Daniel Hestroffer

Introduction: The Gaia space mission of ESA has collected extremely precise data on the astrometry of asteroids. These data can be used to measure the periodic oscillation of the photocenter of binary asteroids and thus to discover their binarity [1]. Using this method for the Gaia DR3 catalogue, Liberato et al. [2] has so far discovered hundreds of candidates in binaries (the so-called Gaiamoons), and most of them are expected to be confirmed in the following years. Observations: We have implemented the observation program of Gaia binary asteroids to reveal their binary nature and determine their physical parameters using precise photometry. For this purpose, we cooperate with different observatories in France, Slovakia, Bulgaria, Georgia, Uzbekistan, Kazakhstan, Taiwan, and Chile. The observatories are distributed across different locations, allowing observations to continue even when weather conditions are poor at some sites. In addition, their coverage across multiple longitudes makes it possible to obtain long, continuous lightcurves, which are essential for the reliable interpretation of the photometric data. The observations are carried out with 0.6 - 2 m telescopes. If possible, the predicted timing of mutual events in the Gaiamoons binary system is used to plan photometric observations. Thus, we expect to obtain photometric lightcurves with the occultation/eclipse events, which could confirm binarity and improve the parameters of mutual orbits. Results and prospects: We present the results of our photometric observations of the Gaiamoons binary candidates, which fall into two categories: known binary asteroids — (87) Sylvia, (90) Antiope, (121) Hermione, (317) Roxane, (542) Susanna, and (4337) Arecibo — and asteroids suspected of being binary systems — (605) Juvisia, (712) Boliviana, (1127) Mimi, (1967) Menzel, and (4209) Briggs. In addition, we include in the analysis the photometric data for Gaiamoons binary asteroids obtained from various sky survey data, including TESS and others. In particular, using photometric data from the TESS survey, we confirmed the predicted occultation/eclipse events in the well-known binary asteroid (22) Kalliope [3]. The obtained observations are supposed to be used to conduct numerical modeling of the binary systems using the astrometric and photometric data. Our numerical code generalizes our previously devised algorithm for shape modeling of contact-binary asteroids [4]. It represents a model of a binary system as two ellipsoid-shaped objects orbiting each other. By comparing simulated brightness and position of the photocenter of the binary system with real observations, we estimate the most likely shapes, sizes, masses, and densities of the asteroid components, together with uncertainties of these values.Acknowledgments: The Kharkiv team is thankful to Ukrainian soldiers who defend our lives and freedom against russian aggression. YK thanks the French PAUSE program for its support of scientists at risk. References[1] Pravec, P., & Scheirich, P. 2012, Planet. Space Sci., 73, 56 [2] Liberato L., Tanga P., Mary D., et al., A&A, 2024, 688, A50.[3] Emelyanov, N. V., Kovalev, M. Y., & Varfolomeev, M. I. 2023, MNRAS, 522,165 [4] Krugly Y., Golubov O., Kyrylenko I., et al., A&A, 2025, 702, A17

