Experimental validation of a high accuracy test of the equivalence principle with the small satellite "Galileo galilet"

The small satellite "Galileo Galilei" (GG) has been designed to test the equivalence principle (EP) to 10(-17) with a total mass at launch of 250 kg. The key instrument is a di. fferential accelerometer made up of weakly coupled coaxial, concentric test cylinders rapidly spinning around the symmetry axis and sensitive in the plane perpendicular to it, lying at a small inclination from the orbit plane. The whole spacecraft spins around the same symmetry axis so as to be passively stabilized. The test masses are large (10 kg each, to reduce thermal noise), their coupling is very weak (for high sensitivity to differential effects), and rotation is fast (for high frequency modulation of the signal). A 1g version of the accelerometer ("Galileo Galilei on the Ground"-GGG) has been built to the full scale-except for coupling, which cannot be as weak as in the absence of weight, and a motor to maintain rotation (not needed in space due to angular momentum conservation). GGG has proved: (i) high Q; (ii) auto-centering and long term stability; (iii) a sensitivity to EP testing which is close to the target sensitivity of the GG experiment provided that the physical properties of the experiment in space are going to be fully exploited.