Science Case for LISA Pathfinder

Paul McNamara

LISA Pathfinder Project Scientist

30th & 31st January 2007

LISA Pathfinder

SSAC 30th/31st January 2007

Why LISA Pathfinder?

The science case for LISA is extremely compelling and has continually been highly ranked by independent review boards

However, both ESA and NASA believed that the risk was too high to fly LISA with an unproven measurement concept

LISA Pathfinder was instigated by ESA to test the concept of low-frequency gravitational wave detection

The LPF development has shown that the technologies required for LISA are difficult, but not impossible– LPF has already solved many of the challenges associated with

low frequency gravitational wave detection

With LISA Pathfinder, ESA paves the way for LISA and With LISA Pathfinder, ESA paves the way for LISA and future fundamental physics missionsfuture fundamental physics missions

LISA Pathfinder

SSAC 30th/31st January 2007

Foundations of General Relativity

The most basic assumption of General Relativity is that free-particles follow geodesics unless acted upon by an unbalanced force

The definition of geodesics in Einstein’s “The Meaning of Relativity”

ALL missions aimed at demonstrating an aspect of GR rely on geodesicsLISA Pathfinder will be the first mission to demonstrate that free particles do indeed follow geodesics at such an unprecedented level– Requires that the free-particle is measured with respect to another

free-particle!

LISA Pathfinder

SSAC 30th/31st January 2007

Mission Concept

ParticleGeodesic

ParticleGeodesic

Light Ray

Light Ray

LISA Pathfinder

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Science demonstrated with LPF

Geodesic motion of free particlesPico-meter/nano radian laser metrology to free falling mirrorsNano-metre three-body “formation flying”– 2nd TM and spacecraft servoed to follow primary TM (to better than

10pm/Hz and 2nm/Hz at 1mHz respectively)

Materials physics– Test mass magnetic properties – Patch field effects– Thermal effects

Precision modelling of cosmic ray charging– Correlate test mass charge with models and on-board particle

detector

LISA Pathfinder

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Goal of LPF

To put test mass into (almost) perfect free-fall

Spurious accelerations kept to less than

Current best “on-orbit” performance by GRACE:

LPF will demonstrate ~TWO orders of magnitude better performance than has been, or is currently planned to be (with the exception of LISA), demonstrated on-orbit!– MICROSCOPE should demonstrate ~10-12ms-2/Hz at 1mHz

HzmsmHz

fxfSa

22

1421

31103

HzmsfSa2102

110

LISA Pathfinder

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Performance of LPF

LISA Pathfinder

SSAC 30th/31st January 2007

Comparison of missions

Mission Test-MassTest-Mass

EnvironmentTracking Method Orbit

Test-Mass geodesic motion

performance

(ms-2/Hz at 1mHz)

Drag-free (Residual

spacecraft accn)

(ms-2/Hz at 1mHz)

GRACEAccelerometer test-

masses (<100g)

<200mm gaps from electrodes.

Mechanical contact via grounding wire

Radio-link plus capacitive sensing

Low Earth orbit ~10-9 No

GOCEAccelerometer test-

masses (320g)

~300mm gaps from electrodes.

Mechanical contact via grounding wire

Capacitive sensing relative to s/c

Low Earth orbit. Drag-free 3x10-12 3x10-8

MicroscopeDifferential

accelerometer test-masses (<500g)

~200mm gap from electrodes.

Mechanical contact via grounding wire

Capacitive sensing relative to s/c

Low Earth orbit. Drag-free ~10-12 3x10-10

LISA PathfinderGravity reference

sensor test masses (Au-Pt 2kg)

4mm gaps from electrodes.

No mechanical contact

High resolution TM-TM interferometry

Interplanetary (L1). Drag-free 3x10-14 3x10-13

LISA Pathfinder

SSAC 30th/31st January 2007

LPF Legacy

The main output of LPF is the physics model of the free falling test mass and its environment– Model of the electromagnetic and gravitational interactions at play

when dealing with extremely small forces and low acceleration levels

This physical model will be used to extrapolate to the parameter ranges relevant to a host of other missions – e.g. LISA, Pioneer Anomaly, SSI, etc

The models (updated with on-orbit data) are designed to be plugged into the spacecraft/environment simulators of future missions

LISA Pathfinder

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Missions: Fundamental Physics [1]

LISA LATOR (Cassini)ASTROD Microscope (FEEPS)

LISA Pathfinder

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Future missions: Fundamental Physics [2]

Probable submission to the Cosmic Vision AO (among others in FP):

GAUGE

Pioneer Anomaly?

Fundamental Physics Explorer

LISA Pathfinder

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Missions: Earth Observation

GRACE (Launched 17/3/2002)GOCE (Launch 2007)

SSI (GRACE/GOCE follow-on mission)– Re-use of LTP inertial sensor AND

laser interferometry

LISA Pathfinder

SSAC 30th/31st January 2007

Conclusions

LISA Pathfinder will be the first mission with the specific goal of demonstrating geodesic motion of a free-particleLISA Pathfinder is the first step in the development of low frequency gravitational wave telescopesThe science and technology knowledge gained from LISA Pathfinder will be re-used in future missionsWith the descope of the NASA provided DRS, the LTP is the ONLY inertial sensor planned to be flown, and gives Europe a significant lead in this and related fields.

The LISA Pathfinder platform allows future fundamental physics missions to become reality

More information on the Science Case for LPF can be found in:LISA Pathfinder: Einstein’s Geodesic Explorer [ESA-SCI(2007)1]

LISA Pathfinder

SSAC 30th/31st January 2007

LTP on LPF