LISA Pathfinder Mission Research Articles

The UF Torsion Pendulum, a LISA Technology Testbed: Sensing System and Initial Results

The upcoming LISA Pathfinder mission will test the Gravitational Reference Sensor and the Disturbance Reduction System for a future LISA-like space mission. While LISA Pathfinder is expected to show that the technology for LISA exists and meets the LISA requirements, it is likely that LISA Pathfinder will also reveal areas where future improvements can be made and might be necessary. Some of these are already well known (such as the discharging system). After all, the technology for LISA Pathfinder was frozen about 10 years ago or about 30 years before a LISA-like mission will be launched. The case for continued testing and development of the technology is clear. The University of Florida is currently building a torsion pendulum-based test facility to explore new techniques and also to develop a base in the United States for state-of-the-art Gravitational Reference Sensor technologies.

The LISA Pathfinder Mission

LISA Pathfinder (LPF), the second of the European Space Agency's Small Missions for Advanced Research in Technology (SMART), is a dedicated technology validation mission for future spaceborne gravitational wave detectors, such as the proposed eLISA mission. LISA Pathfinder, and its scientific payload - the LISA Technology Package - will test, in flight, the critical technologies required for low frequency gravitational wave detection: it will put two test masses in a near-perfect gravitational free-fall and control and measure their motion with unprecedented accuracy. This is achieved through technology comprising inertial sensors, high precision laser metrology, drag-free control and an ultra-precise micro-Newton propulsion system. LISA Pathfinder is due to be launched in mid-2015, with first results on the performance of the system being available 6 months thereafter.The paper introduces the LISA Pathfinder mission, followed by an explanation of the physical principles of measurement concept and associated hardware. We then provide a detailed discussion of the LISA Technology Package, including both the inertial sensor and interferometric readout. As we approach the launch of the LISA Pathfinder, the focus of the development is shifting towards the science operations and data analysis - this is described in the final section of the paper

Measuring test mass acceleration noise in space-based gravitational wave astronomy

The basic constituent of interferometric gravitational wave detectors -- the test mass to test mass interferometric link -- behaves as a differential dynamometer measuring effective differential forces, comprising an integrated measure of gravity curvature, inertial effects, as well as non-gravitational spurious forces. This last contribution is going to be characterised by the LISA Pathfinder mission, a technology precursor of future space-borne detectors like eLISA. Changing the perspective from displacement to acceleration can benefit the data analysis of LISA Pathfinder and future detectors. The response in differential acceleration to gravitational waves is derived for a space-based detector's interferometric link. The acceleration formalism can also be integrated into time delay interferometry by building up the unequal-arm Michelson differential acceleration combination. The differential acceleration is nominally insensitive to the system free evolution dominating the slow displacement dynamics of low-frequency detectors. Working with acceleration also provides an effective way to subtract measured signals acting as systematics, including the actuation forces. Because of the strong similarity with the equations of motion, the optimal subtraction of systematic signals, known within some amplitude and time shift, with the focus on measuring the noise provides an effective way to solve the problem and marginalise over nuisance parameters. The $\mathcal{F}$-statistic, in widespread use throughout the gravitation waves community, is included in the method and suitably generalised to marginalise over linear parameters and noise at the same time. The method is applied to LPF simulator data and, thanks to its generality, can also be applied to the data reduction and analysis of future gravitational wave detectors.

Tests of modified gravity theories in the Solar System

We review the case for testing preferred acceleration scale theories of gravity (sometimes falling under the guise of modified Newtonian dynamics) in the Solar System using the forthcoming LISA Pathfinder (LPF) mission. Using a combination of analytical and numerical results, we suggest that different types of theory should be detectable using the predicted anomalous tidal stresses effects around the saddle points of the Newtonian gravitational field. The saddle point bubbles’ expected extent of ∼400 km are to be contrasted with potential miss parameters of ≤10 km, making such a test in easy reach of LPF. We also consider routes to constraining our theories from data, based on scenarios of both null and positive results.

Kolmogorov-Smirnov like test for time-frequency Fourier spectrogram analysis in LISA Pathfinder

A statistical procedure for the analysis of time-frequency noise maps is presented and applied to LISA Pathfinder mission synthetic data. The procedure is based on the Kolmogorov-Smirnov like test that is applied to the analysis of time-frequency noise maps produced with the spectrogram technique. The influence of the finite size windowing on the statistic of the test is calculated with a Monte Carlo simulation for 4 different windows type. Such calculation demonstrate that the test statistic is modified by the correlations introduced in the spectrum by the finite size of the window and by the correlations between different time bins originated by overlapping between windowed segments. The application of the test procedure to LISA Pathfinder data demonstrates the test capability of detecting non-stationary features in a noise time series that is simulating low frequency non-stationary noise in the system.

