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Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact., accepted by Nature

The NASA Double Asteroid Redirection Test (DART) mission performed a kinetic impact on asteroid Dimorphos, the satellite of the binary asteroid (65803) Didymos, at 23:14 UTC on September 26, 2022 as a planetary defense test. DART was the first hypervelocity impact experiment on an asteroid at size and velocity scales relevant to planetary defense, intended to validate kinetic impact as a means of asteroid deflection. Here we report the first determination of the momentum transferred to an asteroid by kinetic impact. Based on the change in the binary orbit period, we find an instantaneous reduction in Dimorphos's along-track orbital velocity component of 2.70 +/- 0.10 mm/s, indicating enhanced momentum transfer due to recoil from ejecta streams produced by the impact. For a Dimorphos bulk density range of 1,500 to 3,300 kg/m$^3$, we find that the expected value of the momentum enhancement factor, $\beta$, ranges between 2.2 and 4.9, depending on the mass of Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of 2,400 kg/m$^3$, $\beta$= 3.61 +0.19/-0.25 (1 $\sigma$). These $\beta$ values indicate that significantly more momentum was transferred to Dimorphos from the escaping impact ejecta than was incident with DART. Therefore, the DART kinetic impact was highly effective in deflecting the asteroid Dimorphos., Comment: accepted by Nature

While no known asteroid poses a threat to Earth for at least the next century, the catalog of near-Earth asteroids is incomplete for objects whose impacts would produce regional devastation. Several approaches have been proposed to potentially prevent an asteroid impact with Earth by deflecting or disrupting an asteroid. A test of kinetic impact technology was identified as the highest priority space mission related to asteroid mitigation. NASA's Double Asteroid Redirection Test (DART) mission is the first full-scale test of kinetic impact technology. The mission's target asteroid was Dimorphos, the secondary member of the S-type binary near-Earth asteroid (65803) Didymos. This binary asteroid system was chosen to enable ground-based telescopes to quantify the asteroid deflection caused by DART's impact. While past missions have utilized impactors to investigate the properties of small bodies those earlier missions were not intended to deflect their targets and did not achieve measurable deflections. Here we report the DART spacecraft's autonomous kinetic impact into Dimorphos and reconstruct the impact event, including the timeline leading to impact, the location and nature of the DART impact site, and the size and shape of Dimorphos. The successful impact of the DART spacecraft with Dimorphos and the resulting change in Dimorphos's orbit demonstrates that kinetic impactor technology is a viable technique to potentially defend Earth if necessary., Comment: Accepted by Nature

Statler et al., NASA’s Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ∼10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos’s response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor β, showing that a particular direction-specific β will be directly determined by the DART results, and that a related direction-specific β is a figure of merit for a kinetic impact mission. The DART β determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos’s near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction., This work was supported in part by the DART mission, NASA contract No. 80MSFC20D0004 to JHU/APL. Some of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). S.C. acknowledges funding through the Crosby Fellowship of the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. E.D., S.I., A.L., M.P., A.R., and P.T. are grateful to the Italian Space Agency (ASI) for financial support through agreement No. 2019-31-HH.0 in the context of ASI’s LICIACube mission, and agreement No. 2022-8-HH.0 for ESA’s Hera mission. R.L. appreciates the funding from the European Union’s Horizon 2020 research and innovation program, grant agreement No. 870377 (project NEO-MAPP). P.M. acknowledges funding support from the French space agency CNES, ESA and the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP). R.M. acknowledges support from a NASA Space Technology Graduate Research Opportunities (NSTGRO) Award (80NSSC22K1173). R.N. acknowledges support from NASA/FINESST (NNH20ZDA001N). J.O. and I.H. were supported by the Spanish State Research Agency (AEI) project No. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”—Centro de Astrobiología (CSIC-INTA). They are also grateful for all logistical support provided by Instituto Nacional de Técnica Aeroespacial (INTA). S.R.S. acknowledges support from the DART Participating Scientist Program, grant No. 80NSSC22K0318. J.M.T.R. was funded by FEDER/Ministerio de Ciencia e Innovación—Agencia Estatal de Investigación of Spain (grant No. PGC2018-097374-B-I00).

