L’observation sur le long terme des corps du Système solaire est essentielle à la compréhension de leur évolution passée et future. Il s’agit là d’ailleurs de l’unique moyen pour détecter et quantifier des modifications orbitales, des effets séculaires, et des signatures de dissipation d’énergie. Dans ce cadre, la recherche en bases de données d’images CCD peut être utile, dès lors que l’on utilise le catalogue Gaia pour une nouvelle réduction qui garantirait la meilleure précision, même dans le passé. Cependant, les observations d'avant les années 90 ont été réalisées sur plaques photographiques, et les mesures ne sont plus disponibles. De plus, une grande partie des objets présents sur les plaques n’étaient pas connus à ces époques, et doivent donc être re-découverts. Nous proposons l’utilisation d’un nouveau numériseur sub-micrométrique pour produire des images adaptées à une nouvelle réduction astrométrique et/ou photométrique. Le choix des supports à numériser est libre, des plus petites plaques aux plaques de Schmidt. Nous présenterons ainsi le programme NAROO (New Astrometric Réduction of Old Observations) depuis sa genèse au dernier bilan d’avancée, ainsi qu’une partie de la prospective scientifique associée. Programme (à ajuster en fonction du temps des discussions, et du choix de l’heure de la pause café) : 14h00/14h30 : Le programme NAROO — V. Robert 14h30/14h45 : NAROO et dynamique des astéroïdes — J. Desmars 14h45/15h00 : NAROO et spectroscopie — C. Neiner 15h00/15h15 : Visiteurs interstellaires du Système solaire — J. Schneider 15h15/15h30 : Astéroïdes et test de physique fondamentale — C. Le Poncin-Lafitte Le but de faire un séminaire élargi est de continuer à former une communauté NAROO au sein de l’Observatoire, autour d’un instrument qui permet d’aborder nombre de thématiques présentes dans les laboratoires de l'Observatoire. Dans ce cadre, une diffusion multi site du séminaire via renavisio pourra être prévue. Toute idée ou proposition de participation supplémentaire est bienvenue.

The enormous development of astrometry in the years since 1925 is the subject, astrometry being the branch of astronomy for high-accuracy measurement of stellar positions, distances and motions. -- As student of 21 years in 1953 I began to work at the new meridian circle at Brorfelde located 50 km from Copenhagen. I became fascinated by the instrument and saw the great importance of astrometry for astronomy and astrophysics. As a stipendiary in Hamburg, I proposed in 1960 a new method of astrometry by photon counting, using a photo-multiplier as detector and the then novel electronic computers for data reduction. The idea fitted well with plans for an expedition with the Hamburg meridian circle to Perth in Western Australia and we implemented the new method 1960-67 on the instrument. -- The new method was adopted by Pierre Lacroute in France in his design 1964-74 of a scanning satellite. Pierre Lacroute became the father of space astrometry. In 1975 ESA began a study of this idea. I became involved and have been able to contribute to space astrometry since then. The study led to the first satellite for space astrometry, Hipparcos, launched in 1989 on a three year mission and it became a great success. In 2013 a new astrometric satellite, Gaia, was launched by ESA and the first results have now shown that it is, as expected, a million times more powerful than Hipparcos. Plans for a Gaia successor with Near Infrared capabilities, GaiaNIR, for launch about 2035 have been worked out with partners in Europe, USA, Japan and Australia. We have proposed to ESA in Hobbs+2019 arXiv:1907.12535 that GaiaNIR be studied.

