolution porosity in a 3D SPH Hydrocode. Icarus 198, 242–255 (2008).[5] Kiuchi, M. et al. Impact experiments on granular materials under low gravity: Effects of cohesive strength, internal friction, and porosity of particle layers on crater size. Icarus 404, 115685 (2023).[6] Michel, P. et al. The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos. Sci. J. 3, 160 (2022).[7] Luther R., et al. Impact Experiments and Model Validation in the frame of the Hera mission. EPSC (2024).

Impacts into layered targets may generate "concentric craters" where a wider outer crater in the top layer surrounds a smaller, nested crater in the basement, which itself may be complex or simple. The influence of target on cratering depends on the ratio of target strength to lithostatic stress, which, in turn, is affected by gravity, target density, and crater diameter. When this ratio is large, the crater size is primarily determined by target strength, whereas gravitational forces dominate when the ratio is small. In two-layer targets, strength may dominate in one or both layers, whereby the outer crater develops in the weaker top layer and the nested crater in the stronger substrate. However, large natural craters that should be gravity-dominated in both cover strata and substrate may be concentric, the reasons for which are not yet fully understood. We performed qualitative impact experiments at 10-502 G and 1.8 km/s with the Boeing Corp. Hypervelocity centrifuge gun, and at 1 G and 0.4 km/s with the CAB CSIC-INTA gas gun into layered sand targets of different compositions and grain densities but similar granulometry to analyze gravity-dominated cratering. The results are compared with iSALE-2D numerical simulations and natural craters on Earth and Mars. We show that target layering also affects the excavation process and concentric crater formation in gravity-dominated impacts. The most important factors are the density and internal friction of each target layer, respectively. We propose that this is also valid for natural craters of sizes that should make their formation gravity-dominated. Concentric impact craters show a "soup-plate" or "inverted sombrero" shape that forms on planetary surfaces with distinct subsurface layers. Generally, a deep inner crater forms in the substrate and a shallower, broader outer crater forms in the upper layer. The shape is obtained either from extensive post-impact collapse of the upper, often weaker layer, or already during crater excavation, which is the focus of this study. The ratio of outer to inner diameter in the latter craters can be used to probe the depth of the upper layer and the contrast in material properties to the substrate. However, the controls on the relative size of the inner and outer craters are not well understood. While the formation of natural concentric craters is often attributed to a change in cohesive strength between the upper and lower layers, we show through impact experiments and numerical simulations that concentric craters can also form in cohesionless targets with a contrast in both density and friction coefficient between the layers. This provides an additional mechanism for concentric crater formation that may explain the development of some large, gravity-dominated, naturally occurring concentric craters on Earth, Mars and elsewhere. Concentric craters are not only due to strength differences between layers but also observed in cohesionless, gravity-dominated targets This may explain the occurrence of large gravity-dominated concentric craters on Earth, Mars and elsewhere Key factors affecting the concentric growth in these cases are the density and internal friction of each target layer, respectively

NASA's Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the natural satellite of (65803) Didymos, on 2022 September 26, as a first successful test of kinetic impactor technology for deflecting a potentially hazardous object in space. The experiment resulted in a small change to the dynamical state of the Didymos system consistent with expectations and Level 1 mission requirements. In the pre-encounter paper Richardson (2022), predictions were put forward regarding the pre- and post-impact dynamical state of the Didymos system. Here we assess these predictions, update preliminary findings published after the impact, report on new findings related to dynamics, and provide implications for ESA's Hera mission to Didymos, scheduled for launch in 2024 with arrival in late December 2026. Pre-encounter predictions tested to date are largely in line with observations, despite the unexpected, flattened appearance of Didymos compared to the radar model and the apparent pre-impact oblate shape of Dimorphos (with implications for the origin of the system that remain under investigation). New findings include that Dimorphos likely became prolate due to the impact and may have entered a tumbling rotation state. A possible detection of a post-impact transient secular decrease in the binary orbital period suggests possible dynamical coupling with persistent ejecta. Timescales for damping of any tumbling and clearing of any debris are uncertain. The largest uncertainty in the momentum transfer enhancement factor of the DART impact remains the mass of Dimorphos, which will be resolved by the Hera mission.

