Space debris—the thousands of pieces of human-made objects abandoned in Earth’s orbit—pose a risk to humans when they fall to the ground. To locate possible crash sites, a Johns Hopkins University scientist has helped to devise a way to track falling debris using existing networks of earthquake-detecting seismometers. 

The new tracking method generates more detailed information in near real-time than authorities have today—information that will help to quickly locate and retrieve the charred and sometimes toxic remains. 

“Re-entries are happening more frequently. Last year, we had multiple satellites entering our atmosphere each day, and we don’t have independent verification of where they entered, whether they broke up into pieces, if they burned up in the atmosphere, or if they made it to the ground,” said lead author Benjamin Fernando, a postdoctoral research fellow who studies earthquakes on Earth, Mars, and other planets in the Solar System. “This is a growing problem, and it’s going to keep getting worse.”

Fernando and colleague Constantinos Charalambous, a research fellow at Imperial College London, used seismometer data to reconstruct the path of debris from China’s Shenzhou-15 spacecraft after the orbital module entered the Earth’s atmosphere on April 2, 2024. Measuring roughly 3.5 feet wide and more than 1.5 tons, the module was large enough to potentially pose a threat to people, the researchers said. 

Space debris entering the Earth’s atmosphere moves faster than the speed of sound and, consequently, produces sonic booms, or shock waves, similar to those produced by fighter jets. As the debris streaks toward the Earth, vibrations from the shockwave trail behind, rumbling the ground and pinging seismometers along the way. Mapping out the activated seismometers allows researchers to follow the debris’ trajectory, determine which direction it’s moving, and estimate where it may have landed. 

By analyzing data from 127 seismometers in southern California, the researchers calculated the path and speed of the module. Cruising at Mach 25-30, the module streaked through the atmosphere traveling northeast over Santa Barbara and Las Vegas at roughly 10 times the speed of the fastest jet in the world. 

The researchers used the intensity of the seismic readings to calculate the module’s altitude and pinpoint how it broke into fragments. Then, they used trajectory, speed, and altitude calculations to estimate the module was traveling approximately 25 miles north of the trajectory predicted by U.S. Space Command based on measurements of its orbit.

Engulfed in flames, falling debris sometimes produces toxic particulates that can linger in the atmosphere for hours and waft to new parts of the planet as weather patterns change. Knowing the trajectory of the debris will help organizations track where those particulates go and who might be at risk of exposure, the researchers said.

Near-real time tracking will also help authorities quickly retrieve objects that make it to the ground, the researchers said. Such rapid retrievals are especially important because debris can carry harmful substances.

“In 1996, debris from the Russian Mars 96 spacecraft fell out of orbit. People thought it burned up, and its radioactive power source landed intact in the ocean. People tried to track it at the time, but its location was never confirmed,” Fernando said. “More recently, a group of scientists found artificial plutonium in a glacier in Chile that they believe is evidence the power source burst open during the descent and contaminated the area. We’d benefit from having additional tracking tools, especially for those rare occasions when debris has radioactive material.” 

Previously, scientists had to rely on radar data to follow an object decaying in low Earth orbit and predict where it would enter the atmosphere. The trouble, the researchers said, is that re-entry predictions can be off by thousands of miles in the worst cases. Seismic data can complement radar data by tracking an object after it enters the atmosphere, providing a measurement of the actual trajectory.

“If you want to help, it matters whether you figure out where it has fallen quickly—in 100 seconds rather than 100 days, for example,” Fernando said. “It’s important that we develop as many methodologies for tracking and characterizing space debris as possible.”