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(10) Hygiea is the fourth largest main belt asteroid and the only known asteroid whose surface composition appears similar to that of the dwarf planet (1) Ceres 1 , 2 , suggesting a similar origin for these two objects. Hygiea suffered a giant impact more than 2 Gyr ago 3 that is at the origin of one of the largest asteroid families. However, Hygeia has never been observed with sufficiently high resolution to resolve the details of its surface or to constrain its size and shape. Here, we report high-angular-resolution imaging observations of Hygiea with the VLT/SPHERE instrument (~20 mas at 600 nm) that reveal a basin-free nearly spherical shape with a volume-equivalent radius of 217 ± 7 km, implying a density of 1,944 ± 250 kg m − 3 to 1 σ . In addition, we have determined a new rotation period for Hygiea of ~13.8 h, which is half the currently accepted value. Numerical simulations of the family-forming event show that Hygiea’s spherical shape and family can be explained by a collision with a large projectile (diameter ~75–150 km). By comparing Hygiea’s sphericity with that of other Solar System objects, it appears that Hygiea is nearly as spherical as Ceres, opening up the possibility for this object to be reclassified as a dwarf planet. SPHERE at the VLT observed Hygiea, the fourth largest body in the main belt and the parent body of a big asteroid family, at unprecedented spatial resolution. Its unexpected spherical shape without any impact crater is explained by numerical simulations with a big impact that fluidized the body, reassembling it in a rotational equilibrium regime.

Planetary rings are observed not only around giant planets 1 , but also around small bodies such as the Centaur Chariklo 2 and the dwarf planet Haumea 3 . Up to now, all known dense rings were located close enough to their parent bodies, being inside the Roche limit, where tidal forces prevent material with reasonable densities from aggregating into a satellite. Here we report observations of an inhomogeneous ring around the trans-Neptunian body (50000) Quaoar. This trans-Neptunian object has an estimated radius 4 of 555 km and possesses a roughly 80-km satellite 5 (Weywot) that orbits at 24 Quaoar radii 6 , 7 . The detected ring orbits at 7.4 radii from the central body, which is well outside Quaoar’s classical Roche limit, thus indicating that this limit does not always determine where ring material can survive. Our local collisional simulations show that elastic collisions, based on laboratory experiments 8 , can maintain a ring far away from the body. Moreover, Quaoar’s ring orbits close to the 1/3 spin–orbit resonance 9 with Quaoar, a property shared by Chariklo’s 2 , 10 , 11 and Haumea’s 3 rings, suggesting that this resonance plays a key role in ring confinement for small bodies. The authors report observations of a dense and inhomogeneous ring at a surprisingly large distance from the trans-Neptunian body Quaoar.

According to adaptive-optics observations by Ferrais et al., (22) Kalliope is a 150-km, dense and differentiated body. Here, we interpret (22) Kalliope in the context of bodies in its surroundings. While there is a known moon Linus, with a 5:1 size ratio, no family has been reported in the literature, which is in contradiction with the existence of the moon. Using the hierarchical clustering method (HCM) along with physical data, we identified the Kalliope family. Previously, it was associated to (7481) San Marcello. We then used various models (N-body, Monte-Carlo, SPH) of its orbital and collisional evolution, including the break-up of the parent body, to estimate the dynamical age of the family and address its link to Linus. The best-fit age is (900+-100) My according to our collisional model, in agreement with the position of (22) Kalliope, which was modified by chaotic diffusion due to 4-1-1 three-body resonance with Jupiter and Saturn. It seems possible to create Linus and the Kalliope family at the same time, although our SPH simulations show a variety of outcomes, for both satellite size and the family size-frequency distribution. The shape of (22) Kalliope itself was most likely affected by gravitational reaccumulation of `streams', which creates characteristic hills observed on the surface. If the body was differentiated, its internal structure is surely asymmetric.

The Ch-type asteroid (130) Elektra is orbited by three moons, making it the first quadruple system in the main asteroid belt. We aim to characterise the irregular shape of Elektra and construct a complete orbital model of its unique moon system. We applied the All-Data Asteroid Modelling (ADAM) algorithm to 60 light curves of Elektra, including our new measurements, 46 adaptive-optics (AO) images obtained by the VLT/SPHERE and Keck/Nirc2 instruments, and two stellar occultation profiles. For the orbital model, we used an advanced \\(N\\)-body integrator, which includes a multipole expansion of the central body (with terms up to the order \\(\\ell = 6\\)), mutual perturbations, internal tides, as well as the external tide of the Sun acting on the orbits. We fitted the astrometry measured with respect to the central body and also relatively, with respect to the moons themselves. We obtained a revised shape model of Elektra with the volume-equivalent diameter \\((201\\pm 2)\\,{\\rm km}\\). Out of two pole solutions, \\((\\lambda, \\beta) = (189; -88)\\,{\\rm deg}\\) is preferred, because the other one leads to an incorrect orbital evolution of the moons. We also identified the true orbital period of the third moon S/2014 (130) 2 as \\(P_2 = (1.642112 \\pm 0.000400)\\,{\\rm d}\\), which is in between the other periods, \\(P_1 \\simeq 1.212\\,{\\rm d}\\), \\(P_3 \\simeq 5.300\\,{\\rm d}\\), of S/2014 (130) 1 and S/2003 (130) 1, respectively. The resulting mass of Elektra, \\((6.606 \\substack{+0.007 \\\ -0.013}) \\times 10^{18}\\,{\\rm kg}\\), is precisely constrained by all three orbits. Its bulk density is then \\((1.536 \\pm 0.038)\\,{\\rm g\\,cm}^{-3}\\). The expansion with the assumption of homogeneous interior leads to the oblateness \\(J_2 = -C_{20} \\simeq 0.16\\). However, the best-fit precession rates indicate a slightly higher value, \\({\\simeq}\\,0.18\\).

No less than 15% of large (diameter greater than 140 km) asteroids have satellites. The commonly accepted mechanism for their formation is post-impact reaccumulation. However, the detailed physical and dynamical properties of these systems are not well understood, and many of them have not been studied in detail. We aim to study the population of large binary asteroid systems. To do so, we compare the gravitational fields predicted from the shape of the primary body with the non-Keplerian gravitational components identified in orbital models of the satellites of each system. We also aim to contextualize these systems in the greater population of large binary systems, providing clues to asteroid satellite formation. We reduce all historical high-angular-resolution adaptive-optics (AO) images from ground-based telescopes to conduct astrometric and photometric measurements of each system's components. We then determine orbital solutions for each system using the genoid algorithm. We model the shapes of the system primaries using lightcurve-inversion techniques scaled with stellar occultations and AO images, and we develop internal structure models using SHTOOLS. Finally, we compare the distribution of the physical and orbital properties of the known binary asteroid systems. We find that differences between studies binary systems reflect an overall dichotomy within the population of large binary systems, with a strong correlation between primary elongation and satellite eccentricity observed in one group. We determine that there may be two distinct formation pathways influencing the end-state dichotomy in these binary systems, and that (762) Pulcova and (283) Emma belong to the two separate groups.

The largest asteroids in the Koronis family (sizes \\(\\geq 25\\) km) have very peculiar rotation state properties, with the retrograde- and prograde-rotating objects being distinctly different. A recent e-analysis of observations suggests that one of the asteroids formerly thought to be retrograde-rotating, 208~Lacrimosa, in reality exhibits prograde rotation, yet other properties of this object are discrepant with other members this group. We seek to understand whether the new spin solution of Lacrimosa invalidates the previously proposed model of the Koronis large members or simply reveals more possibilities for the long-term evolutionary paths, including some that have not yet been explored. We confirm and substantiate the previously suggested prograde rotation of Lacrimosa. Its spin vector has an ecliptic longitude and latitude of \\((\\lambda,\\beta)=(15^\\circ \\pm 2^\\circ, 67^\\circ\\pm 2^\\circ)\\) and a sidereal rotation period \\(P=14.085734\\pm 0.000007\\) hr. The thermal and occultation data allow us to calibrate a volume equivalent size of \\(D=44\\pm 2\\) km of Lacrimosa. The observations also constrain the shape model relatively well. Assuming uniform density, the dynamical ellipticity is \\(\\Delta=0.35\\pm 0.05\\). Unlike other large prograde-rotating Koronis members, Lacrimosa spin is not captured in the Slivan state. We propose that Lacrimosa differed from this group in that it had initially slightly larger obliquity and longer rotation period. With those parameters, it jumped over the Slivan state instead of being captured and slowly evolved into the present spin configuration. In the future, it is likely to be captured in the Slivan state corresponding to the proper (instead of forced) mode of the orbital plane precession in the inertial space.