Binary asteroids are fundamental probes of the physical and collisional evolution of the Solar System. Their mutual orbits provide one of the few direct ways to estimate asteroid masses, bulk densities, and internal structures. However, the population of known binary systems is small. It remains strongly biased toward small bodies in the near-Earth and inner main-belt populations, while binaries in the intermediate-size regime and in the outer main belt remain poorly characterised. In Liberato et al. (2024), we introduced a new method to identify binary asteroid candidates. We searched for astrometric wobble signatures in Gaia Data Release 3, producing the first-ever list of astrometric binary asteroid candidates, several of which have already been independently confirmed. In Liberato et al. (2026), we further refined the method by accounting for specific statistical properties of the Gaia astrometric data and by implementing a selection procedure designed to control the false-detection rate. We then explored the Gaia Focused Product Release (FPR), which contains more than 150,000 objects observed over 66 months. Among them, 47,896 objects had sufficient data for us to investigate the possibility of an undetected companion. Extensive validation tests demonstrated the robustness of the detections: simulations based on noise-only scenarios produced, on average, a number of candidates approximately 80% lower than that obtained from the Gaia FPR data. In addition, astrometric wobble signatures were successfully recovered for nine previously known binary systems, confirming our method's capacity in detecting genuine binary-induced signals. Our analysis identified 343 astrometric binary candidates.Now, we expand the analysis on the results from Liberato et al. (2026). The FPR candidate population spans a broad range of dynamical classes, with the majority located in the main asteroid belt, but also includes objects associated with the Cybele and Hilda groups, as well as several Jupiter Trojans. The distribution of candidates differs significantly from that of currently known binaries, revealing a larger fraction of objects in the middle and outer main belt. This suggests that our method, associated with the Gaia astrometry precision, is sensitive to binary systems that are difficult to detect with conventional photometric or direct-imaging techniques, reducing the strong observational biases affecting the currently known population.A substantial fraction of the candidates are associated with asteroid families, indicating that collisional environments may play a major role in binary formation and survival. We identify 95 candidates, as well as one previously known binary, associated with asteroid families. Their distribution in proper element space suggests that binaries may be considerably more common in outer-belt families than currently known. The presence of candidate binaries among family parent bodies, including (369) Aeria, (780) Armenia, and (1303) Luthera, is particularly significant because these systems could provide direct constraints on the masses, densities, and reaccumulation histories of poorly characterised collisional families.The physical properties of the candidates indicate a diverse population. Approximately 60% of the objects have estimated diameters below ~15 km, consistent with binary formation through rotational fission driven by the YORP effect (Walsh et al. 2008). In contrast, several candidates exceed diameters of ~50 km, a size range where YORP-driven spin-up becomes inefficient and collisional formation mechanisms are expected to dominate. Some intermediate-sized objects, known as Escaping Ejecta Binaries (EEBs; Durda et al. 2004), may form through sub-catastrophic collisions. These EEBs are severely underrepresented in the known binary asteroid population compared with theoretical expectations (Pravec & Harris 2007). Our results show that 27% of our candidates are within the EEB size range, and several are synchronous. This suggests that we may be revealing a significant fraction of a previously hidden population of binary asteroids across the main belt.The taxonomic distribution of the candidates also differs slightly from that of known binaries. While the fraction of S-type asteroids remain the highest in our sample, the fraction of C-type candidates is significantly enhanced relative to the currently known binary population. This trend is consistent with the larger number of outer main-belt objects recovered by Gaia astrometry. It may suggest that the apparent dominance of S-type binaries in previous surveys (Minker \& Carry 2023) is largely driven by observational bias that favours objects in the inner main belt.Finally, the availability of multiple Gaia observation windows for several candidates allows independent estimates of orbital parameters and provides an internal consistency check on the astrometric solutions. A significant fraction of the multiple detections show compatible or aliased solutions, reinforcing the physical plausibility of the binary interpretation. Confirmation through stellar occultation observations, or photometric light curves, could substantially increase the number of known binaries in the middle and outer main belt, improve estimates of binary occurrence rates in asteroid families, and provide new constraints on the formation, evolution, and internal structure of small bodies.These results show the strong potential of Gaia astrometry to reveal a previously hidden population of binary asteroids across the Solar System and to improve the binaries' physical characterization. Some Gaia Data Release 4 results will also be presented to support these findings and prepare the community to the upcoming full release in December 2026.References:1. D. D. Durda et al. (2004). Icarus 167.2, pp. 382–396.2. L. Liberato et al. (2024). Astronomy & Astrophysics, 688, A50.3. L. Liberato et al. (2026). Astronomy & Astrophysics, Under...