Introduction: Asteroids smaller than about 50 km in diameter are the result of the break-up of a larger parent body [1]. They are often considered to be rubble-pile objects, aggregates held together only by self-gravity or small cohesive forces [2, 3], and have highly heterogeneous surfaces. Recently, the artificial impact experiment (SCI) of JAXA’s Hayabusa2 mission on the surface of asteroid Ryugu [4] created a relatively large crater (~14 m diameter) despite the presence of large boulders close to the impact location [5]. Post-impact images of the SCI impact site revealed that the boulders had different motion mechanisms depending on their size and initial position relative to the impact point. 1 m-sized boulders were ejected several metres outside of the crater, a 5 m boulder was moved about 3m, while a large, possibly deeply rooted boulder (“Okamoto”) was not moved [4]. Impact cratering on weak, heterogeneous targets is still poorly studied, both by means of laboratory experiments and numerical simulations. For example, it is not yet known how the boulders affect the crater size or how the boulders motion is affected by their mass, size, shape or initial location. This is also important in context of NASA’s Double Asteroid Redirection Test (DART) impact on the surface of Dimorphos (the secondary of the 65803 Didymos asteroid system) on the 26th of September 2022. The impact will demonstrate the controlled deflection capabilities of near-Earth asteroids by a kinetic impactor [6]. ESA’s Hera mission [7] will arrive at Dimorphos several years after the DART impact and provide a detailed characterisation of the impact outcome. Recent impact experiments and numerical studies [8, 9, 10, 11] have shown that the kinetic impact efficiency depends strongly on the target properties and structure. The understanding thereof is imperative for a successful interpretation of the DART impact outcome. Most previous impact experiments and subsequent validation work of numerical models have focused on homogeneous targets [e.g, 12, 13, 14]. However, it is unlikely that Dimorphos is homogeneous at the scale of DART impact. In our previous work we studied impacts into granular targets with embedded porous, spherical boulders and their effect on cratering process, final crater morphology, boulder emplacement, and ejecta distribution [15, 16]. We showed that crater diameters are only slightly affected by heterogeneities as long as the energy required for crushing is small compared to impact energy. However, less mass is ejected at higher velocities compared to homogeneous targets, which reduces the momentum enhancement. In this study we present results from recently conducted experiments at the Experimental Projectile Impact Chamber (EPIC) at Centro de Astrobiología CSIC-INTA, Spain, which focused on the motion mechanism of large boulders placed close to the impact site (Fig. 1). We also study the effect of density and surface roughness of boulders located on the target surface on their mobilisation and ejection, as well as on the excavation flow and ejection of fine-grained matrix. Methods: The EPIC utilises a 20 mm calibre compressed N2 (300 bar) cannon that launches projectiles at velocities up to ≈420 m/s [13]. The experiments, half- or quarter-space, can be recorded with two high-speed cameras. In the three experiments presented here we have kept the impact velocity constant at approximately 400m/s. The projectiles were 20mm massive, spherical delrin balls. All shots were vertical and in half space. We performed three shots into a loosely packed beach sand in which a single, relatively large rectangular boulder was placed (Fig. 1). Coloured sand on the surface allows studies of ejecta distribution. In the first shot (Scenario 1) the boulder had relatively high bulk density (quartzite, ~ 2.8 g/cm3), in the second shot (Scenario 2) the boulder had relatively low density (highly porous pumice, ~ 0.27 g/cm3) and a glossy surface (the boulder was covered in a plastic film). In the last shot (Scenario 3) the same object was used, but now the glossy plastic film had been removed revealing a rough, high-friction surface (Fig. 1A). In all experiments the point of impact was located as shown in Figure 1B. Results and discussion: We observe a significant difference in the ejection behaviour between the cases with boulders of different mass. In Scenario 1, the boulder was moved a few cm by the excavation flow and deposited at the crater rim. The ejecta curtain passes around the boulder, creating a “forbidden zone” behind the boulder (Fig. 2A). Similar ejecta behaviour was seen during the SCI impact around the Okamoto boulder [4]. In Scenarios 2 and 3, the boulder was excavated with the ejecta curtain, rotated around its length axis, and landed about one crater diameter away from the crater. The ejecta curtain was again disrupted by the presence of the boulder (Fig. 2 B, C). The final rim-diameter of the crater produced in Scenario 1 was 27cm and in the other two scenarios 29 cm, suggesting that the presence of a dense (i.e., mainly stationary) boulder close to the impact point does not greatly affect the crater size. However, we can clearly see a significant influence of the boulder on the ejecta curtain behaviour. The dense boulder efficiently block the ejecta while the low-density boulders to great extent travel with the ejecta mainly affecting the distal parts of the continuous ejecta layer. The surface roughness had no obvious effect on boulder movement. Conclusions and future work: Here we continue our study of impacts into heterogeneous targets (The first study was presented at EPSC2021 [15]) and work towards understanding the importance of target heterogeneities in the cratering process and impact momentum transfer. Numerical simulations reproducing these impact experiments are underway and preliminary results from the early cratering process show good agreement with the experiments. These results and further numerical studies are vital for understanding cratering processes on rubble-pile asteroids and will help interpret results from the DART impact and its imaged ejecta plume.