Every population of small bodies in the Solar system contains a sizable fraction of multiple systems. Among these, the Jupiter Trojans have the lowest number of known binary systems and the least characterized. We aim at characterizing the reported binary system (17365) Thymbraeus, one of the only seven multiple systems known among Jupiter Trojans. We conducted light curves observing campaigns in 2013, 2015, and 2021 with ground-based telescopes. We model these lightcurves using dumbbell equilibrium figures. We show that Thymbraeus is unlikely a binary system. Its light curves are fully consistent with a bilobated shape: a dumbbell equilibrium figure. We determine a low density of 830 +/- 50 kg.m-3 , consistent with the reported density of other Jupiter Trojan asteroids and small Kuiper-belt objects. The angular velocity of Thymbraeus is close to fission. If separated, its components would become a similarly-sized double asteroid such as the other Jupiter Trojan (617) Patroclus.

Context. The sizes of many asteroids, especially slowly rotating, low-amplitude targets, remain poorly constrained due to selection effects. These biases limit the availability of high-quality data, leaving size estimates reliant on spherical shape assumptions. Such approximations introduce significant uncertainties propagating, e.g. into density determinations or thermophysical and compositional studies, affecting our understanding of asteroid properties. Aims. This work targets poorly studied main-belt asteroids, most of which lacked shape models. Using only high-quality dense light curves, thermal IR observations (incl. WISE), and stellar occultations, we aimed to produce reliable shape models and scale them via two independent techniques, allowing size comparison. We conducted two campaigns to obtain dense photometric light curves and to acquire multi-chord stellar occultations. Methods. Shape and spin models were reconstructed using lightcurve inversion. Sizes were determined by (1) thermophysical modeling with the Convex Inversion Thermophysical Model (CITPM), optimizing spin and shape models to visible lightcurve and IR data, and (2) scaling shape models with stellar occultations. Results. We obtained precise sizes and shape models for 15 asteroids. CITPM- and occultation-derived sizes agree within 5% in most cases, demonstrating the modeling's reliability. Larger discrepancies usually stem from incomplete occultation chord coverage. The study also gives insights into surface properties incl. albedo, roughness and thermal inertia. Conclusions. Using high-quality data and an advanced TPM integrating thermal and visible data with shape adjustment enabled precise size estimates comparable to those from multi-chord stellar occultations. We resolved major inconsistencies in previous size estimates, providing solid input for future studies on asteroid densities and surfaces.

The satellite Linus orbiting the main-belt asteroid (22) Kalliope exhibited occultation and transit events in late 2021. A photometric campaign was organized and observations were taken by the TRAPPIST-South, SPECULOOS-Artemis, OWL-Net, and BOAO telescopes, with the goal to constrain models of this system. Our dynamical model is complex, with multipoles (up to the order \\(\\ell = 2\\)), internal tides, and external tides. The model was constrained by astrometry (spanning 2001--2021), occultations, adaptive-optics imaging, calibrated photometry, as well as relative photometry. Our photometric model was substantially improved. A new precise (\\({<}\\,0.1\\,{\\rm mmag}\\)) light curve algorithm was implemented, based on polygon intersections, which are computed exactly -- by including partial eclipses and partial visibility of polygons. Moreover, we implemented a `cliptracing' algorithm, based again on polygon intersections, in which partial contributions to individual pixels are computed exactly. Both synthetic light curves and synthetic images are then very smooth. Based on our combined solution, we confirmed the size of Linus, \\((28\\pm 1)\\,{\\rm km}\\). However, this solution exhibits some tension between the light curves and the PISCO speckle-interferometry dataset. In most solutions, Linus is darker than Kalliope, with the albedos \\(A_{\\rm w} = 0.40\\) vs. \\(0.44\\). This is confirmed on deconvolved images. A~detailed revision of astrometric data allowed us to revise also the \\(J_2 \\equiv -C_{20}\\) value of Kalliope. Most importantly, a~homogeneous body is excluded. For a differentiated body, two solutions exist: low-oblateness (\\(C_{20} \\simeq -0.12\\)), with a~spherical iron core, and alternatively, high-oblateness (\\(C_{20} \\simeq -0.22\\)) with an elongated iron core. These correspond to the low- and high-energy collisions, respectively, studied by means of SPH simulations in our previous work.