LISA Pathfinder

The LISA Pathfinder mission

LISA Pathfinder launched on December 3rd, 2015 and operated between March 1st, 2016 and July 17th, 2017.

UNITN LISA PATHFINDER : missione compiuta 12-09-2018 al Muse©2018 foto Romano Magrone

The LISA Pathfinder collaboration

Sixteen institutions, tens of scientists, one successful mission.

LISA Pathfinder at the University of Trento

The Experimental Gravitation Laboratory at the University of Trento worked towards the development of an orbiting gravitational wave observatory since the joint NASA – ESA studies for LISA in the 1990s.  We were the leading group for a metrology demonstrator mission since the approval of the LISA Technology Package onboard the original SMART-2 ESA mission in 2002, and S. Vitale played a role as the LISA Pathfinder mission Principal Investigator (PI). 

Gravitational wave observation from space and LISA

Observation of low-frequency gravitational waves from a space observatory such as LISA — proposed for the ESA Gravitational Universe  large mission (L3) theme selected for the 2030 time frame — will revolutionize astrophysics, opening a whole new window of discovery for the physics and astronomy of massive black holes at galactic centers, stellar mass compact objects, and the interactions among and between these disparate astrophysical populations. 

The gravitational waves produced by these distant sources produce a measurable time-varying differential force that on a constellation of free-falling test particles, a tidal deformation that alternately shrinks and expands their effective separation.

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As for terrestrial gravitational wave observatories like LIGO and VIRGO, an orbiting observatory will detect the relative acceleration of the test particles using laser interferometry, in the minute variations of the Doppler shift of a light beam passing between the test particles. 

In the proposed LISA observatory, test masses – roughly kg-sized metal cubes – will orbit inside three spacecraft at the vertices of an orbiting triangle with side length of 2.5 million km, with the relative changes in the triangle side length measured with Michelson laser interferometry.  The large interparticle separation – which amplifies the gravitational wave tidal acceleration – is only possible in space, which also isolates the test masses from the noisy gravitational forces near the Earth.  The test masses play the role of “end mirrors” in the interferometer as well as references of purely free-falling geodesic motion, allowing the gravitational wave accelerations to dominate over any other “spurious” disturbance forces. 

LISA seeks to detect test particle relative acceleration at the level of 10-16 m/s2 or smaller – 17 orders of magnitude smaller than g, the gravitational acceleration on the Earth. This corresponds, at the higher frequencies, to relative displacements of less than 1 picometer (1 pm or 10-12 m), or relative changes in the triangle side length – known as strain – of order 10-20.  These numbers represent the experimental challenge of gravitational wave astronomy. 

Einstein’s Geodesic Explorer: LISA Pathfinder

LISA Pathfinder was an ESA mission dedicated to performing a relative acceleration measurement between two geodesic reference test-masses in near-perfect free-fall.  The two free-falling test masses were inside a single spacecraft at 40 cm separation, and an optical interferometer measured their differential displacement.  

Its target precision of 3 x 10-14 m/s2/Hz1/2  – 30  fm/s2/Hz1/2 with 1 fm = 1 femto-meter – at 1 mHz, was within an order of magnitude of the LISA sensitivity goal and guarantees most of the LISA science return.  This also was a several order of magnitude improvement on the current state of the art for gravitational gradient measurements, provided by the ESA geodesy mission GOCE

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LISA Pathfinder represents an in-flight experimental verification of most aspects of the LISA relative acceleration measurement, as well as an orbiting laboratory for understanding the limits of the most ambitious measurements in space-time physics and of small forces.  Specifically, LISA Pathfinder demonstrated: