Sonic booms could save Earth from dangerous space junk
Scientists are using technology developed to study earthquakes to solve an out-of-this-world threat
By mapping areas where seismometers in Southern California detected sound waves, researchers at Johns Hopkins University and Imperial College London were able to track the Shenzhou-15 orbital module’s path after re-entering Earth’s atmosphere on April 2, 2024.
Benjamin Fernando, Johns Hopkins University
As the global number of space launches continues to skyrocket, so does the amount of hazardous space debris that re-enters the atmosphere and falls back to Earth, increasing the likelihood that, sooner or later, disaster will strike. Most space debris is so small that it burns up completely as soon as it falls. Large objects from NASA and most other space agencies typically follow “controlled” reentry: Pushed down by rocket motors, they head toward remote and desolate areas of the planet. But due to the ongoing boom in space activity, the number of risky uncontrolled re-entry is increasing.
Now scientists have discovered a new way to monitor such potentially dangerous objects falling into the Earth’s atmosphere. It turns out that sonic booms, picked up by an already existing network of seismometers, can reconstruct descent paths and locate crash sites for abandoned spacecraft and large pieces of debris. A study detailing the results was led by Benjamin Fernando, a postdoctoral fellow at Johns Hopkins University, in collaboration with Constantino Charalambous, a research fellow at Imperial College London. published today In Science.
“This is a very useful additional tool in our toolbox,” says astrophysicist Jonathan McDowell of the Center for Astrophysics. Harvard and Smithsonian and a space flight tracker, which were not part of the study. He noted that optical telescopes and radar systems regularly monitor space junk, but both struggle to track debris as it disintegrates during re-entry — and optical systems really only work at night. “The sonic boom should work whether it’s day or night,” says McDowell. “And since these seismic networks are already operational, you can get it almost for ‘free’ once you know how to do the analysis.”
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The origins of the study date back to April 2, 2024, when a 1.5-metric-ton module was launched into orbit by a Chinese crew in 2022. Shenzhou-15 The mission underwent uncontrolled atmospheric reentry at supersonic speed. Passing over major population centers on six continents, the large, heavy module’s decaying orbit had caused international concern – and even prompted U.S. Space Command to predict that re-entry debris would eventually fall into the North Atlantic. This prediction proved to be thousands of kilometers away. Although much of the module burned up while roaming the skies over Southern California and no confirmed debris was found, whatever reached Earth probably fell in the Pacific Ocean or the western US.
Fernando, who also studies extraterrestrial earthquakes on Mars and other worlds, decided to take a closer look after realizing that the shock waves coming from them were supersonic. Shenzhou-15 The debris should have manifested as a sonic boom in the dense network of seismometers spread across earthquake-prone Southern California. When he and Charalambous manually examined the network’s publicly available, open-source data, they found re-entry registered at more than 120 monitoring stations. Together they analyzed the arrival times of the strongest shock waves at each location.
“From that, we were able to work out (the module’s) speed, descent, angle and trajectory – and also investigate how it separated in the atmosphere,” says Fernando. He said that if the technology were scaled up and automated, it could work “in almost real time”, requiring precise tracking peaks within minutes or seconds of the first sonic boom of a re-entry event. “Once an object is burning up and breaking up within the atmosphere, it becomes really quite difficult to track it,” he says, “which also makes it difficult to understand its effects on the atmosphere, the threat it poses to aviation and the threat it might pose to the ground wherever it hits.”
None of these matters are trivial. Many atmospheric scientists are concerned about increasing levels of vaporized aerospace-derived materials in the upper atmosphere, some of which may damage Earth’s protective ozone layer. Air travelers have already suffered near misses, such as when a test flight of SpaceX’s Starship vehicle last year scattered debris over a part of the Caribbean and The aircraft was forced to take evasive action. List of massive debris that have fallen too close to Earth to be convenient for populated areas worryingly long. And besides the sheer impact hazards, this debris contains some materials, such as Radioactive isotopes for nuclear reactors or volatile, toxic rocket fuels, which are very dangerous environmentally.
The sonic boom is unlikely to provide enough lead time for a passenger jet to avoid a collision course with a descending piece of space junk. But the method could prove essential to locating dangerous debris on the ground to aid recovery and treatment efforts. It could also be a game changer for improving models of how breakups occur. “It’s really important,” McDowell says, “to design spacecraft to break up more effectively upon re-entry and to understand how much of a spacecraft’s flow into the atmosphere potentially changes atmospheric chemistry.”
The big question is whether significant investments will be made to change the long-standing status quo. “For 60 years, we’ve been letting things re-enter uncontrolled, knowing that for the bigger ones, some fraction will reach the surface,” says McDowell. “We’re just hoping it doesn’t hit anyone on the head or cause any other harm. But ultimately we’re going to be out of luck.”
As for the technology, Fernando envisions two paths forward, both of which would treat the challenge of sonic boom tracking as a “big data” problem. The first will take advantage of existing seismic networks, particularly on the US West Coast, where such networks are already established and where orbital dynamics dictate that more re-entry events occur. The second will focus on new, custom-built networks to combat the increasing space debris in other parts of the world. “Take, for example, the ecologically sensitive Great Barrier Reef off the northeastern coast of Australia,” says Fernando. “A lot of Chinese rockets land there (from a launch site on the Chinese island of Hainan). It would be extremely cheap to set up a seismic network there compared to alternatives like building a network of radar stations.”
Experts hope the public and policy makers will pay attention to the growing problem of space debris. “It’s going to get worse,” says Fernando. “I fear that space debris will not get the attention it deserves unless a truly catastrophic event occurs – and I think there is a 100 percent chance of that happening.”