Funding Information: To achieve these objectives, Milani is carrying two scientific payloads, the ASPECT visual and near-infrared (Vis-NIR) imaging spectrometer and the VISTA thermogravimeter aimed at collecting and characterizing volatiles and dust particles below 10 μm. Additionally, navigation payloads include a visible navigation camera and lidar. The Milani consortium is composed of entities and institutions from Italy, the Czech Republic, and Finland. The consortium Prime is Tyvak International, responsible for the whole program management and platform design, development, integration, testing, and final delivery to the customer. Politecnico di Torino is tasked with defining requirements and performing thermal, radiation, and debris analysis. Politecnico di Milano is responsible for mission analysis and GNC. Altec will support the Ground Segment architecture and interface definition. Centro Italiano per la Ricerca Aerospaziale (CIRA) is responsible for the execution of the vehicle environmental campaign. HULD contributes to developing the mission-specific software. VTT is the main payload (ASPECT hyperspectral imager) provider and is supported by the following entities dealing with ASPECT-related development: University of Helsinki (ASPECT calibration); Reaktor Space Lab (ASPECT Data Processing Unit development), Institute of Geology of the Czech Academy of Sciences (ASPECT scientific algorithms requirements and testing); and Brno University of Technology (ASPECT scientific algorithms development). INAF-IAPS is the secondary Payload (VISTA, dust detector) provider. Funding Information: The Mission PI is appointed by ESA and is the primary interface to ESA. The Hera SMB consists of the ESA Hera Project Scientist (ESA PS), the Mission PI, and the Hera Advisory Board, consisting of four mission advisors. The Mission PI chairs the HIT and is supported by the Hera Advisory Board. The tasks of the Hera SMB are 1. advising the Hera mission project team on all aspects related to the Hera mission objectives; 2. ensuring that the WGs’ activities cover the needs of the Hera mission; 3. providing recommendations to ESA concerning the membership in the HIT; and 4. implementing the Publication Policy. Funding Information: Hera is the ESA contribution to the AIDA collaboration. Hera, Juventas, Milani, and their instruments are developed under ESA contract supported by national agencies. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP), the CNRS through the MITI interdisciplinary programs, ASI, CNES, JAXA, the Academy of Finland project no. 335595, and was conducted with institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. M.L., E.P., P.T .and E.D. are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2022-8-HH.0 in the context of ESA’s Hera mission. We are grateful to the whole Hera team, including Working Group core members and other contributors for their continuous efforts and support. Their names can be found on the following website: https:// www.heramission.space/team. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society. Hera is a planetary defense mission under development in the Space Safety and Security Program of the European Space Agency for launch in 2024 October. It will rendezvous in late 2026 December with the binary asteroid (65803) Didymos and in particular its moon, Dimorphos, which will be impacted by NASA’s DART spacecraft on 2022 September 26 as the first asteroid deflection test. The main goals of Hera are the detailed characterization of the physical properties of Didymos and Dimorphos and of the crater made by the DART mission, as well as measurement of the momentum transfer efficiency resulting from DART’s impact. The data from the Hera spacecraft and its two CubeSats will also provide significant insights into asteroid science and the evolutionary history of our solar system. Hera will perform the first rendezvous with a binary asteroid and provide new measurements, such as radar sounding of an asteroid interior, which will allow models in planetary science to be tested. Hera will thus provide a crucial element in the global effort to avert future asteroid impacts at the same time as providing world-leading science.

