Modélisation du champ de gravité en harmoniques ellipsoïdales et application à l'astéroïde Eros dans le cadre de la mission NEAR

Many bodies of the solar system have elongated shapes, and the usual spherical harmonic expansion of the gravitational potential is not well suited for the modeling of their gravity fields : the series may be divergent inside the smallest ellipsoid enclosing the whole body, and it is impossible to compute landing trajectories inside this sphere. As the sphere is a poor fit of an elongated body, the series converge very slowly and a high accuracy in the modeling means using a high degree expansion. In this thesis, we used a method specially tailored to this kind of geometry : the ellipsoidal harmonic expansion of the potential. The gravitational potential is represented along an ellipsoidal harmonic expansion instead of a spherical harmonic expansion. The ellipsoidal harmonics are a generalization of the spherical harmonics and keep equivalent mathematical properties. In the thesis, we present the mathematical properties and the computational algorithms for these ellipsoidal harmonics. Then we apply this method to the computation of the landing trajectory of a lander on the surface of comet Wirtanen (Rosetta Mission). The last part is devoted to the gravity field modeling of the asteroid 433 Eros from the radiometric tracking of the NEAR spacecraft. We show that Eros seems homogeneous but there are indications of small density variations as well as a small offset between the center of mass and the center of figure. Moreover, we see positive gravity anomalies located near the main craters of Eros, suggesting a compaction process, and two negative anomalies, located at the ends of Eros but with an unknown origin.A l’image de l’astéroïde Eros, la plupart des comètes et des astéroïdes sont très asphériques. La méthode traditionnelle de modélisation du champ de gravité, un développement du potentiel en séries d’harmoniques sphériques, se révèle mal adaptée aux corps allongés : les séries peuvent être divergentes à l’intérieur de la plus petite sphère englobant le corps, ce qui interdit par exemple le calcul de trajectoires d’atterrissage à la surface du corps. De plus la sphère est une piètre approximation d’un corps allongé et les séries convergent lentement. Une bonne précision de modélisation est donc synonyme d’un haut degré de développement. Le sujet de la thèse a consisté à développer et appliquer une méthode mieux adaptée aux corps allongés : le développement en séries d’harmoniques ellipsoïdales. Le potentiel gravitationnel est alors représenté suivant une base d’harmoniques ellipsoïdales au lieu d’une base d’harmoniques sphériques. Les harmoniques ellipsoïdales sont en fait une généralisation des harmoniques sphériques et conservent les principales propriétés des harmoniques sphériques. Dans cette thèse, nous présentons les propriétés mathématiques ainsi que les algorithmes permettant le calcul des harmoniques ellipsoïdales, du potentiel et de l’accélération. Cette méthode est ensuite appliquée au calcul de trajectoire d’atterrissage à la surface de la comète Wirtanen (Mission Rosetta). Dans une dernière partie, nous présentons une modélisation du champ de gravité de l’astéroïde Eros à partir des données radiométriques transmises par la sonde NEAR. Ce champ de gravité a permis d’établir qu’EROS est pratiquement homogène en densité. On a toutefois mis en évidence un léger décalage du centre de gravité par rapport à celui d’un modèle de forme de densité constante, ainsi que deux défauts de masse localisés aux extrémités d’Eros et certaines anomalies positives liées aux principaux cratères, indiquant une possible compaction du matériau à ces endroits

Etude de la survie et des facteurs pronostiques d'une population d'obèses hypercapniques

STRASBOURG-Medecine (674822101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

The MASCOT Separation Mechanism - A Reliable, Low-Mass Deployment System for Nano-Spacecraft

The Mobile Asteroid Surface Scout (MASCOT), an Asteroid Lander carried by the Hayabusa2 spacecraft, successfully landed on the Near-Earth Asteroid (162173) Ryugu on October 03, 2018. Hereby accomplishing the first ever landing of a European spacecraft on the surface of this type of celestial body. MASCOT was a prototype design of a new class of nano-size surface science packages for the exploration of small solar system bodies. The very low gravity (thus, very low escape velocity) of the target body required the design of a miniaturized deployment mechanism with a relatively small, well reproducible separation velocity. In addition, the mechanism also had to safely restrain the lander to the mother spacecraft during the launch and its 3.5 years cruise phase. In this paper, we describe in detail the design, numerical analysis and test of this newly developed separation mechanism. Furthermore, we compare the mechanism to other existing deployment systems and verify its performance with two independent analysis methods using actual flight data taken during the ultimate flight activation event, which initiated the successful delivery and surface operation of the MASCOT asteroid lande

Reconstruction of the flight and attitude of Rosetta's lander Philae

International audienceSince Rosetta's lander Philae touched down on comet 67P/Churyumov-Gerasimenko on November 12, 2014, many tools have been applied to reconstruct Philae's flight path and attitude between separation, the touchdowns, collision and the final landing at Abydos. In addition to images from the cameras onboard both orbiter and lander (;OSIRIS;, ;CIVA; and ;ROLIS;), radio tracking results, solar array and radio data link housekeeping data, one of the major sources for timing and attitude information were two point magnetic field measurements by the magnetometers ;ROMAP; and ;RPC-MAG; aboard Philae and Rosetta. In this study all the different results are combined to determine in further detail what happened to Philae during its travel above the surface of 67P/Churyumov-Gerasimenko. In addition to a description of the descent dynamics and the attitude during rebound, the approximate coordinates for the collision at 16:20 UTC with the rim of the Hatmehit crater and the second touchdown are estimated. It is also shown, that Philae did not change attitude between the end of the first-science sequence and September 2, 2016

