The perihelion of comet 1P/Halley in 2061 is an excellent chance to revisit this object of outstanding scientific and cultural relevance. During its 1986 approach to the Sun, it was targeted by several flyby missions. Due to its retrograde, highly-inclined orbit, the relative velocities during the encounter were large, limiting the scientific return due to the short time spent inside the coma. A rendezvous trajectory would overcome this limitation, but the design is challenging due to the limitations of current propulsion technology. Given the lead times of spacecraft development and the long duration of the interplanetary transfer to the comet, it is imperative to start mission planning as soon as possible. We present a novel rendezvous strategy, combining unpowered Jupiter and Saturn gravity-assists with deep-space low-thrust arcs. It minimizes launch energy and propellant budget, constraining the arrival to occur before the onset of high activity. The double flyby strategy reduces the otherwise prohibitive cost of the plane change maneuver. Closed expressions for the optimal flyby geometry, together with an explicit low-thrust transcription technique, reduce the number of design parameter to three, improving computational efficiency. Crucially, this is the first rendezvous mission concept achievable with well-proven technology (standard radioisotope thermoelectric generators and a Hall-effect thruster) and compatible with several existing launchers. We describe the trajectory optimization strategy and perform a comprehensive exploration of the design space. Finally, we present two promising proof-of-concept trajectories in detail.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of similar to 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of similar to 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at similar to 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of eta greater than or similar to 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, and z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (greater than or similar to 8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time will provide after it begins in early 2026.

Comets are the most pristine planetesimals left from the formation of the Solar System. They carry unique information on the materials and the physical processes which led to the presence of planets and moons. Many important questions about cometary physics, such as origin, constituents and mechanism of cometary activity, remain unanswered. The next perihelion of comet 1P/Halley, in 2061, is an excellent opportunity to revisit this object of outstanding scientific and cultural relevance. In 1986, during its latest approach to the Sun, several flyby targeted Halley's comet to observe its nucleus and shed light on its properties, origin, and evolution. However, due to its retrograde orbit and high ecliptic inclination, the quality of data was limited by the large relative velocity and short time spent by the spacecraft inside the coma of the comet. A rendezvous mission like ESA/Rosetta would overcome such limitations, but the trajectory design is extremely challenging due to the shortcomings of current propulsion technology. Given the considerable lead times of spacecraft development and the long duration of the interplanetary transfer required to reach the comet, it is imperative to start mission planning several decades in advance. This study presents a low-thrust rendezvous strategy to reach the comet before the phase of intense activity during the close approach to the Sun. The trajectory design combines a gravity-assist maneuver with electric propulsion arcs to maximize scientific payload mass while constraining transfer duration. A propulsive plane change maneuver would be prohibitive. To keep the propellant budget within reasonable limits, most of the plane change maneuver is achieved via either a Jupiter or a Saturn flyby. The interplanetary low-thrust gravity-assisted trajectory design strategy is described, followed by the presentation of multiple proof-of-concept solutions.

Among solar system objects, comets coming from the Oort Cloud are an elusive population, intrinsically rare and difficult to detect. Nonetheless, as the more pristine objects we can observe, they encapsulate critical cues on the formation of planetary systems and are the focus of many scientific investigations and science missions. The Legacy Survey of Space and Time (LSST), which will start to operate from the Vera C. Rubin Observatory in 2025, is expected to dramatically improve our detection ability of these comets by performing regular monitoring of the Southern sky deep down to magnitude 24.5 with excellent astrometry. However, making straightforward predictions on future LSST detection rates is challenging due to our biased knowledge of the underlying population. This is because identifications to date have been conducted by various surveys or individual observers, often without detailed information on their respective selection functions. Recent efforts (see e.g. Vokrouhlicky` et al., 2019) to predict incoming flux of Long Period Comets still suffer of the lack of systematic, well-characterized, homogeneous cometary surveys. Here, we adopt a different point of view by asking how much earlier on known comets on long-period or hyperbolic orbits would have been discovered by a LSST-like survey if it was already in place 10 years prior to their perihelion epoch. In this case, we are not simulating areal flux of incoming comet, as all comets in our sample reach the perihelion simultaneously, but we can analyze the impact of a LSST-like survey on individual objects. We find that LSST would have found about 40% of comets in our sample at least 5 years prior to their perihelion epoch, and at double (at least) the distance at which they were actually discovered. Based on this approach, we find that LSST has the potentiality to at least twofold the current discovery rate of long-period and hyperbolic comets.

