Philippe Jetzer, Physik Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich
LISA Pathfinder (LPF) is a European Space Agency (ESA) mission launched on 3 December 2015 from the European Spaceport in Kourou, French Guiana with a Vega rocket (Fig. 1). Following six apogee-raising manoeuvres, the spacecraft reached its final science orbit around the first Sun-Earth Lagrange point L1, 1.5 million km from Earth, on 22 January 2016 (Fig. 2).
LPF goal is to place two test masses in a nearly perfect gravitational free-fall, and control and measure their relative motion with unprecedented accuracy, at the level required for a future space-based gravitational wave (GW) observatory, such as LISA. This requirement is achieved through innovative technologies comprising inertial sensors, an optical metrology system, a drag-free control system and micro-Newton thruster system. All these technologies are not only essential for LISA, but they are also of crucial relevance for any future space-based test of Einstein’s General Relativity (GR). Much of the experiments in gravitational physics require measuring the relative acceleration between free-falling, geodesic reference test particles. For instance, in lunar laser ranging experiments, the test particles are the Earth and the Moon. In Earth-based gravitational wave measurements, the test particles are the pendulum-suspended mirrors of a Michelson interferometer. For LISA, the test masses will consist, similarly to LPF, of cubes of about 2 kg weight, housed in separate spacecrafts several million km apart. Indeed, the present design of LISA is aiming at a distance between the spacecrafts in the range of 1 to 5 million km.
The LISA Technology Package (LTP) is the main payload onboard of LPF, which was developed jointly by several European institutes and industries. It contains two identical cubic test masses of 1.9 kg and 46 mm in size made of gold-platinum, each suspended in its own vacuum vessel, capacitive sensors to monitor the relative position of the test masses with respect to the satellite, laser interferometry to determine the relative positions and attitudes of the two test masses, and the drag-free control system to adjust the relative alignment of the satellite and test masses through a mixture of micro-Newton (cold gas) thrusters and capacitive actuation. The cubes serve both as mirrors for the laser interferometer and as inertial references for the drag-free control system of the spacecraft, exactly as will be used for LISA. The LPF drag-free control system onboard the spacecraft monitors the micro motions of the test masses. When one of the test-masses moves away from its reference position, a signal is sent to the control system which activates the micro-propulsion thrusters to enable the spacecraft to remain centered on this test mass. The second test mass is made to follow the first by control forces applied via the capacitive actuator (Fig. 3).
The precise inter-test-mass tracking is achieved by optical interferometry and determines the relative acceleration of the two test masses. This configuration mimics one arm of LISA by shrinking the million kilometer scale arm-length down to only 38 cm, the distance of the two test masses in LPF. The primary mission performance requirement is to track, using picometer resolution laser interferometry, the two test masses nominally in free fall, and to show that their relative acceleration, at frequencies around 1 mHz, is within one order of magnitude of that required by a future LISA mission. For that purpose the aim of LPF consists of measuring and developing a physical model for all the spurious effects that limit the ability to create, and measure, the perfect constellation of free-falling test particles needed for LISA.
The LTP was built by a consortium of European space companies from France, Germany, Great Britain, Italy, Netherlands, Spain and Switzerland led by the industrial partner Astrium GmbH (an EADS company), Friedrichshafen, Germany. Switzerland contributed in a significant way to the mission. Since 2003 scientists from ETH and University of Zürich are actively involved in the project. The ETH Zürich group of Domenico Giardini, professor of seismology and geodynamics, with the engineers Peter Zweifel and Davor Mance, working with RUAG Space, developed the electronics for the measurement and control of the detection device that ensures that the cubes float freely inside the spacecraft. The other swiss industries involved are APCO, based in Aigle, who provided mechanical ground support equipment; HES-SO Valais-Wallis performed tests on electronic elements, and RUAG Space also provided the rockets payload fairing. My group at University of Zürich is particularly involved in theoretical investigations, such as the study of the GW waveforms expected from different astrophysical sources, in particular from the coalescence of supermassive black holes, within the framework of GR or alternative theories. The results of these studies are of direct relevance for the design of the LISA mission.
The second payload package, the Disturbance Reduction System (DRS), was developed in the United States by JPL (Jet Propulsion Laboratory) under the leadership of NASA. It provides a system of micro-Newton thrusters complementary to the onboard LTP technology with dedicated control electronics.
