256488 research outputs found
Sort by
Shape Modeling of Asteroid (1036) Ganymed: Searching for Large-Scale Remnant Impact Structures on a Large S-Type NEA
No full textAsteroids preserve critical evidence about early solar system processes, including accretion, thermal metamorphism, and impact history. This project focuses on asteroid (1036) Ganymed, a large S-type near-Earth asteroid (NEA), with the goal of generating a 3D shape model using radar observations and optical lightcurves, analyzed via the SHAPE software (Magri et al., 2007). As part of the 2025 Asteroid Radar Modeling Workshop, this work will serve as a year-long scientific and technical study of asteroid surface evolution. The specific science goal is to search for large-scale remnant impact structures preserved on Ganymed's surface, which may record ancient collisional events and offer insight into the fragmentation history of S-type asteroid parent bodies. Asteroid (1036) Ganymed is one of the largest known NEAs, with an effective diameter of ~35 kilometers and a rotation period of 10.31 hours. It follows an Amor-type orbit with perihelion near 1.24 au. Despite its current NEA classification, its size and dynamical history suggest that it likely spent most of its lifetime in the main asteroid belt, only more recently transitioning into a near-Earth orbit. Its spectral classification as an S-type (Bus-DeMeo taxonomy) indicates a silicate-rich composition dominated by olivine and pyroxene. S-types are widely believed to be the parent bodies of ordinary chondrite meteorites—the most common meteorites found on Earth (Dunn et al., 2010; McGraw et al., 2025). Studying surface morphology on such bodies helps constrain the geological context for meteorite samples and their shock histories. This project uses radar data collected from Arecibo Observatory and the Goldstone Deep Space Communications Complex during Ganymed's 1998 and 2011 apparitions, as well as optical lightcurves from eight apparitions between 1985 and 2024. These data provide strong constraints on the asteroid's shape, spin state, and surface roughness. Using the SHAPE software (Magri et al., 2007), we will iteratively refine the model through inversion of radar echoes and photometric data. While radar imaging has revealed concavities on Ganymed's surface at scales of ~10 km (Figure 1), it is difficult to resolve smaller features with confidence. One specific goal of this project is to test the resolution limits of our shape models by quantifying the smallest concavity sizes that appear uniquely and robustly across modeling iterations. By exploring how model features vary with input assumptions and data subsets, we aim to establish a practical threshold for interpreting large-scale impact structures. Large impact structures can persist for billions of years on low-gravity bodies, although modified over time by regolith movement and secondary cratering (Bierhaus et al., 2005; Marchi et al., 2012). If Ganymed preserves such ancient features, it could provide a unique record of early main-belt collisional history. In this context, Ganymed complements broader research efforts aimed at linking asteroid surfaces to meteoritic samples and understanding the evolution of ordinary chondrite source bodies (Vernazza et al., 2014). The modeling workflow includes processing radar data; investigating shape, spin, and scattering properties; and iteratively adjusting the model to minimize residuals. Surface concavities and basin-like structures will be examined both visually and via automated shape analysis tools, as used in prior studies (Benner et al., 2015). While we will assess candidate features, we emphasize that this model's resolution limits what can be robustly interpreted as impact structures. The result will be the best shape model of Ganymed produced to date, incorporating more radar and lightcurve data than any prior study. This project builds on previous shape modeling by Medina et al. (2023) and includes new observations and refinements. The resulting model will be used to examine large-scale surface morphology and investigate whether preserved craters can be linked to specific collisional epochs. This abstract reflects ongoing collaborative work supported by the Asteroid Radar Modeling Workshop. The final model and crater analysis will inform future studies of asteroid surface evolution and meteorite analogs, while contributing to the broader planetary defense community's understanding of large NEA structure. References Benner, L. A. M., et al. (2015). Radar observations and physical modeling of near-Earth asteroid (162421) 2000 ET70. Icarus, 245, 362-378.Bierhaus, E. B., et al. (2005). Cratering on asteroids: Reconciling regolith processes and crater populations. Icarus, 175(2), 486-500.Dunn, T. L., McCoy, T. J., Sunshine, J. M., & McSween, H. Y. (2010). A coordinated spectral, mineralogical, and compositional study of ordinary chondrites. Icarus, 208(2), 789-797.Magri, C., et al. (2007). Radar observations and a physical model of asteroid 1580 Betulia. Icarus, 186(1), 152-177.Marchi, S., et al. (2012). The violent collisional history of asteroid 4 Vesta. Science, 336(6082), 690-694.McGraw, A. C., Reddy, V., & Sanchez, J. A. (2025). The Gefion Asteroid Family: Parent body puzzles and ordinary chondrite pieces. Monthly Notices of the Royal Astronomical Society, 537(4), 3145-3159. Medina, V. A., Marshall, S. E., Devogèle, M., Taylor, P. A., Brozović, M., Ferrais, M., & Jehin, E. (2023). Shape modeling of 1036 Ganymed from radar and lightcurve data. EPSC-DPS Joint Meeting 2023. Vernazza, P., et al. (2014). Multiple and fast: The accretion of ordinary chondrite parent bodies. The Astrophysical Journal, 791(2), 120. Figure 1: Views of the current best-fit shape model of asteroid (1036) Ganymed along each principal axis, based on radar and lightcurve data. The model reveals large-scale surface concavities and asymmetries that may be remnant impact features. Scale bar = 50 km
