HYPER: A POTENTIAL ESA FLEXI-MISSION IN THE FUNDAMENTAL PHYSICS DOMAIN i. Giorgio Bagnasco 1, Stephen Airey 2

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1 HYPER: A POTENTIAL ESA FLEXI-MISSION IN THE FUNDAMENTAL PHYSICS DOMAIN i Giorgio Bagnasco 1, Stephen Airey 2 1 Scientific Projects Department, ESA/ESTEC, Noordwijk, The Netherlands 2 Electrical Engineering Department, ESA/ESTEC, Noordwijk, The Netherlands Giorgio.Bagnasco@esa.int ; Stephen.Airey@esa.int ABSTRACT HYPER stands for Hyper-precision cold atom interferometry in space. This paper presents an overview of HYPER: a potential Future ESA Fleximission in the fundamental physics domain, based around a cold atom interferometry payload capable of measuring accelerations and rotations with extremely high accuracy. A summary of the history of the mission is provided, followed by a quick introduction to the mission objectives (such as the measurement of the Lense-Thirring effect) and to cold atom interferometry. The preliminary mission architecture, as developed by ESA s Concurrent Design Facility, is outlined along with an overview of the payload design and the concept of operations. The initial study conducted by ESA demonstrated the feasibility of the HYPER mission both from a technical and a cost point of view. The next steps, planned for the beginning of 2002, for further consolidation of the overall mission design are described. HYPER will compete for selection as one of the next group of Flexi-missions which are planned to be launched in the timeframe. INTRODUCTION Flexi-missions (often simply called F-missions) were introduced in the ESA Horizon 2000 Science Programme in 1997 and replaced the socalled Medium-size missions. The F-missions were allotted a ceiling price of 176 MEUROs, at 1999 economic conditions, which corresponded to only half of the quota allotted to their predecessor Medium-size missions. The main reason for this change was the need to enhance the flexibility of the Science Programme by doubling the number of missions. In October 1999, ESA released a Call for Mission Ideas for two Flexi-missions (F2 and F3) to be launched in the period From the responses received, sixteen proposals were reviewed by the Fundamental Physics Advisory Group in February 2000, which identified HYPER 1 as: the mission with the highest scientific potentials in the domain of fundamental physics in space, thanks to the proposed use of the novel matter-wave interferometry techniques, applied to the testing of General Relativity and the measurement of the fine structure constant HYPER was thus selected for a feasibility assessment study, which was carried out between March and July 2001 in the Concurrent Design Facility (CDF) at ESTEC. This study was supported by the HYPER Science team, headed by Prof. W. Ertmer of the Institut für Quantenoptik in Hanover. The team of ESA engineers who performed the study, were requested to assess the feasibility of the Hyper mission objectives and at the same time to elaborate a preliminary mission architecture, complemented by a mission cost analysis. Although this first assessment study did not find any specific show-stopper for the HYPER mission, the Study Reports 2,3 identified a series of engineering and technology issues, which needed further detailed investigation and development. On the basis of the CDF results, the Fundamental Physics Advisory Group recognised in September 2000 that, as the HYPER mission had not yet reached the necessary maturity, it could not be selected as an F2 or F3 missions. At the same time it strongly recommended to continue work on HYPER in order to define the mission architecture more in depth, to solve the issues raised in the Study Reports and also to initiate the required technology developments mainly in the payload area. This article aims to provide a somewhat detailed overview of the scientific objectives, the novel payload technology and the first results reached by the Concurrent Design Facility study. MISSION OBJECTIVES The primary scientific objectives of the HYPER mission are: - to map, for the first time, the latitudinal structure (magnitude and sign) of the Lense- Thirring effect induced by the Earth s rotation and predicted by the General Relativity with a measurement accuracy of 3-5 % - to determine, independently from Quantum Electro-Dynamics theories, the fine structure constant, = 2%e 2 / hc, with a measurement accuracy improvement of one order of magnitude with respect to the present experimental results 1

2 - to investigate various distinct sources of matter-wave de-coherence effects, predicted by quantum gravity theories Furthermore, as a secondary objective the successful completion of this mission would demonstrate for the first time the superior performance of cold atom matter-wave interferometer instruments, which are extremely sensitive to rotations and accelerations. HYPER might therefore lead the way to a novel generation of inertial sensors and gyroscopes that would be needed to enable other future missions. The scientific objectives given above are only feasible in space, where the potential of cold-atom matter-wave interferometry sensors can be fully exploited thanks to the provision, via drag free satellite control, of an environment almost totally free of disturbing accelerations, such