As more than 15,000 tons of human-made objects orbit Earth, the increasing threat of falling space debris is prompting scientists to seek new ways to track and mitigate risk. While the odds of being struck are exceedingly low, larger fragments pose a potential hazard not from direct impact, but from the powerful shockwaves generated during atmospheric re-entry. Researchers are now leveraging existing seismic networks – traditionally used to monitor earthquakes – to pinpoint the landing zones of these objects,a method recently tested with the descent of a Chinese spacecraft module.
More than 15,000 tons of human-made objects are currently orbiting Earth, according to the latest figures from the European Space Agency. This includes approximately 15,860 satellites, around 3,000 of which are no longer operational, not to mention millions of pieces smaller than 10 centimeters that continue to circle the planet. The increasing amount of space debris poses a growing threat, not just to active satellites, but to life on Earth.
Re-entries into Earth’s atmosphere are becoming more frequent. “Last year, several satellites were entering our atmosphere each day, and we don’t have independent verification of where they entered, if they fragmented, if they burned up in the atmosphere, or if they reached the ground,” said Benjamin Fernando, a postdoctoral researcher at Johns Hopkins University who studies earthquakes on Earth and other planets in the solar system. “This is a growing problem and will only get worse.”
While the chance of being struck by falling space debris is less than one in a trillion, the risks associated with larger fragments are more concerning. As noted in previous reporting, most debris is small and burns up upon entering the atmosphere. You’re far more likely to be struck by lightning or win the lottery multiple times than to be injured by space junk.
However, the most significant danger isn’t a direct hit, but the effects of larger debris falling to Earth. These objects re-enter the atmosphere at supersonic speeds, creating sonic booms and shockwaves similar to those produced by fighter jets. As debris descends, the vibrations from these shockwaves can cause structures and the ground to reverberate, potentially leading to substantial damage. This highlights the need for improved tracking and mitigation strategies as space activity increases.
Another potential hazard is the toxic materials released during re-entry. “In 1996, debris from the Russian Mars 96 spacecraft fell out of orbit,” Fernando recalled. “It was believed to have burned up, but its radioactive power source landed intact in the ocean. Attempts were made to locate it at the time, but its position was never confirmed. More recently, scientists found artificial plutonium in a glacier in Chile, which they believe is evidence that the power source disintegrated during descent and contaminated the area.”
To prevent contamination from hazardous debris, Fernando emphasizes the importance of rapid recovery. “If you want to help, it’s important to figure out where things have fallen quickly – in 100 seconds rather than 100 days, for example. It’s crucial that we develop as many methodologies as possible to track and characterize space debris.”
Tracking the Fallout
Benjamin Fernando and his colleague, Constantinos Charalambous, a researcher at Imperial College London, have developed a method for tracking space debris by analyzing the vibrations in the Earth’s crust that seismographs can precisely record. Existing seismic networks contain dozens, or even hundreds, of stations constantly monitoring Earth’s vibrations to study earthquakes and geological activity. By combining signals from multiple stations, scientists can triangulate the location of a debris re-entry and reconstruct the object’s trajectory, allowing them to estimate where and how it fragmented during descent.
As a case study, the researchers applied their method to the descent of the orbital module of the Chinese Shenzhou-15 spacecraft, which re-entered Earth’s atmosphere on April 2, 2024, near Las Vegas. Using data from 124 instruments within the Southern California Seismic Network and an additional station in Nevada, the researchers were able to not only track the descent of a 1.5-ton module falling at ten times the speed of the fastest aircraft, but also pinpoint when fragments separated and followed distinct trajectories before completely disintegrating as they approached the surface.
The results of this analysis were published this week in the journal Science.
