Seminars Temps-Espace-Société

Philippe de La Hire (1640 – 1718), après des études consacrées principalement aux mathématiques, s’est investi dans de nombreux domaines de la science. Contemporain de J.D. Cassini et de Picard, il a en particulier participé aux missions géodésiques dans le but de déterminer les contours des côtes de France et la poursuite du méridien. Il s’est installé à l’observatoire de Paris en 1682, et y a alors effectué des observations astronomiques journalières pendant 35 ans. On analysera les archives de ces observations, et leur exploitation dans le cadre de l’astronomie moderne, en particulier celles relatives aux transits des étoiles et du Soleil au méridien, aux taches solaires, à l’occultation d’Aldébaran par la Lune, aux passages de comètes, et aux planètes.  

The Cassini spacecraft's observations of Saturn's rings revealed that they are remarkably clean, consisting mainly of pure water ice with only minimal contamination from cumulative micrometeoroid impacts. This led to the conclusion that the rings are extremely young (about 100 million years old), which is significantly younger than the age of our solar system (~4.6 billion years). In this talk, I will challenge this common understanding.
 

Une revue de projets menés récemment, en lien avec l'Action Pluri-annuelle Incitative ESTERS2 : "Environnement Spatial de la Terre : Recherche, Surveillance, Société" 

Préparée par le Bureau ESTERS2.

Matin 10h00 - 12h 

The proliferation of artificial light at night, and the light pollution it yields, is a global challenge that relates not only to cities, but also distant places otherwise being considered to be dark. It is because the urban light emissions extend many hundreds of kilometers away from their origins, crossing jurisdictional boundaries and impacting distant, protected areas such as nature reserves and national parks. 

There are two main factors that affect the behavior of emitted light and its potential to cause skyglow over a city. One is the Earth's atmosphere, which is the most variable element in prediction models.  The other factor is information about the emitting light source itself. When this information is incomplete or absent, it leads to approximations and assumptions whose significance scales up with increasing size of the examined areas.

The knowledge of the hemispherical night sky brightness produced by the ever-growing expansion of outdoor lighting systems is a necessary step for characterizing the nighttime environment and monitoring the evolution of the night sky quality, which is directly linked to what happens on the ground in terms of how outdoor lighting is used. Obtaining the all-sky, hemispherical radiance distribution produced by artificial lights is however a more demanding challenge, requiring the radiative transfer equation to be solved subject to boundary conditions.

The seminar will cover topics such as skyglow modeling and measurements, including the presentation of two powerful modeling tools.

Rubble pile asteroids are thought to form in the aftermath of cataclysmic collisions between proto-planets. The details of how the detritus from such collisions reaccumulate to form these bodies are not well understood, yet can play a fundamental role in the subsequent evolution of these bodies in the solar system. Simple items such as how particle sizes and porosity is distributed within a body can have a significant influence on how they subsequently evolve. Current space missions are just starting to gain limited insight into these fundamental questions, but require a better theoretical understanding to fully explain their observations.


To that end, this work studies how the initial energy and angular momentum of a random collection of gravitating bodies is partitioned and redistributed between escaping components and bound multiple body systems. A generic initial distribution of N bodies will naturally lose many components due to multi-body dynamical interactions. If the bodies have finite density, some components will also form condensed distributions. We find and apply rigorous results from the Full N-body problem to place limits and constraints on how the energy and angular momentum of such systems can evolve, which may control the formation of stable rubble pile asteroids.


We are able to establish some of our constraints analytically, providing unique insight into this process. Ultimately, however, we require numerical simulations to elucidate certain aspects of the ejection process. As will be shown, these gravitational ejections will always reduce the system energy yet can cause significant fluctuations in the total angular momentum of the remaining bodies. Some possible implications of these trends will be discussed.
 

(rescheduled) - Over the previous century, scientists have collected >60,000 meteorites. These meteorites originally come from asteroids, primarily in the main asteroid belt between Jupiter and Mars. However, that is where things become unclear. Despite this vast collection, we still struggle to identify the asteroids or regions where these meteorites are derived. Current modeling and meteorite recovery networks have significantly helped us piece together the story of meteorites and, consequently, our solar system. Nevertheless, these models rely on rare cases where meteorites were precisely observed and orbital information was collected. Currently, only ~40 meteorites have been recovered in this manner. Instead, this project aims to create a novel numerical model of solar system debris that could be calibrated with data from tens of thousands of meteorites. Scientists can infer how long ago any meteorite was ejected from its parent asteroid by analyzing isotopic variations  - referred to as the cosmic-ray exposure age (CRE). Concurrently, our new model would be able to predict the transfer times from different regions of the main asteroid belt, enabling us to compare to the CRE ages found in meteorites and draw important conclusions about meteorite source regions.

The hardware characterstics of Graphical Processing Units (GPUs) have advanced to the point that two order of magnitude performance improvements can be obtained in problems that are of general utility. 
I will describe high-level design of the code GLISSE (GPU Longterm Integrator in Solar System Evolution), and use it to illustrate scientific exploration of the stability of orbits (1) between Uranus and Neptune, and (2) in Kuiper Belt resonances. 
For these applications GLISSE is 300-1000 times faster than a CPU core, and thus the hardware cost makes it vastly more financially effective  to use GPU based hardware rather than clusters of nodes.
We are extending the integrator's capabilities to handle small bodies encountering planets, handling such relatively rare encouters on CPU  cores while the GPU deals with huge numbers of test particles.  
I will discuss the application of this GLISSER code (the R standing for 'regularized') the the orbital evolution of the Kuiper Belt region under the existence of additional planets in the early Solar System.