A test of the equivalence principle using a space-borne clock

An experimental verification of Einstein's equivalence principle has been made using an atomic hydrogen maser in a space probe attaining an altitude of 10,000 km above the earth's surface. At the present stage of the data reduction, confirmation is at the 2 x 10-4 level of accuracy. The experiment and the resulting data are described including a comment on the limits to the anisotropy of the velocity of light. It is believed that this is the first direct, high-accuracy test of the symmetry of the propagation of light and a beginning in the use of high-accuracy clocks in space to measure relativistic phenomena. (author)