Calathus: A sample-return mission to Ceres

Ceres, as revealed by NASA's Dawn spacecraft, is an ancient, crater-saturated body dominated by low-albedo clays. Yet, localised sites display a bright, carbonate mineralogy that may be as young as 2 Myr. The largest of these bright regions (faculae) are found in the 92 km Occator Crater, and would have formed by the eruption of alkaline brines from a subsurface reservoir of fluids. The internal structure and surface chemistry suggest that Ceres is an extant host for a number of the known prerequisites for terrestrial biota, and as such, represents an accessible insight into a potentially habitable “ocean world”. In this paper, the case and the means for a return mission to Ceres are outlined, presenting the Calathus mission to return to Earth a sample of the Occator Crater faculae for high-precision laboratory analyses. Calathus consists of an orbiter and a lander with an ascent module: the orbiter is equipped with a high-resolution camera, a thermal imager, and a radar; the lander contains a sampling arm, a camera, and an on-board gas chromatograph mass spectrometer; and the ascent module contains vessels for four cerean samples, collectively amounting to a maximum 40 g. Upon return to Earth, the samples would be characterised via high-precision analyses to understand the salt and organic composition of the Occator faculae, and from there to assess both the habitability and the evolution of a relict ocean world from the dawn of the Solar System.The attached document is the authors’ final accepted version of the journal article provided here with a Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) Creative Commons Licence. You are advised to consult the publisher’s version if you wish to cite from it.

Imagerie SAR du régolithe d'un astéroïde : simulation et traitement des données

In recent years, surface-penetrating radars were part of several space missions dedicated to study the bodies of the solar system. These radars, associated with repeat-pass observations and processing, can lead to 3D radar tomography, which results in 3D imagery of the first tens of meters of the sub-surface of planetary bodies. This technique can be used to better understand and model the formation processes and post-accretion evolution of asteroids. However, even though spaceborne SAR is a classical technique used for the detection and reconstruction of planetary structures, the small body geometry of observation reconsiders the hypotheses usually formulated for Earth observation. Furthermore, in order to achieve the metric-resolution necessary to study kilometric-sized asteroids with sufficient precision, the radar has to be ultra-wideband in range and in Doppler, which also question the SAR synthesis models established for narrow band signals. As the radar geometry of study and configuration drives the instrument performance, and thus the mission science return, simulation of the radar signal and the SAR synthesis needs to be developed while taking into account the specificity of the small body geometry. Thus, my thesis aims at assessing the performances of the UWB SAR HFR, which is dedicated to the study of small bodies, with frequencies ranging from 300 to 800 MHz, by simulating the radar's return.By creating firsly realistic asteroid digital terrain models (DTM), several surface scattering models were studied in order to select the model most suited to simulate the field scattered by the surface of an asteroid. The Kirchhoff Approximation (KA) was selected and applied on the generated DTM, and was used to build SAR images which correctly locate the DTM studied, and which differenciate the terrain’s rough areas from the smooth ones. Then, the Born Approximation (BA) was selected to model the field reflected by the asteroid subsurface and was found out to correctly locate an inclusion below the surface of an asteroid. With a multipass geometry, tomography algortihms were applied to the BA results in order to improve the resolution of the results in the third dimension of space, as well as the precision of the localisation of the inclusion. Finally, the performances of UWB scattering were studied, and, unlike what was foreseen, UWB scattering generates only a small degradation of the resolution in range and in azimuthLes radars pénétrant sont des instruments embarqués à bord de multiples missions spatiales depuis plusieurs années. En mettant à profit des observations obtenues depuis différentes orbites séparées spatialement, ils peuvent mener à l’utilisation de techniques de tomographie radar en 3D afin d’imager la structure interne de corps planétaires, notamment des astéroïdes, et améliorer les modèles de formations et d’évolution des astéroïdes. Cependant, même si les radars à synthèse d'ouverture (SAR) spatiaux sont classiques afin de détecter et reconstruire des structures planétaires, pour une telle étude effectuée à quelques kilomètres d’un astéroïde, la taille, la vitesse de rotation et au final la géométrie d’observation elle-même remets en question les hypothèses habituellement formulées pour l'observation de la Terre. De plus, afin d’atteindre la résolution métrique nécessaire afin d'observer des astéroïdes kilométriques, le radar doit être à très large bande (UWB) en range et en doppler, ce qui remets également en cause les modèles de synthèse SAR. Puisque les performances du radar, et donc le retour scientifique de la mission, dépendent de la géométrie d’observation et de la configuration du radar, des simulations du signal radar et de la synthèse SAR dans la géométrie d’un petit corps doivent être mis en place. Le but de ma thèse est d'étudier les performances du radar UWB HFR, développé afin d’étudier les petits corps avec des fréquences allant de 300 à 800 MHz, en étudiant les retours du radar.En créant dans un premier temps des modèles numériques de terrains réalistes d’astéroïdes, plusieurs modèles de diffusions ont été étudiés afin de sélectionner le modèle le plus apte à modéliser le champ réfléchi par la surface d’un astéroïde. L’approximation de Kirchhoff (KA) a été sélectionnée et appliquée sur les modèles de terrain créés, et a permis de construire des images SAR qui localisent correctement le terrain simulé, et qui différencient les zones rugueuses des terrains des zones plus planes. Ensuite, l’approximation de Born (BA) a été sélectionnée afin de modéliser le champ réfléchi par la sous-surface d’un astéroïde, permettant de construire des images SAR qui localisent correctement une inclusion enfouie sous la surface. Avec une géométrie d’observation multipass, des algorithmes de tomographie ont été appliqués à partir des résultats obtenus avec BA, afin d’améliorer leur résolution dans la troisième dimension de l’espace, ainsi que la précision de la localisation de la cible enfouie. Enfin, l’étude des performances de la diffusion UWB a permis d’évaluer que, contrairement à ce qui était pressenti, la diffusion UWB n’entraine qu’une dégradation limitée de la résolution en range, et en azimuth