Gaia is an ESA space mission primarily designed to perform high precision astrometric observations of stars, but it also observes more than 350,000 small bodies in the Solar System. Its measurements have revealed signatures of binarity such as in the (4337) Arecibo system, demonstrating Gaia's ability to detect binary motion [1]. Furthermore, under certain assumptions, these data allow us to derive the mass ratio and flux ratio of these close-by systems (unresolved systems) [2].In this study, we explore Gaia data for widely separated binary systems where Gaia can partially or fully resolve the components. We combine Gaia astrometry with complementary observational datasets to derive the relative orbits and physical properties of a set of seven binary systems: (22) Kalliope and Linus; (317) Roxane and Olympias; (1509) Esclangona and S/2003 (1509) 1; (617) Patroclus and Menoetius; (90482) Orcus and Vanth; (174567) Varda and Ilmarë; and (136199) Eris and Dysnomia.Our methodology consists of two steps. First, we determine the heliocentric orbit of each system using Gaia astrometry, which corresponds to the motion of the system’s center of mass. We then analyze the residuals from this solution, which reveal the motion of the photocentre of the most massive component. The offset between the center-of-mass orbit and the photocentre directly imposes constraints on the mass ratio of the binary system. Thanks to the Gaia data, our results demonstrate that, for the first time using this astrometric approach, we can directly infer, with good precision, the mass ratio and thus determine the individual masses and densities of the components.We will present the derived orbital solutions, total masses, mass ratios, and individual component masses of our test objects, in addition with the future observation events (stellar occultations and mutual events) for constraining the size of the bodies, and hence their densities. 1. Tanga, P., Pauwels, T., Mignard, F., et al. 2023, A&A, 674, A122. Liu, Z., Hestroffer, D., Desmars, J., and David, P. 2024, A&A, 688, L23.Acknowledgements. This work has received support from France 2030 through the project named Académie Spatiale d'Île-de-France (https://academiespatiale.fr/) managed by the National Research Agency under bearing the reference ANR-23-CMAS-004

In a previous study, we leveraged the astrometric accuracy of to obtain the first list of astrometric binary asteroid candidates. Some of these candidates have now been confirmed. However, that work did not provide the details of the statistical methods. Our first aim is to provide methodological details and a performance evaluation of the approach used for detecting binaries. Our second aim is to establish an updated list of binary asteroid candidates from astrometric residual exploration, accounting for the statistical properties of the data. We accounted for the astrometric uncertainties from and refined the statistical model of the data. We used this model in Monte Carlo simulations to evaluate the strength of the individual detections. We implemented a trend-detection method in the residuals and applied a dedicated period-search algorithm. In addition, we updated the statistical selection process to build the list of candidates. We introduced a method for detecting objects in multiple windows of consecutive observation and refined the estimation of confidence intervals for these parameters, thereby better constraining the physical parameter selection. We detect 343 binary asteroid candidates, corresponding to 410 windows of consecutive observations, in the astrometric data. We show that in noise-only control simulations, the typical number of detections is 88% lower than in the data. We also detect nine known binaries, 25 candidates that overlap with the Pan-STARRS survey, and 99 candidates that overlap with our previous binary search in Finally, we report 45 objects exhibiting residual trends suggestive of wide binary systems. Our results and analyses demonstrate that although detecting binary asteroids is a difficult problem due to their low signal levels, the proposed method provides a reliable list of detections, including systems that are poorly accessible to conventional techniques. This set of targets is valuable for future confirmation with stellar occultations, light curves, and forthcoming LSST data.

