Tracking the untrackable: Reconstructing a Soyuz-2.1b re-entry trajectory using inter-disciplinary observations

When launch vehicles rise through the atmosphere and jettison their stages to place spacecraft in orbit, the first few stages often fall back down to Earth into areas bounded by Notice to Air Missions (NOTAMs). These are known as scheduled space debris re-entries. The amount of scheduled debris, and therefore NOTAMs, depends on the launch vehicle and target orbit. The true impact areas of scheduled debris are often unconfirmed due to a global lack of observational coverage below 200 km. A suite of observational capabilities exist within Australia, though none are specifically designed or designated for re-entries, they incidentally observe these events as they are carrying out their primary functions. At 13:20:00z on 7 August 2023, a Soyuz-2.1b was launched from Plesetsk Cosmodrome, carrying a first-of-its-kind COSMOS 2569 navigation satellite. 36 min after launch, the Desert Fireball Network observed a fireball over Victoria, Australia. Seismic stations from the Australian National Seismograph Network and infrasound arrays from the Australian infrasound network observed ballistic trajectory signals over Victoria and Tasmania. This collection of interdisciplinary observations provided a unique opportunity to reconstruct the trajectory and assess energy deposition of a scheduled re-entry using our methods designed for meteors, such as the Dynamic Trajectory Fit method. We estimated an overall average surviving mass of 1980 kg, with gradual fragmentation over the Bass Strait that separated the object into at least three main fragments. We verified that the object splashed down within the NOTAM danger area, 24 km off the Tasmanian coast. • Scheduled space debris re-entries are poorly understood due to limited global observations below 200 km. • A Soyuz-2.1b second-stage re-entry was incidentally observed over Australia using optical, seismic, and infrasound networks. • Existing Australian infrastructure and methods can be repurposed to detect and analyze space debris re-entries. • Optical, seismic, and infrasound data enabled reconstruction of the Soyuz-2.1b re-entry trajectory and energy deposition assessment using methods designed for meteors. • Observations allowed multiple source verification of the accuracy and reliability of a Notice to Air Mission (NOTAM) danger area.