I. Mateos

LISA Pathfinder: mission and status

LISA Pathfinder, the second of the European Space Agencys 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 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.

Beyond the required LISA free-fall performance: new LISA Pathfinder results down to 20 μHz

In the months since the publication of the first results, the noise performance of LISA Pathfinder has improved because of reduced Brownian noise due to the continued decrease in pressure around the test masses, from a better correction of noninertial effects, and from a better calibration of the electrostatic force actuation. In addition, the availability of numerous long noise measurement runs, during which no perturbation is purposely applied to the test masses, has allowed the measurement of noise with good statistics down to 20u2009u2009μHz. The Letter presents the measured differential acceleration noise figure, which is at (1.74±0.05)u2009u2009fmu2009s^{-2}/sqrt[Hz] above 2xa0mHz and (6±1)×10u2009u2009fmu2009s^{-2}/sqrt[Hz] at 20u2009u2009μHz, and discusses the physical sources for the measured noise. This performance provides an experimental benchmark demonstrating the ability to realize the low-frequency science potential of the LISA mission, recently selected by the European Space Agency.

LISA Pathfinder data analysis

As the launch of LISA Pathfinder (LPF) draws near, more and more effort is being put in to the preparation of the data analysis activities that will be carried out during the mission operations. The operations phase of the mission will be composed of a series of experiments that will be carried out on the satellite. These experiments will be directed and analysed by the data analysis team, which is part of the operations team. The operations phase will last about 90 days, during which time the data analysis team aims to fully characterize the LPF, and in particular, its core instrument the LISA Technology Package. By analysing the various couplings present in the system, the different noise sources that will disturb the system, and through the identification of the key physical parameters of the system, a detailed noise budget of the instrument will be constructed that will allow the performance of the different subsystems to be assessed and projected towards LISA. This paper describes the various aspects of the full data analysis chain that are needed to successfully characterize the LPF and build up the noise budget during mission operations.

On the role of radiation monitors on board LISA Pathfinder and future space interferometers

LISA (Laser Interferometer Space Antenna) and its precursor mission LISA Pathfinder (LISA-PF) will carry particle monitors for noise diagnostics. It was proposed to build and place radiation detectors on board the ASTROD missions as well. We present here a study of the solar energetic particle (SEP) events that the LISA-PF radiation monitors are able to detect above the galactic cosmic-ray (GCR) background predicted at the time of the mission data taking in 2015. In order to optimize the correlation between radiation monitor measurements and gravitational sensor test-mass charging, the energy threshold for particles traversing both detectors should be approximately the same. In LISA-PF, the radiation monitor particle energy cut-off was conservatively set at 75 MeV per nucleon (MeV/n) for protons and ion normal incidence, while the minimum energy of the same particles reaching the test masses is 100 MeV/n. We find that SEP events detectable on LISA-PF are characterized by peak fluxes and fluences at energies >75 MeV/n larger than about 45%, on average, with respect to those at energies >100 MeV/n. We conclude that for an accurate correlation between radiation monitor count rates and test-mass charging, it is mandatory to benefit from absolute flux measurements of both galactic and high-energy solar particles provided by experiments carrying magnetic spectrometers in space at the time of LISA-PF (PAMELA, AMS). On the other hand, the role of the radiation detectors on board LISA-PF is crucial allowing for SEP event onset and dynamics monitoring.

Galactic cosmic-ray energy spectra and expected solar events at the time of future space missions

Galactic cosmic-rays (GCRs) and solar energetic particles (SEPs) affect observations on board long-lived space missions. We developed a parameterization of proton and helium fluxes for various levels of solar modulation during opposite polarity periods. In addition to long-term variations (decades), short-term fluctuations (minutes to days) were considered as well. In particular, we focused on data from experiments carrying magnetic spectrometers in space. The shortest GCR variations we were able to study are of the order of hours. We point out that GCR variations and fluctuations are strongly energy dependent. The detector charging onboard space experiments is also energy dependent. The measurements of energy differential fluxes and their variations are needed in order to evaluate properly the performance of future space missions. We present here the projections for the GCR fluxes and solar events at the time of LISA (Laser Interferometer Space Antenna) Pathfinder (LISA-PF).

Charge-induced force-noise on free-falling test masses: results from LISA Pathfinder

We report on electrostatic measurements made on board the European Space Agency mission LISA Pathfinder. Detailed measurements of the charge-induced electrostatic forces exerted on free-falling test masses (TMs) inside the capacitive gravitational reference sensor are the first made in a relevant environment for a space-based gravitational wave detector. Employing a combination of charge control and electric-field compensation, we show that the level of charge-induced acceleration noise on a single TM can be maintained at a level close to 1.0u2009u2009fmu2009s^{-2}u2009Hz^{-1/2} across the 0.1-100xa0mHz frequency band that is crucial to an observatory such as the Laser Interferometer Space Antenna (LISA). Using dedicated measurements that detect these effects in the differential acceleration between the two test masses, we resolve the stochastic nature of the TM charge buildup due to interplanetary cosmic rays and the TM charge-to-force coupling through stray electric fields in the sensor. All our measurements are in good agreement with predictions based on a relatively simple electrostatic model of the LISA Pathfinder instrument.

LISA-PF radiation monitor performance during the evolution of SEP events for the monitoring of test-mass charging

Cosmic rays of solar and galactic origin at energies >100 MeV/n charge and induce spurious forces on free-floating test masses on board interferometers devoted to gravitational wave detection in space. LISA Pathfinder (LISA-PF), the technology testing mission for eLISA/NGO, will carry radiation monitors for on board test-mass charging monitoring. We present here the results of a simulation of radiation monitor performance during the evolution of solar energetic particle (SEP) events of different intensity. This simulation was carried out with the Fluka Monte Carlo package by taking into account for the first time both energy and spatial distributions of solar protons for the SEP events of 23 February 1956, 15 November 1960 and 7 May 1978. Input data for the Monte Carlo simulations was inferred from neutron monitor measurements. Conversely, for the SEP event of 13 December 2006 observed by the PAMELA experiment in space, we used the proton pitch angle distribution (PAD) computed from the Particle Acceleration and Transport in the Heliosphere (PATH) code. We plan to adopt this approach at the time of LISA-PF data analysis in order to optimize the correlation between radiation monitor observations and test-mass charging. The results of this work can be extended to the future space interferometers and other space missions carrying instruments for SEP detection.

Short-term forecasting of solar energetic ions on board LISA

LISA (Laser Interferometer Space Antenna) and LISA Pathfinder (LISA-PF) free-falling test-masses are charged by galactic and solar energetic particles. This process generates spurious forces on the test masses which appear as noise in the experiments. It was shown that relativistic solar electron detection can be used for up-to-one-hour forecasting of incoming energetic ions at 1 AU. Warning of incoming solar energetic particle events could allow us to optimize the test-mass discharging. The current LISA-PF radiation monitor design needs to be upgraded if solar electron detection is to be implemented in LISA.

Bayesian statistics for the calibration of the LISA pathfinder experiment

The main goal of LISA Pathfinder (LPF) mission is to estimate the acceleration noise models of the overall LISA Technology Package (LTP) experiment on-board. This will be of crucial importance for the future space-based Gravitational-Wave (GW) detectors, like eLISA. Here, we present the Bayesian analysis framework to process the planned system identification experiments designed for that purpose. In particular, we focus on the analysis strategies to predict the accuracy of the parameters that describe the system in all degrees of freedom. The data sets were generated during the latest operational simulations organised by the data analysis team and this work is part of the LTPDA Matlab toolbox.