Gravity, Geodesy and Fundamental Physics with BepiColombo’s MORE Investigation

Space Sci Rev (2021) 217:21 https://doi.org/10.1007/s11214-021-00800-3 Gravity, Geodesy and Fundamental Physics with BepiColombo’s MORE Investigation L. Iess1 · S.W. Asmar2 · P. Cappuccio1 · G. Cascioli1 · F. De Marchi1 · I. di Stefano1 · A. Genova1 · N. Ashby3 · J.P. Barriot4 · P. Bender5 · C. Benedetto6 · J.S. Border2 · F. Budnik7 · S. Ciarcia8 · T. Damour9 · V. Dehant10 · G. Di Achille11 · A. Di Ruscio1 · A. Fienga12 · R. Formaro5 · S. Klioner13 · A. Konopliv2 · A. Lemaître14 · F. Longo5 · M. Mercolino7 · G. Mitri15 · V. Notaro1 · A. Olivieri6 · M. Paik2 · A. Palli16 · G. Schettino17 · D. Serra18 · L. Simone8 · G. Tommei18 · P. Tortora16 · T. Van Hoolst10 · D. Vokrouhlický19 · M. Watkins2 · X. Wu2 · M. Zannoni16 Received: 13 October 2020 / Accepted: 20 January 2021 © The Author(s) 2021 Abstract The Mercury Orbiter Radio Science Experiment (MORE) of the ESA mission BepiColombo will provide an accurate estimation of Mercury’s gravity field and rotational The BepiColombo mission to Mercury Edited by Johannes Benkhoff, Go Murakami and Ayako Matsuoka B L. Iess 1 Dipartimento di Ingegneria Meccanica e Aerospaziale, Università La Sapienza, Rome, Italy 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA 3 National Institute of Standards and Technology, Gaithersburg, MD, USA 4 University of French Polynesia, Puna’auia, French Polynesia 5 University of Colorado Boulder, Boulder, CO, USA 6 Agenzia Spaziale Italiana, Rome, Italy 7 European Space Agency, Darmstad, Germany 8 Thales Alenia Space, Rome, Italy 9 Institut des Hautes Études Scientifiques, Bures sur Yvette, France 10 Royal Observatory of Belgium, Uccle, Belgium 11 INAF - Osservatorio Astronomico d’Abruzzo, Teramo, Italy 12 GéoAzur, CNRS, Observatoire de la Côte d’Azur, Université Côte d’Azur, Valbonne, France 13 Technische Universität Dresden, Dresden, Germany 14 Department of Mathematics, University of Namur, Namur, Belgium 15 Università D’Annunzio, Chieti, Pescara, Italy 16 Dipartimento di Ingegneria Industriale, Alma Mater Studiorum – Università di Bologna, Forlì, Italy 21 Page 2 of 39 L. Iess et al. state, improved tests of general relativity, and a novel deep space navigation system. The key experimental setup entails a highly stable, multi-frequency radio link in X and Ka band, enabling two-way range rate measurements of 3 micron/s at nearly all solar elongation angles. In addition, a high chip rate, pseudo-noise ranging system has already been tested at 1-2 cm accuracy. The tracking data will be used together with the measurements of the Italian Spring Accelerometer to provide a pseudo drag free environment for the data analysis. We summarize the existing literature published over the past years and report on the overall configuration of the experiment, its operations in cruise and at Mercury, and the expected scientific results. Keywords Mercury · Radio science · Planetary geodesy · Relativistic gravity · Spacecraft tracking systems 1 Introduction Mercury is one of the most interesting objects in the solar system, and, till the arrival of NASA’s MESSENGER spacecraft, one of the least known. The 3:2 resonance between its rotational and orbital periods, its large density and the unexpected discovery of a substantial magnetic field were and still are some of the most fascinating problems in planetary science. However, Mercury was visited only twice by a spacecraft since the large energy required to reach the planet and the harsh thermal and radiation environment represent formidable obstacles to its exploration. Mariner 10 flybys in 1974–1975 provided a first glimpse on the planet, revealing the main characteristics of the surface morphology. But much was yet to be discovered. The interior structure and the gravity field were essentially unknown. The magnetic field measured by Mariner 10 puzzled the planetary community, as the geophysical models predicted a solid core for the planet. S. Peale (1976) defined the internal structure of Mercury and the physical state of the core as one of the most important science themes regarding the formation and evolution of the Solar System. Either a new mechanism other than a planetary dynamo was at play or the models for the composition and thermal history of Mercury were inaccurate. The estimation of Mercury’s rotation through Earth-based radar measurements (Margot et al. 2007) provided crucial experimental evidence that Mercury has a partially molten core. The real breakthrough in our understanding of the planet came from the results of the NASA mission MESSENGER, whose planetary phase lasted four years (from April 2011 to April 2015). An in-depth characterization of Mercury’s surface and interior through the analysis of the MESSENGER data revealed a dynamic past, made up probably by an early magma ocean and billions of years of active volcanism (Johnson and Hauck 2016). Gravity (Smith et al. 2012; Genova et al. 2013, 2019; Mazarico et al. 2014) and rotation measurements (Margot et al. 2012; Stark et al. 2015) unveiled the broad structure of Mercury’s interior, whose outer part was at least few Ga ago robustly convective (Breuer et al. 2007; Tosi et al. 2013). The orbital evolution of Mercury about the Sun has also been an important objective of a long and fascinating scientific investigation to study theories of gravitation. The anomalous 17 IFAC, Istituto di Fisica Applicata, CNR, Florence, Italy 18 Università di Pisa, Pisa, Italy 19 Charles University, Prague, Czech Republic Gravity, Geodesy and Fundamental Physics. . . Page 3 of 39 21 precession of its perihelion was difficult to explain in the framework of classical physics. In the attempt to reconcile Newtonian mechanics with observations, Le Verrier suggested the existence of Vulcan, an inner planet closer to the sun than Mercury (Hanson 1962). It was soon recognized that only a belt of many small bodies, the Vulcanoids, could explain the astrometric observations, but neither Vulcan nor the Vulcanoids were ever discovered. Rather, the orbit of Mercury gave General Relativity its first experimental success. Solving the equation of motion in a relativistic formalism, Einstein and de Sitter computed a perihelion drift incredibly close to the observed one. It was a brilliant success for the new theory of gravity. Today GR has passed many experimental tests, such as the measurement of the parameterized post-Newtonian (PPN) parameter γ obtained in 2003 by Doppler tracking of the Cassini spacecraft (Bertotti et al. 2003). But it is very unlikely that GR is the ultimate theory of gravity. The intrinsic incompatibility of GR with quantum mechanics has motivated a long lasting, but still unsuccessful search for violations of Einstein’s paradigm. Recent theoretical developments have suggested that the field equations might be changed to accommodate the contribution from a small scalar field, remnant of the age of inflation. Such a scalar field enta (...truncated)