LISA Pathfinder

at the University of Trento

An orbiting experiment to demonstrate sub-femto-g free-fall

How can an experiment demonstrate that sub-femto-g differential acceleration measurements at 1 mHz will be reliably achieved with the LISA observatory?  And, equally important, how can we demonstrate that all realistic noise sources are under control even at the lower frequencies (0.1 mHz and below) and higher sensitivity needed for LISA? 

As LPF approached its December 2015 launch from the ESA Kourou site towards its science orbit around the L1 Lagrange point the PI team in Trento is focused on the final experimental design and analysis techniques needed to answer these questions.  In addition to the main measurement of the acceleration noise between the two TM, we are a wide array of dedicated tests to quantitatively estimate different effects that can limit a precision differential acceleration measurement, including:

Together with our colleagues around Europe and in the US, we are preparing the measurement procedures and “real time” data analysis tools needed to run LPF as an orbiting laboratory and maximize the science return for LISA and precision gravitational experimentation in the future. 

Designing and prototyping Gravitational Reference Sensors for LISA and LISA Pathfinder

In LISA and LISA Pathfinder, the test masses are surrounded by electrostatic sensing and actuation devices, known collectively as the Gravitational Reference Sensor or GRS.  The GRS has three main responsibilities in the science phase of the mission:

The GRS is a contribution from the Italian Space Agency.  At the University of Trento we developed the original sensor design, and we built and tested several prototypes, before collaborating with the CGS (Milano) aerospace company in their successful realization of the complete flight GRS hardware. 

The GRS design includes:

The LPF GRS was designed for the demanding needs of the LISA observatory, and LPF thus represents a flight verification of a crucial part of the LISA apparatus.

Probing the limits of free-fall in the lab with femtoNewton torsion pendulums

A ground test of free-fall cannot reach the full LISA/LPF sensitivity at the relevant time scales of 1000 or more seconds.  However, a lightweight (hollow) LISA-like TM suspended as a torsion pendulum element inside an LISA-like GRS electrode housing provides a test bench for surface forces, which are perhaps the most threatening to the LISA sensitivity and hardest to analyse from first principles.

In Trento we pioneered efforts to measure the fN surface forces relevant to free-fall inside realistic GRS hardware prototypes and have reached sensitivities that allow placing an upper limit of roughly 2 fm/s2 for the random rms amplitude of TM acceleration from surface forces at 1 mHz.  This is within a factor 2 of the LPF goal, and was critical in demonstrating the readiness for a space mission.  It also represents the current state of the art in small force measurement for the class of 100 g test-bodies. 

December 10th, 2001: Kip Thorne and other representatives of ESA and NASA during a visit at our torsion pendulum facility at the Physics Department of the University of Trento

Our lab measurements have also measured a number of key force noise sources for LISA test masses, including:

The torsion pendulum lab represent a critical ground companion to complete the physical model of small force disturbances that will be a legacy of the LISA Pathfinder mission.