At 1.5 astronomical units from the Sun, the Trojans of Mars represent the innermost stable group of asteroids in the solar system and a natural laboratory to study asteroid evolutionary processes. Embedded wiithin this population is the Eureka family, recently found to share a common and unusual composition (Borisov et al, MNRAS, 2017) that may represent material excavated from deep within Mars itself (Polishook et al, Nat. Astron., 2017). The family probably formed not by collisions, like most asteroid families, but by YORP-induced rotational spinup & fission (Christou et al, Icarus, 2017) and has, therefore, more in common with small asteroid clusters in the Main Belt (Pravec et al, Nature, 2010; Pravec et al, Icarus, 2017). Two other Trojans, of similar size to Eureka, are not associated with families of asteroids, begging the question of what makes Eureka special. I will present a population evolution model of the Trojans where the production of new asteroids through the YORP effect is balanced by their eventual escape from the Trojan clouds through Yarkovsky-driven orbital evolution. Applying the model allows to explain the observations in terms of the different physical evolution of the asteroids and their dynamical environment. In addition to these findings, I will discuss currently open problems and the potential for new discoveries from future facilities such as the Large Synoptic Survey Telescope.

The CHAMP, GRACE and GOCE satellites have provided a wealth of high-precision accelerometer observations, allowing to study the thermosphere with unprecedented spatial and temporal resolution. Continuity of observation time records is guaranteed for the upcoming years by the Swarm and GRACE-FO satellites. Detailed and precise information about e.g. the thermospheric neutral density and winds finds its application in studies of climatology and weather in the thermosphere, and in space mission planning, re-entry prediction, collision avoidance, etc. This presentation focuses on the goal to create an accurate long-term, multi-mission thermosphere data set with high temporal and spatial resolution.

The early age of the solar system has seen numerous giant impacts, on a scale energetic enough to make and unmake planetesimals and moons. The Earth did not escape to this pattern; the last giant impact must have been large enough to transform the proto-Earth and the impactor, Theia, into a protolunar disk from which later on the actual Earth and the Moon condensed. Some of the most important thermodynamic parameters that are necessary in the modeling of such and impact, and that are also missing today, are the positions of the critical points, the liquid-vapor equilibria, and the equation of state for both liquids and the supercritical fluids. To study this region, we employ large-scale first-principles molecular dynamics simulations. We consider the average composition of the Earth’s mantle and we cover the 0.75 – 7.5 g/cm3 density range and 2000 – 10000 K temperature range. This allows us to investigate the thermodynamics conditions ranging throughout the entire disk, from the interior of the molten core to the outer regions of the vaporized disk, to identify and characterize the supercritical region and to discuss the effect of a selected collection of volatiles on the structure and properties of the disk.

By now it has been accepted that most of the small asteroids in the Solar System are granular aggregates kept together by gravitational and possibly, cohesive forces. These aggregates can form, deform and disrupt over millennia subjected to different internal and external factors that would ultimately determine how they evolve over time. Parameters such as porosity, cohesive and tensile strength, angles of friction, particle size distributions, stress states and heterogeneity among others, determine how these bodies will react when subjected to different, changing, external factors. These external factors include solar photon momentum, gravitational tides, micro- and macro-impacts and are believed to have produced and shaped the current asteroid population. In short, it is a competition between the internal strength of the asteroid and the external factors that try to break it apart and this is what we want to understand. In our research we use a combination of Soil Mechanics theory, Soft-Sphere Discrete Element Method (SSDEM) Simulations and Orbital Mechanics in order to understand how simulated, homogeneous and heterogeneous, ellipsoidal and spherical gravitational aggregates, a crude but useful representation of an asteroid, evolve when rotated to the point of disruption. Then, we compare our results to the shapes of observed asteroids as well as to the disruption patterns of a few active asteroids. Our results lead us to believe that the different shapes of observed asteroids as well as their unique disruption patterns could give us clues about their internal structure, strength and geophysical properties in general. In this talk, I will also touch on why the dynamics of granular matter in micro-gravity conditions is important for present and future space missions to small bodies.

It is well known that appropriate time reparametrizations of the equations of motion can be advantageous for the numerical integration of N-body problems. This is particularly so in the presence of close encounters. In that case, accurate numerical integration requires either (i) decreasing the time-step when approaching a close encounter, or (ii) applying a suitable time reparametrization (time regularizations) that allows one to use constant time-step without degrading the accuracy of the computed trajectories. In the second case, the new fictitious time s and the original time t are usually related through a differential equation dt/ds=g(u), where u is the vector of state variables. We will focus on the choice of appropriate time regularization functions g(u). Our approach is based on obtaining lower bounds for the radius of convergence of the solutions of N-body problems.

