We report the first results of the LISA Pathfinder in-flight experiment. The results demonstrate that two free-falling reference test masses, such as those needed for a space-based gravitational wave observatory like LISA, can be put in free fall with a relative acceleration noise with a square root of the power spectral density of 5.2±0.1  fm s^{-2}/sqrt[Hz], or (0.54±0.01)×10^{-15}  g/sqrt[Hz], with g the standard gravity, for frequencies between 0.7 and 20 mHz. This value is lower than the LISA Pathfinder requirement by more than a factor 5 and within a factor 1.25 of the requirement for the LISA mission, and is compatible with Brownian noise from viscous damping due to the residual gas surrounding the test masses. Above 60 mHz the acceleration noise is dominated by interferometer displacement readout noise at a level of (34.8±0.3)  fm/sqrt[Hz], about 2 orders of magnitude better than requirements. At f≤0.5  mHz we observe a low-frequency tail that stays below 12  fm s^{-2}/sqrt[Hz] down to 0.1 mHz. This performance would allow for a space-based gravitational wave observatory with a sensitivity close to what was originally foreseen for LISA.

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 20  μHz. The Letter presents the measured differential acceleration noise figure, which is at (1.74±0.05)  fm s^{-2}/sqrt[Hz] above 2 mHz and (6±1)×10  fm s^{-2}/sqrt[Hz] at 20  μHz, 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.

The LISA Pathfinder charge management device was responsible for neutralising the cosmic ray induced electric charge that inevitably accumulated on the free-falling test masses at the heart of the experiment. We present measurements made on ground and in-flight that quantify the performance of this contactless discharge system which was based on photo-emission under UV illumination. In addition, a two-part simulation is described that was developed alongside the hardware. Modelling of the absorbed UV light within the Pathfinder sensor was carried out with the GEANT4 software toolkit and a separate MATLAB charge transfer model calculated the net photocurrent between the test masses and surrounding housing in the presence of AC and DC electric fields. We confront the results of these models with observations and draw conclusions for the design of discharge systems for future experiments like LISA that will also employ free-falling test masses.

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 freefalling 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 1.0 fm s −2 Hz −1=2 across the 0.1-100 mHz 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 PRL 118, 171101 (2017) P H Y S I C A L R E V I E W L E T T E R S week ending 28 APRIL 20170031-9007=17=118(17)=171101 (7) 171101-1 © 2017 American Physical Society 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.

Capacitive sensing of test mass motion with nanometer precision over millimeter-wide sensing gaps for space-borne gravitational reference sensors. Physical Review D, 96(6), 062004.There may be differences between this version and the published version. You are advised to consult the publisher's version if you wish to cite from it.http://eprints.gla.ac.uk/148989/ We report on the performance of the capacitive gap-sensing system of the Gravitational Reference Sensor (GRS) onboard the LISA Pathfinder (LPF) spacecraft. From in-flight measurements, the system has demonstrated a performance, down to 1 mHz, that is ranging between 0.7 aF Hz −1/2 and 1.8 aF Hz −1/2 . That translates into a sensing noise of the test mass motion within 1.2 nm Hz −1/2 and 2.4 nm Hz −1/2 in displacement and within 83 nrad Hz −1/2 and 170 nrad Hz −1/2 in rotation. This matches the performance goals for LPF and it allows the successful implementation of the gravitational waves observatory LISA. A 1/f tail has been observed for frequencies below 1 mHz, the tail has been investigated in detail with dedicated in-flight measurements and a model is presented in the paper. A projection of such noise to frequencies below 0.1 mHz shows that an improvement of performance at those frequencies is desirable for the next generation of GRS sensors for space-borne gravitational waves observation. 2PACS numbers: 04.80. Nn, 95.55.Ym