Characterization of the DART Impact Ejecta Plume on Dimorphos from LICIACube Observations

Introduction Hayabusa2, the Japanese asteroid exploration mission, conducted an impact experiment on the surface of asteroid Ryugu to create an artificial crater. An observed crater measuring 14 m in size, filled with boulders, formed in the gravity-dominated regime, even in the microgravity environment of ~10-5 G on Ryugu [1]. The study revealed that the artificial crater’s size was well reproduced by the crater size-scaling law for dry sand, suggesting that dry sand can simulate the surface of small rubble-pile bodies rich in boulders in terms of the crater formation process. NASA’s DART mission recently conducted a successful large-scale impact experiment on Dimorphos, a small satellite of asteroid Didymos (~200 m in diameter). The mission demonstrated a significant momentum transfer efficiency exceeding =3 during the impact of the DART spacecraft [2]. Considering Dimorphos’s boulder-covered surface, similar to Ryugu, the resulting crater may grow in the gravity-dominated regime. The  is theoretically sensitive to the momentum of ejecta during cratering and if the value is larger than 1, it strongly depends on the amount of ejecta and the velocity-mass relationship. In addition to the ground-based observation of the orbital period of Dimorphos [2], LICIA-Cube, following the impacting spacecraft, successfully made an in-situ observation of the DART impact event [3], and more the Hubble space observatory observed the ejecta curtain growth after the DART impact [4].

Electron field emission limiting the accelerating gradient in superconducting cavities remains the dominant setback in cavity production. The need to understand and control the field emission has become increasingly important because of the prospect of using high-gradient structures in linear colliders. Since building an accelerator structure is a complicated and costly process, elimination of unnecessary steps has priority. In this paper an analysis of the influence of the enhanced field emission in superconducting radio frequency cavity together with modal field calculations by using COMSOL finite elements package has been presented. The obtained results reveal that the electric field required for the field emission is generated in the cavity irises. The imperfection of the cavity surface leading to very high fields is modelled by a simple cone. The estimated value of the enhancement factor for the cone tip of around 4 is in a good agreement with the data found in the literature. In addition, from the slopes and the intercepts of the Fowler-Nordheim plots, a dependence of the enhancement factor and the effective area on the work function has been estimated.

A dynamic category of asteroids, known as near-Earth asteroids, includes a group of potentially hazardous asteroids (PHAs) with orbits that have an increased likelihood of colliding with Earth. One effective strategy to avert such collisions involves deflecting a PHA, ensuring it deviates from a collision course with Earth. Calculations have demonstrated that one promising deflection method is the kinetic mitigation approach, wherein a rapidly moving human-made object impacts the PHA, deflecting it from the collision course. The Double Asteroid Redirection Test (DART) mission, led by NASA’s Planetary Defense Coordination Office, was specifically designed to assess the kinetic mitigation method. It impacted the moon in the binary asteroid system (65803) Didymos. The mission would be considered successful if the DART kinetic impact caused at least a 73-second change in the binary orbital period, measured with an accuracy of 7.3 seconds. The mission objectives also included measuring the efficiency of momentum transfer to Didymos’s moon, Dimorphos, by estimating the momentum enhancement factor β, determining the location and surface characteristics of the spacecraft impact site, and assessing the dynamical changes in the Didymos system resulting from the DART impact. Launched on November 24, 2021, the DART spacecraft impacted Dimorphos on September 26, 2022. A CubeSat named LICIACube (Light Italian CubeSat for Imaging of Asteroids), supplied by Agenzia Spaziale Italiana, separated from the DART spacecraft 15 days before the impact. LICIACube provided crucial information about the initial minutes following the impact. A network of ground-based and space-based telescopes also monitored the Didymos system. The mission results demonstrated the high efficiency of the kinetic mitigation method, with the impact altering the binary orbital period by −33.24 minutes, determined with an accuracy of less than 2 seconds. The enhancement factor β was estimated to be between 2.4 and 4.9 (the range resulting from different estimations of Dimorphos’s mass), thus significantly exceeding the minimal estimated value equal to unity. The mission yielded valuable information about impact dynamics and the formation of impact ejecta, as well as about the shapes, surface, and interior characteristics of asteroids in binary systems and small asteroids in general. Extensive spectral, photometric, and polarimetric observations by telescopes on Earth and in space allowed for the characterization of dust in the impact ejecta and its long-term evolution, as well as physical changes in the Didymos system. Remaining gaps in understanding DART’s kinetic impact consequences are slated to be addressed by the European Space Agency’s Hera mission, scheduled for launch in 2024.

