Scientists find unlikely new way to track space junk: sonic booms in the atmosphere

Space junk falling out of orbit can be a threat -- now scientists say they have found a new way to track it as it speeds through the atmosphere. (Credit: Getty Images via CNN Newsource)

Space junk falling out of orbit and crashing toward Earth is a growing threat. Indeed, old satellites and spacecraft parts reenter the planet’s atmosphere more than three times a day.

When these objects burn through the atmosphere, they can release harmful substances, and if they reach Earth’s surface, they can contaminate the environment as well as collide with buildings, other infrastructure and possibly even people.

However, tracking falling debris in an effort to mitigate its impact is complicated because space junk can deorbit suddenly as it travels at speeds up to 18,000 miles per hour. Current methods to monitor falling space junk use radar and optical tracking but they struggle to accurately predict where most objects could land, especially if the debris breaks up during reentry into Earth’s atmosphere. This lack of precise location data can delay or prevent the recovery of dangerous toxic space residue.

Now, researchers from the Johns Hopkins University and Imperial College London say they’ve found a new way to help spot space junk during reentry. Their approach uses seismometers, the instruments that normally detect earthquakes in the ground.

The trick is to look for data indicating a sonic boom — the shock wave produced when an object exceeds the speed of sound — which the falling debris generates as it tumbles through the atmosphere.

“We’ve known for a long time that space debris reentering the atmosphere produces sonic booms, exactly the same way as natural meteoroids or supersonic aircraft produce sonic booms,” said Benjamin Fernando, a postdoctoral research fellow at Johns Hopkins, who studies earthquakes on Mars, Earth and other planets in our solar system.

“I did a lot of work on a NASA mission called InSight, where we tried to use meteoroids as seismic sources on Mars, with a single seismometer,” added Fernando, who coauthored a paper with Constantinos Charalambous, a research fellow at Imperial College London, on the new method that published Thursday in the journal Science.

The InSight lander, which touched down on Mars in 2018, has detected more than 1,300 marsquakes, a handful of which were produced by meteoroids striking the surface, rather than by the movement of rocks within the planet. InSight was able to “hear” the shock waves that the meteoroids produced as they entered Mars’ thin atmosphere and then pinpoint the location of the impact. NASA’s Mars Reconnaissance Orbiter was subsequently flown over these craters to study and photograph them, revealing important insights about the red planet’s surface.

“The big step in this paper was taking some of the techniques we developed for studying natural meteoroids on Earth and Mars and applying them to the study of space debris on Earth,” Fernando said.

“But in many ways, space debris is quite different to natural space objects — it tends to enter the atmosphere more slowly and at a much shallower angle. It also tends to break up in a much more complicated fashion as well and indeed pose a much bigger risk to people on the ground,” Fernando added.

A different prediction

To test their method, the researchers used the uncontrolled reentry of China’s Shenzhou-15 spacecraft, a 2022 mission to the Tiangong space station. The spacecraft’s orbital module, measuring 3.5 feet (about 1 meter) wide and weighing more than 1.5 tons, reentered the atmosphere in April 2024 over California.

As the spacecraft burned up through the atmosphere, the sonic booms it produced reached the ground, creating vibrations that seismometers picked up but that didn’t look like earthquakes. The study looked at data from 125 such instruments, using the intensity of the readings to reconstruct the object’s path in the sky.

Compared with a projection by the U.S. space force using radar data, the sonic booms method returned a trajectory that was 25 miles (40 kilometres) farther south. “There are no debris fragments that have been recovered,” Fernando said, “so all we can say is that we see something that is different to the space force prediction.”

The researchers now need more tests to verify the method’s viability. “Our end goal is to produce a tool that we can integrate into a civil monitoring pipeline,” Fernando said. “Say you’re worried that something has fallen out of the sky over California or over London — you’d have a tool, based on open source data, which can help you locate where that’s happened and potentially, inform recovery efforts,” he added, referring to the data from seismographers, which is usually publicly available.

The sonic booms would be detected automatically, Fernando said, allowing people to track falling debris within seconds or minutes from the start of reentry and gather important data about the location of potential atmospheric contamination. Estimating an impact site would take slightly longer because other variables such as the wind would need to be considered, but the tool would still be able to suggest a location quickly enough to support a rapid response.

Fernando cited two examples of environmental concerns from falling debris. One is the 1978 reentry of Soviet satellite Kosmos 954, which dispersed radioactive debris over northern Canada. “Most of it has never been recovered,” he said. “It’s still radioactive.”

The other is the early 2025 explosion of a SpaceX Starship rocket over the Caribbean, which affected civil aviation and scattered debris and heavy metals into marine environments and residential areas.

“Another thing that we’re becoming more aware of is that all of these reentries are beginning to change the composition of the atmosphere,” he added. “A lot of the chemicals contained within spacecraft are quite toxic. Some of them have a clear ozone-depleting potential. So it’s pretty serious stuff, and we’re not exactly sure what the impact is, because this is still a relatively new problem.”

An exciting new development

Hugh Lewis, a professor of astronautics at the University of Birmingham in England, noted that using an existing network of seismic sensors makes the new method a “scalable, low-cost, and exciting new development.” Lewis was not involved with the research.

“It describes an approach that helps us to understand what happens when a spacecraft or rocket stage re-enters the atmosphere — a process that historically has been very difficult to observe and measure, due to the limitations of existing radar systems used to track these objects in orbit and often because of the remoteness of the re-entry location,” Lewis said in an email.

Moriba Jah, a professor of aerospace engineering and engineering mechanics at the University of Texas at Austin, said that using seismic networks to extract information from atmospheric reentries is a good example of how “serendipitous” data can be repurposed. Jah, who also did not participate in the study, noted that the data can be used to learn more about objects scientists otherwise lose track of during the most chaotic phase of their return to Earth.

However, he warned about possible limitations. “This method relies on strong shock waves, essentially sonic booms, coupling into the ground,” Jah wrote in an email. “Many reentering objects are too small or disintegrate too high in the atmosphere to produce signals like this at all. So it won’t detect most debris, and it isn’t a standalone solution to the space debris problem.”

There will also be the challenge of distinguishing reentry signals caused by space debris from those that come from other sources such as aircraft, explosions or natural phenomena, he said. With careful validation and integration alongside radar, optical and satellite tracking, the method could become “a useful complementary tool rather than a universal fix.”

Improving information gathering on objects reentering the atmosphere is crucial not only for timely recovery operations but also for building a deeper understanding of how space activities affect society on Earth, according to Davide Guzzetti, an associate professor of aerospace engineering at Auburn University, who was not involved with the study.

“What I find especially fascinating is that these measurements may also provide insight into the fragmentation dynamics occurring during re-entry, not just the re-entry trajectory,” Guzzetti said in an email. “With the right tools and access to seismic data, it’s easy to imagine citizen-science projects emerging, where people help track and identify debris through the detection of sonic booms.”