  In 1859, Le Verrier discovered the Mercury perihelion advance anomaly. This
anomaly turned out to be the first relativistic-gravity effect observed. During
the 157 years to 2016, the precisions and accuracies of laboratory and space
experiments, and of astrophysical and cosmological observations on relativistic
gravity have been improved by 3-4 orders of magnitude. The improvements have
been mainly from optical observations at first followed by radio observations.
The achievements for the past 50 years are from radio Doppler tracking and
radio ranging together with lunar laser ranging. At the present, the radio
observations and lunar laser ranging experiments are similar in the accuracy of
testing relativistic gravity. We review and summarize the present status of
solar-system tests of relativistic gravity. With planetary laser ranging,
spacecraft laser ranging and interferometric laser ranging (laser Doppler
ranging) together with the development of drag-free technology, the optical
observations will improve the accuracies by another 3-4 orders of magnitude in
both the equivalence principle tests and solar-system dynamics tests of
relativistic gravity. Clock tests and atomic interferometry tests of
relativistic gravity will reach an ever-increasing precision. These will give
crucial clues in both experimental and theoretical aspects of gravity, and may
lead to answers to some profound issues in gravity and cosmology.