Hydroxide catalysis bonding for astronomical instruments

Abstract Hydroxide catalysis bonding (HCB) as a jointing technique has been under development for astronomical applications since ∼1998 (patented by D.-H. Gwo). It uses an aqueous hydroxide solution to form a chemical bond between oxide or oxidisable materials (e.g., SiO2, sapphire, silicon and SiC). It forms strong, extremely thin bonds, and is suitable for room temperature bonding, precision alignment, operation in ultra-low vacuum and down to temperatures of 2.5 K. It has been applied in the NASA satellite mission Gravity Probe B and in the ground-based gravitational wave (GW) detector GEO600. It will soon fly again on the ESA LISA Pathfinder mission and is currently being implemented in the Advanced LIGO and Virgo ground-based GW detectors. This technique is also of considerable interest for use in other astronomical fields and indeed more broadly, due to its desirable, and adjustable, combination of properties. This paper gives an overview of how HCB has been and can be applied in astronomical instruments, including an overview of the current literature on the properties of hydroxide catalysis bonds.

Bayesian model selection for LISA pathfinder

The main goal of the LISA Pathfinder (LPF) mission is to fully characterize the acceleration noise models and to test key technologies for future space-based gravitational-wave observatories similar to the eLISA concept. The data analysis team has developed complex three-dimensional models of the LISA Technology Package (LTP) experiment onboard the LPF. These models are used for simulations, but, more importantly, they will be used for parameter estimation purposes during flight operations. One of the tasks of the data analysis team is to identify the physical effects that contribute significantly to the properties of the instrument noise. A way of approaching this problem is to recover the essential parameters of a LTP model fitting the data. Thus, we want to define the simplest model that efficiently explains the observations. To do so, adopting a Bayesian framework, one has to estimate the so-called Bayes factor between two competing models. In our analysis, we use three main different methods to estimate it: the reversible jump Markov chain Monte Carlo method, the Schwarz criterion, and the Laplace approximation. They are applied to simulated LPF experiments in which the most probable LTP model that explains the observations is recovered. The same type of analysis presented in this paper is expected to be followed during flight operations. Moreover, the correlation of the output of the aforementioned methods with the design of the experiment is explored.

Common mode noise rejection properties of amplitude and phase noise in a heterodyne interferometer

High precision metrology systems based on heterodyne interferometry can measure the position and attitude of objects to accuracies of picometer and nanorad, respectively. A frequently found feature of the general system design is the subtraction of a reference phase from the phase of the position interferometer, which suppresses low frequency common mode amplitude and phase fluctuations occurring in volatile optical path sections shared by both the position and reference interferometer. Spectral components of the noise at frequencies around or higher than the heterodyne frequency, however, are generally transmitted into the measurement band and may limit the measurement accuracy. Detailed analytical calculations complemented with Monte Carlo simulations show that high frequency noise components may also be entirely suppressed, depending on the relative difference of measurement and reference phase, which may be exploited by corresponding design provisions. While these results are applicable to any heterodyne interferometer with certain design characteristics, specific calculations and related discussions are given for the example of the optical metrology system of the LISA Pathfinder mission to space.

Parametrized free functions and saddle stresses in modified gravity

Building on previous work, we explore the parameter space of free functions in non-relativistic modified gravity theories more widely, showing that in fact the two broad regimes present have similar functional forms between different models. Using different parameterisations, we investigate the effects on scaling tidal stresses as well as attempt to constrain the (hitherto poorly understood) deep MONDian scaling C. We also consider a new intermediate MOND limit in these theories and what it tells us about the transition between these regimes. Finally we suggest a model independent framework, with the aim of constraining the MONDian parameter space using future data, such as the forthcoming LISA Pathfinder mission.

Construction of rugged, ultrastable optical assemblies with optical component alignment at the few microradian level

A method for constructing quasimonolithic, precision-aligned optical assemblies is presented. Hydroxide-catalysis bonding is used, adapted to allow optimization of component fine alignment prior to the bond setting. We demonstrate the technique by bonding a fused silica mirror substrate to a fused silica baseplate. In-plane component placement at the submicrometer level is achieved, resulting in angular control of a reflected laser beam at the sub-10-μrad level. Within the context of the LISA Pathfinder mission, the technique has been demonstrated as suitable for use in space-flight applications. It is expected that there will also be applications in a wide range of areas where accuracy, stability, and strength of optical assemblies are important.

