High-Accuracy Mass Estimation of Small Asteroids for Planetary Defense using the Gravity Imaging Radio Observer (GIRO)

Accurate mass determination of small, sub-kilometer asteroids remains a critical yet unsolved challenge in planetary defense, particularly for newly discovered Potentially Hazardous Asteroids (PHAs) requiring rapid response. This work investigates the Gravity Imaging Radio Observer (GIRO), an innovative instrument concept based on inter-satellite Doppler tracking between a host spacecraft and small radio probes, combined with high-precision optical imaging using the Advanced Pointing Imaging Camera (APIC). We assess GIRO’s performance in a simulated flyby mission targeting asteroid 2024 YR4, showing that a compact, beacon-based architecture can achieve formal 1$\sigma$ uncertainties better than 20% in mass estimation for a 60-meter-class object, assuming a Bennu-like density and a flyby at 50 m altitude and 5 km/s relative velocity. The analysis shows that most of the information content arises from post-encounter differential deflections between the host spacecraft and beacons, allowing robust estimation even in scenarios with limited data availability around closest approach. Sensitivity studies confirm that the technique is resilient to variations in flyby geometry and beacon release strategy, enabling accurate mass recovery for a wide range of asteroid sizes, flyby velocities, and operational constraints. These findings position GIRO, in combination of a capable imager, as a highly promising candidate for planetary defense and reconnaissance missions requiring rapid deployment, minimal complexity, and reliable scientific and engineering return.