EnVision is a proposed Venus orbiter mission that will determine the nature and current state of geological activity on Venus, and its relationship with the atmosphere, to understand how and why Venus and Earth evolved so differently. It is one of three finalists in ESA’s M5 selection process and is currently in phase A study; final mission selection is expected in 2021. The mission is studied in collaboration with NASA, with the potential sharing of responsibilities currently under assessment. If selected, EnVision will launch by 2032 into a six month cruise to Venus, followed by aerobraking, to achieve a near-circular polar orbit. The science phase is planned for a nominal duration of 4 Venus cycles (2.66 years) plus an extended duration of 2 more cycles. EnVision will use a number of different techniques to search for active geological processes, measure changes in surface temperature associated with active volcanism, characterise regional and local geological features, determine crustal support mechanisms and constrain mantle and core properties. EnVision scientific mission is carried out by its four experiments : - S-band Synthetic Aperture Radar (VenSAR) - Subsurface Radar Sounder (SRS) - Suite of 3 Spectrometers and Spectro-imagers (VenSpec) - Radio Science Experiment (RSE) Venus (a = 0.73 AU, R = 0.95 RT, Torb = 0.61 yr) differs from Earth in fundamental ways: it lacks the plate tectonic system that shapes Earth’s topography, geology, climate, and long-term habitability. Yet Venus and Earth may have key similarities. Its original atmosphere was probably volatile-rich, similar to that of early Earth, with abundant water that would have been liquid under the young Sun’s fainter output. Venus’ average surface may be as young as 150 m.y. - about the age of Earth’s seafloor. What can be learned from Venus about the life story of terrestrial planets near their habitable zone, in an era of discovery of Earth-sized exoplanets? Were the radically different evolutionary paths of Earth and Venus driven solely by distance from the Sun, or do internal dynamics, geological activity, volcanic outgassing and weathering also play an important part? More information : www.envisionvenus.eu

Since the detection of absorption features in spectra of beta Pictoris in 1990 (Beust et al., 1990) observations of absorption lines varying on short time scales in spectra of stars accumulate. Scientists refer to these findings as features caused by objects evaporating material on their orbit when they come close to the star and thus call them comets in exoplanetary systems -- exocomets. This theory is based on the knowledge about the Solar System. The aim of our dynamical investigation is to find a statistical model for the scattering of small bodies which shows the most probable whereabouts of these objects after the gravitational interaction with the planets. We will show results from our n-body simulations including a star, a Jupiter-like planet and a disk of planetesimals (testparticles). As a consequence of the migration of the gas giant planetesimals are scattered either inward or outward. The outward scattered objects will form analogues to the Kuiper belt respectively the Oort Cloud in our Solar System. The created reservoirs are different depending on the initial conditions of the planetesimal disk, migration of the Jupiter-like planet as well as mass and orbit of the giant planet. We measure semi-major axis, eccentricity, perihel, aphel, inclination and orbital period after 4byr of integration of each testparticle and compare the outcomes of the computations with different acting forces applied -- as there are gravitational influence of the galactic tide and passing stars. Keeping in mind the differences of the confirmed exoplanetary systems and the architecture of our own Solar System we change the number of planets interacting with the small bodies. We show that the scattering process of the testparticles, involving more than one gas giants, is more efficient in putting the planetesimals to further out distances. In order to complete our studies we, too, start with different masses of the central star and different initial conditions for the planets (mass, orbit...) as given from observations. Statistics of the orbits of small bodies after being scattered by the planets are shown for some sample confirmed planetary systems. This might give a clue to observers on where to look for features being produced by cometary bodies in extrasolar systems. The gained knowledge can be used to generate a general model for the formation of cometary reservoirs in extrasolar systems with respect to the system architecture which can be used to predict the location of cometary reservoirs in extrasolar systems.

