Le lundi à 14h00 - Salle de l´Atelier / Observatoire de Paris - 77 Av. Denfert-Rochereau, F-75014 PARIS

Sébastien Lambert (SYRTE) ,
Florent Deleflie (IMCCE) , Mirel Birlan (IMCCE) ,

Saison 2017 - 2018

Lundi 6 novembre 2017, 14h00
Salle Jean-François Denisse, Paris
Collisions with Centaurs
Mattia Galiazzo
TU Wien, Austria
résumé :
Centaurs are objects whose orbits are contained between those of Jupiter and Neptune. Typical Centaurs' lifetime is about ~20 Myrs. They are mostly in very chaotic regions perturbed by the giant planets and they are scattered in the inner solar system and also in the outer one.
In this work we study their dynamical evolution for 30 Myrs. Since Centaurs can enter in the inner solar system and they can be relatively massive (m ? 10-9 m?), we ponder whether close encounters and in particular impacts, and thus water transportation with terrestrial planets have happened in the recent solar system and we compute the amount of these events statistically.
Lundi 20 novembre 2017, 14h00
Salle Danjon, Paris
Les mouvements du noyau terrestre comme vous ne les avez jamais vus !
Nathanael Schaeffer
Institut des Sciences de la Terre CNRS UMR 5275, Grenoble, France
résumé :
Lundi 4 décembre 2017, 14h00
Salle Jean-François Denisse, Paris
TBD (mais déjà connu)
Bruno Sicardy
résumé :
Vous avez aimé les précédents, vous adorerez celui-là (résumé provisoire)
Lundi 18 décembre 2017, 14h00
Salle Jean-François Denisse, Paris

Séminaires précédents (ordre chronologique inverse)

