““Galileo Galilei” (GG)Galileo Galilei” (GG)

GG Phase A-2 Study funded by ASI ongoing at TAS-I (TO) with support from GG/GGG scientists

GG target: test the Equivalence Principle to 10-17 (4 orders of magnitude gain)

Low altitude equatorial orbit. Operation from Italian station in Malindi (Kenya)

Passive stabilization by 1-axis rotation (cylinder height: 1.5 m)

GG inside Italian Vega launcher (double launch possible; Kourou launch site)

TAS-I has built GG Drag Free Control System + Space Experiment Simulator based on heritage as prime contractor of ESA mission GOCE - now flying and performing at 2x10 -9 ms-2/Hz (from 2 mHz to 0.2 Hz)

+ relevant input data taken from GGG lab experiment

GG Phase A-2 Study Report under completion (to include all mission costs except launch)

Strategy: Exploit Italian launcher, rely on TAS-I heritage from GOCE, rely on GGG lab prototype, keep satellite cost very low….

GGG prototype: results till 2008GGG prototype: results till 2008__ July 2005

__ Sept 2007

__ June July 2008

__20-21 Oct 2008

Good sensitivity at very low frequencies

Improvement in sensitivity

10-8 m GG in space must sense about 10-12 m relative displacements at 1.75x10-4 Hz to reach target (EP test to 10-17).

GGG has measured 3x10-9 m at 1.7x10-4 Hz and 6x10-9 m at diurnal frequency. With GGG coupling (not as weak as it will be in possible in absence of weight) these results correspond to:

sun=2.5x10-7 Earth GG orbit=7.7x10-11

Vacuum chamber (second hand..) has wrong symmetry…

New chamber (built) for an advanced GGG (under construction) - ASI fundsNew chamber (built) for an advanced GGG (under construction) - ASI funds

New chamber has the right symmetry and has been designed to minimize disturbances on GGG

New GGG will be suspended inside chamber by cardanic joint (not rotating) to reduce low frequency terrain tilts passively, in addition to active tilt control now in use (Note: active tilt control is limited by thermal effects on tilt sensor and requires good thermal stabilization to be effective)

An Experiment Simulator will be built by Thales Alenia Space-Italy for the new GGG, similarly to the Simulator built for the space experiment, to be compared with experimental measurements …

Old GGG mounted in new chamber

Assessing the lower platform noise provided by the new chamberAssessing the lower platform noise provided by the new chamber

May 2007: 2-day run in old chamber; no thermal stabilization, no tilt control.

Relative displacements of test cylinders: 40 m peak-to-peak

May 2009: 4-day run in new chamber; no thermal control, no tilt control. Relative displacements of test cylinders: 6 m peak-to-peak

The new vacuum chamber provides a more stable and quite environment for the GGG rotating differential accelerometer by a factor 6 to 7

Time (sec)

Thermal stabilization of new chamberThermal stabilization of new chamber

Thermal stability is required primarily because the tilt sensor used in the control loop to reduce low frequency (diurnal) terrain tilts is affected by thermal variations

The body of the chamber is stabilized by heating only (to 30 °C)

FFT of temperature inside chamber at tiltmeter level: diurnal variation reduced to 0.01 °C

First runs in new chamberFirst runs in new chamber

Old chamber, thermal stabilization by heating + cooling; tilt control with online compensation for temperature effects on

tilt sensor

New chamber, thermal stabilization by heating only; tilt control without online compensation for temperature effects on tilt sensor

Results are already comparable but runs in new chamber are still being improved

Direct measurement of electric patch effects on GGG Direct measurement of electric patch effects on GGG

Patches of charges on the surface of test masses are known to be a threat for gravity experiments. Have affected GP-B mission preventing so far the recovery of Lense-Thirring effect (GP-B masses not grounded: UV electric discharging, similar in LISA and LISA PF)

4 cm2 Al plate used to excite patch (at about 10-2 Hz); no gold coating….

GG/GGG masses grounded; GG co-rotating makes patch effects mostly DC

We have implemented a method to measure patches of charge in GGG during experiment runs: patch charges (if present) are excited to produce a differential displacement of the test cylinders at a desired frequency. The method allows us to measure Vpatch rigorously and to asses the effect on the test masses

Measured: Vpatch= 0.3 V

x= 0.0275 micron

y = 0.006 micron

In GGG, beware of patch effects between rotating and non-rotating parts, as they could produce low frequency effects….

Development of a high sensitive double pendulum tiltmeter for low frequency tilt controlDevelopment of a high sensitive double pendulum tiltmeter for low frequency tilt control

Simple pendulum and inverted pendulum aligned, pendulum masses connected by thin cantilever, knife edge suspensions long period, high sensitivity

T

Capacitance pick up

Symmetry, same material (pendulum + structure), thermal expansion/contraction effects dominant in one (vertical) direction should minimize distortions and spurious tilts

22

10

100 (50) 2500

1 10

dp eq sT s L L m

x m rad

11min minmin min

4 1

0.06 2.5 102

eq

C pF d mm

C xx d m rad

C L

(Construction drawings and electronics ready)