Listening to Fireballs: Infrasound Sensing with Iona Clemente
Desert Fireball Network
Building 314 is as imposing as it is old. The hallways are thin, and the carpet is threadbare. In the dusty recesses of the building lay the Desert Fireball Network (DFN) laboratory coated in red dust and smelling of the Australian outback, a delightful earthy fragrance. Near the laboratory is the office, where I run into the tres merveilleuse Iona Clemente from France, a bright student completing a PhD on seismoacoustic signal detection and source identification related to atmospheric entries, such as meteoroids and spacecraft re-entry events.
What is the DFN? It’s not a bird and it isn’t a plane. It’s not Superman, either, though the people at the DFN are super at what they do. The DFN and Global Fireball Observatory (GFO) track fireballs with remote camera systems. This helps scientists to determine where meteors come from in the solar system and recover fallen meteorites. However, that is not all they do. The DFN has and shares seismic and infrasound data from around the world, listening to the sub-audible sounds that meteorites make as they travel through the air and land on Earth.
Storytelling Through Data
A seismoacoustic sensor is a broad term that encapsulates sensors that detect ground vibrations and sound waves in the air or water. An infrasound sensor is a type of seismoacoustic sensor used in studies that combine seismic and acoustic data. Iona clarifies what infrasound is. Infrasound is ‘below the human audio. It’s below 20 Hertz. Infrasound sensors are measuring the pressure fluctuations in the air.’
Iona says she uses ‘seismoacoustic data to try to retrieve meteoroids’ physical properties. With this signal, then we can try to estimate the energy. Because we have the trajectory, we know already the velocity from the DFN and GFO cameras. Using the kinetic energy relation, we get mass. If we have the mass, we can get a radius estimate.’ The kinetic energy relation describes the energy an object possesses due to its motion. If the object wasn’t moving, there would be no kinetic energy. Instead, it has potential energy.
All Fireballs Great and Small
What is fireball? ‘A fireball is a very, very bright meteor, lower than mangitude minus four, approximately the brightness of Venus. When a meteor enters the Earth’s atmosphere, if it’s large enough, greater than 10 centimeters in diameter, it will generate shock wave.’ Shock waves can be picked up by infrasound sensors as they generate a change in pressure in the air. When a meteoroid travels in the atmosphere at hypersonic velocity, from 11.2 to 72 km/s, this will generate shock wave as the velocity of the object is far greater than the local sounds speed (approximately 340 m/s).’
If and how the meteorite breaks up, including where it breaks up tells scientists about what the meteorite is made of. For instance, iron meteorites don’t fragment as much as rocky or ‘chondritic meteorites’ do. A small iron meteorite has a greater chance of landing on Earth, intact, versus a rockier meteorite that is more likely to fragment on atmospheric descent.
Another interesting term Iona introduced me to was ‘ablation coefficient. It’s the rate of disintegration of the body during its path through the atmosphere.’ It’s important to note that ablation is different from fragmentation. ‘Fragmentation events are the breakup of the meteorite’s body into different pieces due to air pressure exerted.’ Both processes deal with how a meteorite loses mass, but ablation refers to gradual surface erosion from heat, while fragmentation involves sudden structural failure.
There are also ‘airbursts that are explosions, signaling, most of the time, the complete disintegration of the body in the air.’ Airbursts happen when a meteoroid entering Earth’s atmosphere breaks apart explosively in mid-air due to extreme pressure and heat. ‘Most of the time, after an airburst, we don’t have any fragments left. The idea after is to use either seismic or infrasound sensors to try to find where along this trajectory this shock wave has been emitted. That’s really interesting.’
Airbursts are explosions, signaling, most of the time, the complete disintegration of the body in the air.
The Heartbeat of Meteorites
Every time a meteoroid hits our atmosphere, it leaves its acoustic signature. How it breaks up will change the writing. ‘If it’s the hypersonic entry or the fragmentation or an airburst, we’ll have a different shock wave signature, because they’re not propagating the same way. If it’s the hypersonic entry, we’ll have something parallel to the trajectory. In the infrasound records, we end up with something like a simple pulse, an N-wave shaped pulse, something really short in duration and quite strong. In comparison, if it’s a fragmentation event or an airburst, we’ll have something less like a pulse and more like a more complex signal, and that’s something that is also usually longer in duration. We’re trying to identify both and then we’re trying to find along the trajectory where it could have been emitted.
If the meteorite has a big enough body that impacts the ground, the meteorite can generate a seismic surface wave. If it’s really big it will generate body waves. Surface waves are just some waves that propagate along the surface, body waves are the ones that go deeper into the surface/ground. In the case of very large fireball (Chelyabinsk-like), the shock waves can be so energetic that they couple with the ground and convert into either seismic surface waves or even sometimes into seismic body waves. For most of the fireballs, we only record shock waves (no coupling). Then, in the extremely rare case of an impact (from a very very large or very dense meteoroid/asteroid, the kind of object that leaves an impact crater), we can record seismic surface and body directly from this impact.’
Why Fireball Pulses?
The interview isn’t complete without an understanding of why this intelligent young woman globe-trotted halfway around the world to study in an isolated city. ‘When I was doing earth physics, it was a lot of seismology. When I did my bachelor’s degree, we were doing geology, volcanology, some astrophysics and earthquakes. Earthquake is seismology. I was really interested in that, so I did an internship looking at earthquakes. Now I had to choose a master’s degree, and I think that’s what I really like, seismology, so I went to geophysics.
I’m in the DFN, which really, the main goal is finding the rocks and being able to identify the type of rock. I’m interested in what’s the entry of this rock generate, and how we can infer property of the fireball.’ Being part of the DFN team allows her to chase space rocks using data through seismoacoustic sensors. Despite her understanding of data from infrasound and her expert use in the sensors, she has no plans to invent her own. Iona mentions that ‘Koshi University are developing some infrasound sensors. They’re doing it themselves. We borrow them, or they ask us to put them outside sometimes.’
Iona is deeply passionate about her PhD, placing a metaphorical stethoscope on the pulse of fireballs as they enter our atmosphere. She is shaking and earthquaking the ground on our understanding of how we use infrasound data to understand meteorites. Our conversation took me on an incredible journey through space, revealing how scientists can decode the mysteries of meteorites—their properties, trajectories, and impact—simply by listening to them. I’m grateful for Iona’s generous time today, helping me unravel heavy scientific concepts and making them understandable for the everyday reader.
Written by Louise Kaestner, 2025. For more, visit LinkedIn or head to her website: https://www.louisekaestnerwriter.com/