Hanns Selig

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.

Testing the Universality of Free Fall for Charged Particles in Space

At first a short analysis of the notion of the Universality of Free Fall (UFF) for charged matter is given. Even if neutral bound systems of charged particles are in full accordance with the UFF, there is still a possibility that an isolated charge couples anomalously to gravitational fields. The experiment of Witteborn and Fairbank aimed at testing the UFF for electrons is shortly reviewed emphasizing the various additional disturbing gravity induced electromagnetic fields. Since these additional gravity induced fields are not very well under control, a space borne version of this experiment will reduce these disturbances considerably. The corresponding estimates for these kinds of tests in space are presented. As a result, gravity–induced stray field can be reduced considerably. Furthermore, also patch–effects can be reduced efficiently due to novel coating techniques. Therefore, due to microgravity conditions and new techniques the UFF for charged particles may be tested with much higher accuracy than in previous experiments.

ASTRODYNAMICAL SPACE TEST OF RELATIVITY USING OPTICAL DEVICES I (ASTROD I) — MISSION OVERVIEW

ASTROD I is the first planned space mission in a series of ASTROD missions for testing relativity in space using optical devices. The main aims are (i) to test general relativity with an improvement of three orders of magnitude compared to current results, (ii) to measure solar and solar system parameters with improved accuracy, (iii) to test the constancy of the gravitational constant and in general to get a deeper understanding of gravity. The first ideas for the ASTROD missions go back to the last century when new technologies in the area of laser physics and time measurement began to appear on the horizon. ASTROD is a mission concept that is supported by a broad international community covering the areas of space technology, fundamental physics, high performance laser and clock technology and drag-free control. While ASTROD I is a single-spacecraft concept that performs measurements with pulsed laser ranging between the spacecraft and earthbound laser ranging stations, ASTROD-GW is planned to be a three spacecraft mission with inter-spacecraft laser ranging. ASTROD-GW would be able to detect gravitational waves at frequencies below the eLISA/NGO bandwidth. As a third step Super-ASTROD with larger orbits could even probe primordial gravitational waves. This paper gives an overview on the basic principles especially for ASTROD I.

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.