Manuel Rodrigues

The MICROSCOPE space mission

The MICROSCOPE mission aims to test the equivalence principle (EP) up to an accuracy of 10-15 using its well known manifestation: the universality of free-fall. The mission, implemented in the Cnes programme of 2000, schedules the launch of the microsatellite for 2004. The satellite payload comprises four gravitational sensors operating at finely stabilized room temperature. The masses of the sensors are controlled to the same orbital motion on-board the satellite, which compensates external surface forces in real time by actuation of electrical thrusters. Accurate measurements of the electrostatic forces applied to the masses, so that they follow the same gravitational orbit, are processed in order to reject any common effects on the masses; then the differential outputs are observed with high precision along the Earth-pointing axis, with an expected resolution of 5×10-15 m s-2. The quasi cylindrical test masses are concentric in order to reject gravity gradient effects, and are made of platinum or titanium alloys. The instruments concept and design are presented, and the rationale of the space experiment is explained.

MICROSCOPE, testing the equivalence principle in space

The test of the equivalence principle can be performed in space with orders of magnitude better resolution than in the laboratory, because of the outstanding steady and soft environment of the in-orbit experiment. The expected new experimental results will contribute to the unification of the four interactions, demonstrate the existence of extra scalar interaction or participate in the research for a quantum gravity theory. The MICROSCOPE space mission is being developed within the framework of the Cnes scientific program with the objective of testing the universality of free fall with a 10−15 accuracy. The concept and the design of the experiment are discussed and the major performance drivers of the room temperature instrument are pointed out. The launch of the drag-free satellite is scheduled for late 2004. By its specific technology demonstration, the mission will open the way to even more accurate acceleration measurements for other space missions in fundamental physics.

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.

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.

Electrostatic accelerometers for the equivalence principle test in space

The concept of the three-axis electrostatic accelerometers based on the full electrostatic suspension of one unique proof mass is very suitable for space applications requiring very high resolution of acceleration measurement or drag-free control of satellite. This concept has been tested in orbit with the accelerometer CACTUS from ONERA in the late seventies and recently with the accelerometer ASTRE on board Columbia shuttle in June 1996. The accelerometer outputs are derived from the measurement of the electrostatic forces, necessary to maintain the mass motionless at the centre of the accelerometer cage. The relative test-mass position and attitude are servo-controlled from measurements of capacitive sensors exhibiting resolutions of better than depending on the geometrical configuration. The test of the weak equivalence principle can be performed in orbit on board a drag-free satellite with two concentric electrostatic accelerometers including two cylindrical test masses made of different materials. The measured common acceleration is controlled to null along the three directions by the drag compensation system of the satellite. The differential acceleration is detected at the orbital frequency (or around the satellite spin frequency) along the common revolution axis with an expected resolution of . The differential disturbing acceleration induced by magnetic, electric and thermal disturbances must be limited to this value thanks to the 4 K environment of the sensor-head. The present definition of such an instrument is presented and the expected performances are detailed.

The inertial reference sensor CAESAR for the laser interferometer space antenna mission

The interferometer of the LISA mission is realized with V-formations of drag-free spacecraft in heliocentric orbit. Each spacecraft will have at its centre a cubic proof mass made in gold - platinum alloy, that defines one end of the interferometer arms. These masses are also those of the inertial reference sensors used for the drag compensation control of the satellites. The goal of the LISA sensor is to obtain a proof mass free of any parasitic forces, at a level of in the very low frequency domain from up to several . Furthermore, the compensation of the satellite drag must limit its relative motion to less than , thus reducing the disturbances that may be induced as variations of the satellite self-gravity effects. The sensor proposed by ONERA is derived from the space electrostatic accelerometer GRADIO and ASTRE, the last one flew in Spacelab during a shuttle mission in June 1996. The challenge of the LISA inertial sensor is to exploit the existing concept and technologies with the best care in order to preserve the capacitive sensor resolution while limiting to a minimum the disturbing electrical effects and measurement backactions. The non-direct demonstration of the expected flight performances of such a sensor should be considered in detail in the future.

