LESIA Observatoire de Paris

Meudon, France

LESIA Observatoire de Paris

Meudon, France
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Harris A.W.,German Aerospace Center | Barucci M.A.,LESIA Observatoire de Paris | Cano J.L.,Deimos Space | Fitzsimmons A.,Queen's University of Belfast | And 10 more authors.
Acta Astronautica | Year: 2013

Although discussions are underway within the Action Team 14 of the United Nations COPUOS, there is currently no concerted international plan addressing the impact threat from near-Earth objects (NEOs) and how to organize, prepare and implement mitigation measures. We report on a new international project to address impact hazard mitigation issues, being the subject of a proposal submitted to the European Commission in response to the 2011 FP7 Call "Prevention of impacts from near-Earth objects on our planet". Our consortium consists of 13 research institutes, universities, and industrial partners from 6 countries and includes leading US and Russian space organizations. The primary aim of the project, NEOShield, is to investigate in detail the three most promising mitigation techniques: the kinetic impactor, blast deflection, and the gravity tractor, and devise feasible demonstration missions. Furthermore, we will investigate options for an international strategy for implementation when an actual impact threat arises. The NEOShield project was formally accepted by the European Commission on 17 November 2011 and funded with a total of 5.8 million Euros for a period of 3.5 years. The kick-off meeting took place at the DLR Institute of Planetary Research, Berlin, in January 2012. In this paper we present a brief overview of the planned scope of the project. © 2012 IAA. Published by Elsevier Ltd. All rights reserved.


Ehrenfreund P.,Leiden Institute of Chemistry | Ehrenfreund P.,Space Policy Institute | Ulamec S.,German Aerospace Center | Barucci M.A.,LESIA Observatoire de Paris | And 9 more authors.
Proceedings of the International Astronautical Congress, IAC | Year: 2012

MarcoPolo-R is a sample return mission to a primitive Near-Earth Asteroid (NEA) selected in February 2011 for the Assessment Study Phase in the framework of ESA's Cosmic Vision 2 program. MarcoPolo-R is a European mission and takes advantage of three completed industrial studies. MarcoPolo-R will rendezvous with a unique kind of target, the primitive binary NEA (175706) 1996 FG3. The MarcoPolo mission will scientifically characterize the binary NEA system at multiple scales, and return a unique pristine sample to Earth unaltered by the atmospheric entry process or terrestrial weathering. The binary target provides enhanced science return: precise measurements of the mutual orbit and rotation state of both components can be used to probe higher-level harmonics of the gravitational potential, and therefore the internal structure. The main goal of the MarcoPolo-R mission is to return unaltered NEA material for detailed analysis in ground-based laboratories which will allow scientists to study the most primitive materials available to investigate early Solar System formation processes. Copyright © (2012) by the International Astronautical Federation.


Lammer H.,Austrian Academy of Sciences | Blanc M.,French National Center for Scientific Research | Benz W.,University of Bern | Fridlund M.,European Space Agency | And 20 more authors.
Astrobiology | Year: 2013

A scientific forum on The Future Science of Exoplanets and Their Systems, sponsored by Europlanet* and the International Space Science Institute (ISSI)† and co-organized by the Center for Space and Habitability (CSH)‡ of the University of Bern, was held during December 5 and 6, 2012, in Bern, Switzerland. It gathered 24 well-known specialists in exoplanetary, Solar System, and stellar science to discuss the future of the fast-expanding field of exoplanetary research, which now has nearly 1000 objects to analyze and compare and will develop even more quickly over the coming years. The forum discussions included a review of current observational knowledge, efforts for exoplanetary atmosphere characterization and their formation, water formation, atmospheric evolution, habitability aspects, and our understanding of how exoplanets interact with their stellar and galactic environment throughout their history. Several important and timely research areas of focus for further research efforts in the field were identified by the forum participants. These scientific topics are related to the origin and formation of water and its delivery to planetary bodies and the role of the disk in relation to planet formation, including constraints from observations as well as star-planet interaction processes and their consequences for atmosphere-magnetosphere environments, evolution, and habitability. The relevance of these research areas is outlined in this report, and possible themes for future ISSI workshops are identified that may be proposed by the international research community over the coming 2-3 years. © Copyright 2013, Mary Ann Liebert, Inc. 2013.