The SPICA Infrared Observatory has been selected by ESA on May 7, 2018 for further study as part of the M5 competition. It carries 3 instruments, SMI, B-BoP, and SAFARI, covering the 17-230 µm range, with a wavelength resolution R between a few hundred and a thousand, combined with various efficient large area imaging and polarimetry modes. The SPICA telescope will be cooled down to about 8K, effectively suppressing most of the satellite's infrared thermal background, which will allow us to reach down to very low fluxes. SPICA will provide spectroscopic capabilities at a high sensitivity of 2-15 x1e-20 W/m2 (5?/1hr), much deeper than Herschel. The scientific key objectives of SPICA are i) to reveal the physical processes that govern the formation and evolution of galaxies and black holes, ii) to resolve for the first time the far-infrared polarization, and therefore the magnetic field, of galactic filaments, and iii) to understand the formation and evolution of planetary systems. I will present the SPICA mission, its capabilities and the key scientific objectives.

The aim of this work is to define the dynamical parameters of asteroids and their satellites based on their astrometric observations. We used the model of motion that took into account the gravitational influence of a primary's axisymmetric non-sphericity. This factor leads to precession of the line of apsides as well as to precession of a satellite's orbit around primary's axis of symmetry. The most appropriate object for our model is the asteroid (22) Kalliope with the satellite Linus. In addition to previously accumulated observations we used a new series of speckle interferometric observations of this system performed in 2017-2018. These observations extended the time interval of observations, thus enabling to detect the precession of the satellite's orbit at 216 degrees. This allowed to find all parameters of the satellite's orbit, the total mass of the system, non-sphericity of Kalliope and the direction of the orbit precession axis. The detected orbit precession is due to the attraction of an extended axisymmetric body formed by rapidly rotating Kalliope. These are the first results obtained on the basis of dynamical study of the primary's extension, unlike earlier studies where results were obtained by using photometric observations which give only apparent external shape of Kalliope that does not necessarily reflect its gravitational field. The paper gives values of all dynamical parameters of the system as well as the results of new positional observations of the Kalliope-Linus system.

Suite au succès de la visite et de la collecte d'échantillons de l’astéroïde de type-S Itokawa par la sonde spatiale Hayabusa-1, la JAXA a décidé d'initier l'exploration et le retour d’échantillons d'un autre astéroïde par une seconde mission Hayabusa-2. La cible retenue est un astéroïde de type-C nommé Ryugu. Si le satellite est très similaire à celui d'Hayabusa-1, cette mission est encore plus ambitieuse et propose une diversité unique de moyens d'exploration avec notamment plusieurs atterrisseurs dont un de conception franco-allemande (Mascot) qui a pu se poser avec succès sur la surface. La collecte de données et leurs analyses sont en cours mais elles s'avèrent déjà remarquables. Je présenterai donc cette mission, les opérations qui se sont parfois révélées aventureuses en raison de la surface surprenante de Ryugu et les résultats majeurs obtenus par certains des instruments à bord de Mascot et de Hayabusa-2.

Since the 1970s Very Long Baseline Interferometry (VLBI) has proven to be a main space geodetic technique by determining Terrestrial Reference Frame (TRF), monitoring the variable Earth rotation and orientation with highest precision, and by deriving many other parameters of the Earth system. Further, geodetic VLBI is the only space-geodetic technique able to realize the International Celestial Reference System (ICRS). Accurate Reference Frames (TRF) are indispensable for many applications, e.g. navigation and mapping on Earth, precise orbit determination for satellites, and geophysical applications, such as monitoring tectonic motions, and observing the variations of the sea level. Especially for the latter the requirements for the accuracy and long term stability are very high, i.e. better than 1 mm and 0.1 mm per year. This goal has not yet been achieved. Main obstacles are the inconsistencies between the different techniques which contribute to their determination and their dependencies on each other. The current realization of the ICRS, the third International Celestial Reference Frame (ICRF3, Charlot et al. 2018), is the main product of the astrometric and geodetic VLBI and its major contribution to fundamental astronomy. It was realized as three independent CRFs at three frequencies from one analysis center, in contrast to the ITRF which combine all space geodetic techniques. Further, the current coordinate model of the ICRF3 sources consists of time-invariant coordinates in right ascension and declination. Hence, any deviation from a constant position is categorized as noise. In reality, sources can have structures differing from the assumed point like ideal and their position can be subjected to significant changes. If unaccounted for, these changes will be absorbed by other parameters, leading to systematic effects in the Earth orientation parameter (EOP) estimates. This talk will give a short overview of geodetic VLBI and an insight to my recent work on the determination source positions and CRFs.

