Planetary Radar Ranging Residuals

High-precision radar ranging to solar system bodies — Mars, Venus, Mercury, and the asteroid belt — provides the most accurate measurements of solar system distances and the astronomical unit, underpinning the entire terrestrial calibration of the distance ladder. The VLBI and ranging programs at JPL, ESA, and others have achieved centimeter-level precision on planetary distances over decades of monitoring. These measurements are analyzed with solar system ephemerides that assume standard Newtonian gravity plus post-Newtonian corrections from general relativity. When the residuals between observed and modeled ranges are examined in detail, small but systematic trends appear at the level of a few centimeters — deviations that are not fully accounted for by known perturbers, non-gravitational forces, or systematic calibration errors.

While these residuals are small in absolute terms and have not yet been attributed to cosmological causes, they probe the same fundamental question: whether the local gravitational framework matches the predictions of ΛCDM-embedded general relativity to the highest currently achievable precision. Any unexplained secular drift in planetary distances would suggest either a time variation in Newton's gravitational constant G, a modified gravitational theory, or a coupling between local dynamics and the cosmic expansion — all of which are outside the standard ΛCDM framework. The solar system is often treated as a "safe" regime where ΛCDM is not being tested, but the increasing precision of ranging measurements means that even picometer-level systematics can probe cosmological-scale physics through the consistency of the overall framework.