The dust ejected by cometary nuclei encodes valuable information on the formation and evolution of the early Solar System. Multiple short-period comets have been studied in situ, but several perihelion passages considerably modified their pristine condition. Comet Interceptor is the first space mission selected by the European Space Agency to study a pristine dynamically new comet in situ. During a fast flyby through the comet coma, hypervelocity impacts with dust particles will represent not only an important source of information, but also a serious hazard to the spacecraft and its payload. Here we discuss the assessment tests performed on the dust shield of the Dust Impact Sensor and Counter instrument (DISC), part of the Comet Interceptor payload, which will be directly exposed to the cometary dust flux. Using a Light-Gas Gun, we shot mm-sized particles at similar to 5 km/s, transferring momenta and kinetic energies representative of those foreseen for the mission. The impact effects on the DISC breadboard were compared to theoretical predictions by a ballistic limit equation for hypervelocity impacts. We find that, with a simple improvement in the dust shield design, DISC is compatible with the expected cometary environment.

The return of Comet 1P/Halley will promote a worldwide interest for ground and space observations of a celestial body of outstanding scientific and cultural interest. In addition to remote observations, space will open the possibility of in situ study, similarly to what was done during the passage of 1986. In this paper, we first discuss the scientific motivations for a rendezvous mission capable to overcome the limitations of the flyby missions that took place at that time. In the second part, we describe an example of a rendezvous trajectory that can be carried out with existing power and propulsion technologies, i.e., with radioisotope thermoelectric generators and a Hall effect thruster. Furthermore, the transfer is made possible by the gravitational assistance of a giant planet. The resulting mission concept, nicknamed HCREM (Halley Comet REndezvous Mission), selected from a number of cases treated in a previous paper of ours (Beolchi et al., 2024), will be capable to reach the comet beyond the distance of Saturn, when the sublimation of super-volatile species (e.g. CO and CO2) will be ongoing, and well before the onset of the sublimation of water (expected to occur around 4 AU, namely at larger heliocentric distance than Mars). Following a direct transfer from Earth, a gravity assist with Jupiter inserts the spacecraft into the cometary orbital plane with retrograde motion. Electric propulsion modifies the trajectory so that the spacecraft reaches the target with zero relative velocity. After the rendezvous, the spacecraft will accompany the comet before, around and after perihelion, which will happen in July 2061, until the outbound crossing of the ecliptic and possibly even later. Given the large heliocentric distances reached by the spacecraft, our concept mission does not foresee the implementation of solar panels. In this way, some shortcomings deriving from the adoption of this technology onboard the Rosetta mission to comet 67P are avoided and operations can occur even inside the dense dust coma at short distance from the nucleus. In the third part of the paper, an innovative imaging system with a very large field of view of approximately 100 degrees is proposed. This optical system allows the simultaneous capture of both details of the cometary surface and the surrounding space within a single image frame. For several degrees outside the borders of the nucleus, it allows following the trajectories of chunks and clouds ejected by pits or fractures, all phenomena crucial to the understanding of the cometary activity. In the conclusions, we stress that a concerted effort is needed in the current decade to plan and approve a rendezvous mission to 1P. Indeed, the scenario here described requires launching before 2040, less than 15 years from now. Later launches with existing rockets imply a severe loss of scientific knowledge, because the spacecraft will not be able to reach the comet before the onset of water sublimation.

