Alberto Anselmi

BepiColombo, ESA's Mercury Cornerstone mission

Abstract The paper presents the results of the definition studies performed for the European Space Agency (ESA) on system architectures and enabling technologies for “BepiColombo”, a Cornerstone class mission to be launched in the 2007–2009 time frame. The scientific mission comprises 1-year observations by a Mercury Planetary Orbiter (MPO), dedicated to remote sensing, and a Mercury Magnetospheric Orbiter (MMO), dedicated to particles and fields, plus short-duration in situ analysis by a Mercury surface element (MSE). A flexible approach to the programme has been developed, comprising two alternative launch scenarios. In the first option (2009), the 2500-kg class satellite composite, including two propulsion modules and three scientific modules, is launched by an Ariane-5. The trajectory design is based on Venus and Mercury gravity assists plus the thrust provided by a Solar Electric Propulsion Module (SEPM), that is jettisoned before being captured into Mercury orbit. Capture and orbit insertion, executed by successive manoeuvres of a Chemical Propulsion Module (CPM), occur less than 2.5 yr after launch. In the second scenario, the mission is split into two launches of a small launch vehicle. Two 1200-kg class composites are launched either in the same one-month window or at an interval of 1.6 yr . One composite comprises the SEPM, CPM, MMO and MSE and the other comprises duplicate SEPM+CPM and the MPO. The trajectory design follows the same principles as the Ariane-5 mission, with the SEPM thrust reduced by half and cruise duration ranging between 2.3 and 3.5 yr . Whatever be the implementation, the mission is expected to return about 1700 Gbit of scientific data during the one-year observation phase. The crucial aspects of the spacecraft design are associated with, and constrained by, the high-temperature and high-radiation environment. Basic feasibility has been demonstrated by an extensive design and analysis exercise, and the focus of the programme has now moved to a 3-year preparatory programme dedicated for developing the enabling technologies.

'Galileo Galilei' (GG): space test of the weak equivalence principle to 10^(−17) and laboratory demonstrations

The small satellite ‘Galileo Galilei’ (GG) will test the universality of free fall and hence the weak equivalence principle which is the founding pillar of general relativity to 1 part in 10 17 . It will use proof masses whose atoms differ substantially from one another in their mass energy content, so as to maximize the chance of violation. GG will improve by four orders of magnitude the current best ‘E¨ ot-Wash’ tests based on slowly rotating torsion balances, which have been able to reach their thermal noise level. In GG, the expected violation signal is a relative displacement between the proof masses of � 0.6 pm caused by a differential acceleration aGG � 8 × 10 −17 ms −2 pointing to the center of mass of the Earth as the satellite orbits around it at νGG � 1.7 × 10 −4 Hz. GG will fly an innovative acceleration sensor based on rapidly rotating macroscopic test masses weakly coupled in 2D which up-converts the signal to νspin � 1H z, a value well above the frequency of natural oscillations of the masses relative to each other νd = 1/Td � 1/(540 s). The sensor is unique in that it ensures high rotation frequency, low thermal noise and no attenuation of the signal strength (Pegna et al 2011 Phys. Rev. Lett. 107 200801). A readout based on a very

On the universality of free fall, the equivalence principle, and the gravitational redshift

Through the contributions of Galileo, Newton, and Einstein, we recall the universality of free fall (UFF), the weak equivalence principle (WEP), and the strong equivalence principle (SEP), in order to stress that general relativity requires all test masses to be equally accelerated in a gravitational field; that is, it requires UFF and WEP to hold. The possibility of testing this crucial fact with null, highly sensitive experiments makes these the most powerful tests of the theory. Following Schiff, we derive the gravitational redshift from the WEP and special relativity and show that, as long as clocks are affected by a gravitating body like normal matter, measurement of the redshift is a test of UFF/WEP but cannot compete with direct null tests. A new measurement of the gravitational redshift based on free-falling cold atoms and an absolute gravimeter is not competitive either. Finally, we compare UFF/WEP experiments using macroscopic masses as test bodies in one case and cold atoms in the other. We con...

The Euclid mission design

Euclid is a space-based optical/near-infrared survey mission of the European Space Agency (ESA) to investigate the nature of dark energy, dark matter and gravity by observing the geometry of the Universe and on the formation of structures over cosmological timescales. Euclid will use two probes of the signature of dark matter and energy: Weak gravitational Lensing, which requires the measurement of the shape and photometric redshifts of distant galaxies, and Galaxy Clustering, based on the measurement of the 3-dimensional distribution of galaxies through their spectroscopic redshifts. The mission is scheduled for launch in 2020 and is designed for 6 years of nominal survey operations. The Euclid Spacecraft is composed of a Service Module and a Payload Module. The Service Module comprises all the conventional spacecraft subsystems, the instruments warm electronics units, the sun shield and the solar arrays. In particular the Service Module provides the extremely challenging pointing accuracy required by the scientific objectives. The Payload Module consists of a 1.2 m three-mirror Korsch type telescope and of two instruments, the visible imager and the near-infrared spectro-photometer, both covering a large common field-of-view enabling to survey more than 35% of the entire sky. All sensor data are downlinked using K-band transmission and processed by a dedicated ground segment for science data processing. The Euclid data and catalogues will be made available to the public at the ESA Science Data Centre.