as those due to gravity on Earth. SCIENTIFIC BACKGROUND The initial HYPER study concentrated on demonstrating the feasibility of the first two scientific objectives listed in the previous paragraph. A brief description of these is given below. Lense-Thirring effect. The Lense-Thirring effect causes the Gravitational Field surrounding a rotating mass to differ from that surrounding a nonrotating mass. From a mathematical point of view such differences are represented by off-diagonal terms in the metric tensor. A simple analogy can be drawn from Electromagnetic Field theory: a rotating charged sphere will create both an electric and a magnetic field, while a non-rotating charged sphere will produce only an electric field. In general, slowly rotating bodies like the Earth create only a tiny non-relativistic modification to the gravitational field with the consequence that their effects on the motion of a body in its immediate surroundings are almost totally insignificant. However, as demonstrated by Lense and Thirring 4, these tiny modifications to the local gravitational field will cause a gyroscope, for example, placed in a orbit around the Earth to precess, according to the following formula: 6 LT = G/r 3 [3 r / r 2 (S e T r) - S e ] (1) (S e is the spin angular momentum of the Earth.) Although sign and strength of the induced rotation, 6 LT, depends on its latitudinal position along the orbit, the average precessional rate of the gyroscope over 1 orbit, (in a 700 km polar orbit) would be of the order of radians/sec. The fine structure constant. The fine structure a- dimensional constant, = , provides a measure of the strength of the electromagnetic interaction and hence plays a major role in Quantum Electro-Dynamic (QED) theories. As shown in figure 1, the methods so far used for the precise determination of the value of α have led to experimental results, which differ more than their predicted uncertainties. Figure 1: Experimental values of alpha and related errors, determined by various methods It is thus still unclear if the present disagreement is due to unknown experimental imperfections or to more profound theoretical reasons. The method proposed by HYPER, which is based on measurement of the ratio of Planck s constant and the atomic mass, does not rely on QED theories. It will thus furnish an independent test of these theories and at the same time will provide the most accurate measurement of yet. ATOM-WAVE INTERFEROMETRY In the last two decades matter-wave interferometry has been pursued mainly to demonstrate and test the wave character of matter, one of the most puzzling effects in physics, predicted by quantum mechanics. Only recently has the technique of matter-wave interferometry also been used to realise very high-performance instruments for the measurement of accelerations, rotations and frequency shifts 5. The technological advances that make atom interferometry useful as a high accuracy sensor are those that allow very large numbers of atoms with low velocities to be controlled and manipulated. These come mainly from the combination of important developments in two main fields: 2

3 1- Laser-controlled high precision spectroscopy for the manipulation and control of atoms 2- Cooling techniques applied to matter-wave atom clouds to increase their coherent interaction time. These techniques are brought together in an Atomic Sagnac Interferometer (ASI). Here the Coherent splitting of atomic waves by Raman transitions is the tool, which has provided a major break-through in atom interferometry. This laser technique allows the coherent splitting, redirection, recombination and spectroscopic detection of atoms by modifying their internal electronic state. At the same time, it modifies their macroscopic state, by absorbing photon momentum, leading to the formation of a Sagnac loop, as shown in figure 2 pure rotation or the pure acceleration without interference from each other. The conceptual configuration of an ASU is shown in figure 3. detection cold atomic beam 3 D MOM π/2 π π/2 Interferometer detection cold atomic beam 3 D MOM Figure 3: Conceptual configuration of an ASU. In each ASI, as in a classical optical Sagnac interferometer, a phase shift 0 is created. While 0 Sagnac-Optical = 4 % A opt 6 / c (2) It can be shows that the equivalent phase shift in a matter wave interferometer is 0 Sagnac-Atoms = 4 % m atom A atom 6 / h (3) Figure 2: Split, redirection and recombination of atoms by lasers in the two interferometer arms, leading to the formation of a Sagnac Loop In an interferometer experiencing a rotation and/or an acceleration the symmetry between the two matter-wave beams, which travel in opposite directions along the interferometer arms, is broken. This symmetry breakdown causes a measurable interference pattern, formed when the arms re-join. It is this measurement that is used to sense the accelerations and rotations to which the loop has been subjected. As with any Sagnac loop, the ASI can only sense rotations perpendicular to the plane formed by the interferometer arms. Accelerations are only sensed in the direction of the counterpropagating laser beams. In order to discriminate between accelerations and rotations two ASIs, propagating in opposite directions, are combined into a single instrument, called