INERTIAL SENSOR CONCEPT FOR THE GRAVITY WAVE MISSIONS

Abstract Several space missions have been proposed in the present years for the observation of the gravity waves by exploiting laser interferometry between three or six drag-free satellites. The laser links between the satellites constitute the arms of the interferometer of a few million kilometres long. Beside the difficulties of the interferometer exhibiting picometers accuracy, the interferometer mirrors are obtained with the proof-masses of inertial sensors that must exhibit outstanding accuracy. The development of such an instrument has been undertaken. It is derived from existing space accelerometers. The mission requirements are presented and the approach to reach the performances is detailed. A sensor prototype, which has been defined on the basis of theoretical analysis and on the experience already acquired with similar accelerometers, is presently produced in view of ground and in orbit design evaluations. This prototype is also described in the paper.

Towards low-temperature electrostatic accelerometry

Ultrasensitive accelerometers with electrostatic suspension use a technology allowing operation at ambient temperature. However, their resolution is theoretically limited to around 10−12 m s−2/Hz1/2 by thermal noise. To overcome this limit, and in the perspective of future space missions, an electrostatic sensor compatible with operation at cryogenic temperatures is currently under development. It should ultimately reduce thermodynamic noise and improve the instrument characteristics. The perspectives for a rectangular proof mass electrostatically suspended with six degrees of freedom at liquid-helium temperature are presented and discussed, as are tests and test data.

Determination of the Equivalence Principle Violation Signal for the MICROSCOPE Space Mission: Optimization of the Signal Processing

The MICROSCOPE space mission aims at testing the Equivalence Principle (EP) with an accuracy of 10−15. The test is based on the precise measurement delivered by a differential electrostatic accelerometer on-board a drag-free microsatellite which includes two cylindrical test masses submitted to the same gravitational field and made of different materials. The experiment consists in testing the equality of the electrostatic acceleration applied to the masses to maintain them relatively motionless at a well-known frequency. This high precision experiment is compatible with only very little perturbations. However, aliasing arises from the finite time span of the measurement, and is amplified by measurement losses. These effects perturb the measurement analysis. Numerical simulations have been run to estimate the contribution of a perturbation at any frequency on the EP violation frequency and to test its compatibility with the mission specifications. Moreover, different data analysis procedures have been considered to select the one minimizing these effects taking into account the uncertainty about the frequencies of the implicated signals.

The LISA accelerometer

In the frame of investigating the fundamental nature of gravity, the Laser Interferometer Space Antenna (LISA) mission could open the way to a new kind of observations unreachable from ground. The experiment, based on a V-formation of six drag-free spacecraft, uses the cubic proof masses of inertial sensors to reflect the laser light, acting as reference mirrors of a 5 × 109 m arm length interferometer. The proof masses are also used as inertial references for the drag-free control of the spacecraft which constitute in return a shield against external forces. Derived from space electrostatic accelerometers developed at ONERA, such as GRADIO for the ESA ARISTOTELES and now GOCE mission(Bernard and Touboul, 1991), the proposed LISA sensor should shield its proof mass from any accelerometric disturbance at a level of 10−15ms−2Hz−12. The accurate capacitive sensing of the mass provides its position relative to the satellite with a resolution better than 10−9m Hz−12 in order to control the satellite orbit and to minimise the disturbances induced by the satellite self gravity or by the proof mass charge. The sensor configuration and accomodation has to be specifically optimised for the mission requirements. Fortunately, the sensor will benefit from the thermal stability of the LISA optical bench environment, i.e. 10−6K Hz−12, and of the selected materials that exhibit a very low coefficient of thermal expansion (CTE), ensuring a high geometrical stability. Apart from the modeling and the evaluation of the flight characteristics, the necessary indirect ground demonstration of the performance and the interfaces with the drag-free control will have to be considered in detail in the future.