Seismometers can track falling space junk

As part of the Chinese spacecraft Shenzhou-15 tumbled back to Earth, its disintegration was tracked by a surprising source: seismometers.

Seismic networks in southern California picked up ground vibrations induced by shock waves as the spacecraft entered Earth’s atmosphere on April 2, 2024. Using that data, scientists were able to track the trajectory of spacecraft bits more accurately than relying on existing ways to predict it, the team reports January 22 in Science. That suggests that networks designed to detect earthquakes can also track falling space junk — 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, setting off ripple effects below that were detectable by seismometers. By analyzing the intensity of those signals, as well as the precise timing when they reach the 127 seismometers in the network, researchers could estimate the debris’ altitude and trajectory. They could even track how the spacecraft broke down into several pieces, each one producing their own cascading shock waves.

Space debris is typically monitored while in orbit using ground-based radar, which can follow objects as small as about 30 centimeters across. But once fragments descend into the upper atmosphere, interactions with the air cause them to break apart, slow down and change direction in complex ways. As a result, predicted reentry paths can be off by hundreds of kilometers. For Shenzhou-15, seismic data showed that it passed about 30 kilometers south of the trajectory predicted by U.S. Space Command.

The work was inspired by 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 working seismometer on the surface of Mars. “A lot of what we did in this paper is essentially taking techniques developed for Mars and reapplying them to Earth.”

The precision of the detection depends on the density of seismometer networks, since sonic booms propagate through the atmosphere for only about 100 kilometers. Urban areas often have dense coverage, but sparsely populated areas in seismically quiet areas do not. This might limit the usefulness of the technique at a global scale, says Daniel Stich, a seismologist at the University of Granada in Spain who was not involved with the study.

Uncontrolled reentries are becoming more frequent as the number of spacecraft in orbit grows unchecked. Falling fragments can hurt people or damage infrastructure, and debris often contains toxic fuels, flammable materials or, in rare cases, radioactive power sources. While seismic monitoring is unlikely to provide advance warning, it could help rapidly assess where debris fell and narrow down areas at risk of contamination.

The study fits into 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 concerts, says Jordi Díaz Cusí, a seismologist at the Geosciences Institute of Barcelona who was not involved with the new work. Tracking the reentry of space debris, he says, “is a good example of how seismic data … can be used for things very far removed from their original objective.”