Stefanie Bremer

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.

New powerful thermal modelling for high-precision gravity missions with application to Pioneer 10/11

The evaluation of about 25 years of Doppler data has shown an anomalous constant deceleration of the deep space probes Pioneer 10 and 11. This observation became known as the Pioneer anomaly (PA) and has been confirmed independently by several groups. Many disturbing effects that could cause a constant deceleration of the craft have been excluded as possible source of the PA. However, a potential asymmetric heat dissipation of the spacecraft surface leading to a resulting acceleration still remains to be analysed in detail. We developed a method to calculate this force with very high precision by means of finite element (FE) modelling and ray tracing algorithms. The elaborated method is divided into two separate parts. The first part consists of the modelling of the spacecraft geometry in FE and the generation of a steady state temperature surface map of the craft. In the second part, this thermal map is used to compute the force with a ray-tracing algorithm, which gives the total momentum generated by the radiation emitted from the spacecraft surface. The modelling steps and the force computation are presented for a simplified geometry of the Pioneer 10/11 spacecraft including radioisotope thermoelectric generators (RTG), equipment/experiment section and the high gain antenna. Analysis results how that the magnitude of the forces to be expected are non-negligible with respect to the PA and that more detailed investigations are necessary. The method worked out here for the first time is not restricted to the modelling of the Pioneer spacecraft but can be used for many future fundamental physics (in particular gravitational physics) and geodesy missions like LISA, LISA Pathfinder or MICROSCOPE for which an exact disturbance modelling is crucial.

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].

Modelling of Solar Radiation Pressure Effects: Parameter Analysis for the MICROSCOPE Mission

Modern scientific space missions pose high requirements on the accuracy of the prediction and the analysis of satellite motion. On the one hand, accurate orbit propagation models are needed for the design and the preparation of a mission. On the other hand, these models are needed for the mission data analysis itself, thus allowing for the identification of unexpected disturbances, couplings, and noises which may affect the scientific signals. We present a numerical approach for Solar Radiation Pressure modelling, which is one of the main contributors for nongravitational disturbances for Earth orbiting satellites. The here introduced modelling approach allows for the inclusion of detailed spacecraft geometries, optical surface properties, and the variation of these optical surface properties (material degradation) during the mission lifetime. By using the geometry definition, surface property definitions, and mission definition of the French MICROSCOPE mission we highlight the benefit of an accurate Solar Radiation Pressure modelling versus conventional methods such as the Cannonball model or a Wing-Box approach. Our analysis shows that the implementation of a detailed satellite geometry and the consideration of changing surface properties allow for the detection of systematics which are not detectable by conventional models.

DEVELOPMENT OF MODELS FOR HIGH PRECISION SIMULATION OF THE SPACE MISSION MICROSCOPE

MICROSCOPE is a French space mission for testing the Weak Equivalence Principle (WEP). The mission goal is the determination of the Eotvos parameter with an accu- racy of 10 15 . This will be achieved by means of two high-precision capacitive differential accelerometers, that are built by the French institute ONERA. At the German institute ZARM drop tower tests are carried out to verify the payload performance. Additionally, the mission data evaluation is prepared in close cooperation with the French partners CNES, ONERA and OCA. Therefore a comprehensive simulation of the real system in- cluding the science signal and all error sources is built for the development and testing of data reduction and data analysis algorithms to extract the WEP violation signal. Cur- rently, the High Performance Satellite Dynamics Simulator (HPS), a cooperation project of ZARM and the DLR Institute of Space Systems, is adapted to the MICROSCOPE mission for the simulation of test mass and satellite dynamics. Models of environmen- tal disturbances like solar radiation pressure are considered, too. Furthermore detailed modeling of the on-board capacitive sensors is done.

Microscope – A space mission to test the equivalence principle

MICROSCOPE is a ESA/CNES space mission for testing the validity of the weak equivalence principle. The missions goal is to determine the Eotvos parameter η with an accuracy of 10 −15 . The French space agency CNES is responsible for designing the satellite which is developed and produced within the Myriade series. The satellites payload T–SAGE (Twin Space Accelerometer for Gravitation Experimentation) consists of two high–precision capacitive differential accelerometers and is developed and built by the French institute ONERA. As a member of the MICROSCOPE performance team, the German department ZARM performs free fall tests of the MICROSCOPE differential accelerometers at the Bremen drop tower. The projects concepts and current results of the free fall tests are shortly presented.