The environmental relaxation allows the LPF spacecraft to be in orbit around L1, as opposed to LISA's heliocentric Earth-trailing orbit. In particular, this relaxation is most evident on the thermal stability, gravitational balancing and magnetic cleanliness of the spacecraft. The relaxation in the acceleration noise requirement significantly reduces the environmental requirements levied on the spacecraft. The line labelled LPF CBE is the measured performance of the EM interferometer (optical bench, laser, phase meter). As can be seen, the LISA Pathfinder requirement on the performance of the readout interferometer is approximately equal to the performance of the LISA local (test mass readout) interferometer. The graph on the right side is showing the Level One Displacement Noise requirements.The gap between the LPF CBE line and the LISA Requirement represents the extrapolation required to transfer the LPF technology to LISA. The line labelled LPF CBE shows the Current Best Estimate of the expected performance of the mission. The line labelled LISA Pathfinder Requirement shows the value as listed in the Science Requirements Document.

The graph on the left side is showing the Level One acceleration noise requirement of LISA Pathfinder and LISA.7) 8)įigure 5: Relative acceleration noise requirements of the LISA Pathfinder and LISA test masses (image credit: NASA, ESA) Plans call for LISA's trio of spacecraft to orbit the sun at the same distance as Earth, but trailing about 50 million km in orbit behind our planet (representing an angle of about 20º with respect to the Earth-Sun direction). In this concept, the three LISA spacecraft constellation form an equilateral triangle (a giant interferometer) with an armlength of 5 million km, inclined by 60º against the ecliptic (Figure 2). With the detection of these waves, which for example originate from black holes in close orbit around each other, an entirely new window to the universe will be opened. The goal of the LISA fundamental physics mission is to detect low-frequency gravitational waves in the range of 10 -4 to 10 -1 Hz (according to Einstein's Theory of General Relativity the force of large gravity changes, generated by a collapsing star or an entire galaxy, will produce tiny ripples of gravity waves) - requiring technologies that have so far never been tested. LISA Pathfinder is a necessary precursor mission for LISA, the joint ESA and NASA formation-flying mission (launch planned for 2034) of three spacecraft. LISA Pathfinder will not only pave the way for LISA, but will also demonstrate the main assumption of the thought experiment: that free particles follow geodesics. This is the basis for all gravitational wave detectors, e.g. Legend to Figure 1: The top left image shows the classical Einstein thought experiment to measure the spacetime curvature. Figure 1: LISA Pathfinder experiment concept (image credit: ESA)

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