Seismic sensors detect the effects of sonic booms when spacecraft enter Earth’s atmosphere
As part of the Chinese spacecraft Shenzhou-15 returned to Earth, its disintegration was tracked by a surprising source: seismometers.
Seismic networks in Southern California captured ground vibrations induced by shock waves when the spacecraft entered Earth’s atmosphere on April 2, 2024. Using this data, scientists were able to track the spacecraft’s bit trajectory more accurately rather than relying on existing means to predict it, the team reported January 22 in Science. This suggests that networks designed to detect earthquakes can also track falling space debris – defunct spacecraft or abandoned launch hardware that can pose risks to people and infrastructure.
As space debris plunges toward Earth, it travels faster than the speed of sound, generating shock waves, triggering ripple effects below that were detectable by seismometers. By analyzing the intensity of these signals, as well as the precise moment when they reach the 127 seismometers in the network, the researchers were able to estimate the altitude and trajectory of the debris. They were even able to track how the spacecraft broke into several pieces, each producing its own cascading shock waves.
Space debris is typically monitored in orbit using ground-based radar, which can track objects as small as about 30 centimeters in diameter. But once the fragments descend into the upper atmosphere, interactions with the air cause them to break up, slow down and change direction in complex ways. As a result, planned re-entry trajectories can deviate by several hundred kilometers. For Shenzhou-15, seismic data showed it passed about 30 kilometers south of the trajectory predicted by U.S. Space Command.
The work draws inspiration from techniques used to track meteoroids using seismic and acoustic data, both on Earth and Mars. “I worked a lot with NASA’s InSight mission, and for us, meteoroids were actually a very useful seismic source,” says Benjamin Fernando, a seismologist and planetary scientist at Johns Hopkins University. InSight put the first functional seismometer on the surface of Mars. “A lot of what we’ve done in this paper is basically taking techniques developed for Mars and reapplying them to Earth.”
Detection accuracy depends on the density of seismometer networks, since sonic booms propagate through the atmosphere only about 100 kilometers. Urban areas often have dense coverage, but sparsely populated areas in seismically quiet areas do not. That could limit the technique’s usefulness on a global scale, says Daniel Stich, a seismologist at the University of Granada in Spain, who was not involved in the study.
Uncontrolled returns are becoming more and more frequent as the number of spacecraft in orbit is increasing uncontrollably. Falling fragments can injure people or damage infrastructure, and debris often contains toxic fuels, flammable materials, or, in rare cases, radioactive energy sources. Although seismic monitoring is unlikely to provide advance warning, it could help quickly assess where debris has fallen and reduce areas at risk of contamination.
The study is part of a recent trend known as environmental seismology, which uses seismic data to monitor phenomena beyond earthquakes – from storms and avalanches to explosions, road traffic during COVID or even Taylor Swift concertsexplains Jordi Díaz Cusí, a seismologist at the Barcelona Institute of Geosciences, who was not involved in the new work. He said tracking the reentry of space debris “is a good example of how seismic data… can be used for things that are very far from their original purpose.”
