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the creation of a physical body that provides the best possible approximation to the free motion of a test particle space - time geodesic is a profound goal for experiments dedicated to the study of space - time geometry in the vicinity of external gravitating bodies , this enabling high precision tests of general relativity and alternative theories of gravity to be carried out .
there are important issues regarding the approximation to a geodesic that are being addressed by the motion of an actual extended body . on the one hand , in general relativity @xcite ,
the problem of an extended body is subtle due , not only to the non - linearity of the equations of motion , but also because of the need to deal with the internal structure of a compact body , constructed of continuous media , where kinetic variables and thermodynamic potentials , are involved , and where there may be intrinsically non - local effects arising from the internal structure of an extended body , such as tidal ones .
moreover , there are problems concerning approximations that need to be made in order to describe a given extended body as a test particle moving along a geodesic , these being related with the fact that many of the common newtonian gravitational concepts such as centre of mass , total mass , or size of an extended material body do not have well defined counterparts in general relativity @xcite .
the ehlers - geroch theorem @xcite , which generalized an earlier result by geroch and jang @xcite , provides a justification , under appropriate conditions , for the trajectory of an extended body , having a limitingly small gravitational field of its own , to be a geodesic :    _ a timelike curve @xmath0 on a 4-manifold of lorenzian metric @xmath1 must necessarily be a geodesic , if for each closed neighborhood @xmath2 of @xmath0 there exists , within each neighborhood of the metric @xmath1 ( with accompanying connection ) in @xmath2 , a metric @xmath3 whose the einstein tensor @xmath4 ( @xmath5 being the ricci tensor and @xmath6 the scalar curvature of @xmath3 ) satisfies the dominant energy condition in @xmath2 , is non - zero in @xmath2 , and vanishes on @xmath7 . _
the conditions of the theorem are not specific to any equations of state that the material of a test body might be subject to , beyond its satisfaction of the dominant energy condition .
this theorem , asserting that _ small massive bodies move on near - geodesics _ , thus achieves a rigorous bridge from general relativity to satellite experiments .
this suggests a high level of suppression of non - gravitational and self - gravitational effects from the satellite s own small gravitational field , enabling us to consider that it has a very nearly geodesic motion and , hence , providing a genuine testing ground for general relativity effects .
laser relativity satellite ( lares ) @xcite was launched on february 13 , 2012 from the european space agency s spaceport in kourou , french guiana .
that launch also marked the first qualification flight of the new vehicle - rocket vega of the european space agency .
the satellite is a tungsten alloy sphere of 18.2 cm radius and mass 386.8 kg ( fig .
1 ) , covered by 92 retro - reflectors to reflect the laser signals from the international laser ranging service stations .
the orbit is circular to high accuracy , the eccentricity is 0.0007 , and inclination 69@xmath8.5 at semi - major axis 7820 km .
the efficiency of lares is in the simplicity of its design , aimed at the suppression of non - gravitational orbital perturbations including atmospheric drag and the anisotropic thermal radiation from its surface due to the anisotropic heating of the sphere due to the solar and earth s radiations ( yarkovsky effect ) : it has the smallest ratio of cross - sectional area to mass of any other artificial satellite , including the previous two laser geodynamics satellites ( lageos ) and a single piece structure .
( lageos had a heterogeneous structure . )
the experience with the lageos satellites had proved the efficiency of laser ranging passive satellites for testing the frame - dragging effect predicted by general relativity @xcite .
an important fact was the availability of the improved earth s gravity models of grace , providing the possibility of the accurate evaluation of their spherical harmonic coefficients .
the obtained behavior for the residuals of the nodes of the lageos 1 and 2 confirmed the lense - thirring effect @xcite , i.e. the rate of drag of the longitude of the nodes @xmath9 to an accuracy of the order of 10% @xcite , where * j * is the angular momentum of the central mass , @xmath10 and @xmath11 are the semi - major axis and the eccentricity of the orbit of the test particle , respectively .
the analysis of the first months of measurements @xcite confirmed the efficiency of the design of lares , even though its orbit is lower than that of both lageos : the acceleration due to the non - gravitational perturbations for lares achieves 2 - 3-times improvement with respect to lageos .
it has to be stressed that until the launch of lares , the two lageos satellites had the smallest residual acceleration of any other artificial satellite .
the creation of a general relativistic test particle moving along a geodesic in the earth s space - time gravitational field has to be considered as a principal achievement for high accuracy testing of any relativistic effect .
these studies are able to constrain possible extensions of general relativity such as the chern - simons modified gravity ( e.g. @xcite ) , thus bridging earth - vicinity measurements to issues of dark energy and cosmology .
penrose r. , structure of spacetime , in : battelle rencontres , ( c.dewitt and j.a.wheeler , eds . ) , new york , 1968 .
ciufolini i. , wheeler j.a . , gravitation and inertia , princeton university press , princeton , 1995 .
rindler , w. , relativity : special , general , and cosmological , oxford university press , oxford , 2001 .
ehlers j. in : relativity , astrophysics and cosmology , ( w.israel ed . ) , 1 - 125 , reidel publ .
ehlers j. , geroch r. , annals of phys . 309 ( 2004 ) 232 .
geroch r. , jang p.s .
, j.math.phys . 16
( 1975 ) 65 ciufolini i. , paolozzi a. , pavlis e.c . , ries j.c . , koenig r. and matzner r. , in : general relativity and john archibald wheeler , ( i.ciufolini and r.matzner eds . ) , 467 - 492 , springer , 2010 .
ciufolini i. , nature , 449 ( 2007 ) 41 .
lense j. , thirring h. , phys.zeitschr .
19 ( 1918 ) 156 .
ciufolini i. , paolozzi a. , pavlis e. , ries j. , gurzadyan v.g .
, koenig r. , matzner r. , penrose r. , sindoni g. , lares , the test - particle for gravitational physics and general relativity , eur .
j. plus 127 ( 2012 ) 133 ; arxiv:1211.1374 .
yagi k. , yunes n. , tanaka t. arxiv:1206.6130 .
according to general relativity , as distinct from newtonian gravity , motion under gravity is treated by a theory that deals , initially , only with test particles . at the same time , satellite measurements deal with extended bodies . 
 we discuss the correspondence between geodesic motion in general relativity and the motion of an extended body by means of the ehlers - geroch theorem , and in the context of the recently launched laser relativity satellite ( lares ) . 
 being possibly the highest mean density orbiting body in the solar system , this satellite provides the best realization of a test particle ever reached experimentally and provides a unique possibility for testing the predictions of general relativity .