Comets represent the most preserved planetesimals we can nowadays study and dust is one of their major components. Once emitted in the coma, cometary dust particles represent anisotropic scatterers of the incident solar light and their nature can be investigated with remote sensing studies. Among them, the measurement of the phase function curve has a key importance in several scientific aspects. It can be inverted with theoretical and laboratory studies to derive hints on the intimate nature of the emitted dust. It is also needed in adjusting cometary dust production rates for phase angle effects when data obtained throughout large time intervals are correlated. Finally, it is useful for space instruments planning since it provides inputs for optimal exposure times for remote sensing sensors which observe the coma spanning a large range of phase angles during close approaches. This will be particularly valuable in the framework of the future ESA Comet Interceptor mission which is going to fly-by a Dynamically New Comet entering our Inner Solar System for the very first time, carrying instruments which will image the coma with different observing geometries and phase angles in a short amount of time. In order to provide an useful tool to address the aforementioned scientific topics, we used available literature data to build a new composite phase function for cometary dust comae. This was obtained fitting Henyey-Greenstein functions to the original data of 11 comets and then connecting them in a continuous way as all data values were coming from a single average comet. We then fitted our result with a compound Henyey-Greenstein curve and compared it with previous models which were not including recent literature data constituting fine follow-ups of comets at small and large phase angles. The main difference is found in the description of the forward scattering surge, where our model depicts intensity one order of magnitude larger than previous ones. This finding is extremely important since it shows that the choice of the model may have severe consequences when interpreting, or instrumentally planning, forward scattering data.

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.

The Dust Impact Sensor and Counter (DISC), part of the payload of Comet Interceptor mission, will determine the coma dust features of the mission target comet. DISC sensing plate will be exposed to cometary dust hypervelocity impacts (HVI), because of the high flyby speed (10– 60 km/s) foreseen for the space probe. For this measurement configuration, an efficient dust shield is mandatory, to protect the electronics of the detector and the housing S/C. Aiming at withstanding the predicted HVI without affecting DISC measurements and significantly increasing the payload mass, we designed a light aerogel-stuffed Whipple shield. In this work, we present an analysis of the dust-shield performances by computing the related Ballistic Limit Equations (BLEs) with the support of laboratory tests and numerical simulations. Our results reveal that the DISC dust shield exhibits resilience against particles up to 7 mm in size impacting DISC at 20 km/s—resulting in a momentum 100 times larger than impact predicted during the Comet Interceptor flyby. In addition, we find that the DISC dust shield withstands, for fixed impact velocity, critical particle diameters about four times larger than those stopped by a classic Whipple shield, with a Bumper and Rear wall configuration of equal areal density.

In the last decade an increasing number of CubeSat-class spacecraft have been involved in the execution of space missions, operating at distances from our home planet ranging between Low-Earth Orbit to Mars. In particular, the Light Italian CubeSat for Imaging of Asteroids (LICIACube) mission of Agenzia Spaziale Italiana (ASI, Italian Space Agency) has contributed to the Double Asteroid Redirection Test (DART) mission of National Aeronautics and Space Administration (NASA) by witnessing, on September 26th, 2022, the successful impact of DART spacecraft with the asteroid (65803) Didymos I Dimorphos, and revealing the immediate aftermaths of the event while performing a quick flyby, successfully participating in the first Planetary Defense real-scale test ever conducted by humanity. LICIACube mission, leveraging on national competences and together with the international collaboration with the DART mission, has been able to improve the operational, technical, managerial and scientific know-how at national level, together with creating new records in interplanetary exploration. The concept of modularity in space segments, for the achievement of ever complex mission goals, has recently found favourable conditions thanks to the increasing capabilities of smaller platforms. In particular, the present work aims also at highlighting and describing the value of the LICIACube contribution in validating and verifying the outcome of the kinetic impact method with the presence of a proximity observer spacecraft.