Du prescrit légal aux supports d’enseignement : analyse comparée des orientations de l’enseignement de la littérature dans quatre contextes territoriaux pour la classe d’entrée au secondaire
No full textpeer reviewe
Improved meteoroid trajectory and speed reconstruction with BRAMS: pre-t0 phase technique and uncertainty quantification
No full textThis study presents a significant advancement in reconstructing meteoroid trajectories and speeds using the Belgian RAdio Meteor Stations (BRAMS) forward scatter radio network. We introduce an improved method based on a novel extension of the pre-t0 phase technique, initially developed for backscatter radars, and adapt it for continuous wave forward scatter systems. This approach leverages phase information recorded before the meteoroid reaches the specular reflection point t0 to enhance speed estimations. Furthermore, we combine this newly determined pre-t0 speed with time of flight measurements to reduce uncertainties in the reconstructed meteoroid paths and velocities. The robustness of our method is assessed using Markov Chain Monte Carlo techniques and validated against optical observations from the CAMS-BeNeLux network.Measurement uncertaintiesA critical aspect of reliable trajectory reconstruction is the accurate characterization of measurement uncertainties, particularly for the times of flight (Δt) between receiving stations. The uncertainty σΔt is closely tied to the uncertainty in determining the specular timing t0 at each station. We developed a method to determine the uncertainty σt0 as a function of two parameters: the rise time of the meteor amplitude curve (trise) and the signal-to-noise ratio (SNR). To derive this relationship, we performed a series of Direct Monte Carlo (DMC) simulations. For each combination of trise and SNR, ideal meteor echoes were generated using the Cornu Spiral model, including some diffusion. For each SNR value, a large number of noisy clones of the ideal echo were created by adding Gaussian noise. The t0 values were extracted from these noisy echoes using the same post-processing chain as real observations, and the statistical spread σt0 was computed. Solver improvementBuilding on the measured uncertainties, we integrate them directly into the trajectory reconstruction process by redefining the cost function used by the solver:where w is a weight parameter balancing the influence of time of flight (Ltof) and pre-t0 speed (Lpt0) measurements:Optimizing this cost function across different values of w leads to the creation of a Pareto front representing the trade-off between minimizing the two components. The optimal solution is chosen at the "knee" of the curve, corresponding to the maximum curvature point.Validation against optical observationsThe reconstructed trajectories and speeds are compared to CAMS-BeNeLux optical data, which shows good agreement when a combination of time of flight and pre-t0 information is used. The differences are of the order of 5 % on the speed and 2-4° on the inclination.Uncertainty propagationTo accurately quantify uncertainties on the reconstructed parameters, we employ a Markov Chain Monte Carlo approach. Assuming independent, Gaussian-distributed errors and uniform priors, the cost function L is proportional to the logarithm of the posterior probability. Thus, minimizing L is equivalent to maximizing the posterior. We use a Single Component Adaptive Metropolis-Hastings algorithm to efficiently explore the parameter space as well as to determine uncertainties and correlations
Building Immune Digital Twins: An International and Transdisciplinary Community Effort
No full textpeer reviewe
Long-term monitoring of a dynamically new comet C/2020 V2 (ZTF)
No full textpeer reviewedComet C/2020 V2 (ZTF, Zwicky Transient Facility) is categorized as a dynamically new long-period comet, making its first approach to the inner Solar system. We have observed this comet for around 32 months (from 2022 January to 2024 July) at heliocentric distances from 5.41 au (pre-perihelion) to 5.26 au (post-perihelion) through various telescopes, employing photometric (60 epochs) and spectroscopic techniques (5 epochs). Using these observations, we derived the production rates of different molecules such as CN, C2, and C3 and calculated the production rate ratios. The values of the ratios closest to the perihelion are found to be log (C2/CN) =−0.04±0.03 and log (C3/CN) =−0.70±0.04, which implies a typical carbon composition. The mean photometric broad-band colours are found to be B−V=0.77±0.04, V−R=0.43±0.04, R−I=0.42±0.06, and B−R=1.19±0.04. The stability of the molecular production rate ratios and mean photometric broad-band colours, pre- and post-perihelion, implies a homogeneous composition. The mean reflectivity gradient for B−V colour is 10.90±3.62%/1000 Å; V−R colour is 6.15±3.51%/1000 Å; and for R−I colour is 4.94±3.56%/1000 Å which is similar to the mean value of the dynamically new comets. . Additionally, using an asymmetric non-gravitational force model, we report the comet's nuclear radius to be km. Our results are expected to provide inputs to the selection of a potential dynamically new comet as a target for the Comet Interceptor mission
Modélisation de la Situation Océanographique Actuelle des Golfes de Patras et Corinthe
