At least three large pieces of space debris — old satellites and spent rocket stages — fall back to Earth every day on average, but researchers have only a very limited understanding of where these potentially dangerous fragments land and what happens to them in the atmosphere. A new method, based on sonic boom tracking by earthquake sensors, could provide real-time information about the hurtling debris fragments' whereabouts.
In November 2022, Spain and France shut parts of their airspace for about 40 minutes as a giant piece of a Chinese rocket was predicted to potentially come crashing down in southern Europe. The shutdown diverted or delayed hundreds of flights and cost millions of dollars. The rocket body eventually reentered on the other side of the globe, reentering over the Pacific Ocean.
The space community predicts paths of reentering space debris based on measurements from a global network of radars and optical telescopes. This approach is sound, but it has limitations.
"The space situational awareness radars and optical tracking are great when the object's in orbit," study lead author Benjamin Fernando, a postdoctoral fellow at Johns Hopkins University, told Space.com. "But once you're below a couple of hundred kilometers in altitude, the interactions with the atmosphere become quite chaotic, and it's not always apparent where the piece of debris will re-enter."
Ground-based radars are sparsely distributed around the globe and struggle to monitor the disintegration of the returning space object, Fernando added. Moreover, the measurements are not immediately available to everyone who might need them.
On the contrary, large parts of the globe are quite densely dotted with seismic sensors designed to detect earthquakes, and those measurements are mostly openly available online. In addition to tremors emanating from within the planet, these sensors detect explosions, traffic vibrations and even the vocalizations made by whales in the oceans.
In the new study, Fernando and his colleagues used data from such seismic sensors to reconstruct the path of an orbital module that detached from China's Shenzhou 17 crew capsule and fell to Earth in April 2024.
The 1.5-ton piece of junk was expected to come crashing down in the South Pacific or the North Atlantic, Fernando said. But both of these predictions "were completely wrong," he added.
The researchers analyzed data from 127 earthquake sensors spread across California to discern the propagation of the sonic boom produced by the Shenzhou 17 module as it hurtled through Earth's atmosphere at up to 30 times the speed of sound. They saw it travel about 25 miles (40 kilometers) north of the trajectory predicted by U.S. Space Command, with some fragments possibly crashing down somewhere between Bakersfield, California, and Las Vegas, Nevada.
"There's 50 million people living under that flight path," Fernando said. "It doesn't necessarily appear that any debris reached the surface from that event, but it could have done."
Fernando says that, while the data cannot forecast where a piece of space debris will crash-land, it can help accurately track down the impact locations, allowing ground teams to retrieve any possibly toxic fragments that could pose a hazard to the environment.
"A supersonic object will always outrun its own sonic boom," Fernando said. "You're always going to see it before you hear it. If it's going to hit the ground, there is nothing we can do about that. But we can try to reduce the time it takes to find fragments from days or weeks down to minutes or hours."
He mentioned a 1978 incident when a re-entering Russian satellite broke apart over Canada, scattering radioactive debris from its onboard nuclear reactor. Most of that toxic junk was never found, Fernando added.
The new tracking method may also help answer the big unknown of how much space debris actually reaches the surface of Earth. For example, SpaceX claims that the satellites in its Starlink internet-beaming megaconstellation completely evaporate during their fiery reentry, but many experts question this assessment, claiming that some components, such as fuel tanks and batteries, are made of extremely sturdy materials and are therefore likely to survive. Having a better understanding of how completely satellites burn up in the atmosphere will help experts better assess the risk these objects pose to people and property on Earth, as well as to cruising aircraft.
"In this study, we show that the seismic network we have in the U.S. is able to track the sonic booms from the debris, which allows us to identify the trajectory, speed and descent angle and also characterize some of the breakup process," said Fernando. "This kind of detailed probing of the reentry process is really interesting, because it allows us to understand a little bit more about how the object is interacting with the atmosphere and whether or not any fragments will reach the ground."
The seismic sensors used in this study turn ground vibrations into electrical signals that can detect sonic booms at distances of several hundred miles, said Fernando. But a different kind of sensor network, based on acoustic measurements, exists, which could further increase the method's reach. In future studies, the team wants to analyze outputs of these other sensors and potentially track space debris across even larger areas.
"The acoustic sensors sense things from thousands of miles away," Fernando said. "They are able to pick up the launch of the Starship rockets [from Texas] from Alaska. They potentially can listen out over the open ocean for the noise of reentries, which would be great because there's no seismic data and very little radar data over the ocean. So SpaceX may claim that Starlink satellites demise over the Pacific, but there is no way to actually check that out."
The study was published on Thursday (Jan. 22) in the journal Science.
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