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Update on the LISA Pathfinder mission

LISA Pathfinder is a mission of the European Space Agency (ESA), which will test the technology needed to develop future space-borne gravitational wave detectors. LISA stands for 'Laser Interferometer Space Antenna'. One day it is hoped that the eLISA mission will observe gravitational waves in the quiet environment of space.

The core of the test package consists of two test masses, each 2 kg. To help ESA verify that such test masses are sufficiently non-magnetic, their d.c. volume magnetic susceptibility was measured in Sèvres in 2007 and again in 2012 using the BIPM susceptometer (Fig 1). We reported this in a previous BIPM news item which appeared shortly after the mission was launched on 3 December 2015.

Now the science mission has begun. This means that the science module is in its wide orbit around the L1 Earth-sun Lagrange point, about 1.5 million km from the Earth. Crucially: inside the module, the test masses have been released from their cages. The position of one of the test masses will serve as a reference which the surrounding module is made to follow (this requires actively cancelling out the extraneous external forces to which the module is subjected so that it will be "drag free"; the distance between the two test masses is unconstrained and will be monitored optically as a function of time. The goal is to set limits on the differential acceleration between the test masses, separated by 38 cm, to unprecedented accuracy in a region of space where the gravitational acceleration of Earth cancels the opposing gravity of the sun.

Prof. Stefano Vitale, Principal Investigator for the LISA PF technology package has summarized the present status of the mission for this issue of BIPM news: "We have now completed the commissioning of the LISA Pathfinder spacecraft and payload and are now in the Science Operations phase of the mission. The commissioning demonstrated that all subsystems are fully functional (all prime units are currently in use), and initial system level performance is fully in line with the commissioning-level expectations."

"Preliminary results from the science operations (i.e. residual differential acceleration between the test masses) are looking very good, and initial observations of the system behaviour are consistent with our expectations from ground testing. The science team are now in the process of running experiments to better understand the physics of the forces acting on a free-floating test mass. These investigations are expected to last for approximately 6 weeks, with initial results on the overall performance of LISA Pathfinder being made available by mid-April."

He added that "until we have consolidated the data analysis, we are not in a position to provide quantitative results."

Prof. Vitale visited the BIPM in 2006 to arrange our collaboration on the measurement of the magnetic susceptibility of the test masses.

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