Testing Gravity at Encounter Speed: Geometric Prediction of Earth Flyby Anomalies and the GRACE Absorption Hypothesis

When spacecraft swing past Earth to change their trajectory (a routine maneuver called a gravity assist), precise radio tracking sometimes reveals that the spacecraft's speed after the encounter does not match what physics predicts. This mismatch, called the flyby anomaly, has been observed in several missions since 1990 but remains unexplained after 35 years of investigation. We analyze all twelve documented Earth flybys (1990–2013) using a geometric scoring function, S, that combines three measurable quantities: how asymmetrically the spacecraft's path crosses Earth's equatorial plane, how close the spacecraft comes to Earth, and how fast it is traveling. This score uses only pre-encounter orbital geometry with no mission-specific tuning. Three globally fixed parameters are applied identically to all twelve flybys. The score correctly classifies eleven of the twelve: every confirmed null is predicted null, and every confirmed anomaly is predicted anomaly. It also predicts the correct sign (speed increase or decrease) for all five detected cases. However, the score produces one apparent contradiction: the 2013 Juno flyby scores higher than the 1990 Galileo I flyby, yet Juno showed no anomaly while Galileo I showed a clear one. We propose a testable explanation. The high-precision gravity maps of Earth built from the GRACE satellite mission (2002–2017) may have unknowingly absorbed the flyby effect into their mathematical description of Earth's gravity field. This would cause modern trajectory analyses to subtract it out as "normal gravity." We describe four concrete tests that could confirm or refute this hypothesis and note that the critical test, reprocessing historical flyby data with pre-GRACE gravity models, has never been performed.