Seismic Sensors Can Track Falling Space Junk During Uncontrolled Reentry

Scientists have identified a new method for monitoring the uncontrolled reentry of falling space junk.

When pieces of space debris tear into the atmosphere, they generate sonic booms. These can be picked up by ground instruments normally used to study activity beneath the surface: seismic sensors that record the internal tremors of our active planet.

Seismic sensors and uncontrolled space junk reentry

This is more than a theoretical possibility. Planetary scientist Benjamin Fernando of Johns Hopkins University and engineer Constantinos Charalambous of Imperial College London put the idea to the test using the 2024 reentry of the Shenzhou-15 orbital module.

Readings gathered by seismic sensors delivered precise information not only about the reentry event, but also about its speed, altitude range, size, angle of descent, and the timing of fragmentation during its fall.

"Observations of cascading, multiplicative fragmentation offer insight into debris disintegration dynamics, with clear implications for space situational awareness and debris hazard mitigation," the researchers write in their paper.

Space debris is becoming an ever greater problem. An April 2025 European Space Agency report estimates that 1.2 million potentially dangerous pieces of space junk orbit Earth – and that total will continue to grow as more satellites reach the end of their working lives.

A ‘dead’ spacecraft of this kind can no longer be contacted or controlled. If it strikes another item of junk, or its orbit deteriorates enough to trigger reentry, there is little that can be done beyond observing it.

Fernando and Charalambous argue, however, that this observation can be far more effective than previously recognised. Establishing where a reentering object breaks up, along with its height, speed and manner of fragmentation, could improve understanding of atmospheric reentry dynamics and help predict where its pieces may land.

How sonic booms reveal falling space debris

A sonic boom occurs when an object moves through a medium faster than sound can travel within it. The term is slightly deceptive: rather than a single isolated boom, it is more like a wake – a shock wave created as outward-moving pressure waves are compressed into a cone behind the fast-moving object.

Objects arriving in Earth’s atmosphere from space frequently descend faster than sound, attaining supersonic and even hypersonic speeds. As they pass through the air, they leave behind a cone of acoustic energy that listeners along its route can hear as a boom.

Seismic sensors are built to identify acoustic signals originating deep within Earth. The researchers nevertheless proposed that the same instruments could also follow the acoustic Mach cone produced by falling space debris.

On 2 April 2024, the discarded Shenzhou-15 orbital module reentered Earth’s atmosphere above southern California. At 2.2 metres (7.2 feet) long and weighing 1.5 metric tonnes, it was sufficiently large and heavy to endanger aviation and ground infrastructure – making it an ideal case for testing this tracking approach.

The team examined publicly available records from the Southern California Seismic Network and the Nevada Seismic Network for signs of the module’s transit. They identified patterns matching the Mach cone striking Earth’s surface and used them to reconstruct the object’s final journey and destruction.

Shenzhou-15’s final flight reconstructed

Seismic readings indicated that the module was travelling at approximately Mach 25 to 30. This matched the object’s pre-entry orbital characterisation, which put its velocity at roughly 7.8 kilometres (4.8 miles) per second.

The researchers further determined that the early stage of the descent created one large boom signal. Later, that signal developed into a complicated sequence of several smaller booms, consistent with ground observations that the object had fragmented.

The module ultimately burned up safely in the atmosphere during its fall. Even so, the findings demonstrate that seismic stations can accurately and effectively trace the characteristics of a reentry flight. For objects that do not burn up as completely, the approach could eventually assist in identifying the most probable debris field for fragments that reach the ground.

"Because these objects necessarily reenter the atmosphere at supersonic speeds, if the largest fragments impact the ground, they will do so before their sonic booms are detected," the researchers write. "However, detection and tracking based on seismoacoustic methods enable debris to be more rapidly and precisely located on the ground than could otherwise be achieved."

A further issue is the release of potentially hazardous aerosol-sized particles as an object burns and breaks apart. Understanding how these failure states unfold could help scientists model the locations and ways in which such clouds spread.

For the moment, uncontrolled reentries remain just that: uncontrolled. Although they may not be preventable, the new study demonstrates that publicly available tools can be used to observe and understand how these objects fall.

The research has been published in Science.