IntroductionThe NASA Double Asteroid Redirection Test (DART) mission will be the first test to check an asteroid deflection by a kinetic impactor. The target of DART mission is Dimorphos the secondary element of the (65803) Didymos binary asteroid system, and the impact is expected in late September – early October, 2022 [1] The DART S/C will carry a 6U cubesat called LICIACube (Light Italian Cubesat for Imaging of Asteroid) [2], provided by the Italian Space Agency, with the aim to collect pictures of the impact’s effects. On board LICIAcube will be hosted 2 camera payloads: LEIA a panchromatic (400-900nm Filter, 2.9x2.9° FOV) Narrow Angle Camera and LUKE a RGB (Bayer color filter, 4.8 x 9.15° FOV). LICIACube will be able to acquire the structure and evolution of the DART impact ejecta plume and will obtain high-resolution images and 2 colours data (B-G, G-R) of the surfaces of both bodies and the plume.In order to check the imaging capability and to optimize the fast scientific phase of LICIACube, the LICIACube team performed simulations of pictures’ acquisition. In these simulations, considering the specifications of the 2 optical payloads and the foreseen mission design, we reconstructed synthetic images mainly of the plume. Since the study of the plume and its evolution is one of the main scientific goal of the mission we performed a scattering modelling of the ejecta in order to invert the future photometric data deriving hints on the intimate nature of the dust particles released by the impact.Plume simulated Images and column densityWith the two-fold aim of set the operative parameters for the Payloads and to understand the information retrievable by the images of the evolving plume we started an imaging simulation activities taking into account:LICIAcube mission design [3] (Trajectory, Speed, illumination conditions) Payloads optical characteristics The plume evolution was simplified assuming:Non colliding particles during the plume evolution; A speed distribution in the plume given by eq: Where x is the distance on Dimorphos surface from the DART impact point and the other parameters used, considering as main material of asteroid system the cemented basalt, are reported in table:We considered the most representative 3 size bins for what concerns the ejected mass, the expected total number of particles are reported in table:In Figure 1 is reported the simulated image obtained considering the LICIACube trajectory 50s before the close approach (about 110 s after the DART impact).Figure 1 Plume simulated image relative values for irradianceOnce the simulated column density image was obtained, we added a scattering simulation considering spherical dust particles and using a Mie code well suited for large particles approaching the geometric optics regime [4]. In this way we were able to translate column densities in luminous fluxes measured by the instrument using a methodology described in the next section.Plume colours scattering modellingRGB data of the ejecta plume can be used to derive hints on the physical properties of the ejected particles through scattering modelling of the measured two colours (B-G, G-R) and the phase function versus the phase angle of observation α.Given the intensity of solar light incident on the plume’s single particle Iinc,, considering the incident solar light as unpolarized, the intensity of light scattered by the particle at α, Isca is given by [5]:where S11(α) is the first element of the 4X4 scattering Müller matrix, k=2π/λ is the wave number, and r is the distance between the particle and the observer. In this case: being FSun the solar flux at 1 AU, rh the heliocentric distance of the dust particle, and a its radius.The Mie code provides the complete scattering matrix once the dimension of the particle and its composition in terms of the complex refractive index of the material at the considered wavelength are given as input. We used largely referenced laboratory data on basaltic materials to obtain the optical properties of the dust particles [6]. This composition is used to model the dust particles residing on the asteroid surface [1], [2].Then, in order to find the intensity due to the scattering of a single particle measured by the instrument at phase angle α, we convolved Isca with the photometric response of the instrument. For a generic filter, such measured intensity is where Resp is the photometric response of the instrument extended throughout the bandpass of the filter. This response is a known product of several factors as the entrance pupil of the system, the reflectivity of the optics, the transmission curve of the filter, the quantum efficiency of the detector, and the exposure time.Synthetic colours of the dust particles can therefore being computed being the generic color A-B = -2.5log(IA/IB). We performed sample scattering colour calculations varying the particle size from 0.1 micron to 1 cm.Small particles provide extremely variable colours due to the strong influence of scattering resonances being the incident wavelength comparable with the size of the particles themselves. Colours get stable for a larger interval of phase angle proportionally to the increase of the size. Observations of stable colours in the plume during LICIACube flyby will be indicative of particles larger than 100 micron. At the same time, large basalt particles provide a flatter phase function at intermediate and small phase angles than smaller particles.Combined observations of the plume phase function and colour will therefore effectively constrain the size of the ejected particles providing theoretical inputs to the dynamical models.Acknowledgements: The LICIACube team acknowledges financial support from Agenzia Spaziale Italiana (ASI, contract No. 2019-31-HH.0 CUP F84I190012600).References[1] Cheng et al. P&SS 121 (2016).[2] Dotto et al. 2020, " LICIACube - the Light Italian Cubesat for Imaging of Asteroids In support of the NASA DART mission towards asteroid (65803) Didymos” P&SS, submitted.[3] Capannolo et al. 70th IAC Conference Paper (2019).[4] Wolf and Voshchinnikov. Computer Physics Communications 162 (2004).[5] Bohren & Huffman. Absorption and Scattering of Light by Small Particles. Wiley (1983).[6] Pollack et al. Icarus 19 (1973).