SAR imaging of an asteroid's regolith : simulation and data processing

Les radars pénétrant sont des instruments embarqués à bord de multiples missions spatiales depuis plusieurs années. En mettant à profit des observations obtenues depuis différentes orbites séparées spatialement, ils peuvent mener à l’utilisation de techniques de tomographie radar en 3D afin d’imager la structure interne de corps planétaires, notamment des astéroïdes, et améliorer les modèles de formations et d’évolution des astéroïdes. Cependant, même si les radars à synthèse d'ouverture (SAR) spatiaux sont classiques afin de détecter et reconstruire des structures planétaires, pour une telle étude effectuée à quelques kilomètres d’un astéroïde, la taille, la vitesse de rotation et au final la géométrie d’observation elle-même remets en question les hypothèses habituellement formulées pour l'observation de la Terre. De plus, afin d’atteindre la résolution métrique nécessaire afin d'observer des astéroïdes kilométriques, le radar doit être à très large bande (UWB) en range et en doppler, ce qui remets également en cause les modèles de synthèse SAR. Puisque les performances du radar, et donc le retour scientifique de la mission, dépendent de la géométrie d’observation et de la configuration du radar, des simulations du signal radar et de la synthèse SAR dans la géométrie d’un petit corps doivent être mis en place. Le but de ma thèse est d'étudier les performances du radar UWB HFR, développé afin d’étudier les petits corps avec des fréquences allant de 300 à 800 MHz, en étudiant les retours du radar.En créant dans un premier temps des modèles numériques de terrains réalistes d’astéroïdes, plusieurs modèles de diffusions ont été étudiés afin de sélectionner le modèle le plus apte à modéliser le champ réfléchi par la surface d’un astéroïde. L’approximation de Kirchhoff (KA) a été sélectionnée et appliquée sur les modèles de terrain créés, et a permis de construire des images SAR qui localisent correctement le terrain simulé, et qui différencient les zones rugueuses des terrains des zones plus planes. Ensuite, l’approximation de Born (BA) a été sélectionnée afin de modéliser le champ réfléchi par la sous-surface d’un astéroïde, permettant de construire des images SAR qui localisent correctement une inclusion enfouie sous la surface. Avec une géométrie d’observation multipass, des algorithmes de tomographie ont été appliqués à partir des résultats obtenus avec BA, afin d’améliorer leur résolution dans la troisième dimension de l’espace, ainsi que la précision de la localisation de la cible enfouie. Enfin, l’étude des performances de la diffusion UWB a permis d’évaluer que, contrairement à ce qui était pressenti, la diffusion UWB n’entraine qu’une dégradation limitée de la résolution en range, et en azimuth.In recent years, surface-penetrating radars were part of several space missions dedicated to study the bodies of the solar system. These radars, associated with repeat-pass observations and processing, can lead to 3D radar tomography, which results in 3D imagery of the first tens of meters of the sub-surface of planetary bodies. This technique can be used to better understand and model the formation processes and post-accretion evolution of asteroids. However, even though spaceborne SAR is a classical technique used for the detection and reconstruction of planetary structures, the small body geometry of observation reconsiders the hypotheses usually formulated for Earth observation. Furthermore, in order to achieve the metric-resolution necessary to study kilometric-sized asteroids with sufficient precision, the radar has to be ultra-wideband in range and in Doppler, which also question the SAR synthesis models established for narrow band signals. As the radar geometry of study and configuration drives the instrument performance, and thus the mission science return, simulation of the radar signal and the SAR synthesis needs to be developed while taking into account the specificity of the small body geometry. Thus, my thesis aims at assessing the performances of the UWB SAR HFR, which is dedicated to the study of small bodies, with frequencies ranging from 300 to 800 MHz, by simulating the radar's return.By creating firsly realistic asteroid digital terrain models (DTM), several surface scattering models were studied in order to select the model most suited to simulate the field scattered by the surface of an asteroid. The Kirchhoff Approximation (KA) was selected and applied on the generated DTM, and was used to build SAR images which correctly locate the DTM studied, and which differenciate the terrain’s rough areas from the smooth ones. Then, the Born Approximation (BA) was selected to model the field reflected by the asteroid subsurface and was found out to correctly locate an inclusion below the surface of an asteroid. With a multipass geometry, tomography algortihms were applied to the BA results in order to improve the resolution of the results in the third dimension of space, as well as the precision of the localisation of the inclusion. Finally, the performances of UWB scattering were studied, and, unlike what was foreseen, UWB scattering generates only a small degradation of the resolution in range and in azimut

