Testing Relativistic Gravity to One Part per Billion

In 1859, Le Verrier discovered the mercury perihelion advance anomaly. This is a 10(-7)-part deviation from Newtonian gravity. This together with new theoretical ideas prompted the formulation of general relativity. In a similar way, will a test of relativistic gravity to 10(-7)-10(-9) prompt a new theory of gravity and probe significantly into the microscopic origin of gravity? We envisage that the time is ripe to test relativistic gravity to 10(-9) level of accuracy (1 ppb) within the next 25 years. Deep-space pulse and interferometric ranging holds such a key. This will ensure a revival of interest both in experimental and theoretical aspects of gravitation, and may lead to answer some profound questions about the origin of gravity and the cosmos.