When a brilliant fireball streaked across the bright skies of Alaska last spring, conventional tracking tools—cameras and satellites—failed to capture a clear picture because the event occurred in broad daylight. However, the meteoroid left behind something else: low-frequency sound waves that traveled hundreds of kilometers and were recorded by a dense network of ground-based sensors designed to monitor earthquakes and volcanoes. Using those signals, a team of researchers, students, and citizen scientists led by Sandia National Laboratories reconstructed the object's path through the atmosphere, identified where it fragmented, and determined where debris likely fell.
In a study published in the Journal of Geophysical Research: Planets, the team demonstrated how combining low-frequency sound waves, subtle ground vibrations, weather radar data, and publicly shared videos can reconstruct a fireball's trajectory even when optical coverage is sparse or absent. This achievement is significant for planetary defense, as quickly and reliably reconstructing an object's atmospheric entry can help scientists understand what happened, where debris may have landed, where the object originated, and whether any risks exist.
The investigation began on the same day the fireball was spotted. Logan Scamfer, then an intern at the University of Alaska Fairbanks, reviewed seismic stations across the region and noticed an unusual signal appearing repeatedly. He then checked data from an array of sensors south of Anchorage and found a clear N-wave, a waveform often associated with decaying shock waves. This led him to suspect the signals came from a meteor. Later that afternoon, reports of a fireball sighted over Alaska confirmed his initial suspicion.
About a month later, Scamfer arrived at Sandia for a summer internship with physicist Elizabeth Silber, whose research focuses on using infrasound—sounds at frequencies too low for human hearing—and seismic data to study meteors and other fast-moving objects in the atmosphere. Because this fireball had not been clearly detected by satellites or all-sky cameras, the pair decided to test whether infrasound and seismic signals could provide the missing information. The challenge, Silber noted, was that this was not a planned measurement campaign with cameras and sensors positioned in advance; the team had to reconstruct the fireball's passage from whatever data was available.
In a broad 2023 study, Silber had recorded and characterized infrasound and seismic waves generated by the reentry of NASA's Osiris-Rex mission capsule—experience that proved valuable for the Alaska fireball investigation. When a meteoroid travels through the sky, it generates a powerful shock wave similar to a sonic boom, but produced at high altitude and often along an extended path. As the shock wave propagates, it can transform into infrasound. Part of that energy can also transfer to the ground, where pressure waves reaching the surface create tiny vibrations recorded by seismic sensors.
The fireball generated low-frequency sound waves that traveled hundreds of kilometers across Alaska. The signals were recorded by 57 different sensors used for earthquake and volcano monitoring, providing the team with enough data to begin reconstructing the fireball's trajectory, even without the optical recordings scientists would normally prefer. Among the instruments were 37 seismic stations, 16 infrasound sensors, and four infrasound sensor arrays. Some detected the event from as far as 580 kilometers away.
Using those ground-based recordings, the team reconstructed the fireball's flight path, identified the point where it fragmented, and narrowed down the area where debris likely fell. They then shared the approximate location with a colleague at NASA, who used Doppler weather radar to search for—and find—the signature of falling debris. Radar typically does not see the bright fireball itself, Silber explained, but in some cases it can detect radio waves reflected from the debris cloud as fragments fall. The team then compared their reconstruction with the radar data to validate their method.
Alaska's exceptionally dense network of infrasound and seismic stations, originally deployed for earthquake and volcano monitoring, proved crucial. Silber noted that the same infrastructure also records pressure waves and ground motion coupled to them when a meteoroid generates shock waves during hypersonic entry. This study demonstrates that even when optical observations are unavailable, existing ground-based monitoring networks can provide valuable data for tracking meteoroids and improving planetary defense capabilities.