The process for the selection of MASCOT landing site on Ryugu: design, execution and results

International audienceMASCOT, the Mobile Asteroid Surface SCOuT, is a small lander jointly developed by the German and French space agencies [Ho et al., 2017], that travelled on board of the JAXA Hayabusa2 spacecraft for over 3 years to the C-type asteroid Ryugu. The goal of MASCOT was to perform in situ measurements on the surface of the asteroid by means of its four scientific instruments, substantially contributing in this way to the overall scientific return of Hayabusa2 mission. The objective of the paper is to provide a detailed overview of the Landing Site Selection Process (LSSP) for MASCOT, from the preliminary design phase that started several years before launch, up to the actual execution of the selection process and its operational implementation. The effort that was put on the LSSP by all the teams involved over all these years was one of the key elements, leading to the unprecedented success of this mission

Rosetta Lander - Philae: Operations on comet 67P/Churyumov-Gerasimenko, analysis of wake-up and final state

The Lander Philae, part of the ESA Rosetta mission successfully landed on comet 67P/Churyumov- Gerasimenko on November 12th, 2014. After several (unplanned) bounces it performed a First Scientific Sequence (FSS), based on the energy stored in its on board batteries. All ten instruments of the payload aboard Philae have been operated at least once. Due to the fact that the final landing site was poorly illuminated, Philae went into hibernation on November 15th. Signals from the Lander were received again in June and July 2015, which indicated multiple awakening episodes of the lander. However, various attempts to re-establish reliable and stable communications links failed. Based on the analysis of the data gained during FSS, and during the contacts in June and July 2015 we draw conclusions on the state of Philae. In addition, images from the OSIRIS camera aboard the Rosetta Orbiter have allowed the identification of the exact position of Philae and its attitude, relative to the local surface terrain. This paper also gives an overview of the implications of Philae results for future engineering comet models, required particularly for the design of in-situ (landing) or sample return missions. Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae Lander is provided by a consortium led by DLR, MPS, CNES and ASI with additional contributions from Hungary, UK, Finland, Ireland and Austria

Quick-look results for the surface/regolith mechanical properties of Ryugu based on MASCOT bouncing analyses

We present quick-look results and constraints on the mechanical properties of the regolith on asteroid 162173 Ryugu, based on expected data regarding the mechanical interactions by the lander MASCOT as well as data from MINERVA nano-landers and possibly from the first sampling touchdown by Hayabusa2. MASCOT is going to be deployed from an altitude of ~55 m and has a touch-down velocity of ~0.2 m/s. It is expected to bounce several times before coming to rest [3]. The descent trajectory and the larger bouncing arcs can be captured by optical imaging from the spacecraft (ONC [5]) giving constraints on MASCOT in-flight trajectories. Moreover, direct images of footprints as well as data from MASCOT’s magnetometer MASMAG [4] on bounce times (and possibly rotation rates or changes thereof), images by MASCOT’s camera MASCAM [2] during bouncing and their fusion with ONC images projected to the shape, and finally MASCAM images after rest offer a rich database that allows us to constrain Ryugu’s surface mechanical properties, with implications on the asteroid’s surface history. Variations of the radio-frequency signal all along MASCOT’s trajectory and day/night detection by MASCOT’s photoelectric cell sensors can also contribute to the analysis. The measured total linear energetic coefficient of restitution (CoR), i.e. the fraction of energy dissipated at each bounce, can be compared to the CoR values measured for the MASCOT structure bouncing against a hard wall [6] and soft-sphere DEM simulations of MASCOT landing on a bed of granular material [7,8]. Footprint images of the bounce imprints in loose granular material also constrain the granular frictional properties and the regolith depth. Preliminary conclusions on the mechanical properties of Ryugu’s surface material will be draw

Rosetta lander Philae: Flight dynamics analyses for landing site selection and post- landing operations

International audienceOn the 12th of November 2014, The Rosetta Lander Philae became the first spacecraft to softly land on a comet nucleus. Due to the double failure of the cold gas hold-down thruster and the anchoring harpoons that should have fixed Philae to the surface, it spent approximately two hours bouncing over the comet surface to finally come at rest one km away from its target site. Nevertheless it was operated during the 57 hours of its First Science Sequence. The FSS, performed with the two batteries, should have been followed by the Long Term Science Sequence but Philae was in a place not well illuminated and fell into hibernation. Yet, thanks to reducing distance to the Sun and to seasonal effect, it woke up at end of April and on 13th of June it contacted Rosetta again. To achieve this successful landing, an intense preparation work had been carried out mainly between August and November 2014 to select the targeted landing site and define the final landing trajectory. After the landing, the data collected during on-comet operations have been used to assess the final position and orientation of Philae, and to prepare the wake-up. This paper addresses the Flight Dynamics studies done in the scope of this landing preparation from Lander side, in close cooperation with the team at ESA, responsible for Rosetta, as well as for the reconstruction of the bouncing trajectory and orientation of the Lander after touchdown