MG13 - Talk detail

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BepiColombo, the forthcoming ESA (cornerstone) mission to Mercury, will include a comprehensive set of experiments -- denominated Radio Science Experiments (RSE) -- in order to measure the gravitational field of the planet, its rotation and to perform precise tests of Einstein's theory of general relativity versus other metric theories of gravity. Among the onboard instruments, a fundamental role in the RSE will be played by ISA (Italian Spring Accelerometer). This is a three–axis high-sensitivity accelerometer, characterized by an intrinsic noise level of about 10^-10g/sqrt(Hz) in the frequency band 3×10^-5 -- 1×10^-1 Hz. The main goal of ISA it to measure the very strong non–gravitational accelerations acting on the MPO (Mercury Planetary Orbiter) spacecraft, which are an important source of error in the RSE measurements. Indeed, in order to reach the ambitious objectives of the RSE, the a posteriori reconstruction of the MPO orbit should reach the 10^-8 m/s^2 level in acceleration over a time span of one orbital revolution of the spacecraft around Mercury. Therefore, the accelerometer measurements have to be integrated with the radar tracking measurements from Earth's stations in a very precise orbit determination procedure. Being the first time for an high-sensitivity accelerometer onboard an interplanetary mission, a number of choices had to be made and several issues had to be faced in the design phases. After a general description of the instrument scientific objectives, its working and operations will be described. Then emphasis will be given on the complex calibration procedures required on ground -- in particular for the measurement of the orientation of the accelerometer sensitive axes with respect a reference system connected with the spacecraft body -- and in flight during the various mission phases (both in cruise and during the nominal phase around Mercury), as well as on the integration of the measurements with the overall RSE operations and data analysis.

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Einstein's theory of general relativity (GR) is the best theory we have for the description of the gravitational interaction and has successfully passed a thorough experimental investigation: from Earth-laboratory experiments to space observations and experiments. However, GR is a classical (not quantum) theory and all efforts to find a common frame with the other fundamental interactions have failed up to now. At the foundation of GR is Einstein Equivalence Principle (EEP) which relies on the Weak Equivalence Principle (WEP) and represents its generalization to all the laws of special relativity. The WEP is based on the principle that the ratio of the gravitational (passive) mass to the inertial mass is the same for all bodies, hence all bodies fall with the same acceleration independently of their mass and chemical composition, i.e., the gravitational force is independent of the composition of the freely falling bodies. This is a unique property of gravity, also known as the Universality of Free Fall (UFF) and is a direct consequence of the WEP. Consequently, tests for the UFF are the most significant, among the others, to be performed to verify Einstein's theory and are, at the same time, the most promising probing tests for the discovery of new physics beyond GR. Presently, the best tests of the UFF have reached an accuracy of about 1 part in 10^13, using rotating torsion balances (in the field of the Earth) and LLR (Lunar Laser Ranging) measurements. GReAT (General Relativity Accuracy Test) is a proposed experiment aimed at testing the UFF or Einstein's WEP with an accuracy equal to a few parts in 10^15 by means of a differential acceleration detector free falling inside a co–moving and cryogenic evacuated capsule released from a stratospheric balloon at an altitude of about 40 km. This factor of about 100 fold improvement in the knowledge of the relative acceleration \Delta a/g of two falling bodies with different composition allows to discriminate among GR and other proposed theories of gravity that admit violations of WEP at these accuracy levels. In the experiment, the differential detector is spun about an horizontal axis during the fall to modulate a possible violating signal at the spin frequency in order to distinguish the possible WEP violation signal from other superimposed perturbations. The high accuracy requires resolving a very small signal out of key components like the instrument intrinsic noise -- mainly related with the Brownian motion -- and the noise components associated with the detector's motion and gravity gradients. The development of a cryogenic version of the differential acceleration detector has been the object of a recent research activity. The talk describes the differential accelerometer prototype that consists of two self-differential accelerometers, capable of rejecting both linear and angular noise while retaining the signal associated with a possible violation of the WEP. Moreover, we briefly discuss the development and testing in the laboratory of the instrument and show preliminary results of the detector differential measurement mode obtained at a cryogenic temperature down to about 11 K, where we reached a quality factor Q of about 130,000.

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The Einstein theory of General Relativity (GR), also known as geometrodynamics, represents a metric theory for the description of the gravitational interaction whose predictions, especially in the Weak Field and Slow Motion (WFSM) limit, have successfully passed a deep experimental investigation during the last 50 years --- from Earth-bound experiments to space observations and measurements with Earth-orbiting satellites and interplanetary probes. Besides GR, other metric theories of gravitation have been developed during the years. Metric theories of gravitation share with GR the same spacetime structure and the same equations of motion for test particles but differ in the field equations form. Moreover, metric theories of gravity are the only theories for the gravitational interaction that fully embody the Einstein Equivalence Principle (EEP), which relies on the Weak Equivalence Principle (WEP) and represents its generalization to all the laws of special relativity. A way to discriminate, from the experimental point of view, among the different metric theories is through the so-called Parametrized Post-Newtonian (PPN) formalism. This formalism uses a set of 10 parameters that arise from a post–Newtonian expansion --- in terms of very well-defined classical potentials --- of both the metric tensor and the energy–momentum tensor. Among the 10 PPN parameters, the most important are beta and gamma. The first is a measure of the non–linearity of the gravitational interaction, while the second is a measure of the space curvature per unit of mass. In GR they are both equal to unity, while all the other PPN parameters are zero. We present the results of a 13 years analysis of the orbit of the two LAGEOS (LAser GEOdynamic Satellite) laser–ranged satellites that has allowed us to measure the PPN parameters b and g in the field of the Earth. We determined the orbit of the two satellites with GEODYN II --- an orbit determination and prediction software developed at NASA/GSFC --- and then calculated the corresponding orbital residuals in their Keplerian elements with the difference–method. Obviously, we did not include in the selected dynamical models the GR corrections. We analyzed and fitted the orbital residuals in the satellites node longitude and argument of perigee. The former are sensitive to the GR precessions of Lense–Thirring and de Sitter, while the latter are mainly sensitive to the Schwarzschild precession. Therefore, we did not fit directly for the two parameters from the equations of motion, but we extracted them from a least–squares fit to the integrated residuals of the satellites node and argument of perigee. In particular, from the argument of perigee of LAGEOS II we estimated the combination (2+2gamma-beta)/3 of the two parameters, while from a suitable combination of the two nodes we obtained gamma. This work also represents the first measurement of the de Sitter precession with Earth–orbiting artificial satellites. The results for the relativistic measurements can be summarized as follow: i) a 3×10-4 measurement of the beta and gamma combination; ii) a 1.5×10-2 measurement of the combined Lense–Thirring and de Sitter precessions; iii) a 2.7×10-2 measurement of gamma and of the de Sitter precession; iv) a 1.4×10-2 measurement of the Lense–Thirring precession; and v) a 9×10-4 measurement of beta when for gamma the result obtained from the CASSINI's Superior Conjunction Experiment is used. Finally, the impact of the main systematic error sources on the above measurements is discussed and their values estimated.

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