Lundi 16 octobre 2017, 14h00
Salle Jean-François Denisse, Paris
High precision optical astrometry of MEO satellites
Bogdan Danila
Astronomical Institute of the Romanian Academy, Cluj-Napoca Observatory
résumé :
As the European Union is preparing for the Galileo GNSS cluster to go online in 2020, existing in-orbit satellites as well as ones that will be launched soon require accurate orbital measurements for calibration and optimal function. Although laser ranging is the preferred technique used in precision orbital tracking for all MEO satellites, some of these objects do not have retroreflector arrays installed or their orbits have large associated uncertainties. Optical observations can help complement the shortcomings that laser ranging has, offering a wide field of view appropriate for tracking objects with large orbital uncertainty. The ESA requirement for a Space Surveillance and Tracking (SST) system is an accuracy better than 3.6 arcseconds. I will present an inexpensive solution that makes use of a small 12 inch telescope with a CCD camera, connected to an FPGA board that is controlling a GPS receiver, in order to reliably trigger the shutter open of the camera regardless of any thread interruptions from the host operating system. The astrometric data reduction process relies on the "Astrometrica" software package, combined with our developed software tools for the satellite streak centroid determination. The latter uses a statistically method for the ellipsoid of greater concentration, and a Laplacian of Gaussian filter for image contour determination, implemented with open source software ImageJ. The combined error for the observation and astrometry process is on the arcsecond level, enough to satisfy the ESA requirement.
Lundi 2 octobre 2017, 14h00
Salle Jean-François Denisse, Paris
Fantastic Icy Worlds and Where to Find Them
Dr Michele Bannister
Queen’s University Belfast
résumé :
The outer Solar System has a wealth of recent discoveries that inform our understanding of orbital dynamics and provoke exciting new questions. The populations of small icy worlds orbiting in the vast volume beyond Neptune are remarkably abundant. A major new observational effort, the Outer Solar System Origins Survey (OSSOS), is revealing the details of an intricate filigree of mean-motion resonant orbits, emplaced by the historic migration of Neptune. OSSOS has also made new discoveries of rare trans-Neptunian objects that orbit even further afield, so far from planetary and Galactic tide influences that they are not thought to be produced in the current known planetary architecture of the Solar System. These have informed the recent debate on the existence of a distant giant planet. The hard-to-observe extreme TNOs require a formation method - and offer tantalizing hints that our Solar System is more complex than our current conception.
Lundi 25 septembre 2017, 14h00
Salle Danjon, Paris
Attention : salle inhabituelle
Revisiting TW Hydrae association in light of Gaia – DR1
Rama Teixeira
résumé :
L’arrivée du catalogue DR1 de Gaia permet de tester les mouvements propres des étoiles cataloguées. On verra ici ce que l’on peut en déduire pour les étoiles de l’association TW Hydrae.
Lundi 18 septembre 2017, 14h00
Salle Jean-François Denisse, Paris
Apogeios, un concept de cité spatiale
Olivier Boisard (1) et Pierre Marx (2)
(1) ingénieur-conseil & enseignant Ecole Centrale de Lille, (2)ancien directeur de la prospective CNES
résumé :
Le physicien Gérard O’Neill imaginait dans les années 70, avec ses étudiants de l’université de Princeton, des « iles spatiales » positionnées sur des points de Lagrange du système Terre-Lune, capables d’accueillir des populations de plusieurs dizaines, voire plusieurs centaines de milliers d’habitants. Près de quarante ans plus tard, quels sont les éléments nouveaux permettant de poursuivre et approfondir ces travaux, d'en identifier les points faibles et les solutions désormais concevables, les enjeux et les conditions de réalisation ? Ce sont les questions que se propose d’aborder notre projet de cité spatiale « Apogeios » ...
Vendredi 23 juin 2017, 14h00
Salle Jean-François Denisse, Paris
-- attention : vendredi --
International Space Station-based Meteor Observation Project: METEOR
Tomoko Arai
Planetary Exploration Research Center/Chiba Institute of Technology
résumé :
The International Space Station (ISS) is an ideal platform for continuous meteor observation without distortion caused by weather and atmospheric disturbances. We are currently conducting two-year long meteor observation project, named METEOR onboard ISS with a high-sensitive, color high definition TV (HDTV) camera and a wide-angle, bright lens (F 0.95, f=10.5mm) through the window of the Window Observational Research Facility (WORF) in the pressurized US Lab module (DESTINY) of the ISS. For the first year, photometric observation is done and for the second year, spectrosopic obseration is done with a transmitted diffraction grating to estimate elemental abundances (Fe, Ca, Mg, Na) of meteoroids. These observation enable us to understand variation of physical and chemical characteristics of meteoroids within a single meteor shower, among meteor showers and possibly of their parent bodies. The onboard METEOR camera is being operated directly from the mission operation center located in PERC, Chitech and the captured data are readily downlinked and analyzed within a day. Here, we report the latest status and outcomes of METEOR.
Lundi 12 juin 2017, 14h00
Salle Jean-François Denisse, Paris
Where Do Meteorites Come From?
Richard P BINZEL
Professor of Planetary Science Joint Professor of Aerospace Engineering Massachusetts Institute of Technology, Chercheur Associé, IMCCE-Observatoire de Paris
résumé :
Correlating meteorites with their exact source bodies has been a long standing challenge. Success toward that goal requires the interplay of telescopic, laboratory, and dynamical research. Astronomical (spectral) measurements of Near-Earth Asteroids, as possible analogs to more frequent meteorite deliveries, have forged ahead to encompass more than 1,000 individual objects. New links and source locations are emerging, anchored in part by the success of the Japanese Hayabusa sample return mission and anticipated results from Hayabusa 2 and OSIRIS-REx. This talk will focus on the intersection of telescopic data with dynamical models with the latest results for major meteorite classes.
Lundi 29 mai 2017, 14h00
Salle Jean-François Denisse, Paris
Clustering of a driven granular gas in microgravity
Eric Opsomer
Université de Liège, Université Paris Diderot
résumé :
Driven granular gases present rich dynamical behaviors. Given the dissipative character of the collisions between the constitutive particles of the medium, dense and slow regions may appear in the system. These clusters emerge naturally during cooling phenomena but another, dynamical, clustering can be observed for continuously excited systems. In our work, the physical processes that trigger the transition from a granular gas to a dynamical cluster are evidenced through molecular dynamics simulations, realized in the frame of ESA’s SpaceGrains project. Physical arguments, based on relaxation times and energy propagation, deliver an analytical prediction for the gas cluster transition of a granular media. This new control of the granular clustering provides an interesting way to gather particles in microgravity and, furthermore, to achieve granular transport. Finally, the morphology of the cluster and its growth have been explored present intriguing analogy to condensation.
Lundi 15 mai 2017, 14h00
Salle Jean-François Denisse, Paris
Modélisation des populations de débris spatiaux
Alexis Petit
University of Namur, Namur, Belgium
résumé :
Depuis le début de l'ère spatiale le 4 octobre 1957, avec le lancement du satellite Spoutnik 1, le nombre de débris spatiaux en orbite autour de la Terre ne cesse de croître. A ce jour nous comptons près de 20 000 objets de plus de 10 cm, plusieurs centaines de milliers de plus de 1 cm et plusieurs dizaines de millions de plus de 1 mm. Les vitesses relatives étant de l'ordre de 10 km/s en orbite basse, un objet de quelques millimètres possède l'énergie d'une balle de fusil 22 Long Rifle (environ 95 Joules). Ainsi, la surveillance et l'estimation des populations de débris est un enjeu crucial pour la sécurité des missions spatiales. Malheureusement, seuls les plus gros objets de plus de 10 cm en orbite basse (en dessous de 2 000 km d'altitude) peuvent être observés et suivis par les réseaux de télescopes optiques et radars.