the Atom Sagnac Unit (ASU). The effects of the rotations will affect these ASIs differently whereas the accelerations will cause the same effect. The signals from the two ASIs can therefore be combined to measure, independently, either the This phase shift between the two waves, travelling along the two arms of the interferometer, is caused by the Doppler effect between the atomic waves and the manipulating lasers, which are essentially fixed. Comparison of equation (2) and (3) suggests a theoretical superiority in sensitivity of the matterwave interferometer, as: 0 Sagnac-Atoms / 0 Sagnac-Optical = m atom c / h (4) For typical experimental parameters ( = 633 nm, m atom = Caesium mass) the increase in sensitivity would result in a factor 10 10, if the areas A were equal. In reality A atom is << A optics It can be shown that A atom in equation (3) is equal to A atom = L 2 A atom V T / V L (5) where L is the length of the interferometer, V T = hk/ 2% m atom is the transverse velocity due to the two-photon momentum transferred to the atomic beam during the Raman transition, V L = (2k B T/ m atom ) 1/2 is the drift velocity of the atomic beam in the interferometer. From equation (3) and (5) it can be seen that the highest potential for further improvements of the sensitivity in the detection of 0 Sagnac-Atoms in an atom-wave interferometer resides in the lowering of the drift velocity V L of the atoms by means of cooling techniques. 3

4 Since 1975 several techniques have been developed to cool a large amount of atoms ( ) hence slowing them down. Table 1 identifies some of these techniques together with the temperatures and drift velocities they can typically achieve. Thermal Atomic beam Magneto-optical trap Sub-Doppler Molasses Bose-Einstein Condensation 300 K m/s mk m/s K 1 10 cm/s nk 1 mm/s Table 1: Atom Cooling Techniques and Performances Laser cooling does not only reduce the mean drift velocity of the atoms but also increases their coherence. Indeed, if all the atoms could be controlled to exactly the same velocity they would then behave as a single coherent wave whose wavelength would be given by the de-broglie relation: d-b = h / m atom V L (6) MODEL PAYLOAD The configuration of the HYPER payload has been mainly driven by the mission objective to measure the Lense-Thirring effect. This effect produces a small rate component in two directions. The payload must therefore be capable of measuring both of these components and of distinguishing the rate measurements from other effects such as accelerations. The coupling of two ASIs into one ASU allows the unambiguous measurement of rates around a single axis. Therefore, two independent and orthogonal ASUs are needed for HYPER. This baseline configuration is presented in figure 4. Figure 4: Conceptual Payload configuration showing two ASUs, each consisting of two counter propagating ASIs. The Lense-Thirring effect may be thought of as the dragging of the local inertial reference frame by the Earth s rotation. However, each ASU measures the relative rotation of the atomic waves with respect to the manipulating lasers. As the lasers and the s/c are contained within the same local inertial frame they too will experience the Lense-Thirring effect. It is therefore fundamental that the manipulating lasers are maintained fixed with respect to a non-local inertial frame. A guide star provides the reference for this non-local inertial frame, tracked by a 20 cm diameter telescope, acting as a high-performance star-tracker. The measurement precision and stability required for the HYPER mission are extremely demanding. In order to reduce to a minimum any disturbing effects, the body of the star-tracker is constructed from a monolithic block of Zerodur, which acts also as the main optical bench for the two ASUs. Three mirrors mounted directly onto the sides of the startracker determine the paths of the manipulating lasers, which in turn determine the sensitive directions of the ASUs. The main ASU reference frame is therefore defined by the line through the centre of these reflectors and is nominally aligned with the star-tracker boresight. The payload configuration is shown in figure 5. Atom Preparation Bench Drag Free Sensor Star-tracker/ Optical Bench Supporting structure Figure 5: Payload configuration MISSION ARCHITECTURE The main drivers for the mission architecture arise mainly from the very high pointing precision required, the need of a non-local reference frame and the fact that the magnitude of the latitudedependent Lense-Thirring effect decreases rapidly with distance from the Earth. Orbit. The desire to measure the latitudinal variation of the effect leads directly to a very highly inclined orbit, which must be as low as possible in 4