Asteroids in our solar system are metallic, rocky and/or icy objects, ranging in size from a few meters to a few hundreds of kilometers. Whereas we now possess constraints for the surface composition of most D>100 km primordial main-belt asteroids, little is known regarding their internal structure. Yet, this is a fundamental property whose characteristics result directly from (a) their formation location, (b) their time of formation, and (c) their collisional history. Characterizing the internal structure of the main compositional classes of asteroids would therefore allow us to address entirely new questions regarding the earliest stages of planetesimal formation and their subsequent collisional and dynamical evolution. To achieve this goal, we carry out - via an ESO Large Program (LP) that was awarded 152h on VLT/SPHERE (the observations are spread over 4 semesters from April 1st, 2017 till March 30, 2019 in service mode) - disk-resolved observations of a substantial fraction of all D>100 km main-belt asteroids (sampling the four main compositional classes) at high angular resolution with VLT/SPHERE throughout their rotation. These observations enable us to derive their volume (via their 3-D shape) which combined with already existing mass estimates allow us to determine their bulk density and hence to characterize their internal structure. The high-resolution images also allow us to detect craters larger than 30 km and use their morphology (crater diameter and depth) to characterize the density of the outer shell. The knowledge of both the bulk density and the density of the outer shell of our targets allows a characterization of their internal structure. This information, in turn, allows us to determine: (a) the nature of their initial building blocks (rock only, or a mixture of ice and rock) and (b) which compositional classes experienced differentiation. These constraints then serve as direct inputs to thermal evolution models for determining the time of formation and the formation location (inward or outward of the snowline) of the main compositional classes of objects in the asteroid belt.

Ce séminaire spécial à deux voix aura un format spécial, comme environ une fois l'an quand le sujet s'y prête. Deux interventions viendront donc ponctuer cet après-midi, avec une pause café méridienne qui facilitera les questions et échanges dans l'assistance. 14h-15h : Estefanía de Mirandés (BIMP). La révision du Système International d’Unités. Résumé : Le 16 novembre 2018 la Conférence Générale des Poids et Mesures a approuvé une révision du système international d’unités (SI) qui entrera en vigueur le 20 mai 2019. Quatre des sept unités de base du SI - le kilogramme, l’ampère, le kelvin et la mole - seront redéfinies par rapport à des valeurs fixées de constantes de la nature – la constante de Planck, la charge élémentaire, la constante de Boltzmann et la constante d’Avogadro -. Cette révision représente l’aboutissement du rêve de Stoney, Planck et d’autres de mettre en place un système naturel d’unités fondé sur des invariants de la nature, car après la révision du SI aucune unité ne sera plus définie à partir d’un étalon matériel. L’exposé présentera le détail de cette refonte, les raisons qui l’ont poussée, ainsi que la méthodologie suivie pour choisir et fixer la valeur de ces constantes. Une perspective sur les conséquences envisageables de cette révision sera également proposée. 15h30-16h30 : Jean-Philippe Uzan (IAP) Les constantes fondamentales - le nouveau SI et la Relativité générale. Résumé : Le nouveau système international d’unités qui a été adopté le 16 novembre 2018 fonde la définitions des unités en fixant la valeur numérique d’un jeu de constantes de la nature. Cet exposé reviendra sur ce choix qui est l’aboutissement d’une longue histoire. Il rappellera le rôle des constantes dans les lois de la nature et soulignera les échanges avec la théorie. Pour finir, nous discuterons de l’hypothèse selon laquelle ces paramètres sont constants, aussi bien d’un point de vue théorique qu’expérimental, ce qui soulignera un lien avec le principe d’équivalence au coeur de la Relativité générale.