Dans cet exposé, je présenterai les différentes populations de débris spatiaux actuellement en orbite et leur origine. J'introduirai les méthodes de modélisation et les conclusions importantes sur l'évolution de ces populations durant les prochaines décennies. Nous verrons l'importance des mesures de prévention et de réduction des débris. Je présenterai quelques spécificités de mon travail pour la modélisation de la dynamique des objets en orbite et de la caractérisation des sources comme les fragmentations.
Mardi 2 mai 2017, 14h00
Salle Jean-François Denisse, Paris
Radio astrometry in post-Gaia epoch
Leonid Petrov
résumé :
Overview of VLBI absolute astrometry survey programs ran in past two decades will be given. Their scientific goals, strategies and main results will be discussed. For last five years Gaia program started to change the shape of VLBI absolute astrometry programs. The Gaia DR1 for the first time produced an independent catalogue with accuracy rival to the accuracy of VLBI catalogue. Approximately one half of VLBI sources have a counterpart in Gaia DR1 catalogues. A cursory comparison of over 6000 matches showed that in general both catalogues agrees at a level of their formal uncertainties. A more detailed comparison revealed that there is a fraction of 6% matches that have statistically significant offsets.
The most stunning results of comparison is that the offsets Gaia minus VLBI are not distributed randomly but aligned along the jet. That means that this systematic error is caused by an intrinsic property of a source, namely the presence of optic jet. Discovery of omnipresent optic jet at milliarcsecond scales has profound astrophysical and astrometric implications and changed the game. Several new VLBI absolute programs are under preparation with the goals from one hand to study optic structure and from the other hand mitigate systematic errors caused by optic source structure to improve astrometry results.
Mardi 18 avril 2017, 14h00
Salle Jean-François Denisse, Paris
Attention : date inhabituelle !
Earth rotation in sight of climate modulations
Leonid Zotov
National Research University Higher School of Economics, Moscow, Russia
résumé :
We draw attention to the length of day (LOD) and amplitude of the Chandler wobble changes, which show 70-year modulations similar to what can be observed in the mean Earth temperature and sea level. We analyze Chandler wobble phase and amplitude changes, as well as LOD, and their excitation sources, trying to bridge this traditional subject of geodesy with contemporary climatological observations over ocean, atmosphere, and land mass transport. We believe, our epoch of precise observations of the Earth system is already providing an evidence of interconnection between climate processes and Earth rotation changes.
Lundi 3 avril 2017, 14h00
Salle de l'atelier, Paris
Spectral characterization of small NEOs in the framework of NEOSHIELD-2 project.
Marcel Popescu
Astronomical Institute of the Romanian Academy & LESIA, Observatoire de Paris, PSL Research University
résumé :
The characterization of NEOs (near-Earth objects) is fundamental to assess and mitigate their impact risk with our planet. Moreover, understanding the physical properties of asteroids can provide crucial information on the origin and early evolution of the solar system. Their composition allows determining the compositional gradient of the solar nebula and the processes that governed the first evolutionary phases at different heliocentric distances. Asteroid studies can help shedding light on topics such as the delivery of water and organic-rich material to the early Earth, and the emergence of life.
One of the main objectives of the NEOShield-2 project is to undertake an extensive observational campaign involving complementary techniques to provide physical and compositional characterization of a large number of NEOs in the hundred-meter size range. Priority has been given to potential space-mission targets, optimized for mitigation or exploration missions. This project has been funded by European Commission (2015-2017) in the framework of the EU H2020 programme.
In the framework of this observational program we obtained 147 new spectra of NEAs for 137 individual objects. A number of 29 asteroids with diameters smaller than 100 meters (thus, we tripled the spectral data for objects of this size range) and 71 with diameters between 100 m and 300 m were characterized. Overall, this represents an increase of 40% compared with the data available in EARN database. We discovered eight new D-type asteroids accessible for a space-mission rendezvous, by using the visible spectral data obtained within the NEOShield2 survey. This result more than double the small number of NEAs classified as D types which satisfy the deltaV requirements.
In this talk I will describe the observations, the data reduction methods and preliminary results regarding their taxonomic classification and compositional distributions across this peculiar group of very small asteroids. I will compare our results with the existing data for the large NEAs. I will discuss about the suitable targets from our sample for a space mission.