THE LISA PATHFINDER MISSION

Laser Interferometer Space Antenna (LISA) Pathfinder (formerly known as SMART-2) is a European Space Agency mission designed to pave the way for the joint ESA/NASA LISA mission by testing in flight the critical technologies required for space borne gravitational wave detection; it will put two test masses in a near-perfect gravitational free-fall and control and measure their motion with unprecedented accuracy. This is achieved through technology comprising inertial sensors, high precision laser metrology, drag-free control and an ultra precise micro-Newton propulsion system. LISA Pathfinder (LPF) essentially mimics one arm of space-borne gravitational wave detectors by shrinking the million kilometer scale armlengths down to a few tens of centimeters, giving up the sensitivity to gravitational waves, but keeping the measurement technology. The scientific objective of the LPF mission consists then of the first in-flight test of low frequency gravitational wave detection metrology.

IMPLICATIONS OF GALACTIC AND SOLAR PARTICLE MEASUREMENTS ON BOARD INTERFEROMETERS FOR GRAVITATIONAL WAVE DETECTION IN SPACE

Test-mass charging due to galactic cosmic rays (GCRs) and solar energetic particles (SEPs) represents one of the major sources of noise for missions devoted to gravitational wave detection in space. Particle detectors on board future space interferometers will help in monitoring the test-mass charging process. Variations and fluctuations of GCRs and evolution of SEP events of different intensities are discussed here for the correlation of SEP radiation monitor observations and particle fluxes charging the test masses. We consider the performance of the radiation monitors designed for the LISA Pathfinder mission for the results presented in this work. We point out that in addition to the primary use of test-mass charging monitoring, particle detectors on board space interferometers will naturally provide SEP observations at different intervals in heliolongitude and distances from Earth for space weather applications.

Saddle stresses for generic theories with a preferred acceleration scale

We show how scaling arguments may be used to generate templates for the tidal stresses around saddles for a vast class of modified Newtonian dynamics theories detached from their obligations as dark matter alternatives. Such theories are to be seen simply as alternative theories of gravity with a preferred acceleration scale, and could be tested in the Solar System by extending the LISA Pathfinder mission. The constraints thus obtained may then be combined, if one wishes, with requirements arising from astrophysical and cosmological applications, but a clear separation of the issues is achieved. The central technical content of this paper is the derivation of a scaling prescription allowing complex numerical work to be bypassed in the generation of templates. We find that LISA Pathfinder could constrain very tightly the acceleration ${a}_{0}$ and the free parameter $\ensuremath{\kappa}$ present in these theories. As an application of our technique we also produce predictions for the Moon saddle (for which a similar scaling argument is applicable) with the result that we recommend that it should be included in orbit design.

Prediction of the LISA-Pathfinder release mechanism in-flight performance

In the LISA Pathfinder mission, the grabbing positioning and release mechanism will perform a critical phase, which is the injection of a test mass into a perfect free-fall condition (also called geodesic trajectory). A possible failure is the test mass remaining adhered to the mechanism or released with an excessive velocity, which would hinder the subsequent capacitive control to the desired geodesic status.In space engineering it is recognized that mechanisms constitute critical subsystems, because they may have single point failures which can seriously affect the mission. In order to limit the risk, a good practice is to test the mechanism critical phases in an environment which simulates the conditions that will be experienced in operation. In the LISA Pathfinder ground testing plan a combined experimental-analytical strategy has been conceived to test this critical phase.The momentum transfer from an actuator to an adhered test mass upon quick separation is measured by means of a dedicated experiment, characterizing the behavior of adhesion under dynamic failure. In parallel, a mathematical model of the release mechanism electro-mechanical dynamics is experimentally validated. The mathematical model of the test mass in-flight injection into geodesic is built, combining the adhesion model with the identified model of the release mechanism. The compliance of the resulting injection with the requirements is presented and discussed.

Improving Markov Chain Monte Carlo algorithms in LISA Pathfinder Data Analysis

The LISA Pathfinder mission (LPF) aims to test key technologies for the future LISA mission. The LISA Technology Package (LTP) on-board LPF will consist of an exhaustive suite of experiments and its outcome will be crucial for the future detection of gravitational waves. In order to achieve maximum sensitivity, we need to have an understanding of every instrument on-board and parametrize the properties of the underlying noise models. The Data Analysis team has developed algorithms for parameter estimation of the system. A very promising one implemented for LISA Pathfinder data analysis is the Markov Chain Monte Carlo. A series of experiments are going to take place during flight operations and each experiment is going to provide us with essential information for the next in the sequence. Therefore, it is a priority to optimize and improve our tools available for data analysis during the mission. Using a Bayesian framework analysis allows us to apply prior knowledge for each experiment, which means that we can efficiently use our prior estimates for the parameters, making the method more accurate and significantly faster. This, together with other algorithm improvements, will lead us to our main goal, which is no other than creating a robust and reliable tool for parameter estimation during the LPF mission.

