Bernard Foulon

MICROSCOPE Mission: First Results of a Space Test of the Equivalence Principle

According to the weak equivalence principle, all bodies should fall at the same rate in a gravitational field. The MICROSCOPE satellite, launched in April 2016, aims to test its validity at the 10^{-15} precision level, by measuring the force required to maintain two test masses (of titanium and platinum alloys) exactly in the same orbit. A nonvanishing result would correspond to a violation of the equivalence principle, or to the discovery of a new long-range force. Analysis of the first data gives δ(Ti,Pt)=[-1±9(stat)±9(syst)]×10^{-15} (1σ statistical uncertainty) for the titanium-platinum Eötvös parameter characterizing the relative difference in their free-fall accelerations.

Odyssey: A Solar System Mission

The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System [(4) in the above list].

Flight experience on CHAMP and GRACE with ultra-sensitive accelerometers and return for LISA

The challenging drag-free sensor of the Laser Interferometer Space Antenna (LISA) mission is derived from electrostatic accelerometers developed for a long time in ONERA. The LISA sensor includes a gold platinum alloy inertial mass free-floating in space and used as reflectors for the laser interferometer. This test mass should not undergo more than 3 × 10−15 m s−2 Hz−1/2 acceleration at 0.1 mHz. This tremendous performance is not close to what has been reached so far, but should be approached within one order of magnitude with the projected SMART-2 ESA mission by 2006. Meanwhile, ONERA has participated in several space missions with the flight of increasingly sensitive accelerometers. The German CHAMP mission aims at mapping the Earths magnetic and gravity fields. More than two years data have been accumulated showing a resolution better than 3 × 10−9 m s−2 Hz−1/2 for the accelerometer. With the JPL/NASA GRACE mission launched in March 2002, even more sensitive measurements have been obtained. From these two flight experiments with electrostatic sensors very similar in concept to that of LISA, the accelerometric environment on board a satellite is discussed at nanogravity levels. It is also shown that these first analyses are compatible with the expected LISA performance when the results are extrapolated to the LISA environment, needing femto-gravity levels.

On Board Evaluation of the STAR Accelerometer

The main results of the on-board evaluation of the STAR accelerometer are presented after 18 months of mission. The instrument demonstrates high performances in terms of resolution and reliability and its contribution to dynamic orbit determination is clear. However, unexplained signal jumps have been detected and analysed. In addition, an anomalous behaviour of the X3 electrode of the accelerometer, affects the Radial, Roll and Pitch accelerations. Nevertheless, corrected observations can be recovered from a new combination of the electrode voltages. The in-orbit calibration will also benefit from the new EIGEN gravity field model that includes CHAMP data.

The microscope mission

Abstract The MICROSCOPE mission had been selected at the end of 1999 by the French space agency Cnes for a launch scheduled in 2004. The scientific objective of the mission is the test of the Equivalence Principle (EP) up to an accuracy of 10−15 with its well-known manifestation, the universality of free fall. This principle, at the origin of general relativity, is only consolidated by experimental results and presently with an accuracy of several 10−13. The micro-satellite developed by Cnes weighs less than 120 kg and is compatible with a low-cost launch like ASAP ARIANE V. The instrument is composed of two differential electrostatic accelerometers operating at finely stabilised room temperature. Each accelerometer includes two cylindrical and concentric test masses, made of platinum or titanium alloys. The experiment consists in controlling the two masses in the same orbital motion. Because of the drag compensation system of the satellite including field effect electrical thrusters, this motion is quite purely gravitational. The electrostatic control forces used in the differential accelerometers are finely measured. The principle of the experiment is presented, the configuration of the instrument and of the satellite is detailed with regard to the present development status. The specifications for the major parameters of the experiment are detailed.

Electrostatic accelerometer with bias rejection for Gravitation and Solar System physics

Radio tracking of interplanetary probes is an important tool for navigation purposes as well as for testing the laws of physics or exploring planetary environments. The addition of an accelerometer on board a spacecraft provides orbit determination specialists and physicists with an additional observable of great interest: it measures the value of the non-gravitational acceleration acting on the spacecraft, i.e. the departure of the probe from geodesic motion. This technology is now routinely used for geodesy missions in Earth orbits with electrostatic accelerometers. This article proposes a technological evolution which consists in adding a subsystem to remove the bias of an electrostatic accelerometer. It aims at enhancing the scientific return of interplanetary missions in the Solar System, from the point of view of fundamental physics as well as Solar System physics. The main part of the instrument is an electrostatic accelerometer called MicroSTAR, which inherits mature technologies based on ONERA’s experience in the field of accelerometry. This accelerometer is mounted on a rotating stage, called Bias Rejection System, which modulates the non-gravitational acceleration and thus permits to remove the bias of the instrument from the signal of interest. This article presents the motivations of this study, describes the instrument, called GAP, and the measurement principle, and discusses the performance of the instrument as well as integration constraints. Within a mass of 3.1 kg and an average consumption of 3 W, it is possible to reach a precision of 1 pm/s 2 for the acceleration measured with an integration time of five hours. Combining this observable

Capacitive sensing and electrostatic positioning of the miniSTEP test masses

Abstract The capacitive position sensing and electrostatic positioning is a multi-purpose subsystem of the miniSTEP instrument. Implemented inside each of the four differential cryogenic accelerometers, it will be designed first to accurately set up the position of the two test masses. In fact, all degrees of freedom of each nearly cylindrical masses, except the rotation about the revolution axis, shall be finely controlled. The system will allow an easy way to calibrate the SQUIDs and the superconductive magnetic suspension and to centre the two masses to satisfy the rejection of the gravity gradient and angular motion effects. During the Equivalence Principle measurement phase, it will provide electronic damping for the differential mode and the dither signal for the test mass charge control system. The position measurements will be also useful for the drag compensation system of the satellite. Taking advantage of highly accurate and low back-action sensors and of the easiness and in quite real time adjustment of the configuration parameters through digital control loops it could even constitute a back-up for the SQUID sensors and the magnetic bearings.

Electrostatic accelerometer as an advantageous component of precise attitude and orbit control system

Abstract Satellite attitude control systems most often use star sensors to measure the satellite attitude. This well known method presents a good resolution for low frequencies; however in some cases it is interesting to have a more precise attitude control by increasing the functioning bandwidth. Angular accelerations delivered by electrostatic accelerometer such as those developed by ONERA may notably complement star sensor measurements. This instrument presents a very good resolution at higher frequencies (typically in the bandwidth 10 -4 – 10 -1 Hz). However, the accelerometer introduces a bias and a scale factor whose limited knowledge affects the accuracy of the measurement. An original method to estimate these parameters, by means of a least square fit to the star sensor measurements is described in this paper. The combination of the star sensor and adjusted accelerometer measurement, with fine actuators, will improve the accuracy of the attitude control.