ORBITAL TESTS OF RELATIVISTIC GRAVITY USING ARTIFICIAL-SATELLITES

We reexamine non-Einsteinian effects observable in the orbital motion of low-orbit artificial Earth satellites. The motivations for doing so are twofold: (i) recent theoretical studies suggest that the correct theory of gravity might contain a scalar contribution which has been reduced to a small value by the effect of the cosmological expansion; (ii) presently developed space technologies should soon give access to a new generation of satellites endowed with drag-free systems and tracked in three dimensions at the centimeter level. Our analysis suggests that such data could measure two independent combinations of the Eddington parameters betaBAR = beta - 1 and gammaBAR = gamma - 1 at the 10(-4) level and probe the time variability of Newton's ''constant'' at the G/G approximately 10(-13) yr-1 level. These tests would provide well-needed complements to the results of the lunar laser ranging experiment, and of the presently planned experiments aimed at measuring gammaBAR. In view of the strong demands these tests make on the level of nongravitational perturbations, further analysis is needed to determine the level of precision which could be realistically achieved in a dedicated mission consisting of an optimized passive drag-free satellite.