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Session GRAAPH

Near Earth Asteroids astrometry with Gaia and beyond
D.Bancelin, D.Hestroffer, W.Thuillot
(IMCCE-Observatoire de Paris)
Gaia is a mission that will be launched in 2012, and will observe a large number of Solar System Objects down to magnitude V lower than 20. The solar system science mission is to map thousands of MBA, NEO, comets (and also planetary satellites) with the principal purpose of orbital determination (better than 5 mas astrometric precision), determination of asteroid mass, spin properties, and taxonomy. Besides Gaia will be able to discover new objects, in particular Near Earth Objects (NEOs) in the region below solar elongation of 90^o, which are harder to detect with current ground-based surveys. During the 5-year mission, Gaia will continuously scan the sky with a specific strategy: objects will be observed from two lines of sight separated with a constant basic angle. Five constants -already fixed- determinate the nominal scanning law of Gaia: The inertial spin rate (1^/min) that describe the rotation of the spacecraft around an axis perpendicular to those of the two fields of view, the solar-aspect angle (45^) that is the angle between the Sun and the spacecraft rotation axis, the precession-period (63.12 days) which is the precession of the spin axis around the Sun-Earth direction. Two other constants are still free parameters: the initial spin phase, and the initial precession angle that will be fixed at the start of the nominal science operations. These latter are constraint by scientific outcome (possibility of performing test of fundamental physics) together with operational requirements (downlink to Earth windows). Several sets of observations of specific NEOs will hence be provided according to the initial precession angle. The purpose is to study the statistical impact of the initial precession angle on the error propagation and on the collision probability, especially for PHAs. We will also analyse the advantage of combining space-based to ground-based observation over long term, as well as in short term from observations in alert.



Accelerometers of the GOCE Mission: return of experience from one year of in-orbit
Bruno Christophe, Jean-Pierre Marque, Bernard Foulon
(ONERA)
The tri-axes Gradiometer of the ESA GOCE Mission is conceived around six electrostatic accelerometers developed by ONERA. The contribution of the accelerometers to the mission is double by providing the Satellite with the combination of linear accelerations as input to the continuous drag compensation and attitude control system and with the scientific data measurements to be on-ground processed for the Earth gravity gradients restitution. The satellite was launched on March 17th, 2009 and the gradiometer was switched on in Science mode on April 7th. Since, the accelerometers are continuously feeding the science channel with data, first during the commissioning and calibration phases, then during the first measurement phase started in September 2009. The presentation will illustrate the in-flight behaviour of the six accelerometers as deduced from the analysis of their output signals.


Evaluation de l'ITRF2008p par telemetrie laser
ILRS GRGS AC
(OCA-IMCCE-GRGS)



Effects of asteroids on spacecrafts orbiting the Lagrangian point L2. Application to Herschel, Planck and Gaia.
P.Kuchynka, J. Laskar, A. Fienga, M. Gastineau et P. Robutel
(IMCCE-Observatoire de Paris)
The Lagrangian point L2 presents many advantages for space observatories. Today it harbors the Herschel and Planck satellites and it has also been selected for the incoming Gaia mission. In this presentation we will give a short overview of the dynamics in the proximity of the L2 point. We will then present results of our investigation of asteroid perturbations on objects placed on quasiperiodic orbits around L2. Given the fact that spacecraft orbits are corrected on a regular basis, the asteroid effects are in general negligible. A few individuals could be an exception to this rule because of their extremely close approaches with L2.


The MICROSCOPE space mission and the in flight calibration plan for its instrument
Agnès Levy, Pierre Touboul, Manuel Rodrigues, Gilles Métris, Alain Robert, Vincent Josselin
(ONERA)
MICROSCOPE is a fundamental physics space mission which aims at testing the Equivalence Principle (EP) with an accuracy of 10$^{-15}$. The gravitational signal is measured precisely by a differential electrostatic accelerometer which includes two cylindrical test masses made of different materials. The accelerometer is onboard a drag-free microsatellite which is controlled Earth pointing or rotating about the normal to the orbital plane with a very stable angular velocity. The accuracy of the measurement used for the EP test is limited by the defects of the instrument's physical parameters and by the environment of the instrument on board the satellite. These defects are partially measured or estimated with ground tests or during the integration of the instrument on the satellite. However, to satisfy the EP test accuracy objective, these evaluations are not sufficient and an in-orbit calibration is therefore needed to characterize finely the defects and to correct the measurements. After an overall presentation of the MICROSCOPE mission and its scientific goal, the presentation will focus on the accelerometer onboard the satellite and will describe the specific procedures which are planned to determine in orbit the exact values of the instrument's defects.



INPOP08 ephemeris: adjustment to Lunar Laser Ranging observations
H. Manche, A. Fienga, J. Laskar, M. Gastineau, S. Bouquillon, G. Francou; P. Kuchynka
(Observatoire de Paris/IMCCE)
INPOP08 is a planetary solution developped at the Paris Observatory. Equations of motion of the planets and more than 300 asteroïds are numerically integrated. Parameters and initial conditions are fitted to planetary observations (Fienga et al.,2009) and to Lunar Laser Ranging measurements. We will briefly present the dynamical model, the physical effects taken into account in the reduction of LLR observations and the residuals obtained.



Dernière nouvelles de L'Action Spécifique GRAM
Gilles METRIS et Peter WOLF
(OCA - Geoazur)



La station de télémétrie Laser Mobile de l'Observatoire de la Côte d'Azur à Grasse (France), l'instrument, ses missions et les projets scientifiques associé.
F.Pierron & All.Observatoire de la Côtte d'Azur-Grasse-France
(Observatoire de la Côte d'Azur)
The Mobile Satellite Laser Ranging System from "Observatoire de la Côte d'Azur" à Grasse (France), technical issues, measurements campaigns and scientific projects linked to this instrument.




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Samuel Boissier 2010-06-16