LISA Pathfinder: First steps to observing gravitational waves from space

LISA Pathfinder, the European Space Agency's technology demonstrator mission for future spaceborne gravitational wave observatories, was launched on 3 December 2015, from the European space port of Kourou, French Guiana. After a short duration transfer to the final science orbit, the mission has been gathering science data since. This data has allowed the science community to validate the critical technologies and measurement principle for low frequency gravitational wave detection and thereby confirming the readiness to start the next generation gravitational wave observatories, such as LISA. This paper will briefly describe the mission, followed by a description of the science operations highlighting the performance achieved. Details of the various experiments performed during the nominal science operations phase can be found in accompanying papers in this volume

LISA Pathfinder closed-loop analysis: a model breakdown of the in-loop observables

This paper describes a methodology to analyze, in the frequency domain, the steady-state control performances of the LISA Pathfinder mission. In particular, it provides a technical framework to give a comprehensive understanding of the spectra of all the degrees of freedom by breaking them down into their various physical origins, hence bringing out the major contributions of the control residuals. A reconstruction of the measured in-loop output, extracted from a model of the closed-loop system, is shown as an instance to illustrate the potential of such a model breakdown of the data

Competing Ideas of Social Justice and Space: Locating Critiques of Housing Renewal in Theory and in Practice

This article considers the experience of the English government's policy of Housing Market Renewal from the perspective of spatial justice. The paper first proposes an analytical framework that situates competing notions of territorial social justice within a space of complex sociospatial relations. The dialectic of two formulations of social justice is first set up, comparing 'procedural' or deontological forms of justice and the distributional justice of outcomes. Soja's formulation of spatial justice is advanced as an appropriate balance between spatial and socio-historic contexts for the justice question. Drawing on the literature on sociospatial relations, concrete critiques and justifications of HMR are then positioned in terms of the intersection of structuring principles and policy fields. The role of demolition in urban restructuring programmes is used to explore the differential spatialities involved in different justicial perspectives. It is concluded that 'gentrification' critiques of HMR are only partial in their evaluation of justice and lack normative power. Some practical implications for the design of urban restructuring policies are offered

Micrometeoroid Events in LISA Pathfinder

The zodiacal dust complex, a population of dust and small particles that pervades the Solar System, provides important insight into the formation and dynamics of planets, comets, asteroids, and other bodies. Here we present a new set of data obtained using a novel technique: direct measurements of momentum transfer to a spacecraft from individual particle impacts. This technique is made possible by the extreme precision of the instruments flown on the LISA Pathfinder spacecraft, a technology demonstrator for a future space-based gravitational wave observatory that operated near the first Sun-Earth Lagrange point from early 2016 through Summer of 2017. Using a simple model of the impacts and knowledge of the control system, we show that it is possible to detect impacts and measure properties such as the transferred momentum (related to the particle's mass and velocity), direction of travel, and location of impact on the spacecraft. In this paper, we present the results of a systematic search for impacts during 4348 hours of Pathfinder data. We report a total of 54 candidates with momenta ranging from 0.2$\,\mu\textrm{Ns}$ to 230$\,\mu\textrm{Ns}$. We furthermore make a comparison of these candidates with models of micrometeoroid populations in the inner solar system including those resulting from Jupiter-family comets, Oort-cloud comets, Hailey-type comets, and Asteroids. We find that our measured population is consistent with a population dominated by Jupiter-family comets with some evidence for a smaller contribution from Hailey-type comets. This is in agreement with consensus models of the zodiacal dust complex in the momentum range sampled by LISA Pathfinder.Comment: 22 pages, 14 figures, accepted in Ap

LISA Pathfinder closed-loop analysis: a model breakdown of the in-loop observables

This paper describes a methodology to analyze, in the frequency domain, the steady-state control performances of the LISA Pathfinder mission. In particular, it provides a technical framework to give a comprehensive understanding of the spectra of all the degrees of freedom by breaking them down into their various physical origins, hence bringing out the major contributions of the control residuals. A reconstruction of the measured in-loop output, extracted from a model of the closed-loop system, is shown as an instance to illustrate the potential of such a model breakdown of the data

Preliminary results on the suppression of sensing cross-talk in LISA Pathfinder

In the original paper describing the first measurements performed with LISA Pathfinder, a bulge in the acceleration noise was shown in the 200 mHz - 20 mHz frequency band. This bulge noise originated from cross-coupling of spacecraft motion into the longitudinal readout and it was shown that it is possible to subtract this cross-talk noise. We discuss here the model that was used for subtraction as well as an alternative approach to suppress the cross talk by realignment of the test masses. Such a realignment was performed after preliminary analysis of a dedicated cross-talk experiment, and we show the resulting noise suppression. Since then, further experiments have been performed to investigate the cross-coupling behaviour, however analysis of these experiments is still on-going

LISA Pathfinder as a Micrometeoroid Instrument

LISA Pathfinder is perhaps the most precise accelerometry instrument ever flown in space. The drag-free control system can sense and react to external disturbances of an extremely small magnitude. One class of such disturbances are the impacts of micrometeoroids or dust. A simple model of the LPF system suggests that individual impacts with transferred momentum exceeding a few tens of nanoNewton-meters are detectable. Furthermore, the ability of LPF to resolve both the linear and angular momentum transfer as vector quantities allows information such as the sky location and the impact location of the impactor to be reconstructed. This novel approach to micrometeoroid detection and characterization, as well as the location of LPF at L1, provide an opportunity to improve our understanding of the dust environment in the inner solar system. Here we present some preliminary findings from LPF, including four candidate impact events

LISA Pathfinder: First steps to observing gravitational waves from space

LISA Pathfinder, the European Space Agency's technology demonstrator mission for future spaceborne gravitational wave observatories, was launched on 3 December 2015, from the European space port of Kourou, French Guiana. After a short duration transfer to the final science orbit, the mission has been gathering science data since. This data has allowed the science community to validate the critical technologies and measurement principle for low frequency gravitational wave detection and thereby confirming the readiness to start the next generation gravitational wave observatories, such as LISA. This paper will briefly describe the mission, followed by a description of the science operations highlighting the performance achieved. Details of the various experiments performed during the nominal science operations phase can be found in accompanying papers in this volume

GCR flux 9-day variations with LISA Pathfinder

Galactic cosmic-ray (GCR) energy spectra in the heliosphere vary on the basis of the level of solar activity, the status of solar polarity and interplanetary transient magnetic structures of solar origin. A high counting rate particle detector (PD) aboard LISA Pathfinder (LPF) allows for the measurement of galactic cosmic-ray and solar energetic particle (SEP) integral fluxes at energies > 70 MeV n−1 up to 6500 counts s−1. Data are gathered with a sampling time of 15 s. A study of GCR flux depressions associated with the third harmonic of the Sun rotation period (∼ 9 days) is presented here

Gravitational Reference Sensor Front-End Electronics Simulator for LISA

At the ETH Zurich we are developing a modular simulator that provides a realistic simulation of the Front End Electronics (FEE) for LISA Gravitational Reference Sensor (GRS). It is based on the GRS FEE-simulator already implemented for LISA Pathfinder. It considers, in particular, the non-linearity and the critical details of hardware, such as the non-linear multiplicative noise caused by voltage reference instability, test mass charging and detailed actuation and sensing algorithms. We present the simulation modules, considering the above-mentioned features. Based on the ETH GRS FEE-simulator for LISA Pathfinder we aim to develop a modular simulator that provides a realistic simulation of GRS FEE for LISA