NASA’s DART spacecraft is planned to reach and impact asteroid Dimorphos, the small moon of binary asteroid (65803) Didymos, at a velocity of 6 km s−1 in late 2022 September. DART will be the first mission to test the “kinetic impactor” technique, aimed at deflecting the orbital path of a potentially hazardous asteroid. The success and effectiveness of this technique resides in the efficiency of momentum exchange between the spacecraft and the impacted target. This depends on many factors, including the cratering process, the formation of ejecta, and their fate, as they remain in the system or escape from it, carrying momentum away. Here we provide an overview of the cratering process, including ejecta formation and their subsequent dynamical evolution. We use different methodologies to model the physics of the problem, including smoothed particle hydrodynamics to model the cratering and ejecta formation process after the hypervelocity impact, N-body granular simulations to model early collisional processes between ejecta fragments right after cratering, and high-fidelity planetary propagation to model the dynamical evolution of ejecta during their purely ballistic phase. We highlight the key features of each phase and their role in defining the dynamical fate of ejecta. We investigate the effect of surface cohesion in the impacted target and identify the qualitative behavior of ejecta particles as a function of the key parameters of the problem. We provide quantitative estimates for the specific case study related to the DART–Dimorphos scenario and a selected range of target properties.

NASA's Double Asteroid Redirection Test (DART) mission is the first full-scale planetary defense mission. The target is the binary asteroid (65803) Didymos, in which the smaller component Dimorphos (∼164 m equivalent diameter) orbits the larger component Didymos (∼780 m equivalent diameter). The DART spacecraft will impact Dimorphos, changing the system’s mutual orbit by an amount that correlates with DART's kinetic deflection capability. The spacecraft collision with Dimorphos creates an impact crater, which reshapes the body. Also, some particles ejected from the DART impact site on Dimorphos eventually reach Didymos. Because Didymos’s rapid spin period (2.26 hr) may be close to its stability limit for structural failure, the ejecta reaching Didymos may induce surface disturbance on Didymos. While large uncertainties exist, nonnegligible reshaping scenarios on Didymos and Dimorphos are possible if certain conditions are met. Our analysis shows that given a surface slope uncertainty on Dimorphos of 45°, with no other information about its local topography, and if the DART-like impactor is treated as spherical, the ejecta cone crosses Didymos with speeds ≳14 m s−1 in 13% of simulations. Additional work is necessary to determine the amount of mass delivered to Didymos from the DART impact and whether the amount of kinetic energy delivered is sufficient to overcome cohesive forces in those cases. If nonnegligible (but small) reshaping occurs for either of these asteroids, the resulting orbit perturbation and reshaping are measurable by Earth-based observations.

The DART (Double Asteroid Redirection Test) mission will impact a spacecraft on the secondary (Dimorphos) of the binary asteroid system Didymos in 2022 September, with the goal of altering the orbital period of Dimorphos about Didymos sufficiently to be observed from ground-based observations. Numerical impact modeling is a crucial component in understanding the outcome of the DART experiment, and while many have investigated the effects of target properties, such as material strength and porosity (which remain unknown), an often overlooked factor is the importance of accurately representing the spacecraft itself in such models. Most impact modeling to date has considered simple impactor geometries such as a solid uniform sphere, but in reality the spacecraft is a complex shape full of different components, open spaces, and thin walled structures. At a minimum, a simple solid representation underestimates the surface area of the impact: for a small body such as Dimorphos (approximately 160 m in diameter), the difference between a spacecraft spanning 20 m (including solar arrays) impacting and a sub-1 m idealized shape may be important. In this paper, we compare models impacting high-fidelity models of the spacecraft based on the CAD geometry with various simplified impactors, in order to assess the potential importance of this effect. We find that the difference between the simplest impactor geometries (such as a uniform sphere) and the real spacecraft is measurable, and has an interesting dependence on the material properties of the asteroid itself.