In summer and autumn 1676 both Cassini and Rømer announced the discovery of the velocity of light to the Académie royale des Sciences and they mentioned a value of some 10-11 minutes/AU. Both Cassini and Rømer ascribed the discovery to Rømer, but it is nevertheless sometimes claimed that Rømer took over the idea from Cassini, who had abandoned it. These claims indirectly accuse Cassini and Rømer of scientific malpractice and leave many open questions. We discuss the background for the discovery, the difficulties involved and why the value was so far from the expected 8.3 minutes/AU. We show that by looking at the events in 1676 in connexion with the scientific priorities of Cassini the mystery of the double announcement can be resolved.

Comets in the Oort cloud evolve under the influence of internal and external perturbations, such as giant planets, stellar passages, and the galactic tide. We aim to study the dynamical evolution of the comets in the Oort cloud, accounting for the perturbation of the galactic tidal field and passing stars. We base our study on three main approaches; analytic, observational and numerical. We first construct an analytical model of stellar encounters. We find that the cumulative effect of passing stars can strip the Oort cloud on a very short time scale(<500 Myr). Using proper motions, parallaxes, and radial velocities from Gaia DR2 and combining them with the radial velocities from external surveys, we construct an astrometric catalogue of the 14,659 stars that are closer than 50pc to the Sun. We calculate the encounters that are closer than 2.5pc and that the Sun has had in the last 10Myr and will have in the near future 10Myr. Finally, we study the dynamical evolution of the comets in the Oort cloud under the effect of multiple stellar encounters from stars that pass within 2.5pc from the Sun and the galactic tide over ±10Myr. For this, we use the Astrophysical Multipurpose Software Environment (AMUSE), and the GPU-accelerated direct N-body code ABIE. We find that the cumulative effect of relative distant stellar encounters together with the galactic tidal field, proves to be an efficient mechanism in the creation of interstellar comets. This raises the question about the formation, evolution and current status of the Oort cloud, as well as, the existence of an Interstellar Oort Cloud. A larger sample of stars constructed through less filtering or an incompleteness correction can only increase the comet loss.

Since the detection of absorption features in spectra of beta Pictoris in 1990 (Beust et al., 1990) observations of absorption lines varying on short time scales in spectra of stars accumulate. Scientists refer to these findings as features caused by objects evaporating material on their orbit when they come close to the star and thus call them comets in exoplanetary systems -- exocomets. This theory is based on the knowledge about the Solar System. The aim of our dynamical investigation is to find a statistical model for the scattering of small bodies which shows the most probable whereabouts of these objects after the gravitational interaction with the planets. We will show results from our n-body simulations including a star, a Jupiter-like planet and a disk of planetesimals (testparticles). As a consequence of the migration of the gas giant planetesimals are scattered either inward or outward. The outward scattered objects will form analogues to the Kuiper belt respectively the Oort Cloud in our Solar System. The created reservoirs are different depending on the initial conditions of the planetesimal disk, migration of the Jupiter-like planet as well as mass and orbit of the giant planet. We measure semi-major axis, eccentricity, perihel, aphel, inclination and orbital period after 4byr of integration of each testparticle and compare the outcomes of the computations with different acting forces applied -- as there are gravitational influence of the galactic tide and passing stars. Keeping in mind the differences of the confirmed exoplanetary systems and the architecture of our own Solar System we change the number of planets interacting with the small bodies. We show that the scattering process of the testparticles, involving more than one gas giants, is more efficient in putting the planetesimals to further out distances. In order to complete our studies we, too, start with different masses of the central star and different initial conditions for the planets (mass, orbit...) as given from observations. Statistics of the orbits of small bodies after being scattered by the planets are shown for some sample confirmed planetary systems. This might give a clue to observers on where to look for features being produced by cometary bodies in extrasolar systems. The gained knowledge can be used to generate a general model for the formation of cometary reservoirs in extrasolar systems with respect to the system architecture which can be used to predict the location of cometary reservoirs in extrasolar systems.