Scientific goals achievable with radiation monitor measurements on board gravitational wave interferometers in space

Cosmic rays and energetic solar particles constitute one of the most important sources of noise for future gravitational wave detectors in space. Radiation monitors were designed for the LISA Pathfinder (LISA-PF) mission. Similar devices were proposed to be placed on board LISA and ASTROD. These detectors are needed to monitor the flux of energetic particles penetrating mission spacecraft and inertial sensors. However, in addition to this primary use, radiation monitors on board space interferometers will carry out the first multipoint observation of solar energetic particles (SEPs) at small and large heliolongitude intervals and at very different distances from Earth with minor normalization errors. We illustrate the scientific goals that can be achieved in solar physics and space weather studies with these detectors. A comparison with present and future missions devoted to solar physics is presented.

The LISA Pathfinder mission

In this paper, we describe the current status of the LISA Pathfinder mission, a precursor mission aimed at demonstrating key technologies for future space-based gravitational wave detectors, like LISA. Since much of the flight hardware has already been constructed and tested, we will show that performance measurements and analysis of these flight components lead to an expected performance of the LISA Pathfinder which is a significant improvement over the mission requirements, and which actually reaches the LISA requirements over the entire LISA Pathfinder measurement band.

Attitude and orbit control systems for the LISA Pathfinder mission

The main purpose of the LISA Pathfinder mission is to provide in-orbit validation of the critical technologies necessary for LISA (Laser Interferometer Space Antenna), aiming at detecting gravitational waves generated by very massive objects such as black holes. The spacecraft consists of a Science Module (SCM) and a Propulsion Module (PRM). The former performs the science experiment, and the later provides the propulsive capability to raise the spacecraft from the injection orbit to the operational orbit at around L1 and is then separated from the former. The Spacecraft Attitude and Orbit Control System (AOCS) is actually composed of three distinct systems to fulfill the needs of the whole mission: – Composite AOCS, used to reach L1, aims at raising the Perigee through a succession of about 10 boosts performed with high thrust chemical propulsion; – Micro-propulsion AOCS takes over once the separation of the SCM from PRM has occurred and is based on micro-propulsion systems (micro-Newton electrical thrusters); – Drag-Free Attitude Control System (DFACS) is then used to perform science experiments. This article provides a comprehensive overview of the AOCS architecture, requirements, selected sensors and actuators, system design & evolution, and achieved performances. It focuses in particular on the Composite AOCS and the Micro-propulsion AOCS and will analyze the challenges of using micro-Newton electric propulsion.

LISA Pathfinder: mission and status

LISA Pathfinder, the second of the European Space Agency's Small Missions for Advanced Research in Technology (SMART), is a dedicated technology demonstrator for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission. The technologies required for LISA are many and extremely challenging. This coupled with the fact that some flight hardware cannot be fully tested on ground due to Earth-induced noise led to the implementation of the LISA Pathfinder mission to test the critical LISA technologies in a flight environment. LISA Pathfinder essentially mimics one arm of the LISA constellation by shrinking the 5 million kilometre armlength down to a few tens of centimetres, giving up the sensitivity to gravitational waves, but keeping the measurement technology: the distance between the two test masses is measured using a laser interferometric technique similar to one aspect of the LISA interferometry system. The scientific objective of the LISA Pathfinder mission consists then of the first in-flight test of low frequency gravitational wave detection metrology.LISA Pathfinder is due to be launched in 2013 on-board a dedicated small launch vehicle (VEGA). After a series of apogee raising manoeuvres using an expendable propulsion module, LISA Pathfinder will enter a transfer orbit towards the first Sun–Earth Lagrange point (L1). After separation from the propulsion module, the LPF spacecraft will be stabilized using the micro-Newton thrusters, entering a 500 000 km by 800 000 km Lissajous orbit around L1. Science results will be available approximately 2 months after launch.

The LISA Pathfinder interferometry—hardware and system testing

Preparations for the LISA Pathfinder mission have reached an exciting stage. Tests of the engineering model (EM) of the optical metrology system have recently been completed at the Albert Einstein Institute, Hannover, and flight model tests are now underway. Significantly, they represent the first complete integration and testing of the space-qualified hardware and are the first tests on an optical system level. The results and test procedures of these campaigns will be utilized directly in the ground-based flight hardware tests, and subsequently during in-flight operations. In addition, they allow valuable testing of the data analysis methods using the MATLAB-based LTP data analysis toolbox. This paper presents an overview of the results from the EM test campaign that was successfully completed in December 2009.