We use a novel approach to model the entire cratering process resulting from impacts on small, weak asteroids that uses shock physics code calculations directly. We found that small-scale impacts (with a projectile size less than 1/150 of the target size) can significantly deform weak asteroids, causing global resurfacing at the same time. As a result, the collisional lifetime of the overall asteroid shapes is significantly lower than the traditionally used lifetime based on catastrophic disruption events. We also show that even very low asteroid cohesions can drastically influence the outcome of an impact. Consequently, if the target is homogeneous and weaker than ≈10 Pa, then NASA’s Double Asteroid Redirection Test impact on Dimorphos may not lead to a cratering event, as originally anticipated. Rather, the impact may change the global morphology of the asteroid. Our results, together with future observations by the ESA’s Hera mission, will provide constraints regarding the evolution of the shapes and structures of small asteroids by subcatastrophic impacts.

Keywords: Impact; asteroid surfaces; SSDEM; SPH. Introduction: Impacts can modify the physical state of a substantial fraction of a target body. Studying the hypervelocity impact process and outcome is crucial in the interpretation of the history of a planetary body (Jutzi et al., 2015) and the design of asteroid deflection strategies based on the kinetic impactor technique (Raducan et al., 2019). Images returned by space missions show that small asteroids have complex surface morphologies with heterogeneous distributions of fine regolith and large boulders (e.g., Watanabe et al., 2019). To properly decipher the crater imprints on asteroid surfaces, we carried out numerical investigations to understand the effect of surface material properties (i.e., friction, cohesion) and the presence of large boulders on cratering processes. Methods: We used a hybrid SPH-SSDEM framework to model the high-speed impact cratering (Zhang et al., 2021). The Smooth Particle Hydrodynamics (SPH) is used to simulate the initial shock propagation and fragmentation stage (Jutzi & Michel, 2015). The outcome is then transferred into a Soft-Sphere Discrete Element Method (SSDEM) code (Zhang et al., 2018), which solves the ejecta evolution and crater growth in the later stages. This modeling framework is capable of simulating impacts from the beginning to the later stages when all ejecta are settled down, allowing capturing the final morphology of the resulting crater. To make comparisons with the first impact experiment performed on an asteroid by the Hayabusa2 Small Carry-on Impactor (SCI; Arakawa et al., 2020), we conducted SCI-like cratering tests using the same impact condition (except using an impact angle of 0º) and Ryugu’s gravity field. The target is modeled as a 15-meter-radius granular bed held by a hemispherical ball. The particle-ball contact parameters are the same as those used for particle-particle contacts. Results: As the SCI cratering analyses show consistencies with a very low-strength scaling law (Arakawa et al., 2020), we considered modeling the surface properties with three types of low cohesion (i.e., 0 Pa, 0.01 Pa, and 0.1 Pa) and four types of low to moderate friction angles (20°, 25°, 30°, and 33°). The results show that, in a monotonic manner, the diameter and depth of the resulting crater and rim decrease with a larger friction or cohesion (Fig. 1). Compared with the crater morphology of the SCI impact (i.e., crater diameter 14.5 ± 0.8 m and depth ~2.3 m, rim diameter 17.6 ± 0.7 m and depth 0.4 m), the case with C = 0.1 Pa and

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.