Ultra-Wideband SAR Tomography on Asteroids

International audienceOur knowledge of the internal structure of asteroids is currently indirect and relies on inferences from remote sensing observations of surfaces. However, it is fundamental for understanding small bodies' history and for planetary defense missions. Radar observation of asteroids is the most mature technique available to characterize their inner structure, and Synthetic Aperture Radar Tomography (TomoSAR) allows 3D imaging of their interior. However, as the geometry of observation of small asteroids is complex, and TomoSAR studies have always been performed in the Earth observation geometry, its results in a small body geometry must be simulated to assess the methods' performances. We adopt here two different tomography algorithms and evaluate their performances in our geometry by assessing the resolution and the difference between the scatterer's position and its retrieved position. The first method, the Frequency Domain Back Projection (FDBP) is based on correcting the Fourier transform of the received signal by a phase function built from the geometry. While it can provide a good resolution, a bias remains in the imaged scatterer's position. Meanwhile, Compressive Sensing (CS) relies on the hypothesis that few scatterers lie in the same direction from the subsurface. Its application in the small body geometry is studied, which results in a slightly impoverished resolution but an improved localization of the scatterer

Performances of the Passive SAR Imaging of Jupiter’s Icy Moons

With the development of the JUpiter ICy moons Explorer (JUICE)/Radar for Icy Moons Exploration (RIME) [European Space Agency (ESA)] and Europa Clipper/Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) [National Aeronautics and Space Administration (NASA)] instruments, designed to study the subsurface of the Galilean moons, interest has been growing to study the performances of sounding radar orbiting these bodies. In the presence of strong Jupiter's radio emissions, probing in a passive mode using these emissions as the emitter is considered. However, radar performances in this bistatic mode are dependent on the entire geometry of observation: the position of the source of emission, the spacecraft trajectory, and on the region probed. The 3-D Simulations are necessary to estimate the performances of these measurements. We analyze the influence of the geometry of observation by approximating Jovian radio bursts as radio impulses, simulating the signal scattered by a point target in a realistic 3-D geometry and computing the resolution. This allows a preliminary identification of scenarios of observation best suited for this radar mode. The influence of the correct localization of Jupiter's emissions on the synthetic aperture radar (SAR) image is investigated as well, and interest in recovering the true position of Jupiter's source is highlighted

A radar package for asteroid subsurface investigations: Implications of implementing and integration into the MASCOT nanoscale landing platform from science requirements to baseline design

International audienceThe internal structure of asteroids is still poorly known and has never been analyzed directly by measurements. Our knowledge relies entirely on inferences from remote sensing observations of the surface and theoretical modeling. Direct measurements are crucial to characterize an asteroid's internal structure and heterogeneity from sub-metric to global scale. The radar package developed in the frame of the phase A/B1 of the Asteroid Impact Mission (AIM) as part of the larger Asteroid Impact & Deflection Assessment (AIDA) mission is a mature instrument suite to answer this question and to improve our ability to understand and model the mechanisms driving Near Earth Asteroids (NEA). It is of main interest for science, exploration and planetary defense. This instrument suite consists of a monostatic high frequency radar (HFR) to investigate the stratigraphy of surface regolith and a bistatic low frequency radar (LFR) to characterize the deep interior. The chosen platform to deliver the surface unit of the LFR and other instruments for a close-up study of the target asteroid is the MASCOT nanolander, which already flies on Hayabusa2 (HY2) in a mineralogy scout configuration. In this paper, we present the chosen instrumentation for radar science, baseline mission requirements and the initial design for integration into the lander platform, including all peculiarities and constraints

Direct Observations of Asteroid Interior and Regolith Structure: Science Measurement Requirements

International audienceOur knowledge of the internal structure of asteroids is, so far, indirect – relying entirely on inferences from remote sensing observations of the surface, and theoretical modeling of formation and evolution. What are the bulk properties of the regolith and deep interior? And what are the physical processes that shape asteroid internal structures? Is the composition and size distribution observed on the surface representative of the bulk? These questions are crucial to understand small bodies’ history from accretion in the early Solar System to the present, and direct measurements are needed to answer these questions for the benefit of science as well as for planetary defense or exploration.Radar is one of the main instruments capable of sounding asteroids to characterize internal structure from sub-meter to global scale. In this paper, we review the science case for direct observation of the deep internal structure and regolith of a rocky asteroid of kilometer size or smaller. We establish the requirements and model dielectric properties of asteroids to outline a possible instrument suite, and highlight the capabilities of radar instrumentation to achieve these observations. We then review the expected science return including secondary objectives contributing to the determination of the gravitational field, the shape model, and the dynamical state. This work is largely inherited from MarcoPolo-R and AIDA/AIM studies