Every day, about three large fragments of decommissioned satellites and used rocket stages make their return into Earth's atmosphere. However, their final trajectory and behavior during descent remain very difficult to predict, generating permanent risks for air safety and populations.
To address this issue, an original approach has been developed by researchers from Johns Hopkins University and Imperial College London. It relies on the use of seismic sensors to track the shock waves produced by these objects: the supersonic 'bang' during atmospheric traversal and the final impact shock. A method likely to provide real-time information on the location of fragments.
Starship debris tearing through the sky
This new technique was tested on the orbital module of the Shenzhou 17 mission, which fell in April 2024. The analysis of data from 127 seismic sensors located in California allowed to reconstruct its trajectory, which ultimately deviated by about 25 miles (40 kilometers) from initial predictions.
According to the scientists, this method does not allow to determine the exact impact point, but it significantly shortens the time required to locate fragments on the ground. This speed facilitates the recovery of potentially toxic debris, thus limiting dangers to the environment and public health.
The future integration of acoustic sensors could further expand the capabilities of this monitoring. These devices, capable of detecting sounds over vast distances, would allow to track re-entries even over oceans, where radar data are scarce.
An incident that occurred in November 2022 illustrated the limits of current systems. At that time, portions of airspace in Spain and France had to be closed due to the passage of a large Chinese rocket element, causing significant flight delays and substantial costs. This episode highlighted the need for more effective tracking tools.
This advancement, whose results are published in the journal Science, allows to better understand how satellites disintegrate upon their return. It helps to better assess risks to property and people, while paving the way for more effective space management.
Shock waves and their detection by seismic sensors
Shock waves, or supersonic sounds, are produced when an object exceeds the speed of sound in air. These vibrations propagate through the atmosphere and can be captured by networks of seismic sensors scattered across the globe.
These sensors, initially designed to detect earthquakes, record ground movements with great precision. They are sensitive to variations caused by events such as explosions or the passage of fast objects, transforming these vibrations into electrical signals.
When space debris enters the atmosphere at high speed, it generates a shock wave that travels for hundreds of miles (kilometers). By analyzing the arrival time of these signals on different sensors, researchers can calculate the object's trajectory and speed.
This technique allows real-time monitoring without relying solely on radars, offering broader coverage and quickly accessible data to improve safety.