5 order to increase the measurable size of the effect. These considerations, together with the desire for a very stable thermal environment and low disturbances led to the identification of a 700 km sun synchronous dawn dusk orbit. Launcher. HYPER will be inserted directly into this orbit by a Rockot vehicle launched from Plesetsk (40.1ºE, 62.8ºN) and using the Breeze upper stage. The combination of launcher and high launch site latitude make for a very cost-effective launch. In this configuration, the launch capacity of 870 kg for the desired orbit provides ample mass margin over the initial design mass of 767 kg (including system margin and launch adapter). Space Segment. Due to the size of the effect to be measured, the spacecraft needs to provide the payload with an extremely stable environment with full six degree-of-freedom control and to minimize every possible source of disturbance. The most demanding areas of the spacecraft design lie within the structure, the Thermal Control Sub-system (TCS) and Attitude and Orbit Control Sub- systems (AOCS). The main driver for the structure relates to the provision of the mechanical stability required by the experiment while retaining the compatibility with the launch vehicle mass and dimensions. A viable design solution was found by adopting a two-structure approach. The main body of the spacecraft is based on a Carbon Fiber Reinforced Plastic load bearing cylinder encased in the center a boxed structure of aluminum honeycomb panels. A second cylindrical payload structure, which ensures minimal distortion, supports the two critical payload elements (the combined star-tracker/optical bench and the atom preparation bench) and is isostatically mounted inside the central cylinder of the primary structure. The TCS must ensure an ultra stable temperature for prolonged periods across the entire payload. For this reason the spacecraft is designed as a sort of onion with thermally de-coupled layers in which the most demanding units from a thermal stability point of view are positioned at the center of the onion. The thermal design is eased by the use of the single circular non-deployable solar array acting as a sun shield. The AOCS must provide very accurate control of the six degree of freedom of the spacecraft in addition to the normal requirements such as offlauncher de-spin and failure detection and correction. To cope with this the AOCS is split into a primary and a secondary sub-system. The primary sub-system, which relies on sun sensors, star trackers and inertial measurement units, performs all off-launcher and safe mode functions. It provides sufficient accuracy and stability to be able to hand over control to the secondary AOCS which then uses the narrow field of view (106 arcseconds) precision star-tracker and two drag free sensors for measurement. The primary AOCS remains tasked with realising the required thrust and torques activities. Actuation is performed, in all cases other than off launcher de-spin, by high dynamic range, high accuracy Field-Effect Electric Propulsion (FEEP) thrusters. The overall configuration of the spacecraft is shown in figure 6. Fixed Solar Array FEEP Cluster Payload section 20 cm Precision Star Tracker Figure 6: HYPER Spacecraft Configuration Mission operations. Mission operations will be performed by ESOC at the Mission Operations Center using the 15 m antenna at Kiruna to provide the required coverage of 7 minutes per day in a single pass. During launch and early orbit phase the available coverage will be augmented by the use of the 15 m stations at both Perth and Kourou. During the nominal operations phase, which lasts two years, a data store and dump strategy has been selected. The 190 Mbits produced every day (including housekeeping and science data) can be downloaded in a single pass via the low gain antennas. The on-board mass memory is sized to enable storage for up to 10 days. The initial study conducted by ESA demonstrated that the HYPER mission is feasible both from a technical and cost point of view. However, due to the very demanding mission requirements and time constraints, the study identified further work in some key areas. FURTHER DEVELOPMENTS The Science Programme Committee has strongly recommended that work on the HYPER mission should continue. The development of the HYPER 5

6 design will be conducted via a dedicated industrial study, which is currently in joint preparation by ESA and Core Science Team members. It will build on the initial work performed so far and will concentrate mainly on areas like the startracker/optical bench, the analysis of the drag free and thermo-elastic performances and the examination of potential disturbance effects generated by both the environment and the spacecraft The results of the industrial study will be presented to the HYPER Symposium in autumn HYPER will compete for selection as one of the next group of Flexi-missions which are planned to be launched in the timeframe. REFERENCES 1. HYPER: Hyper Precision Atom Interferometry in Space A Proposal for Fleximission F2 or F3 in Fundamental Physics Institut für Quantenoptik, January HYPER: Hyper Precision Atom Interferometry in Space ESA CDF-09, September HYPER: Hyper Precision Atom Interferometry in Space - Assessment Study Report ESA-SCI(2000) 10, July J. Lense, H. Thirring, Phys Z. 19, 156 (1918) 5. Ch. Borde : Atom interferometry and laser spectroscopy in Laser Spectroscopy X, World Scientific, 239 (1991) ACKNOWLEDGEMENTS Data included in this paper are largely based on output generated during the HYPER Assessment Study carried out at the ESTEC Concurrent Design Facility by ESA engineers and HYPER Science team members. Their contribution is gratefully acknowledged. 1 Copyright 2001 by European Space Agency. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Released to IAF/IAA/AIAA to publish in all forms. 6