Researchers present a novel way to track errant space debris as it falls to Earth in near-real-time, according to a new study. Their method uses ground-based seismic sensors. Over the last several years, the number of spent spacecraft and other debris reentering Earth’s atmosphere has grown exponentially. These uncontrolled reentries pose increasing risks to human life, infrastructure, and the environment. As Earth’s orbit grows increasingly crowded and reentries become more frequent – potentially involving spacecraft carrying toxic, flammable, or radioactive materials – these risks are expected to become more of a concern. However, predicting an object’s reentry timing and trajectory is extremely difficult, and existing ground-based radar and optical tracking systems struggle to monitor space debris once it begins to disintegrate in the atmosphere. These limitations complicate response planning and mitigation efforts. Thus, there is a need for tools that can rapidly determine the trajectory, size, composition, and potential impact locations of falling space debris in near real time.

To fill this gap, Benjamin Fernando and Constantinos Charalambous demonstrate a new method that uses publicly available data from ground-based seismic sensors to detect the shockwaves, or sonic booms, of reentering debris. Fernando and Charalambous tested their approach using the April 2024 reentry of the large and heavy Shenzhou-15 orbital module, which had been left in a decaying orbit that regularly passed over major population centers across six continents. Using seismic data from sensors across Southern California and Nevada, the authors analyzed the sonic booms from Shenzhou-15’s reentry. (The observed reentry point for Shenzhou-15 was ultimately approximately 8,600 kilometers away from the Tracking and Impact Prediction estimate, which had pointed to reentry in the northern Atlantic Ocean.) By interpolating the arrival times of the largest of the shockwaves at different points across the region, Fernando and Charalambous were able to estimate the spacecraft’s ground track, speed and altitude. Moreover, the pattern of sonic booms revealed that Shenzhou-15 did not fall in a single explosive event but instead likely fragmented progressively into smaller pieces. This matched eyewitness reports and video footage. In addition to tracking inbound space debris, the authors argue that this type of near-real-time tracking could help rapidly determine the location of debris on the ground or the spread of smaller hazardous particles in the atmosphere, crucial for recovery and contamination mitigation. “Further research is needed to reduce the time between an object’s (re)entry in the atmosphere and the trajectory determination,” writes Chris Carr in a related Perspective. “Nonetheless, the method used by Fernando and Charalambous unlocks the rapid identification of debris fall-out zones, which is key information as Earth’s orbit is anticipated to become increasingly crowded with satellites, leading to a greater influx of space debris.”

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