Tracing Starship re-entries with infrasound: moving-source signatures in IMS observations

Summary In this study, we analysed infrasound detections of SpaceX Starship re-entries at International Monitoring System (IMS) stations in the Indian Ocean region, recorded during six controlled re-entries: IFT-5, IFT-6, IFT-10, IFT-11, IFT-12 and IFT-13. For each flight, different IMS stations detect coherent infrasonic energy from spatially separated positions along the descending trajectory, with back-azimuths and arrival times inconsistent with a single fixed acoustic source. Using a trajectory-constrained association framework combining PMCC back-azimuth measurements with publicly available telemetry, we demonstrate that the observed detection geometry is reproduced by treating the re-entry as a spatially extended, moving source. For IFT-5, three stations sample distinct source zones at altitudes of approximately 70, 41 and 18 km along a ∼3200 km trajectory segment. Geometric acoustic ray-tracing and parabolic equation modelling support interpretation of three distinct propagation regimes: thermospheric ducting, Mach-cone coupling and stratospheric waveguide leakage. The same spatiotemporal pattern repeats across these controlled re-entries, demonstrating that the moving-source signature is reproducible and intrinsic to the re-entry geometry. These results show that existing global infrasound infrastructure can resolve the spatial extent of large spacecraft re-entry trajectories.