Asteroid porosity ranges from 0 to >50%, with most >20%, and some asteroids exhibit a water feature in their reflection spectra. Porosity and hydration are expected to influence the momentum transferred in hypervelocity collisions. We conducted a series of measurements of the post-impact momentum, characterized by a factor β, the ratio of the total linear momentum acquired by the target to the momentum of the impactor. We measured β for anhydrous meteorites, samples of their asteroidal parent bodies, spanning a wide range of porosities: 7 samples of the CV3 carbonaceous chondrite Northwest Africa (NWA) 4502 (2.1% porosity), 7 samples of the ordinary chondrite NWA 869 (6.4% porosity), and 4 samples of the ordinary chondrite Saratov (15.6% porosity), as well as 2 samples of terrestrial pumice (80% porosity). We also measured hydrous meteorite analog targets, including 2 samples of terrestrial serpentine (17.9% porosity) and 4 samples of terrestrial montmorillonite (51.5% porosity), the two clay minerals that dominate the composition of the hydrous CI carbonaceous chondrite meteorites, as well as 4 samples of hydrous meteorite analog material prepared by powdering and hydrating an anhydrous carbonaceous chondrite. We found that for both anhydrous and hydrous samples β decreased with increasing porosity, consistent with hydrocode modeling. The β for each target type was >2 demonstrating that crater ejecta makes a significant contribution to recoil in hypervelocity collisions.. The β values we measured for the anhydrous samples are larger, with β = 3.55 for NWA 4502, 2.69 for NWA 869, 2.10 for Saratov, and 2.15 for pumice, than results from hydrocode modeling for 10 km/s impacts into relatively strong, porous rock targets. The momentum enhancement by ejecta (β - 1) for the moderate porosity (17.9%) hydrous serpentine targets (β = 4.70), the highly porous (51.55% porosity) hydrous montmorillonite targets (β = 2.79), and the intermediate porosity (~26%) CI-analogs (β = 2.99) are much larger than β value for anhydrous targets of similar porosity, indicating jetting of water vapor could significantly affect deflection of hydrous asteroids and comets in natural or human-induced collisions.

Insight into the electron–positron correlations in metals through the looking glass

Measuring ejecta characteristics and momentum transfer in experimental simulation of kinetic impact

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.

The NASA Double Asteroid Redirection Test (DART) spacecraft was the first Planetary Defence mission to hit an asteroid, specifically the moon of Didymos, Dimorphos [1]. This impact altered Dimorphos's orbital parameters and excavating significant amount of material. The resultant was observed through limited ground-based telescopes, and for several months after the impact, the asteroid became the first man-made active asteroid, exhibiting mass loss that manifested as a tail, which changed over time [2].The measurements presented here derive from a unique set of photometric data obtained from the ground over several months covering the period before, during and after the impact, between August 2022 and January 2023. The data set taken in collaboration with ESA at Les Makes Observatory (La Réunion) [3], was excellently located to observe the impact and contains more than 8500 images taken over multiple months before and after impact. Another unique data set comes from the TRAPPIST telescopes network [4], two 0.6-m twin telescopes located in Morocco and in Chile. The data from TRAPPIST, contain almost 14 000 images, spanning several months. The images were taken in different filters, for photometric and colour measurements that gave us more information about the asteroid and also the dust composition of the ejecta tail. Here, we present the observations, their analysis and preliminary results on the evolution of ejecta plume and the long-lasting tail.AcknowledgementThis research is funded by the University of Liège and takes place under the COMETA team, and is made possible through TRAPPIST that is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant T.0120.21 and the support and collaboration of the Royal Observatory of Belgium, Les Makes Observatory, and the European Space Agency. Özgur Karateking, acknowledges the support of European Union’s Horizon 2020 research and innovation program, NEO-MAPP project (grant: 870377) as well as the funding support from the PRODEX program managed by the European Space Agency (ESA) with help of the Belgian Science Policy Office (BELSPO). Reference[1] Nancy L. Chabot et al (including Petrescu E.), PSJ 5:49 (24pp), 2024 February 2024;[2] Nicholas Moskovitz et al 2024 Planet. Sci. J. 5 35;[3] https://www.observatoiredesmakes.com/[4] Jehin E. et al, The Messenger, vol. 145, p. 2-6, 2

