PPN Gamma Results from the NEAR Conjunction

On February 19, 1997 the Near Earth Asteroid Rendezvous (NEAR) spacecraft passed solar conjuction en route to an encounter with the asteroid Mathilde. The X-band ranging system on board made it possible to obtain data to within four solar radii and reduced the effect of plasma delay by an order of magnitude relative to the previous best space test of gravitational delay from the Viking landers. The range precision was one meter, though unmodeled plasma fluctuations and spacecraft position noise limited the data to an average accuracy of 25 meters. The data were fit to a trajectory model that included stochastic parameters to account for solar plasma fluctuation and spacecraft jitter. In addition, a parameter that scales the amount of gravitational time delay, PPN parameter gamma, was added to the solution. The Parameterized Post Newtonian (PPN) Formalism is a formalism that encompasses most metric theories of gravity. The PPN parameter gamma, which describes the amount of space curvature produced by a rest mass, is predicted to be 1 by General Relativity. Results for gamma from the NEAR data are consistent with this prediction with an uncertainty less than 0.1 percent.