New Seismic Method Tracks Reentering Space Debris by Detecting Sonic Booms
Researchers from Johns Hopkins University and Imperial College London have developed a groundbreaking method for tracking reentering space debris by detecting sonic booms with ground-based seismometers. This innovative technique was successfully demonstrated using the April 2024 reentry of China's Shenzhou-15 orbital module, showcasing its potential to provide more accurate and near real-time trajectory information than current methods.
The approach aims to significantly improve the prediction of debris landing sites, assist in recovery efforts, and monitor the dispersal of potentially hazardous materials, enhancing overall space situational awareness.
The Growing Challenge of Space Debris
Space debris, consisting of old satellites and spacecraft components, reenters Earth’s atmosphere multiple times daily. These objects pose risks of environmental contamination from harmful substances released during atmospheric burn-up or potential collision if fragments reach the surface.
Current tracking methods, primarily relying on radar and optical tracking, face significant limitations once objects descend below a few hundred kilometers in altitude. At these lower altitudes, interactions with the atmosphere become chaotic, making accurate predictions difficult. Additionally, ground-based radar coverage is geographically sparse, and its data is not always immediately available globally.
Incidents such as inaccurate reentry predictions for a Chinese rocket body in November 2022 led to temporary airspace closures over parts of Spain and France, resulting in flight disruptions. The European Space Agency reported an estimated 1.2 million potentially hazardous pieces of space debris in Earth orbit as of April 2025. Concerns associated with reentering debris include the 1978 Soviet satellite Kosmos 954, which dispersed radioactive debris, and the 2025 SpaceX Starship rocket explosion, which scattered heavy metals. Researchers also highlight that reentries may alter atmospheric composition, as many chemicals within spacecraft are toxic and some possess ozone-depleting potential.
A New Method Using Seismic Sensors
A novel method has been developed to track uncontrolled reentries by utilizing existing networks of ground-based seismometers, instruments typically used for earthquake detection. This approach identifies sonic booms—shock waves produced when an object exceeds the speed of sound—generated by falling debris as it travels at supersonic or hypersonic velocities through the atmosphere.
The method was developed by Benjamin Fernando, a postdoctoral research fellow at Johns Hopkins University, and Constantinos Charalambous, a research fellow at Imperial College London. The technique drew inspiration from the NASA InSight mission, which employed a single seismometer to detect meteoroid impacts on Mars, successfully aiding in pinpointing impact locations.
Testing the Method with the Shenzhou-15 Reentry
To test the method, researchers analyzed the uncontrolled reentry of China’s Shenzhou-15 orbital module on April 2, 2024. The module, weighing over 1.5 metric tons and measuring approximately 2.2 meters (7.2 feet), reentered the atmosphere over Southern California and Nevada. (One source, however, identified the reentering object as an orbital module from China's Shenzhou 17 crew capsule.) Researchers accessed data from 125 to 127 seismometers within the publicly available Southern California Seismic Network and Nevada Seismic Network, which recorded vibrations consistent with sonic booms.
This seismic data allowed researchers to reconstruct the object's trajectory, speed, and altitude. The data indicated the module was traveling at speeds around Mach 25 to 30, aligning with its pre-entry orbital characterization. The intensity of the seismic readings provided insight into the module's altitude and fragmentation dynamics. Initially, the fall produced a single large boom signal, which later fragmented into multiple smaller signals, consistent with ground reports and video footage of the object's disintegration.
When comparing the seismometer-derived trajectory to projections made by the U.S. Space Force or U.S. Space Command based on radar data, conflicting reports emerged. One report stated the seismic-derived trajectory was 25 miles (40 kilometers) farther south than the projection, while other reports indicated the trajectory was approximately 25 miles (40 kilometers) north of the predicted path, passing over a populated region between Bakersfield, California, and Las Vegas, Nevada. The observed reentry point was approximately 8,600 kilometers away from the initial Tracking and Impact Prediction estimate.
Implications and Potential Benefits
This seismic tracking method offers several potential benefits for space situational awareness:
- Improved Accuracy: Provides more detailed information in near real-time, including precise measurements of reentry speed, altitude range, size, descent angle, and fragmentation timing.
- Debris Location and Recovery: Could assist in rapidly determining debris fall-out zones for objects that do not fully burn up, facilitating timely recovery operations. This is critical for incidents involving hazardous materials, such as the 1978 Soviet satellite Kosmos 954 or the 1996 Russian Mars 96 spacecraft, which involved radioactive components.
- Environmental Monitoring: Helps track the dispersal of potentially hazardous aerosol particulates and toxic chemicals released during breakup, some of which may have ozone-depleting potential.
- Understanding Reentry Dynamics: Offers insight into how space objects fragment during reentry, contributing to a deeper understanding of debris hazard mitigation.
- Cost-Effectiveness and Scalability: Utilizes existing, often publicly available, seismic sensor networks, making it a scalable and low-cost development.
- Complementary Tool: The method is envisioned as a complementary tool to existing radar, optical, and satellite tracking systems, enhancing overall safety and response efforts.
- Verification: Can help verify claims regarding the complete disintegration of satellite components, aiding in the assessment of risks to people, property, and aircraft.
Expert Perspectives
Hugh Lewis, a professor of astronautics at the University of Birmingham, who was not involved in the research, characterized the method as a "scalable, low-cost, and exciting new development" with the potential to enhance the understanding of spacecraft reentry.
Moriba Jah, a professor of aerospace engineering at the University of Texas at Austin, also not involved in the study, acknowledged the value of repurposing seismic networks. However, he cautioned that the method relies on strong shock waves and may not detect smaller debris or objects that disintegrate at high altitudes, suggesting it would serve as a useful complementary tool rather than a universal solution.
Davide Guzzetti, an associate professor of aerospace engineering at Auburn University, not involved in the study, underscored the importance of improved information gathering for timely recovery operations and for building a deeper understanding of how space activities affect society on Earth. He also observed that these measurements could provide insight into fragmentation dynamics.
Chris Carr, in a related perspective, noted that while further research is needed to reduce the time between an object's reentry and trajectory determination, the method is key for the rapid identification of debris fall-out zones, especially as Earth's orbit is anticipated to become increasingly crowded with satellites and space debris.
Future Directions
Researchers intend to conduct further tests to verify the method's viability and integrate it into civil monitoring pipelines. Future plans include integrating acoustic sensor networks, which possess an even greater detection range of thousands of miles, to potentially track reentries over vast oceanic areas where seismic and radar data are currently scarce. This expanded capability could help verify the reentry and demise of objects such as Starlink satellites.