The binary asteroid 65803 Didymos-Dimorphos is the target of the first asteroid deflection test (NASA's Double Asteroid Redirection Test, DART) and the first binary asteroid system that will be characterized by a rendezvous mission (ESA's Hera). The cohesive strength of the fast-spin-primary Didymos is a key factor that could affect the impact outcome and stability of this system. To support the preparation and data interpretation of these missions and gain a better understanding of the formation and evolution of this system, we investigate the structural stability and cohesive strength of Didymos based on current observational information. We use the Didymos radar shape model to construct rubble-pile models consisting of ~40,000 to ~100,000 particles with different arrangements and size distributions. To investigate the effect of cohesion on the structural stability and dynamical behaviors of Didymos, we explicitly simulate the YORP spin-up process of these rubble-pile models from a slow spin state to Didymos' current spin state using a high-efficiency soft-sphere-discrete-element-model code, pkdgrav. We test the creep stability of Didymos' rubble-pile representation with different values of cohesion and derive the critical amount of cohesion to maintain stability. The results show that Didymos should at least have a minimum bulk cohesion on the order of 10 Pa to maintain its structural stability if the interparticle tensile strength is uniformly distributed. Since the surface particles are less bonded by cohesive contacts than the interior particles, the internal macroscopic cohesion is about three times the surface macroscopic cohesion. We find that the bulk density and particle arrangement and size distribution of Didymos have significant influences on its critical cohesion and failure behaviors, indicating different binary formation pathways. With the critical cohesion, Didymos is at the edge of maintaining a stable shape, and a rapid small decrease in its spin period would excite its rubble-pile structure and lead to reshaping or mass shedding. Whether the DART impact could partially or globally destabilize this system requires further investigation of the full two-body gravitational dynamics and the ejecta evolution. With the expected measurements returned by DART's onboard cubesat LICIACube in 2022 and Hera in 2027, the correlations between Didymos' physical properties and failure behaviors found in this study may be possible to constrain the mechanical properties and evolutionary history of this binary system.

The Double Asteroid Redirect Test (DART) is NASA’s first kinetic impact–based asteroid deflection mission. The DART spacecraft will act as a projectile during a hypervelocity impact on Dimorphos, the secondary asteroid in the (65803) Didymos binary system, and alter its mutual orbital period. The initial momentum transfer between the DART spacecraft and Dimorphos is enhanced by the ejecta flung off the surface of Dimorphos. This exchange is characterized within the system by the momentum enhancement parameter, β, and on a heliocentric level by its counterpart, β ⊙. The relationship between β and the physical characteristics of Dimorphos is discussed here. A nominal set of Dimorphos physical parameters from the design reference asteroid and impact circumstances from the design reference mission are used to initialize the ejecta particles for dynamical propagation. The results of this propagation are translated into a gradual momentum transfer onto the Didymos system barycenter. A high-quality solar system propagator is then used to produce precise estimates of the post-DART encounters between Didymos and Earth by generating updated close approach maps. Results show that even for an unexpectedly high β ⊙, a collision between the Didymos system and Earth is practically excluded in the foreseeable future. A small but significant difference is found in modeling the overall momentum transfer when individual ejecta particles escape the Didymos system, as opposed to imparting the ejecta momentum as a single impulse at impact. This difference has implications for future asteroid deflection campaigns, especially when it is necessary to steer asteroids away from gravitational keyholes.