Introduction: The NASA Double Asteroid Redirection Test (DART) impact [1], was the first space mission that successfully demonstrated the kinetic impactor technique for planetary defense. It was at the same instant, on 26th September 2022, when ASI/Light Italian Cubesat for Imaging of Asteroids (LICIACube) [2] was the first Cubesat to image the plume coming from Dimorphos, the smaller body of the binary asteroid (65803) Didymos. The DART impact into Dimorphos [3] caused ejecta plume propagation with high velocity and very filamentary structure, composed of dust particles from μm to cm sizes in size [4]. The large aperture and observed spikes did not prevent propagation of larger excavated material, namely, boulders up to ~2 m [5]. Far-field observations such as HST clearly showed dust tail formed from the low-speed ejecta dust due to solar radiation pressure (SRP) [4].The scientific objectives: The estimation of the size distribution and velocity distribution of the plume in close vicinity to Dimorphos, captured in the LICIACube images is still an unanswered question. While the long-term monitoring of the tail can reveal the size distribution up to tens of cm in size, the impact simulations can constrain the initial velocity of the excavated material. Near and mid – field simulations considering different dynamical properties at local scale can address the complex collimated but inhomogeneous distribution of the dust within the plume. Here, we discuss some of the dynamical properties of the plume using the available observational DART and LICIACube data of the plume propagation. We try to constrain the particle sizes within the collimated plume structures.The model: We apply the 3D+t model – LIMARDE [6,7] constrained with laboratory observations [8], impact simulations and near- and far- field observations such as the LICIACube [9] images and HST [2] dust observations, respectively. The model computes single particle trajectories, the dust rotational frequencies and velocity as well as the particle orientation at any time and distance. We compute the dust velocity distribution based on the physical properties (size, mass and shape) derived from the LICIACube observations. The results are useful to check what is the role of the fragmentation of the particles and to constrain the physical properties based on the dynamical properties of the ejected dust in the near- and mid- environment.Fig. 1. The dust speed and rotation frequency of particles with different shapes as constitutes of the dust clumps shown in the observations of ASI/LICIACube/LUKE, 2022-09-26T23:17:04.Discussion: The LICIACube observations suggest that we have the locations of accumulation of different particles along the collimated plume streamers. The latter may contain particles of the same density and shape but with different velocity and rotation due to the initial ejected position and form not-linear motion within the collimated filament – like structures. In Fig. 1 we show LIMARDE simulations with particles of different shapes that result with different velocities suggesting a scenario where the dusty clumps could occur at the same location due to motion of particles with different shapes. The study discusses what is the probability that these dust clumps are formed owing to fragmentation, or their location is a result of their motion history of the ejected particles.Acknowledgements: This research was supported by the Italian Space Agency (ASI) within the LICIACube (ASI-INAF agreement AC n. 2019-31-HH.0).References: [1] Rivkin, A.S. et al. 2021, PSJ, 2, 24pp; [2] Dotto, E. et al. 2021, PSS 199, [3] Daly, R.T. et al. (2023) Nature. [4] Li, J.-Y., et al. (2023) Nature. [5] Farnham et al. LPSC abs. [6] Ivanovski et al. 2023, u.rev.; [7] Fahnestock et al. 2022, PSJ; [8] Ormo et al. 2022, E&PSL [9] Dotto et al. 2023, Nature

We spectroscopically characterized the Didymos system, target of the Double Asteroid Redirection Test (DART)/Light Italian Cubesat for Imaging of Asteroids (LICIACube) space mission, close in time to the DART impact event, during six nights between 2022 August and November at Telescopio Nazionale Galileo. Here, we show that near-infrared (NIR) spectra (0.75-2.25 mu m) look mostly similar within the same night and between different nights. They are in good agreement with the only spectrum previously available in the literature, observed almost 20 years before those reported in this paper. During one of the observing nights we also obtain spectroscopy information on the ejecta tail induced by the DART impact. The spectrum of the ejecta tail is also very similar to Didymos/Dimorphos itself. All of these aspects seem to suggest that the Didymos system in the NIR looks mostly homogeneous, with very subtle spectral variations.

