ИСТИНА

The gravitational redshift effect is one of the cornerstones of the experimental basis of general relativity as well as any metric theory of gravity. We present an experiment to test the redshift effect due to the gravitational fields of the Sun and Moon using the RadioAstron space radio telescope. We intend to extract the solar and lunar redshifts from residual delay rates obtained from correlation and post-correlation analysis of RadioAstron observations of bright extragalactic radio sources (quasars). In order to obtain the required accuracy of delay rate measurements we need to observe quasars with an uncorrelated flux density of at least ~ 100 mJy at the preferred wavelength of 6 cm. The quasars need to be observed at an angular separation of no more than 2-3 degrees off the Earth - RadioAstron line of sight in order to reduce the uncertainty of the spacecraft velocity contribution to the delay rate. For a one year long observational campaign we estimate the accuracy of the solar gravitational redshift test to be 0.2 %, which is a factor of 5 improvement over the current best test performed in 1989-1990 with the Galileo spacecraft. The lunar gravitational redshift, which has never been tested before, can be measured with an accuracy of 30 %. The experiment is complementary to gravitational redshift measurements in the field of the Earth and aims to search for source-dependent violations of Einstein's formula for the gravitational redshift.