Earthquake Sensors Now Detecting Sonic Booms from Falling Space Junk

Scientists have just found a new way to track the uncontrolled reentry of falling space junk. As they punch into the atmosphere, chunks of space debris create sonic booms that can be detected by ground-based instruments that are usually focused on what's happening below: the seismic sensors that monitor the internal rumbles of our restless planet. It's not just theory, either: Planetary scientist Benjamin Fernando of Johns Hopkins University and engineer Constantinos Charalambous of Imperial College London tested their hypothesis on the 2024 reentry of the Shenzhou-15 orbital module. Related: Seismic Waves From Intense Storms Can Ripple Through Earth's Core The data collected by seismic sensors gave precision measurements, not just of the reentry itself, but also its speed, altitude range, size, descent angle, and timing of its fragmentation as it fell. "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 an escalating concern. According to an April 2025 report from the European Space Agency, an estimated 1.2 million pieces of potentially hazardous space junk are in Earth orbit – and that number is only going to increase as more satellites reach the end of their operational lifespans. A 'dead' spacecraft of this nature cannot be communicated with or controlled; if it collides with another piece of junk, or its orbit decays sufficiently for reentry, all we can do is watch. According to Fernando and Charalambous, however, we can do that watching much more effectively than we thought. Knowing where, how high, how fast, and how a reentering piece of space debris broke apart can help us better understand the dynamics of atmospheric reentry and track where the pieces are likely to fall. A sonic boom is what happens when an object travels faster than the speed of sound in a medium. The name is a little misleading – It's not one discrete boom, but more of a wake, a shock wave formed by outward-moving pressure waves that get compressed into the shape of a cone behind the speeding object. Objects entering Earth's atmosphere from space often fall faster than the speed of sound, reaching supersonic and even hypersonic velocities. They stream through the atmosphere, trailing a cone of acoustic energy that can be heard by listeners in its path as a boom. Seismic sensors are built for detecting acoustic signals from deep inside Earth. However, the researchers reasoned these instruments might be able to track the acoustic Mach cone of falling space debris, too. frameborder="0″ allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen> On 2 April 2024, the discarded Shenzhou-15 orbital module reentered Earth's atmosphere over southern California. At 2.2 meters (7.2 feet) and 1.5 metric tons, it was large enough and heavy enough to pose a hazard to both aviation and ground-based infrastructure – the perfect test case for this kind of tracking. The researchers accessed the publicly available Southern California Seismic Network and Nevada Seismic Network and looked for evidence of the module's passage. They found signatures consistent with the booming Mach cone thumping down on Earth's surface and reconstructed the object's final flight and destruction. According to the seismic data, the module was traveling at a speed of around Mach 25 to 30, which was consistent with the object's pre-entry orbital characterization, which determined its velocity at about 7.8 kilometers (4.8 miles) per second. The researchers also found that while the earlier part of the fall produced a single large boom signal, it later decayed into a complex train of multiple, smaller boom signals – consistent with ground reports of the object's fragmentation. Ultimately, the module burned up harmlessly in the atmosphere as it fell, but the results show that the characteristics of a reentry flight can be effectively and precisely tracked by seismic stations. For objects that might not burn so completely, this could one day help locate the most likely debris field for pieces that fall to 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." Related: Strange Metal From Beyond Our Planet Spotted in Ancient Treasure Stash Another concern is the dispersal of potentially hazardous aerosol-sized particulates that may be released as the object burns and breaks apart. Knowing how these failure states play out could help scientists model where and how these clouds disperse. For now, uncontrolled reentries remain precisely that. While we may not be able to prevent them, though, the new research shows a way we can use publicly available tools to watch and understand how they fall.