Vincent J.-B.,Max Planck Institute for Solar System Research | Schenk P.,Lunar and Planetary Institute | Nathues A.,Max Planck Institute for Solar System Research | Sierks H.,Max Planck Institute for Solar System Research | And 11 more authors.
Planetary and Space Science | Year: 2014

Orbiting asteroid (4) Vesta from July 2011 to August 2012, the Framing Camera on board the Dawn spacecraft has acquired several tens of thousand images of the asteroid surface, revealing a complex landscape. The topography is dominated by craters of all sizes and shapes, from fresh, simple, bowl-shaped craters to giant basins, as seen in the southern hemisphere. Craters of different ages or states of degradation can be seen all over the surface; some have very sharp rims and simple morphology, whereas others are highly eroded and have sometimes been filled by landslides and ejecta from nearby craters. The general depth/Diameter (d/D) distribution on Vesta is similar to what has been observed on other small rocky objects in the Solar System with a distribution peaking at 0.168±0.01 in the range 0.05-0.35. However, the global map of d/D reveals important geographic variations across the surface, unlike any other asteroid. The northern most regions of Vesta show d/D values comparable to other asteroid surfaces, with a mean d/D of 0.15±0.01, and a steep cumulative distribution. Craters in the regions affected by the giant southern impacts are deeper (mean d/D=0.19±0.01) and show less erosion. It can be interpreted as the southern surface being younger than the rest of the asteroid, or made of a material which either allows the formation of deeper features or prevents their erosion. This picture is consistent with the idea of a southern Vestan hemisphere resurfaced relatively recently by the giant impact that created the Rheasilvia basin. The analysis of depth-to-Diameter variations over the whole surface also brings some insight into the transition regions between different cratering regimes: about 20 km for the strength-to-gravity dominated regime, and 38 km for the beginning of the simple-to-complex transition. © 2013 Elsevier Ltd. All rights reserved.


Fletcher L.N.,Jet Propulsion Laboratory | Fletcher L.N.,University of Oxford | Achterberg R.K.,University of Maryland University College | Greathouse T.K.,Southwest Research Institute | And 7 more authors.
Icarus | Year: 2010

Five years of thermal infrared spectra from the Cassini Composite Infrared Spectrometer (CIRS) are analyzed to determine the response of Saturn's atmosphere to seasonal changes in insolation. Hemispheric mapping sequences at 15.0cm-1 spectral resolution are used to retrieve the variation in the zonal mean temperatures in the stratosphere (0.5-5.0mbar) and upper troposphere (75-800mbar) between October 2004 (shortly after the summer solstice in the southern hemisphere) and July 2009 (shortly before the autumnal equinox).Saturn's northern mid-latitudes show signs of dramatic warming in the stratosphere (by 6-10. K) as they emerge from ring-shadow into springtime conditions, whereas southern mid-latitudes show evidence for cooling (4-6. K). The 40-K asymmetry in stratospheric temperatures between northern and southern hemispheres (at 1. mbar) slowly decreased during the timespan of the observations. Tropospheric temperatures also show temporal variations but with a smaller range, consistent with the increasing radiative time constant of the atmospheric response with increasing pressure. The tropospheric response to the insolation changes shows the largest magnitude at the locations of the broad retrograde jets. Saturn's warm south-polar stratospheric hood has cooled over the course of the mission, but remains present.Stratospheric temperatures are compared to a radiative climate model which accounts for the spatial distribution of the stratospheric coolants. The model successfully predicts the magnitude and morphology of the observed changes at most latitudes. However, the model fails at locations where strong dynamical perturbations dominate the temporal changes in the thermal field, such as the hot polar vortices and the equatorial semi-annual oscillation (Orton, G., and 27 colleagues [2008]. Nature 453, 196-198). Furthermore, observed temperatures in Saturn's ring-shadowed regions are larger than predicted by all radiative-climate models to date due to the incomplete characterization of the dynamical response to the shadow. Finally, far-infrared CIRS spectra are used to demonstrate variability of the para-hydrogen distribution over the 5-year span of the dataset, which may be related to observed changes in Saturn's tropospheric haze in the spring hemisphere. © 2010 Elsevier Inc.


Thebault P.,LESIA Observatoire de Paris | Marzari F.,University of Padua | Augereau J.-C.,Joseph Fourier University
Astronomy and Astrophysics | Year: 2010

Context. Debris disc analysis and modelling provide crucial information about the structure and the processes at play in extrasolar planetary systems. In binary systems, this issue is more complex because the disc should also respond to the companion star's perturbations. Aims. We explore the dynamical evolution of a collisionally active debris disc for different initial parent body populations, diverse binary configurations, and optical depths. We focus on the radial extent and size distribution of the disc in a stationary state. Methods. We numerically followed the evolution of 10 5 massless small grains, initially produced from a circumprimary disc of parent bodies following a size distribution in dN propto s -3.5ds. Grains were submitted to both stars' gravity and radiation pressure. In addition, particles were assigned an empirically derived collisional lifetime. Results. For all the binary configurations, the disc extends far beyond the critical semi-major axis a crit for orbital stability. This is due to the steady production of small grains, placed by radiation pressure on eccentric orbits reaching beyond a crit. The amount of matter beyond a crit depends on the balance between collisional production and dynamical removal rates: it increases for more massive discs, as well as for eccentric binaries. Another important effect is that, in the dynamically stable region, the disc is depleted from its smallest grains. Both results could lead to observable signatures. Conclusions. We have shown that a companion star can never fully truncate a collisionally active disc. For eccentric companions, grains in the unstable regions can contribute significantly to the thermal emission in the mid-IR. Discs with sharp outer edges, especially bright ones such as HR4796A, are probably shaped by other mechanisms. © ESO, 2010.