Binary asteroids provide key access to fundamental parameters of Solar System remnants and planetary formations. However, the current knowledge of binary asteroids remains strongly biased by observational limitations, and main belt binary systems are still poorly characterised since current techniques preferentially detect either widely separated binaries through direct imaging or close and bright systems via photometry and radar for near-Earth asteroids. In this context, the high-precision astrometry of the Gaia mission has revealed a new population of candidate binaries exhibiting dynamical signatures consistent with unresolved companions. Stellar occultations have therefore emerged as one of the most effective methods to confirm the binary nature of a candidate and improve the current census of intermediate-size systems. This work is part of the GaiaMoons program, and our aim with it was to characterise a sample of 357 potential binary asteroid targets and confirm or refute their binary nature. The properties of these candidates were derived from the high-precision photometric and astrometric observations provided by the Gaia satellite. We adopted stellar occultation as the observational method to study these targets. Between October 2023 and February 2026, we successfully carried out 165 observations for 101 targets. We subsequently analysed these events in the context of the available literature and previously reported observations. Out of the 165 observations, 76 led at least to one positive observation. Among these, 33 had at least two positives for 24 objects that have undergone unprecedented occultation observation campaigns, with four objects showing indications of binary or contact binary features, namely (1127) Mimi, (35420) 1998 AG_6, (206) Hersilia, and (36882) 2000 SW_155. For the vast majority of these objects, the resulting dataset from all reduced observations provides unique physical and astrometric constraints, as they had never been observed through stellar occultations before. In addition, 89 observations with only negatives allowed the near environment of the targets to be probed. GaiaMoons illustrates how stellar occultation campaigns associated with Gaia observations generate a self-improving cycle to find new binary, thereby probing size and shape to constrain future observations. By standardising this approach, we deliver critical data in unexplored parameter spaces, resolving long-standing observational ambiguities.

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 ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼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 η ≳ 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 (≳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.

Context . Robust impact monitoring of near-Earth objects is an essential task of planetary defense. Current systems such as NASA’s Sentry-II, NEODyS’s CLOMON2, and ESA’s Aegis have been highly successful, but independent approaches are essential to ensure reliability and to cross-validate predictions of possible impacts, probabilities, and paths on Earth. Aims . We present NEOForCE (Near-Earth Objects’ Forecast of Collisional Events), a new independent monitoring system for asteroid impact prediction. By relying on orbital solutions from DynAstVO at Paris Observatory and using an original methodology for uncertainty propagation, NEOForCE provides an alternative line of verification for future impact assessments and strengthens the overall robustness of planetary defense. Methods . As other monitoring systems do, NEOForCE samples several thousand so-called “virtual asteroids” from the uncertainty region and integrates their orbits up to 100 years into the future. Instead of searching for close approaches of the virtual asteroids themselves with the Earth, our system looks for times when the Earth comes close to the “realistic” uncertainty regions around them, which are mostly stretched along the osculating ellipses of virtual asteroids. For every virtual asteroid and every possible collision time, we also estimate the maximal impact probability, and only if this value is large enough ( > 5 × 10 ⁻⁸ ) do we continue to the next step. In this second step, we compute how the original asteroid orbit should be slightly modified so that the new trajectory leads to an Earth impact, which allows us to confirm the possible collision and estimate the impact probability. Results . We tested NEOForCE against NASA’s Sentry-II system on five representative asteroids with a high impact probability and significant number of possible collisions: 2000 SG344, 2005 QK76, 2008 JL3, 2023 DO, and 2008 EX5. NEOForCE successfully recovered almost all of the possible collisions reported by Sentry-II with impact probabilities above 10 ⁻⁷ , demonstrating the robustness of our approach. In addition, NEOForCE identified several potential impacts at the 10 ⁻⁷ –10 ⁻⁶ level that Sentry-II did not report.