Introduction On September 30, 2022, NASA’s Double Asteroid Redirection Test (DART, [1]) mission will be the first space mission demonstrating the kinetic impactor method for planetary defense. DART will impact Didymos B (a 164±18 m secondary of asteroid (65803) Didymos [2]) with its mass of 650 kg and at a speed of 6.6 km/s. Such impact is expected to change the secondary orbital period by about 10 minutes. DART will carry as a piggyback the Light Italian Cubesat for Imaging of Asteroids (LICIACube, [3]) which will be released from DART ten days before the impact. LICIACube will provide evidence of the impact and will take multiple images of the target up to a distance of ~55 km from the target. The LICIACube narrow and wide angle cameras - LEIA (LICIACube Explorer Imaging for Asteroid) and LUKE (LICIACube Unit Key Explorer), respectively – will capture the post-impact processes coming from in situ events, such as the newly formed crater, the expanding ejecta and the dynamics of its plume. In particular, the measurement of the motion of the slow ( Dust plume model We use a 3D+t non-spherical dust model that solves the Euler dynamical and kinetic equations. Considering free-collisional dust regime we study the effects on the particle dynamics provided with different shapes, initial particle orientation and velocities as well as torque. Torque is computed from the law of variation of the angular momentum by using the Euler dynamic equations. The particles are assumed to be homogeneous, isothermal convex bodies, having the same physical properties of the target. The dust motion is governed by solar radiation pressure force, initial dynamical parameters (speed, orientation and torque) and gravity of the asteroid binary system. To compute the gravity we take into account that Didymos B is orbiting at distance of ~1.18 km the Didymos primary (780±80 m in size) [6]. As input we use the physical parameters obtained via numerically simulated impact into low-gravity and strength- dominated asteroid surface done with the iSALE numerical code [5]. A detailed description of the parameters set used in the impact modelling are reported in Table 1. As an example, in Fig.1, we show the iSALE simulation of the spatial distribution of tracer particles for 50 ms after the impact. Table 1: Paremeters used for iSAle simulation. A detailed description can be found in [5]. Fig. 1: A snapshot of the formation of the crater resulting from the iSALE modelling performed with parameters reported in Table 1. The colorbar represent the peak pressure of the ejecta tracer particles. Non-spherical dust motion in the plume We have studied the motion of non-spherical particles after the impact assuming various set of initial parameters coming from the iSALE simulations, such as particle mass, shape, initial orientation, launch position and computed velocity. The Didymos system physical parameters and particle temperature were taken from [7]. Here, in Fig. 2, we show the velocity and the distance from the surface where it has been reached for a prolate spheroid (aspect ratio = 2 [4]) with assumed density 2600 kg/m3. The particle mass is 7.45 x10-2 kg, the initial velocity 0.94 m/s, the launch position is 11 m from the center of the crater and the initial tilt of the initial velocity vector away from local surface normal is 46 degrees. We plotted the results for two cases: 1) only the Didymos B gravity field is considered and 2) the gravity of the whole binary is used. Our results show that the particle dynamics is sensitive to parameters such as orientation and ejection velocity. Fig. 2: Velocity and distance at which it has been achieved for a prolate particle (aspect ratio a/b = 2). The complete set of parameters is given in the text. The solid line indicates the case when only gravity of Didymos B was considered and the dashed line when the gravity of the whole binary was used (binary was approximated as a mass point). This study of dust dynamics in expanding ejecta together with LICIACube plume evolution images can further constrain the momentum transfer efficiency estimation as well as the ejecta velocity and dust size distribution in the “far-field” expanding plume. Acknowledgments This research was supported by the Italian Space Agency (ASI) within the LICIACube project (ASI-INAF agreement AC n. 2019-31-HH.0). References [1] Cheng A. F. et al. (2016) Planet. Space Sci., 121, 27-35. [2] de León, J. et al. (2010) Astron. Astrophys, 517, A23. [3] Dotto, E. et al., (submitted), Planet. Space Sci. [4] Ivanovski S. et al. (2017), Icarus 282, p. 333 - 350. [5] Raducan, S. D. et al. (2019), Icarus, 329, p. 282 - 295. [6] Michel, P. et al. (2016) Adv. Space Sci. 57, p. 2529 - 2547. [7] Yu Y. et al. (2017), Icarus 282, p.313 - 325