For a dynamically closed universe governed by either Newtonian gravity or general relativity, I will discuss the consequences of two assumptions: 1) physical quantities are dimensionless ratios; 2) the scale variable of the universe increases monotonically in both temporal directions away from a unique minimum. In a talk given at the Observatory in 2014, I discussed the case of a non-vanishing minimum and its potential to explain the arrows of time, including the second law of thermodynamics. In this talk I will discuss the intriguing and possibly far-reaching consequences of assumption 1) if the scale variable vanishes at its minimum (total collision in the N-body problem and big bang in general relativity).

L'hypothèse d'une matière noire adéquate fut inventée par Fritz Zwicky en 1933 pour stabiliser les mouvements internes, trop rapides pour un équilibre gravitationnel ordinaire, dans les galaxies et les amas de galaxies. Une autre hypothèse explicative fut avancée par Mordehai Milgrom en 1987 : une petite modification des lois de la gravitation pour les accélération très faibles. La question se pose cependant de savoir si un équilibre strict est vraiment nécessaire pour faire admettre ces grandes formations. Déjà, en ce qui concerne les galaxies, il apparaît finalement que l'attraction interne des étoiles est suffisante pour un quasi-équilibre de leur presque totalité et ce sont essentiellement les nuages de gaz alentour qui ont une vitesse trop rapide. Sans doute ces nuages sont-ils en voie d'évasion tout comme le vent solaire (plusieurs millions de tonnes par seconde) s'échappe continuellement du Système solaire sans modifier notablement celui-ci. En dix milliards d'années de la vie du Soleil, la masse du vent solaire émis ne représente que 0,03% de la masse totale du Soleil. Pour les amas de galaxies la question principale n'est pas de savoir s'ils sont gravitationnellement stables : ils ne le sont pas. Ce qui importe est de savoir si leur dispersion est moins rapide que l'expansion de l'Univers. Les amas ayant une énergie cinétique bien supérieure à leur potentiel gravitationnel les galaxies qui les composent se comportent presque comme si elles étaient isolées dans l'espace, mais dans un espace en expansion un corps isolé n'avance pas à vitesse uniforme... Cette question requiert l'utilisation de la Relativité générale et le résultat est probant : dans le système de coordonnées co-mobile les vitesses relatives des galaxies sont en voie de décélération rapide et leurs déplacements sont limités. Bien entendu les (petites) attractions gravitationnelles intergalactiques renforcent cette stabilité. Les amas de galaxies conservent donc leur individualité même en l'absence de matière noire... Ce séminaire discutera l'ensemble de ces arguments, basés sur les lois de la gravitation de la mécanique classique et de la Relativité.

The Time Transfer by Laser Link (T2L2) experiment on-board the Jason-2 satellite (launched the 20 June 2008, at 1335 km of altitude) aims to synchronize remote clocks in the ground with a sub-nanosecond accuracy. Except short interruptions, the experiment worked perfectly from June 2008 to April 2018, for almost 10 years. T2L2 consists in an electronic device for timing, linked to the Doppler Orbitography Radiopositioning Integrated by Satellite (DORIS) Ultra Stable Oscillator (USO), which delivers the frequency reference to the whole satellite, and an optical device that detected the laser pulses coming from the laser stations of the International Laser Ranging Service (ILRS). T2L2 offered a time colocation with three geodetic techniques on-board the Jason-2 satellite (SLR, DORIS and Global Positioning System (GPS)), and with the ground. It is a unique opportunity to reduce the systematics of these techniques, linked to the “time observable” (e.g. clock stability, time biases…) and meet the next requirement for geodesy; “the millimeter level”. After a quick review of the main results obtained during these last 10 years, I will present the impacts and applications of the T2L2 experiment for space geodesy. I will focus on the DORIS and SLR techniques, by two aspects: - The development of an USO model, with the use of T2L2 and its impact on Precise Orbit Determination (POD) and beacons positioning. - The measurement of Time biases in the SLR stations, and their impacts on geodetic products.