Evaluation of a proposed test of the weak equivalence principle using Earth-orbiting bodies in high-speed co-rotation: re-establishing the physical bases

Test masses coupled by weak mechanical suspensions are sensitive to differential forces such as the force due to a possible violation of the equivalence principle (EP). If in addition they are put in rapid rotation, the differential signal is modulated at high frequency, which is beneficial for noise reduction. Galileo Galilei (GG) is a proposed space experiment for testing the equivalence principle to 1 part in 10 17 based on these concepts. A recent paper by Jafry and Weinberger (1998 Class. Quantum Grav.15 481-500) claims that GG can only reach 10 14 . We show that the analysis of this paper is flawed (by several orders of magnitude) because of two misconceptions: one on the physical nature of mechanical damping and the other on active control methods for the stabilization of spinning bodies.

The “Galileo Galilei” (GG) project: Testing the Equivalence Principle in space and on Earth

“GALILEO GALILEI” (GG) is a proposal for a small, low orbit satellite devoted to testing the Equivalence Principle (EP) of Galileo, Newton and Einstein to 1 part in 1017. At the end of 1997 GG has been selected and funded by ASI (Agenzia Spaziale Italiana) for a 1-year Phase A study. The main novelty of GG is that the concentric hollow test cylinders whose relative motion (in the plane perpendicular to the spin axis) would be affected by an EP violation, spin together with the read-out capacitance sensors placed in between them. The nominal spin rate is 2 Hz, and this is the frequency at which the putative EP violation signal is modulated by the sensors. As compared to other experiments the modulation frequency is increased by more than a factor 104, thus reducing 1f (low frequency) electronic and mechanical noise. GG will have FEEP ion thrusters for drag compensation. The required amount of propellant is of a few grams only. The experiment works at room temperature. To demonstrate the feasibility of the space experiment a payload prototype for EP testing on the ground (GGG - GG on the Ground) is under development in the laboratories of Laben. The challenge in this field is to fly an experiment able to improve by many orders of magnitude the current best ground sensitivity (≅10−12). This requires spurious relative motions of the test bodies to be greatly reduced, leaving them essentially motionless. Doing that with more than one pair of bodies appears to be an unnecessary complication. This is why GG is now proposed with a single pair of test masses. Information, research papers and photographs of the ground apparatus are available on the Web (http://tycho.dm.unipi.it/nobili).

Relevance of the weak equivalence principle and experiments to test it: Lessons from the past and improvements expected in space

Abstract Tests of the Weak Equivalence Principle (WEP) probe the foundations of physics. Ever since Galileo in the early 1600s, WEP tests have attracted some of the best experimentalists of any time. Progress has come in bursts, each stimulated by the introduction of a new technique: the torsion balance, signal modulation by Earth rotation, the rotating torsion balance. Tests for various materials in the field of the Earth and the Sun have found no violation to the level of about 1 part in 1013. A different technique, Lunar Laser Ranging (LLR), has reached comparable precision. Today, both laboratory tests and LLR have reached a point when improving by a factor of 10 is extremely hard. The promise of another quantum leap in precision rests on experiments performed in low Earth orbit. The Microscope satellite, launched in April 2016 and currently taking data, aims to test WEP in the field of Earth to 10 − 15 , a 100-fold improvement possible thanks to a driving signal in orbit almost 500 times stronger than for torsion balances on ground. The ‘Galileo Galilei’ (GG) experiment, by combining the advantages of space with those of the rotating torsion balance, aims at a WEP test 100 times more precise than Microscope, to 10 − 17 . A quantitative comparison of the key issues in the two experiments is presented, along with recent experimental measurements relevant for GG. Early results from Microscope, reported at a conference in March 2017, show measurement performance close to the expectations and confirm the key role of rotation with the advantage (unique to space) of rotating the whole spacecraft. Any non-null result from Microscope would be a major discovery and call for urgent confirmation; with 100 times better precision GG could settle the matter and provide a deeper probe of the foundations of physics.

AGP (Astrometric Gravitation Probe) optical design report

This paper describes the current opto-mechanical design of AGP, a mission designed for astrometric verification of General Relativity (GR) and competing gravitation theories by means of precise determination of light deflection on field stars, and of orbital parameters of selected Solar System objects. The optical concept includes a planar rear-view mirror for simultaneous imaging on the CCD mosaic detector of fields of view also from the direction opposite to the Sun, affected by negligible deflection, for the sake of real time calibration. The precision of astrometric measurements on individual stars will be of order of 1 mas, over two fields separated by few degrees around the Sun and observed simultaneously. We describe the optical design characteristics, with particular reference to manufacturing and tolerancing aspects, evidencing the preservation of very good imaging performance over the range of expected operating conditions.