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 β¯≡β-1 and γ¯≡γ-1 at the ${10}^{\mathrm{-}4}$ level and probe the time variability of Newton’s ‘‘constant’’ at the Ġ/G∼${10}^{\mathrm{-}13}$ ${\mathrm{yr}}^{\mathrm{-}1}$ 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 γ¯. 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.