Yang P.,CAS Shanghai Institute of Optics and fine Mechanics | Yang P.,Max Planck Institute for Astronomy | Yang P.,University of Chinese Academy of Sciences | Hippler S.,Max Planck Institute for Astronomy | And 9 more authors.
Optics Express | Year: 2013

The adaptive optics system for the second-generation VLT-interferometer (VLTI) instrument GRAVITY consists of a novel cryogenic near-infrared wavefront sensor to be installed at each of the four unit telescopes of the Very Large Telescope (VLT). Feeding the GRAVITY wavefront sensor with light in the 1.4 to 2.4 micrometer band, while suppressing laser light originating from the GRAVITY metrology system, custom-built optical components are required. In this paper, we present the development of a quantitative near-infrared point diffraction interferometric characterization technique, which allows measuring the transmitted wavefront error of the silicon entrance windows of the wavefront sensor cryostat. The technique can be readily applied to quantitative phase measurements in the near-infrared regime. Moreover, by employing a slightly off-axis optical setup, the proposed method can optimize the required spatial resolution and enable real time measurement capabilities. The feasibility of the proposed setup is demonstrated, followed by theoretical analysis and experimental results. Our experimental results show that the phase error repeatability in the nanometer regime can be achieved. © 2013 Optical Society of America.


Gratadour D.,LESIA Observatoire de Paris | Gendron E.,LESIA Observatoire de Paris | Fusco T.,ONERA | Rousset G.,LESIA Observatoire de Paris | Meimon S.,ONERA
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

In this paper we present the results of a study comparing the performance of various centroiding methods on an extended spot. Various spot shapes, in good agreement with measurements of the Sodium layer density profile are investigated. We compare the performance of center-of-gravity (COG) based ad cross-correlation (CC) based methods and the matched filter (MF) algorithm. While their performance are comparable in the case of a Gaussian Sodium profile, we show that, above 15 to 20m off-axis, substantial differences appear in the case of a known non-Gaussian Sodium density profile. CC based and MF methods seem to have comparable or better performance with various spot shapes while non-Gaussian shapes have dramatic impact on COG based methods. © 2010 SPIE.


Moullet A.,Harvard - Smithsonian Center for Astrophysics | Gurwell M.A.,Harvard - Smithsonian Center for Astrophysics | Lellouch E.,LESIA Observatoire de Paris | Moreno R.,LESIA Observatoire de Paris
Icarus | Year: 2010

Many of the key properties of Io's atmosphere, such as its spatial distribution, temperature, column density and composition, are still not fully assessed despite decades of extensive observations. The contribution of the possible gas sources to the atmospheric replenishment are then still unclear.This paper presents disk-resolved observations performed with the Submillimeter Array (SMA) at 345GHz of atmospheric rotational lines of the main atmospheric species SO2, and, for the first time, of the minor species SO and NaCl. All these species appear concentrated on the anti-jovian hemisphere, but do not share the same spatial distribution. The obtained maps and line-averaged fluxes are compared to realistic models simulating gas sources including volcanic plume outgassing, SO2 frost sublimation and photolysis. Arguments in favor of each sources are examined and compared to observations, putting constraints on their relative roles for each species.While sublimation clearly appears as the favored major source for SO2, SO2 photolysis may account for most of the production of SO. Using constraints on the volcanic plumes distribution from Galileo results, we find that direct volcanic input can only contribute for a minor fraction of atmospheric SO2, but represent a more significant source for SO atmosphere, and is likely to be the only source for NaCl. Temperature and column densities findings are also presented for SO2, and compare well to previously published observations and atmospheric models. © 2010 Elsevier Inc.


Gratadour D.,LESIA Observatoire de Paris | Gendron E.,LESIA Observatoire de Paris | Rousset G.,LESIA Observatoire de Paris
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

Phase local gradient measurements in a Shack-Hartmann (SH) wavefront sensor (WFS) are achieved by tracking the displacements of a spot in each sub-aperture. This can be done by comparing the position of its center-ofgravity (COG) to a reference position. A commonly used method for position tracking is the weighted (W) COG in which the original image is multiplied by a weighting function, before centroiding. However, it can be shown that, if the spot centered on its COG is not even, the WCOG estimator is biased. We propose a new method for spot centroiding with a direct application to wavefront sensing on a LGS: the symmetrically weighted (SW) COG, in which the weighting function is taken as the symmetrical version of the spot, with respect to its COG. © 2010 OSPIE.

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