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 first stages of solids aggregation in the proto–solar nebula start with hit–and–stick collisions of sub–micrometer sized grains which form porous fractal aggregates with low fractal mass dimension Df < 2. Subsequent stages lead to aggregates compaction in non–fractal structures and to the appearance of the so–called bouncing barrier, increasing the impact energy between solid particles and the degree of decoupling with the surrounding gas (Blum & Wurm 2008). The outcome of dynamical simulations points to the concentration of solid material through streaming instability of the gas as the major agent followed by gravitational collapse to overcome the bouncing barrier (Goodman & Pindor 2000). Nevertheless, some of the primitive formed fractals, characterized by fractal dimension slightly lower than 2, survived the subsequent planetesimal formation and were recently found in comet 67P/Churyumov–Gerasimenko after their release from the nucleus due to cometary activity (Mannel et al. 2016; Fulle & Blum 2017).The dynamics of fractal solids and their interaction with a surrounding gaseous medium is therefore of major interest in studying the first phases of solid aggregation in the solar system and the nature of the pristine material released by comets in space.In the present work we study fractal clusters of monodisperse spherical monomers having two different morphologies, one extremely compact and one extremely loose. They are produced through statistical aggregation methods called particle-cluster aggregation and cluster-cluster aggregation, respectively. In the particle-cluster aggregation method a monomer, represented by a single spherical object, is used as a projectile and shot towards the center of mass of a cluster made by identical monomers (the target), sticking with it at the first contact point. No restructuring of the aggregate after the collision is taken into account. The target is therefore growing in mass at every hit and the final result is a porous object characterized by a compact overall shape. In the cluster-cluster aggregation identical cluster of monomers are colliding along a line connecting the two centers of mass, sticking at the contact point without restructuring. The final result is a porous object with a fluffy open appearance. The two processes are iterated until the desired mass is reached. The porosity and fractal mass dimension of the two kinds of aggregates increase with the number of constituent monomers and reach asymptotic values for large aggregates. Cluster-cluster aggregates (CCAs) and particle-cluster aggregates (PCAs) are characterized by porosity p~97% and p~75%, and fractal mass dimension Df~2.0 and Df~3.0 respectively, in the limit of large aggregates (Bertini et al. 2009). PCAs and CCAs resemble morphologically the classical ballistic particle-cluster aggregates (BPCAs) and ballistic cluster-cluster aggregates (BCCAs) used in dust studies in astrophysical environments (Mukai et al. 1992), respectively. Examples of the PCA and CCA structures are shown in Fig.1 with a number of monomers N = 2048, corresponding to the largest size used in our study.Due to a probabilistic nature of the fractal aggregate model, the generated fractal objects are not identical (even constructed with the same number of monomers). We derive statistics on the geometrical (e.g. cross-sections) and dynamical properties (e.g. moments of inertia) of the fractal objects.Assuming the dust particles are submitted in a gas flow of a simplified model of a cometary coma (Ivanovski et al. 2017), we derive asymptotic mean and dispersion in the translational and rotational motion of dust.References:Bertini, I., Gutierrez, P. J., & Sabolo, W. 2009, A&A, 504, 625Blum, J. & Wurm, G. 2008, ARA&A, 46, 21Fulle, M. & Blum, J. 2017, MNRAS, 469, S39Goodman, J. & Pindor, B. 2000, Icarus, 148, 537Ivanovski, S.L., Zakharov, V.V., Della Corte, V., Crifo, J-F., Rotundi, A., Fulle, M. 2017, Icarus 282, 333-350Mannel, T., Bentley, M. S., Schmied, R., et al. 2016, MNRAS, 462, S304Mukai, T., Ishimoto, H., Kozasa, T., Blum, J., & Greenberg, J. M. 1992, A&A, 262, 315

