More space junk is heading toward Earth. Seismic sensors can track it with sonic booms.

In the early morning of April 2, 2024, the sky over Southern California was lit up by flashes of glowing light. The residents were confused. Were they missiles? Plane crashed? This unusual activity confused even experts, until they realized it was a disposable part of China’s Shenzhou-15 spacecraft that burned up in the atmosphere during its return to Earth.

Scientists knew the event was on the horizon, and mapped a possible entry point over the North Atlantic Ocean, thousands of miles from greater Los Angeles. Fortunately, no one was hurt when the unit crashed over the city.

But the incident confirmed an uncomfortable truth. We are not close to being able to accurately predict the path of space debris during a downpour. As more spacecraft launch and re-enter the atmosphere, damage to infrastructure and Earthlings is only a matter of time.

Researchers are looking for a solution from an unexpected source: sensors that measure earthquakes. When space debris descends to Earth at supersonic speeds, it generates a sonic boom. This causes a slight tremor in the ground that sensors can easily register.

Using data from a network of these sensors, Benjamin Fernando of Johns Hopkins University and Konstantinos Charalambous of Imperial College London have developed a system that can reconstruct the path of space debris with unprecedented accuracy. They used the system to map Shenzhou-15’s speed, altitude, gradual disintegration and final destination.

To be clear, this is not an early warning system. Because sonic booms lag behind the objects causing them, this method is similar to reconstructing the final flight of space debris. However, it can quickly identify potential fall areas for faster retrieval and cleanup, which is especially important if the waste is toxic or radioactive.

Chris Carr of Los Alamos National Laboratory, who was not involved in the work, wrote that the work is “a critical step toward near-realtime monitoring of natural and human objects entering from space.”

An embarrassment of riches

Launching satellites was once a massive undertaking. But thanks to the innovations of SpaceX and national space agencies around the world, it has become much more routine.

These spacecraft have already changed life on Earth. Thousands of Starlink satellites beam Internet into dead zones and former disaster areas. Mini-satellites are now an affordable research platform used by scientists to determine the weather, measure the solar wind, and track the effects of microgravity and radiation on living cells. A new space race will only increase the fleets of spacecraft that already cover the Earth.

“The big change we have seen since 2020 is the emergence of massive satellite constellations,” Fernando told Science. “Companies are not putting up dozens of spacecraft, but maybe a thousand or ten thousand over the course of a few years.”

The massive constellations have already caused problems for scientists by polluting astronomical images with bright lines. It may also increase the rate of space debris rain. In a paper describing their system, Fernando and Charalambous wrote that in 2025 there were approximately four to five readmissions per day, and the numbers are likely to grow rapidly.

We are already monitoring spacecraft in orbit. Telescopes provide real-time images. Radar tracks location and speed. But these instruments run into difficulty when the spacecraft drifts into Earth’s upper atmosphere.

The interaction between the fragments and the air becomes “really chaotic,” Fernando said. “We can no longer predict with good accuracy the exact location [and when] A piece of returning space debris will enter the atmosphere.”

Radar can track parts of a spacecraft as it returns to Earth, but the technology is limited to small areas of the world and barely covers oceans. Even when we know the ultimate fate of a piece of debris, it is often difficult to reconstruct its full path.

Supersonic waves

The new work was inspired by the way scientists track meteorites using a dense network of seismic sensors to detect tiny vibrations in the Earth.

The Shenzhou-15 capsule entered the atmosphere at approximately 25 to 30 times the speed of sound. Like a fighter jet, it created a powerful sonic boom about 80 kilometers (50 miles) above the ground. The boom traveled to the Earth’s surface, where it was detected by seismic sensors.

It’s like catching an earthquake, Fernando said, but “in this case the waves come from above versus earthquakes, which tend to come from below.”

Southern California is full of earthquake sensors, each measuring activity in a small area. To model the spacecraft’s path and speed, the team collected the largest sonic boom recorded by each sensor and its arrival time and compiled the data into a map.

The map shows where, when and how the capsule broke apart as it hurtled through the atmosphere. Earlier, sensors recorded large, discrete signals. These later became more scattered and complex, suggesting that the capsule disintegrated gradually rather than exploding all at once.

The findings “are consistent with observations on the ground, including videos and witness reports of multiple fireballs flying across the sky,” Carr wrote. After digging deeper into the data, the team showed that it could also be used to measure the size of each piece of decomposing debris.

The spacecraft’s acoustic signature is different from that generated by meteorites, making it possible to distinguish between man-made objects and those of natural origin.

Distinguishing between the two categories is key. Meteorites pose a “kinetic hazard” as pieces crash into the Earth, damaging cars, homes and other infrastructure. However, human space debris can also contain metals, toxic or flammable materials, or, in rare cases, radioactive components. The model also reconstructed how different parts of the spacecraft broke apart, which may make it easier to predict whether pieces burned up completely in the atmosphere or reached Earth, making them useful for recovery or cleanup missions.

Crashing and burning is not the only fate of the spacecraft. Engineers are also working to move dead satellites into higher orbits that could be stable “for thousands of years,” according to Fernando, although this does not solve the space junk problem. Other researchers are exploring ways to design spacecraft so that they burn up completely safely and predictably.

Currently, this technology works best in places with a lot of seismic sensors, which are rare. But Fernando said there is pressure to add sensors in places at risk due to sensitive environment or geology at prices much lower than building return-tracking radar systems.

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