On 22 February 2016 the two cubes housed in the LPT of LPF were left to move under the effect of gravity alone, and one day later the spacecraft’s main operating mode was switched on for the first time. The LPF scientific mission started officially on 1 March. Based on only 55 days of science operations LPF could already demonstrate that the key technologies needed to build a space-based GW observatory are properly working . It turned out that the two test masses are freely falling under the influence of gravity alone, unperturbed by other external forces, to a precision more than five times better than originally required. The two cubes are almost motionless with respect to each other, with a relative residual acceleration lower than 10-14 g, with g being Earth’s gravitational acceleration. For the remaining forces acting on the test masses, three main sources of noise, depending on the frequency, were identified (see Fig. 4).
At the lowest frequencies accessible to the experiment, below 1 mHz (on the left on the graph in Fig. 4), one measures a small centrifugal force acting on the cubes, which is caused by a combination of the shape of LPF’s orbit and the effect of the noise in the signal of the startrackers used to orient it. In the graph (Fig. 4) the contribution of the centrifugal force to the relative acceleration of the two test masses has been subtracted. Further investigations are under way to better identify the source of the residual noise after subtraction.
At frequencies in the range 1 - 60 mHz (at the centre of the graph in Fig. 4), the control over the test masses is limited by gas molecules bouncing off the cubes: a small number of them remain in the surrounding vacuum. This effect was seen to be reducing as more molecules were vented into space, and it is expected to lower further in time.
At higher frequencies, between 60 mHz and 1 Hz (on the right of the graph in Fig. 4), LPF’s precision is limited only by the sensing noise of the optical metrology system used to monitor the position and orientation of the test masses. Nicely, the performance of this system has already surpassed the level of precision required by a future gravitational-wave observatory by a factor of more than 100. The scientific operations of LPF will continue until 31 May 2017..
The demonstration of the LPF’s key technologies opens the door to the development of LISA, which will be capable of detecting gravitational waves emanating from a wide range of objects in the Universe. In November 2013 ESA has selected The Gravitational Universe  as the science theme to be explored by ESA’s Large class mission L3. The suggested realization of the L3 mission is, after the success of LPF, certainly LISA, which at present is scheduled to be launched in 2034. The scope of LISA is to detect and study low-frequency GW from about 0.1 mHz to 1 Hz, and thus to complement ground-based gravitational observatories. LISA opens new possibilities for astrophysical studies by allowing, for instance, to detect supermassive black holes (typically of 106 - 107 M☉) merging at cosmological distances. Mergers of a supermassive black hole with another compact object (such as another black hole or a neutron star) produce a very clean GW signal which LISA will be able to measure with high precision. Alternative gravity theories influence the dynamics of such mergers and hence LISA is expected either to directly see the imprints of certain alternative theories or to put severe constraints on them. Another class of objects, which will be observed by LISA, are ultra-compact binaries, in particular of white dwarfs in our Galaxy. They are important sources of gravitational waves in the mHz frequency range. Moreover, it will be possible to detect or put strong constraints on the primordial GW background, which is just, as the cosmic microwave background, a leftover from the Big Bang.
On 14 September 2015 the two LIGO detectors simultaneously observed a transient gravitational wave signal, which has been interpreted as due to the merger of two black holes with masses of about 36 M☉ and 29 M☉, respectively. This being the first direct detection of GW , which was announced last 11 February. In the data of the first Advanced LIGO run at least one more GW event has been found [4, 5]. The year 2016, 100 years after Einstein’s first paper on GW as a consequence of his theory of General Relativity [6, 7], has thus seen dramatic advancements in the field of GW. It is likely that given these developments, the discovery of GW and the impressive performance of LPF, the launch of the LISA mission could be anticipated by some years.
 The LISA Pathfinder collaboration, M. Armano et al. Sub-Femto- g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results, Phys.Rev.Lett. 116 (2016) no.23, 231101
 eLISA Collaboration, P. Amaro Seoane et al., The Gravitational Universe, arXiv:1305.5720
 LIGO Scientific and Virgo Collaborations (B. P. Abbott (Caltech) et al.), Observation of Gravitational Waves from a Binary Black Hole Merger, Phys.Rev.Lett. 116 (2016) no.6, 061102
 LIGO Scientific and Virgo Collaborations (B. P. Abbott (LIGO Lab., Caltech) et al.), GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence, Phys.Rev.Lett. 116 (2016) no.24, 241103
 LIGO Scientific and Virgo Collaborations (B. P. Abbott (LIGO Lab., Caltech) et al.), Binary Black Hole Mergers in the first Advanced LIGO observing run, arXiv:1606.06096
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[Released: October 2016]