Gaia provides an exceptional opportunity to explore high-precision astrometric data for a large number of solar system objects. Thanks to its unprecedented precision, Gaia data can reveal the astrometric signatures of binary asteroids. This is the case of the binary system (4337) Arecibo, discovered by occultation. Using Gaia DR3 data [1], an astrometric wobble was clearly detected over several transits over a two days window.In this study, we investigated the Arecibo system, combining all Gaia FPR observations. We first fit the heliocentric orbit, where the residuals contain the binary signal. This signal is proportional to the relative orbit, and a scaling factor related to the flux ratio and mass ratio of the components (see [2], [3] for the analytical formula). We then adjust the relative orbit to derive the key parameters, including full orbital parameters, flux and mass ratio. Based on estimated volumes, we obtain overall densities of around ρ₁ ≈ 1.2 and ρ₂ ≈ 1.6 for the primary and secondary, respectively. These results indicate that this system is composed of ice-rich bodies in the outer main belt [4].Moreover, we are applying this method to other asteroid systems whose orbits are poorly known. Using Gaia observations combined with ground-based data, we have been able to refine their orbits. Furthermore, for trans-Neptunian objects, where the separation between the components is large enough for only the primary body to be observed by Gaia, the astrometric wobble can indicate the mass ratio. In such cases, Gaia data can be crucial in determining individual masses and, hence, density.For binary candidates identified in previous studies [5], it is possible to solely use Gaia data or adding it with primary-only occultation data to perform mutual orbit determination. This can help estimate the position of a potential satellite, guiding future occultation observations for detection and confirmation.Gaia's high-precision astrometric measurements enable us to estimate the individual masses and densities of solar system bodies. This provides valuable insights into the formation and evolution of the solar system. Additionally, by looking at orbital parameters, we can predict future mutual events and stellar occultations, which will provide further constraints on density. We aim to present preliminary results on main belt asteroid systems, TNBs and a few potential binaries, demonstrating how Gaia data contribute to our understanding of these objects and to future observation. [1] Tanga, P., Pauwels, T., Mignard, F., et al. 2023, A&A, 674, A12[2] Pravec, P. & Scheirich, P. 2012, Planet. Space Sci., 73, 56[3] Lindegren, L. 2022[4] Liu, Z., Hestroffer, D., Desmars, J., and David, P. 2024, A&A, 688, L23. [5] Liberato, L., Tanga, P., Mary, D., et al. 2024, A&A, 688, A5

Investigation of dynamical and physical properties of the main-belt and near-Earth asteroids is a priority research area, which has reached a new level in recent years. This progress has been facilitated by space missions to asteroids as well as modern all-sky surveys aimed to discover new objects which also provide highly accurate astrometric and photometric data. However, the role of ground-based programs remains valuable. The long-term monitoring program is crucial for retrieving asteroid physical properties and its evolution. The primary aim of our Project is obtaining new observational data of asteroids by carrying out their spectrophotometric (Shevchenko et al. 2022) and polarimetric (Belskaya, Krugly 2024) observations which allow us to determine absolute magnitudes, rotation periods, surfaces properties, albedo, sizes and shapes of these objects. Our observations are based on cooperation with colleagues from different observatories in several countries (Bulgaria, Georgia, Kazakhstan, Slovakia, Uzbekistan, Chili, USA) with using the 0.6 - 2 m telescopes which are equipped with modern CCD cameras. Our priority targets include newly discovered and potentially hazardous near-Earth asteroids, binary asteroids, especially those observed by Gaia, objects studied through radar observations, and targets of ongoing and future space missions (Hayabusa2#, Hera etc). During January-April 2025, new photometric observations of over 40 NEAs (4 newly discovered, 17 PHAs, for 18 the rotation periods were determined for the first time) and 8 known main-belt binary asteroids were carried out, as well as polarimetric observations of 2 NEAs were obtained at high phase angles. Preliminary results of our new and long-term observations will be presented. AcknowledgmentsThe Kharkiv team is thankful to Ukrainian soldiers who defend our lives and freedom against russian aggression. YK thanks the French PAUSE program for its support of scientists at risk. ReferencesShevchenko, V. G., I. N. Belskaya, I. G. Slyusarev, O. I. Mikhalchenko, Yu. N. Krugly et al. 2022. Kharkiv database of asteroid absolute magnitudes: Comparative analysis with other datasets. 2022A&A...666A.190S Belskaya I., Y. Krugly. 2024. Study of near-Earth asteroids by polarimetric technique. Report on “EU-ESA Workshop on Size Determination of Potentially Hazardous Near-Earth Objects”, 11–13 Nov 2024, Darmstadt, Germany.