NB: We acknowledge the financial support from the NEOShield-2 project, funded by the European Commission's Horizon 2020 program (contract No. PROTEC-2-2014-640351).
Lundi 20 mars 2017, 15h00
Salle Jean-François Denisse (ex Salle de l'Atelier) , Paris
Attention heure inhabituelle
Détection des séismes et des éruptions volcaniques dans l’ionosphère
Elvira Astafyeva
Institut de Physique du Globe
résumé :
Les tremblements de terre et les éruptions volcaniques génèrent des ondes acoustiques et de gravité. Ces ondes se propagent verticalement dans l’atmosphère, où elles augmentent en amplitude suite à la décroissance de la densité atmosphérique avec altitude. Par le couplage entre les particules neutres et les particules chargées dans l’atmosphère, on peut détecter ces perturbations dans des mesures des paramètres ionosphériques. Nous utilisons les données du contenu électronique total pour étudier la réponse ionosphérique aux séismes et aux éruptions volcaniques, et nous démontrons la possibilité d’utilisation des données ionosphériques pour des applications sismologiques et volcanologiques.
Lundi 6 mars 2017, 14h00
Salle de l'atelier, Paris
On the ages of asteroid families
Federica Spoto
Observatoire de la Côte d'Azur
résumé :
The most recent dynamical family classification is based on synthetic proper elements for 510746 numbered/multi-opposition asteroids. I present a review of the method used to compute the age of a family and the latest results obtained working on resonant, eroded and fossil asteroid families. The method has been systematically applied to estimate asteroid family collisional ages, and it solves several problems of collisional models, including cases of complex relationship between dynamical families (identified by clustering in the proper elements space) and collisional families (formed at a single time of collision).
Lundi 20 février 2017, 14h00
Salle de l'atelier, Paris
Secular representations for the long-term dynamics beyond Neptune
Melaine Saillenfest
IMCCE - Observatoire de Paris & Dipartimento di Matematica - Università di Pisa
résumé :
I will present the development and the application of semi-analytical secular models, designed to describe generically the long-term dynamics of transneptunian objects. One-degree-of-freedom systems are obtained in both the non-resonant and resonant cases, allowing to represent every possible trajectory by the level curves of the Hamiltonian. The application to known objects reveals pathways to high perihelion distances. In particular, distant resonant objects can be tracked back to their resonance capture.
Lundi 23 janvier 2017, 14h00
Salle de l'atelier, Paris
Fundamental limits to the precision in astrometry and photometry using array detectors
René Mendez
Departamento de Astronomia & Observatorio Astronomico Nacional / Universidad de Chile
résumé :
Digital detectors have been in use for astronomical research since the early 80´s, with the inception of Charge-Couple Devices (CCDs) for civil applications. Nowadays CCDs are the the most widely used detector at optical wavelengths in all major professional observatories around the globe - as well as in space-based observatories - and the expectation is that this type of detector will accompany us for several more years.
Since CCD detectors have a well-characterized behavior, and motivated by the ever increasing measurement precision achieved by them, we have started to explore ways to characterize the maximum precision limits to astrometry (location) and photometry (brightness) that could possibly be achieved by these type of detectors under realistic experimental (observing) conditions. In this talk I will present some results of these efforts from our group.
Lundi 9 janvier 2017, 14h00
Salle de l'atelier, Paris
Comment El Niño a influencé la rotation de la Terre
Sébastien Lambert
Observatoire de Paris, SYRTE
résumé :
Un récent évènement El Niño a secoué le Pacifique pendant l'hiver 2015-16. Il a été souvent présenté comme le plus important El Niño de ces dernières décennies, certains climatologues allant même jusqu'à le surnommer Godzilla. Pendant les mois qui ont précédé cet hiver, un important volume d'eau chaude a migré depuis l'Océanie, où il était confiné par la poussée des alizés, vers l'est du Pacifique, déséquilibrant alors la circulation atmosphérique intertropicale et modifiant la circulation normale des courants jets subtropicaux.

Outre les impacts sur le climat global et les activités économiques de la zone Pacifique, les épisodes El Niño sont les phénomènes climatiques ayant des conséquences les plus visibles sur la rotation terrestre. En effet, le renforcement des courants jets subtropicaux circulant d'ouest en est entraine alors un ralentissement de la rotation de la Terre. El Niño est même l'un des principaux responsables des irrégularités de la rotation terrestre aux échelles de temps saisonnières.

Dans cet exposé, je présenterai le phénomène El Niño et sa signature dans la rotation terrestre telle que nous l'avons mesurée l'an dernier : un allongement de la longueur du jour de presque une milliseconde. Je montrerai ensuite comment l'atmosphère a interagi avec les zones montagneuses du globe pour forcer cet dernier à ralentir sa rotation.
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