Our knowledge of the internal structure of asteroids relies entirely on inferences from remote sensing observations of the surface and theoretical modeling. Many fundamental questions are still unresolved. Is the body a monolithic piece of rock or a rubble-pile, and how high is the porosity? What is the typical size distribution of the constituent blocks? Are these blocks homogeneous or heterogeneous? Direct measurements of an asteroid’s deep interior structure are needed to better understand asteroid accretion and their dynamic evolution. The characterization of the asteroids’ internal structure is crucial for science, planetary defense and exploration.In orbit sounding Radars are the most mature instruments capable of achieving the objective of characterizing the internal structure and heterogeneity, both for the benefit of science as well as for planetary defense or exploration. It is the goal of JuRa on cruise to probe Didymos binary system in 2027 and aim of two instruments under development for the ESA Ramses Mission and the Caltech Mission is to study the deep interior of Apophis in 2029. JuRaJuRa, the Juventas radar, onboard the ESA HERA mission, is a monostatic radar, BPSK coded and inherited from CONSERT/Rosetta. The center frequency of 60 MHz provides the capability to probe the internal structure to a depth of 100 meters or more, with a limited vertical resolution of about 20 m. Multipass processing makes it possible to create a 3D tomographic image of the interior. HERA was launched in October 2024 to deeply investigate the Didymos binary system and in particular its moonlet Dimorphos, five years after the DART/NASA impact. On the ESA mission HERA, the Juventas 6U CubeSat is carrying the Juventas Radar (JuRa).JuRa maps the backscatter coefficient (sigma zero - σ0) of the surface and of the subsurface, which quantifies the returned power per unit surface or volume. It is related to the degree of heterogeneity on the wavelength scale and to the dielectric contrast of heterogeneities, giving access to both, the sub-meter texture of the constituent material and larger scale structures.The main objective of JuRA is to characterize the asteroid interior, to identify internal geological structure such as layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution from sub-metric to global scale. The second objective is to estimate the average permittivity and to monitor its spatial variation in order to obtain information on its composition and porosity. The characterization of the constitutive material and its spatial organization both on the main and on the moon will help to better model the formation and stability conditions of the binary system and the response of Dimorphos to the DART impact. Radar to ApophisKnowledge of the internal structure of Apophis is crucial to better understand its accretion and dynamical evolution, to better study its stability conditions, and to model its response to the gravitational constraints imposed by its approach to induced by Earth close approach.For the RAMSES mission, the radar is operating in monostatic mode on one of the CubeSats and deeply inheriting from JuRa on Juventas/Hera, with minor modification. The radar is currently under development and under responsibility of Tyvak International, prime contractor for the CubeSat development. For the electronics (1U, 1kg), minor optimizations are proposed based on the experience from JuRa (mainly thermal optimization, adjustment of the network position and software modifications). The interfaces are revised depending on the carrying platform inheriting from Hera/Milani. For the antenna, a crossed dipole is proposed, as for JuRa on Juventas, providing measurements in full polarization. The cross dipole is consisting of 4 booms of 65g each. An optimization of the boom length and matching network will be required depending on the platform, on the antenna accommodation and on the solar array geometry.The Caltech Mission to Apophis is under study by Caltech and JPL. For his mission, the radar is a customized version of JuRa, operating in bistatic mode: the two satellites are maintained opposite from each other around Apophis, using semi-autonomous navigation based on optical cameras. Electronics on the two platforms measure the signal transmitted throughout Apophis, like done for CONSERT on Rosetta/ESA. Using the synchronization provided by the Inter Satellite Link (ISL), the radar provides an absolute measurement of the propagation delay between the platforms through the asteroid. This provides a direct measurement of the dielectric permittivity, which is related to composition and microporosity of the inner structure of Apophis. Partial coverage will provide slices of the body with an average characterization and its spatial variability to characterize large scale structures. Dense coverage will provide a larger diversity of observation angles, the bistatic mode will then allow a complete 3D tomography to recover the permittivity contrast throughout the volume.In addition to modifications depending on the new platform and the antenna accommodation and the adjustments based on JuRa lesson learned, the implementation of the bistatic mode requires a deeper revisit of the software and firmware to preserve synchronization of the two electronics, while the ISL is ensuring the control of the timing and the frequency.In this talk we will present the instruments, their status, performances and goals as well as the science objectives in the context of the different targets.

