Recent determination of Gamma with Cassini M.T. Crosta, F. Mignard CNRS-O.C.A. 5th RRFWG, June, Estec

What is Cassini Huyghens-Cassini, NASA-ESA–ASI mission, launched in 1997 The mission is due to explore the Saturnian system

The determinations of  1. Deflection of the light 2. Time delay of radio signal Doppler frequency shift, interplanetary tracking 3. Doppler frequency shift, interplanetary tracking different manifestations of the same aspects of the solar gravitational field

The Doppler observable The observable is the fractional frequency shift of a stable and coherent two-way radio signal The round-trip time to and from the spacecraft changes as a result of the motion of the Earth and Cassini relativistic correction in the relative velocity along the line of sight Shapiro time delay

The measurement Relativistic contribution to the Doppler shift impact parameter: small values of b are important !

The Cassini experiment Alignment of the Earth, the Sun and Cassini between 2 June to 7 July 2002, distance Cassini-Earth  8.43 AU

The Cassini antennas a.Cassini’s 4-meter antenna b. new ground station at the NASA Deep Space Network (DSS25, 34-meter diameter antenna)

Why the Doppler method Sophisticated radio system, handling at the same time carriers in X ( GHz) and Ka (32-34 GHz) bands (additional Ka-band downlink locked to the X phase on board) A multi-frequency link with which 3 different phases are measured at the ground station Drastical reduction of: 1. the solar coronal plasma noise (four orders of magnitude smaller than ) 2. the noise from the Earth troposphere

Other contributions to y gr Cassini is a stable platform, subject only to small and steady non-gravitational accelerations: the solar radiation pressure the anisotropic thermal emission on board (3 on-board radioisotope generators)

The gravitational signal peak value in the cassini solar conjunction, Two-ways relativistic frequency shift

The result Cassini-Earth-Sun conjunction has allowed to estimate gamma in agreement with GR ( B. Bertotti, L.Iess & P.Tortora, Nature, 425, 2003) The precision is about 10 times better than its present best limit by VLBI measurements (Phys. Rev.D 66, ,2002):

The physical meaning If the universe is evolving, today we can expect a very small discrepancy of the order A remnant of a long range scalar field would violate General Relativity and also the assumptions in the equivalence principle (i.e. the constants of microphysics)

The physical conclusion the exact amount of the violations depends on the particular tensor-scalar theory adopted no violation of General Relativity has been detected The accuracy of the Cassini gamma experiment sets a constraint on the cosmological scenario

Impact of GPB Gravity Probe B was launched on 22 April 2004 Design to evidence : The geodetic precession The frame dragging (Lense Thirring effect) Monitoring of 4 ultrastable gyroscopes : With respect to a star (IM Peg) [  (µ) ~ 0.1 mas/yr !] Direction of the star with respect to local frame Direction of the spin of the gyros with respect to local frame Error budget per gyro 0.1 mas/yr from gyro drift 0.1 mas/yr from star proper motion uncertainty 0.2 mas/yr from readout noise

Geodetic and LT precessions 6.6 ’’/yr 42 mas/yr Frame dragging Geodetic prec. L (orbital) J (Earth) IM Peg gyro

Determination of  Frame dragging : Measurements at 0.1 mas/yr   should come at 3x10 -5 In March, IM Peg comes within 22° from the Sun Deflection of 22 mas Over few weeks one can use the gyros as reference directions   should come at 5x10 -3 Geodesic precession :