The binary asteroid 65803 Didymos-Dimorphos is the target of NASA’s DART (Cheng et al., 2018) and ESA's Hera missions (Michel et al., 2018). Hera will arrive at the asteroid system several years after the DART impact. It will carry out a detailed characterisation of the asteroids’ overall properties and will measure the outcome of the DART impact on Dimorphos. Didymos, the primary body, is a fast spinning asteroid with a period of only 2.26 hours, which is very close to the critical spin limit of 2.2 hours for asteroids bigger than approx. 200 m (Walsh et al., 2018). The interior structure of Didymos is crucial for our understanding of its stability. Without cohesion and with an estimated bulk density of 2.1 g/cc, Didymos is not able to keep its shape stable (Holsapple, 2001; Zhang et al., 2017) and thus, cohesive forces might be present in its structure.The interior strength properties of asteroids determine to a large degree their collisional evolution. For small bodies, even a small level of cohesion can significantly affect the outcome of an impact (Raducan and Jutzi, 2021). Previous studies have applied N-body and soft-sphere discrete element (SSDEM) codes to study the structural stability of rubble pile asteroids (Sanchez and Scheeres, 2012; Zhang et al., 2017, 2021; Ferrari and Tanga, 2020). Recently, the stability and failure modes of such objects have been studied using finite element (FEM) (Hirabayashi et al., 2020). Here we use Bern’s Smooth Particle Hydrodynamics (SPH) code (Jutzi et al., 2008; Jutzi 2015) to simulate the rotating asteroid and to investigate its physical properties. To do this, we set up Didymos according to its recent shape model (provided by the Hera Didymos Reference Model) with a bulk density of 2.1 g/cc and we spin up the body to a rotation period of 2.26 hours.For the preliminary simulations presented here, we assume that Didymos has a homogeneous structure. We perform simulations using a range of values for the cohesion and the internal friction coefficient of the asteroid, and then for each case we evaluate Didymos’ stability.An example of such a simulation is shown in Figure 1. For a cohesion of 100 Pa and a friction coefficient of 0.4, we find that Didymos’ shape is stable and only very little deformation takes place, as indicated by the small values in the total accumulated strain. In Figure 2, we show the case with a lower cohesion of 0.1 Pa and a coefficient of friction of 0.4. In this case, Didymos experiences large-scale deformations. To quantify the degree of deformation, we compute the cumulative strain distributions for both cases (Figure 3). For the 100 Pa cohesion case, > 90 % of the total mass experiences a total strain of < 0.03 and therefore this case is considered to be stable. On the other hand, for the case with 0.1 Pa cohesion, > 50 % of the total mass experiences a total strain of > 0.5, consistent with the large scale deformations observed in Figure 2. Our initial results suggest that a cohesion of ~ 10-100 Pa is required to maintain structural stability, which is consistent with recent SSDEM modeling results (Zhang et al. 2021). However, so far only homogeneous structures have been explored. We plan to investigate also more complex interior structures that may lead to different outcomes in terms of required cohesion to obtain an overall structural stability. Figure 1: Surface of Didymos after spin-up is complete, for a friction coefficient of 0.4 and a cohesion of 100 Pa. The body is shown from the same side at four different times. The body experiences only very little deformation, as indicated by the small amount of accumulated strain.Figure 2: same as Figure 1, but this time for a friction coefficient of 0.4 and a cohesion of 0.1 Pa. The body gets deformed already before the spin-up is completed.Figure 3: Cumulative strain distribution for the two simulations shown in Figures 1 and 2. The strain experienced by the body with a cohesion of 100 Pascal (a) is much smaller than for the body with a cohesion of 0.1 Pa (b). Acknowledgements: This work has received funding from the European Union’s Horizon 2020 research and in- novation programme under grant agreement No. 870377References: Cheng, A. F., et al. (2018) Planet. Space Sci., 157, 104-115.Jutzi, M. et al. (2008) Icarus, 198:242–255Jutzi, M. (2015) Planet. Space Sci., 107:3–9. Ferrari, F. and Tanga P. (2020) Icarus, 350.Hirabayashi et al. (2020) Icarus, 352.Holsapple (2001) Icarus, 154, 432-448.Michel, P., et al. (2018) Adv. in Space Res., 62 (8), 2261-2272.Raducan, S. D. and Jutzi, M. (2021) LPSC, (2548):1900.Sanchez, D. P. and Scheeres, D. J. (2012) Icarus, 218, 876-894.Sugiura et al. (2021) Icarus, https://doi.org/10.1016/j.icarus.2021.114505Walsh, K. J. (2018) Annu. Rev. Astron. Astrophys., 56, 593-624.Zhang, Y., et al. (2017) Icarus, 294, 98-123.Zhang, Y., et al. (2021) Icarus 362, http://doi.org/10.1016/j.icarus.2021.114433

On September 26, 2022, NASA's Double Asteroid Redirection Test (DART) mission successfully impacted Dimorphos, the natural satellite of the binary near-Earth asteroid (65803) Didymos. Numerical simulations of the impact provide a means to explore target surface material properties and structures, consistent with the observed momentum deflection efficiency, ejecta cone geometry, and ejected mass. Our simulation, which best matches observations, indicates that Dimorphos is weak, with a cohesive strength of less than a few pascals (Pa), similar to asteroids (162173) Ryugu and (101955) Bennu. We find that a bulk density of Dimorphos, rhoB, lower than 2400 kg/m3, and a low volume fraction of boulders (<40 vol%) on the surface and in the shallow subsurface, are consistent with measured data from the DART experiment. These findings suggest Dimorphos is a rubble pile that might have formed through rotational mass shedding and re-accumulation from Didymos. Our simulations indicate that the DART impact caused global deformation and resurfacing of Dimorphos. ESA's upcoming Hera mission may find a re-shaped asteroid, rather than a well-defined crater.