The Asteroid Impact and Deflection Assessment (AIDA) is an international collaboration supported by ESA and NASA to assess the feasibility of the kinetic impactor technique to deflect an asteroid, combining data obtained from NASA’s DART and ESA’s Hera missions [1, 2]. In 2022, DART will perform a kinetic impact on the secondary of the binary near-Earth asteroid (65,803) Didymos, recently named Dimorphos. After 2 years, Hera will follow-up with a detailed post-impact survey of Didymos, to fully characterize this planetary defense technique. Additionally, Hera will deploy two CubeSats around Didymos once the Early Characterization Phase has completed, to complement the observations of the mother spacecraft and increase the scientific return of the mission. The first Cubesat, called Juventas, will complete a low-frequency radar survey of the secondary, to unveil its interior, while the second one has not yet been selected.One of the main objectives of Hera is to characterize the mass and mass distribution of both Didymos primary and secondary by means of radio science investigations. This paper describes the concept of the gravity science investigations to be jointly carried out by the three mission elements, i.e. Hera, Juventas and CubeSat-2. The experiment will combine classical ground-based radiometric measurements, spacecraft-based optical images of Didymos, and Satellite-to-Satellite radiometric tracking between Hera and the Cubesats. Finally, our results and achievable accuracy for the estimation of the mass and gravity field of Didymos and Dimorphos are presented.1. IntroductionMost of the information about the formation processes of an asteroid lies in its interior structure. One of the very few constraints of the internal mass distribution of a celestial body is given by its gravity field, even if the inversion process is not unique. First, the bulk density can be inferred by measuring the mass of the body, combined to the volume estimated from optical images. In addition, the higher degrees of the gravity field provide information about the internal distribution of mass, such as the moments of inertia.The main scientific goals of the Hera radio science investigations are:Determine the mass and gravity field of Didymos and Dimorphos; Reconstruct the motion of Dimorphos around Didymos; Contribute to the characterization of the energy transfer between DART and Dimorphos. Such objectives are a valuable contribution to the Hera mission objectives, leading to a better understanding of the formation and evolution processes of the Didymos system.2. TechniqueThe determination of the gravity field of a celestial body is an application of the orbit determination process of deep space spacecraft. In particular, the gravity of Didymos can be estimated precisely reconstructing the trajectory of Hera during a selected number of close encounters (about 10 km at closest approach). The classical observables used in the orbit determination are obtained from the X-band radio link between the spacecraft and the Earth. The microwave signal is sent to spacecraft from a ground antenna and coherently retransmitted back to Earth, where Doppler and range measurements are obtained. A previous study performed for the AIM proposed mission [4] demonstrated that gravity science at Didymos is feasible using radio tracking data only, under realistic assumptions on the technological capabilities of the space and ground segment. Shorter pericenter distances increase the attainable accuracy. However, a significant improvement can be obtained even at relatively large distances processing also optical images of Didymos and Dimorphos taken by the spacecraft.In addition, Hera may track Juventas and CubeSat-2 by means of a space-to-space inter-satellite link (ISL), capable of determining the relative distance (ranging) and the relative line-of-sight velocity (Doppler) between the two bodies. In particular, the latter is expected to represent a very nice add-on to the gravity investigation carried out by the Hera mission, as the Doppler shift that affects the inter-satellite link contains the information on the dynamics of the system, i.e. masses and gravity field of Didymos and Dimorphos.The expected accuracy in the estimation of Didymos gravity fields were obtained through numerical simulations of the orbit determination of Hera and the two Cubesats. Conservative assumptions were made in terms of both radiometric and optical measurement noises, and large a-priori uncertainties for the estimated parameters were used.  3. ResultsAs a result of the numerical simulations, the masses of Didymos and Dimorphos are expected to be estimated with relative uncertainties less than 10-4 and 10-3, respectively. The addition of the ISL measurements improves the achievable accuracies but it is not required to estimate the masses. However, given the relatively large distance of Hera from the system, the higher degree gravity of Didymos and Dimorphos can be estimated only adding the ISL Doppler measurements between the Cubesats and the mother spacecraft. In this case, the gravity field of Didymos can be estimated to at least degree 3, depending on the assumptions about the ISL operations and performance. Similarly, ISL Doppler measurements allows to estimate the extended gravity field of Dimorphos up to degree 2, with an uncertainty of about 10%.AcknowledgementsThis project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP). MZ, IG, ML, EG, RLM, and PT wish to acknowledge Caltech and the Jet Propulsion Laboratory for granting the University of Bologna a license to an executable version of MONTE Project Edition S/W.

