Seismometers, traditionally used to detect earthquakes, have proven capable of tracking the disintegration of falling spacecraft with greater precision than conventional methods. A recent study published in Science on January 22 details how ground vibrations induced by shock waves from the re-entry of China’s Shenzhou-15 spacecraft were picked up by seismic networks in southern California on April 2, 2024.
The Science Behind the Detection
As space debris plummets toward Earth, it exceeds the speed of sound, creating shock waves that register as ripple effects detectable by seismometers. By analyzing the strength and timing of these signals across a network of 127 seismometers, scientists were able to estimate the debris’ altitude and trajectory. The system can even identify how the spacecraft broke apart into multiple pieces, each generating its own shock wave.
This method offers a significant advantage over existing tracking systems. Current space debris monitoring relies heavily on ground-based radar, which struggles to accurately predict reentry paths once fragments enter the upper atmosphere. Interactions with air cause debris to fragment, slow down, and alter direction unpredictably, leading to prediction errors of hundreds of kilometers. In the case of Shenzhou-15, seismic data revealed the spacecraft landed roughly 30 kilometers south of the U.S. Space Command’s predicted trajectory.
From Mars to Earth: Adapting Existing Technology
The approach builds upon techniques already used to track meteoroids using seismic and acoustic data, both on Earth and Mars. Benjamin Fernando of Johns Hopkins University, who worked with NASA’s InSight mission on Mars, explains: “A lot of what we did in this paper is essentially taking techniques developed for Mars and reapplying them to Earth.” The InSight mission demonstrated the usefulness of seismometers for detecting meteoroid impacts on Mars, paving the way for this terrestrial application.
Limitations and Future Implications
The precision of seismic detection is tied to seismometer network density. Sonic booms propagate for only about 100 kilometers, meaning sparse coverage in remote areas limits the technique’s global scalability. Daniel Stich of the University of Granada notes that urban areas with high seismometer concentration offer the best results.
Uncontrolled reentries are increasing as the number of spacecraft in orbit rises unchecked. Falling fragments pose risks to people, infrastructure, and the environment due to toxic fuels, flammable materials, and occasional radioactive components. While seismic monitoring won’t provide advance warning, it could help rapidly assess impact zones and identify contamination risks.
The successful adaptation of Martian seismic tracking techniques to Earth demonstrates a novel and potentially vital tool for managing the growing threat of space debris.
The technology offers a complementary approach to existing radar-based systems, improving overall situational awareness as space traffic continues to expand.