No full textpeer reviewedIn our study we investigated the hydrodynamic circulation of the Gulfs of Patras and Corinth through modelling. To this end, ROMS was used to numerically calculate the parameters of the waters for these peculiar semi-enclosed basins. Several oceanographic forcings were used with an emphasis on the tides and the winds. With several simulations, each focusing on a specific element, we were able to describe more accurately the dynamics under the surface to complete what was previously done. The high velocity currents (0.6 m/s at the Patraic end of the strait) were validated through ADCP and satellite data, proving that modelling can be trusted to fill the gap in the in situ data over these two gulfs. Our simulations, mainly based on the month of May 2023, allowed us to understand the importance of the tides, especially in the Rio–Antirio Strait. There, the bottom currents are the strongest while the center of the Corinthian Gulf remains quiet. The surface dynamics were observed to be sensitive to the tides, the winds and the season, but general patterns were still highlighted for the oceanographic circulation of the gulfs
Thermo-Hydro-Mechanical Behaviour of the Argillaceous Rock in the Context of Nuclear Waste Repository
No full textDeep geological disposal is widely recognized as one of the most safe and feasible strategies for isolating radioactive waste from the biosphere and ensuring its long-term management. The fundamental concept involves the emplacement of radioactive waste within a multi-barrier confinement system, wherein clay formations are often selected as host rocks due to their favourable properties, such as low permeability, self-sealing capacity, and the ability to retain radionuclides on the surface of clay minerals. Among their various coupled multi-physical processes that may influence the long-term safety functions of the geological barrier, the increase in temperature resulting from the decay of radioactive waste induces thermal pressurisation both in the excavation damage zone (EDZ) and the surrounding intact far-field. This thermal pressurisation can significantly affect the \textit{in situ} thermo-hydro-mechanical (THM) behaviour of the host rock, thereby potentially compromising the long-term safety performance of the disposal facility.
Thermal effects on the fracturing behaviour within the EDZ surrounding a supported drift are specifically investigated. Fracturing induced by heating is modelled through strain localization in the form of shear bands. A coupled local second gradient model, incorporating a regularisation technique, is employed with thermoelasticity to account for THM couplings. Among the various parameters considered, the discrepancy in thermal dilation coefficients between the solid and fluid phases is identified as a key factor contributing to excess pore pressure generation. The proposed model is applied to simulate a benchmark exercise in the Callovo-Oxfordian (COx) claystone. The results successfully reproduce shear bands oriented along the direction of the minor principal stress, in agreement with experimental observations during excavation. Furthermore, the nature of the contact between the drift wall and its support is shown to significantly influence the development and pattern of the shear bands.
An advanced THM constitutive model is implemented, featuring stress- and strain-dependent permeability and stiffness properties of the clay formation. The evolution of intrinsic water permeability is described using a strain-dependent formulation, which successfully reproduces the significant increase observed in the EDZ, in agreement with experimental measurements. The stiffness of the clay is characterized using the small strain stiffness theory, enabling the model to capture the degradation of stiffness in the EDZ due to large deformations, while preserving the higher stiffness of the intact far-field. The proposed model is applied to simulate both a laboratory-scale experiment from \textit{in situ} sample extraction to laboratory triaxial testing, and the large-scale PRACLAY heater test.
Thermally induced mechanical behaviour of claystone is specifically investigated, with particular attention paid to the temperature dependence of shear strength. Experimental evidence has demonstrated that elevated temperatures can significantly affect both the shear strength and the development of crack networks in claystone. To capture these effects, a thermo-mechanical constitutive law is implemented, which explicitly incorporates the temperature dependence of cohesion. A key feature of the model is that cohesion evolves as a function of mechanical softening, fabric evolution, and thermal softening. The proposed model is applied to the \textit{in situ} large-scale ALC1605 heating experiment, where the evolution of cohesion is shown to play a critical role in the initiation and propagation of shear bands within the EDZ.
The proposed approach has been progressively enhanced from a thermoelastic to a thermoplastic framework, and from assumptions of constant hydraulic properties and homogeneous stiffness to fully coupled THM interactions. The primary objective of this research is to improve the understanding of thermal effects in clay materials, to accurately reproduce THM experimental observations at large-scale, and ultimately to contribute to the design and long-term safety assessment of underground radioactive waste disposal facilities
TALIS 2024 - Présentation de l'enquête et paysage de l'enseignement
No full textTeaching and Learning International Assessment : TALIS 202