The Hera mission has been approved for development and launch in the new ESA Space Safety Programme by the ESA Council at Ministerial Level, Space19+, in November 2019. Hera will both offer a high science return and contribute to the first deflection test of an asteroid, in the framework of the international NASA- and ESA-supported Asteroid Impact and Deflection Assessment (AIDA) collaboration.The impact of the NASA DART (Doube Asteroid Redirection Test) spacecraft on the natural satellite of Didymos in October 2022 will change its orbital period around Didymos. As Didymos is an eclipsing binary, and close to the Earth on this date, the change can be detected by Earth-based observers. ESA’s Hera spacecraft will rendezvous Didymos four years after the impact. Hera’s instruments will perform the measurements necessary to understand the effect of the DART impact on Didymos’ secondary, in particular its mass, its internal structure, the direct determination of the momentum transfer and the detailed characterization of the crater left by DART. This new knowledge will also provide unique information on many current issues in asteroid science.From small asteroid internal and surface structures, through rubble-pile evolution, impact cratering physics, to the long-term effects of space weathering in the inner Solar System, Hera will have a major impact on many fields. For instance, collisions play a fundamental role in our Solar System history, from planet formation by collisional accretion to cratering of solid surfaces and asteroid family formation by collisional disruption. The fully documented hypervelocity impact experiment provided by DART and Hera will feed collisional models with information obtained at actual asteroid scale and for an impact speed (~6 km/s) that is close to the average impact speed between asteroids in the main belt. Moreover, Hera will perform the first rendezvous with an asteroid binary, characterize the smallest object ever visited (165 m in diameter) and provide the first direct measurement of an asteroid interior. Additionally, studies using Hera data will in turn affect our understanding of the asteroid population as a whole. The scientific legacy of the Hera mission will extend far beyond the core aims of planetary defense.Acknowledgment: The authors acknowledge funding support from ESA and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP), from the European Space Agency and from the French space agency CNES.

The Double Asteroid Redirection Test (DART) mission impacted Dimorphos, the moonlet of the binary asteroid 65803 Didymos, on 2022 September 26 and successfully tested a kinetic impactor as an asteroid deflection technique. The success of the deflection was partly due to the momentum of the excavated ejecta material, which provided an extra push to change Dimorphos’s orbital period. Preimpact images provided constraints on the surface but not the subsurface morphology of Dimorphos. DART observations indicated that Dimorphos contained a boulder-strewn surface, with an impact site located between a cluster of large surface boulders. In order to better understand the momentum enhancement factor (β) resulting from the impact, we performed impact simulations into two types of targets: idealized homogeneous targets with a single boulder of varying size and buried depth at the impact site and an assembly of boulders at the impact site with subsurface layers. We investigated the relative effects of surface morphology to subsurface morphology to put constraints on the modeling phase space for DART following impact. We found that surface features created a 30%–96% armoring effect on β, with large surface boulders measuring on the order of the spacecraft bus creating the largest effect. Subsurface effects were more subtle (3%–23%) and resulted in an antiarmoring effect on β, even when layers/boulders were close to the surface. We also compared our 2D axisymmetric models to a 3D rectilinear model to understand the effects of grid geometry and dimension on deflection efficiency computational results.

An asteroid mitigation demonstration mission is gaining interest among the planetary defense community to better understand the challenges and the dynamics of a small solar system body (SSSB) impact scenario. The Kinetic Impactor (KI) deflection technique, considered the most mature and cost effective approach for deflecting SSSBs, gained more credibility following the numerous studies performed (Don Quijote, NEOShield-2, preparations for DART mission, …) and last but not least the successful targeting of the Deep Impact (DI) spacecraft into comet 9P/Tempel 1. A dual-launch concept Asteroid Impact Deflection Assessment (AIDA) with KI (DART) and an Explorer spacecraft (AIM) is currently under study by the ESA and NASA. While one of the more mature deflection options, there are still a significant number of poorly constrained aspects of the KI deflection technique. Of particular interest are the complex ejecta cloud dynamics that can have a considerable impact on the deflection efficiency — better known as the momentum enhancement effect. Understanding the beta factor is considered paramount as it bears the potential of overall mission reduction cost and is inherently linked to SSSB geotechnical properties. Therefore, measuring this enhancement effect will be one of the top-level scientific requirements for future demonstration missions. This paper discusses the aspects of beta estimation techniques for a mitigation demonstration mission and reveals (1) the geometric considerations of the impact trajectory and the near-Earth object (SSSB) heliocentric orbit, (2) the requirements on the impact spacecraft navigation accuracies and (3) the prerequisites on the SSSB characterisation. An illustrative scenario and results are presented where the work concludes with considerations for an asteroid mitigation demonstration mission design aiming at measuring the beta-factor.