Throughout the initial four years of the operative mission, METIS coronagraph [1] carried out numerous scientific observations, including some focused-on comets. Among the observed cometary targets, there are periodic comets, like 2P/Encke, sunskirters, such as 96P/Machholz (see Figure 1), some sungrazers [2] and even a long period comet, the C/2021 A1 (Leonard) [3], having an orbital period of approximately 80,000 years. Although many of these observations, especially of periodic comets, were specifically planned, some comets were also identified a posteriori on images collected for solar corona studies.Figure 1: UV channel observation for the 96P/Machholz sunskirter comet during its transit in the METIS Field of View on January 30, 2023.Metis is the coronagraph onboard SolO and it has been conceived to acquire images of the solar corona both in linearly polarized visible light (VL, 580–640 nm) and narrow-band (±10 nm) ultraviolet (UV) around the HI Lyman-a (121.6 nm) spectral line. Metis is the first coronagraph able to perform such simultaneous observations.The instrument is designed to image the structure and dynamics of the full solar corona in an annular FoV covering the range from 1.6° to 2.9°, with a plate scale up to 10 “/px in VL channel and up to 20”/px in UV. Owing to the eccentricity of the spacecraft orbit, the heliocentric distances imaged are from 1.6 to 3.1 solar radii at minimum perihelion distance (0.28 au), up to the range from 6.0 to 12.0 solar radii when the spacecraft is around 1.0 au. A sketch of the raytrace of the two channels of the Metis coronagraph, i.e. the UV and VL, is given in Figure 2.Figure 2: Metis layout. On the top: the UV path. On the bottom: the VL path [4]. The ability of METIS to perform simultaneous imaging in a narrow UV band around HI Ly-alpha and in the visible wavelength range can be highly impactful in cometary studies. UV images enable the study of neutral hydrogen coma morphology and the estimation of the water outgassing rate from the comet nucleus. Conversely, visible polarization images allow the derivation of comet parameters correlated with the physical properties (distribution, density, size, ...) of the dust grains in the coma.In this work, a summary of the activities and main results obtained so far is presented, highlighting some original results obtained from METIS comet observations and sharing some valuable “lessons learned" from these four years of activity. AcknowledgementsSolar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. Metis was built and operated with funding from the Italian Space Agency (ASI), under contracts to the National Institute of Astrophysics (INAF) and industrial partners. Metis was built with hardware contributions from Germany (Bundesministerium für Wirtschaft und Energie through DLR), from the Czech Republic (PRODEX) and from ESA. References[1] Antonucci et al, A&A 642, A10 (2020).[2] Bemporad et al, A&A 680, A90 (2023).[3] Corso et al, EPSC2022-901 (2022)[4] Fineschi, S. et al., Exp. Astron. 49, 239-263 (2020).

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.

On 2022 September 26, NASA's Double Asteroid Redirection Test (DART) successfully hit Dimorphos, the smaller companion of the binary system formed with the asteroid (65803) Didymos. Both the binary system and the impact event were imaged by the Light Italian Cubesat for Imaging of Asteroids, detached from DART 15 days before the impact. Images from the onboard LUKE red, green, and blue camera together with ground-based observations enabled the reconstruction of Didymos's brightness phase curve, with phase angles ranging from 2.35° to 107.7°. The opposition effect regime was studied using the exponential-linear equation, the “Shevchenko” function and the linear-by-parts model while the IAU-official HG1G2 magnitude system was applied to the full phase curve. The opposition effect indicates an unusual asteroid surface for an S type, with characteristics similar to M-type asteroids. While the HG1G2 parameters from the full phase curve place Didymos well among asteroids of the taxonomic C complex. Didymos’s phase curve parameters when compared to near-Earth asteroids are very close to the Q type (1862) Apollo, indicating possible depletion of fine submicrometric grains through resurfacing. Didymos's geometric albedo (0.15 ± 0.01) is reported to be 30%–45% smaller than the average geometric albedo for near-Earth S types (0.26 ± 0.04). We propose that Didymos might be an LL ordinary chondrite analog containing albedo-suppressing, shock-darkened/impact melt minerals that have undergone resurfacing processes in the past. A comparison with meteorites indicates that, less likely, Didymos could also contain materials analog to carbon-bearing brecciated L3 ordinary chondrites.