ISA, a High Sensitivity Accelerometer in the Interplanetary Space

Space Sci Rev (2020) 216:145 https://doi.org/10.1007/s11214-020-00768-6 ISA, a High Sensitivity Accelerometer in the Interplanetary Space Updates after the Near-Earth Commissioning Phase of Italian Spring Accelerometer – ISA Francesco Santoli1 · Emiliano Fiorenza1 · Carlo Lefevre1 · David Massimo Lucchesi1 · Marco Lucente1 · Carmelo Magnafico1 · Alfredo Morbidini1 · Roberto Peron1 · Valerio Iafolla1 Received: 3 June 2020 / Accepted: 12 November 2020 / Published online: 8 December 2020 © The Author(s) 2020 Abstract ISA (Italian Spring Accelerometer) is a high sensitivity accelerometer flying, as scientific payload, on-board one of the two spacecraft (the Mercury Planetary Orbiter) of BepiColombo, the first ESA mission to Mercury. The first commissioning phase (performed in the period November 2018 - August 2019) allowed to verify the functionality of the instrument itself as well as of the related data handling and archiving system. Moreover, the acceleration measurements gathered in this time frame allow to envisage the potentiality of such an instrument as a high-accuracy monitor of the spacecraft mechanical environment. Keywords Accelerometer · BepiColombo · Radioscience · Mercury · Precise orbit determination · Non-gravitational perturbations · Space instrumentation · Structural vibrations monitoring · Gravitation 1 Introduction Among the scientific objectives of the ESA mission BepiColombo (BC), an important part is given by those dedicated to the study of the Hermean geophysics, as well as to dedicated tests of the laws of gravitation, by employing the Mercury Planetary Orbiter (MPO, one of the two probes making up the mission) as a test mass in the Mercury gravity field. In order to fulfil these objectives, the Mercury Orbiter Radioscience Experiments (MORE) have been set up (Iess et al. 2020; Genova et al. 2020). The MPO spacecraft hosts a suite of instruments that enable a precise measurement of the MPO orbital motion and its dynamics, which are the key elements to reconstruct the gravitational field and rotation state of the planet and to test selected predictions of General Relativity, along with the ones of alternative theories of gravitation (Milani et al. 2001, 2002; Imperi and Iess 2015; Schettino et al. 2015, 2016; Will The BepiColombo mission to Mercury Edited by Johannes Benkhoff, Go Murakami and Ayako Matsuoka B F. Santoli 1 Istituto di Astrofisica e Planetologia Spaziali–IAPS, Istituto Nazionale di Astrofisica–INAF, Via Fosso del Cavaliere 100, 00133, Rome, Italy 145 Page 2 of 30 F. Santoli et al. 2014). In the procedure of orbit determination and parameter estimation (usually called POD — precise orbit determination), a model for the orbital dynamics of the spacecraft is fitted to the tracking data. The model is required to be sufficiently accurate to describe this dynamics at a level comparable to the information content of tracking. It has to be noticed that, while the majority of the forces acting on the spacecraft once in orbit around Mercury comes from the gravitational attraction of the primary as well as from the attraction of the other bodies in the Solar System, a non-negligible part is constituted by surface forces resultant from the interaction of the spacecraft body with particles and fields in the near-Mercury environment (the so-called non-gravitational perturbations — NGP). A precise modellisation of these forces is not easy (Lucchesi and Iafolla 2006), so that developing a NGP mathematical model with the accuracy needed for the MORE goals has been considered not viable. An effective alternative is the direct measurement of the total resulting acceleration via an onboard accelerometer. This is the solution that has been chosen for BC, employing the ISA (Italian Spring Accelerometer) instrument (Iafolla et al. 2010; Iafolla and Nozzoli 2001; Iafolla et al. 2016). ISA is a three-axis accelerometer: it features three sensing elements arranged in a suitable geometrical configuration, each one measuring a one-dimensional component of the overall signal. These three channels can then be properly combined in order to obtain the total acceleration vector acting on a given reference point (called Vertex). The resulting time series of acceleration values can then be used as an input to the POD, allowing to remove the NGP from the list of the unknowns of the problem and hence allowing a better determination of the other unknowns (first of all the ones related to MORE scientific objectives). After a long development and design phase by the Experimental Gravitation group at IAPS-INAF, the space-qualified version of ISA has been engineered and built by Thales Alenia Space – Italia (Milano) and integrated into MPO spacecraft. Following the successful launch on 19 October 2018 from the spaceport of Kourou (French Guiana), the instrument has been turned on for the first time on 26 November 2018. Subsequently, a first slot of tests has been successfully carried out in the Near Earth Commissioning Phase (NECP). This work is organised as follows. In Sect. 2 the development path of ISA is briefly reviewed, starting from the pioneering work on resonant gravitational wave detectors. Section 3 is devoted to a discussion of the ISA measurements scientific objectives, in particular those linked to MORE, and it is highlighted the role of the acceleration measurements in the POD. In Sect. 4 it is shown the instrument working principle, along with details on measured signals and design choices. In Sect. 5 the instrument calibration procedures, both on ground and in flight, are discussed. Section 6 is dedicated to a presentation of the various data types produced by the instrument, along with the processing pipeline developed to prepare the scientific products. Section 7 is devoted to a first presentation and discussion of selected observations from the commissioning activities after launch. 2 ISA, ab initio The history of ISA accelerometer starts at the beginning of the seventies of the past century through the studies carried out by the Roma Gravitational Waves research group (Guido Pizzella and Edoardo Amaldi, lead), devoted to the search of gravitational waves (GW) (Pizzella 2016). In those studies, one of key steps was the development of transducers able to enhance the subatomic vibrations produced by the interaction between GW and cryogenic gravitational antennas. The basic technology for such transducers is related to the measurement of the displacement of a sensing mass produced by a tiny force acting on ISA, a High Sensitivity Accelerometer in the Interplanetary Space Page 3 of 30 145 it. The capability to sense and measure such small displacements allows to develop a wide range of instrumentation, among which ultra-sensitive accelerometers. For that reason, it is usual that people working on experimental gravitation topics often move to the research in accelerometers development. At the beginning of nineties, the Experimental Gravitat (...truncated)