Sonic booms can protect Earth from dangerous space junk

Sonic booms can protect Earth from dangerous space debris

By Lee Billings edited by Clara Moskowitz

As the global number of space launches skyrockets, the amount of dangerous space debris that re-enters the atmosphere and falls back to Earth also increases, increasing the chances that, sooner or later, disaster will strike. Most space debris is so small that it burns completely as it falls. Larger objects from NASA and most other space agencies typically follow a “controlled” re-entry: pushed by rocket engines, they plunge toward remote, desolate regions of the planet. But the continued resumption of space activity has led to a growing number of riskier uncontrolled re-entries.

Scientists have now found a new way to monitor these potentially dangerous objects moving through Earth’s atmosphere. It turns out that sonic booms picked up by pre-existing networks of seismometers can reconstruct descent trajectories and locate crash sites of derelict spacecraft and large debris. Led by Benjamin Fernando, a postdoctoral researcher at Johns Hopkins University, in collaboration with Constantinos Charalambous, a researcher at Imperial College London, a study detailing the findings was published today in Science.

“This is a very useful additional tool in our toolbox,” says Jonathan McDowell, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and a spaceflight tracker, which was not part of the study. Optical telescopes and radar systems routinely monitor space debris, he notes, but both have trouble tracking debris as it disintegrates during re-entry — and optical systems really only work at night. “Sonic booms should work day or night,” says McDowell. “And since these seismic networks are already operational, you will be able to get this 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-ton module that had been left in orbit in 2022 by the Chinese crew Shenzhou-15 The mission underwent an uncontrolled atmospheric reentry at supersonic speed. Passing ominously over major population centers on six continents, the large, heavy module’s decaying orbit had sparked international concern and even prompted U.S. Space Command to predict that re-entry debris would eventually fall into the North Atlantic. This prediction turned out to be wrong by several thousand kilometers. Although the module largely burned up as it streaked across the skies of Southern California and no confirmed debris was found, any that reached Earth likely landed in the Pacific Ocean or the western United States.

Fernando, who also studies extraterrestrial earthquakes on Mars and other worlds, decided to take a closer look after realizing that shock waves from supersonic Shenzhou-15 the debris should have manifested as sonic booms in the dense networks of seismometers that line earthquake-prone Southern California. When he and Charalambous manually sifted through the networks’ open source data, they found the reentry recorded on more than 120 monitoring stations. Together, the duo analyzed the arrival times of the strongest shock waves at each location.

“From there, we were able to determine [the module’s] “Once an object burns up and breaks up in the atmosphere, it actually becomes quite difficult to track,” he says, “which also makes it harder to understand its impacts on the atmosphere, the risk it poses to aviation and the threat it poses wherever it hits the ground.

None of these questions are trivial. Many atmospheric scientists are increasingly alarmed by high levels of vaporized aerospace-derived materials in the upper atmosphere, some of which could harm Earth’s protective ozone layer. Air travelers have faced near misses before, like when a test flight of SpaceX’s Starship vehicle last year scattered debris across part of the Caribbean and forced planes to take evasive action. The list of significant debris that has fallen to Earth too close to allow the comfort of inhabited areas is worrying. And aside from simple impact threats, some of this debris contains materials, such as radioactive isotopes for nuclear reactors or volatile and toxic rocket fuel, that are very dangerous to the environment.

Sonic booms are unlikely to provide sufficient delay for, say, an airliner to escape a collision course with free-falling space debris. But this method could prove essential for locating hazardous debris on the ground to aid recovery and remediation efforts. It could also be a game-changer in improving disruption patterns at the top. “This is really important,” McDowell says, “both for designing a spacecraft so that it decays more efficiently upon re-entry and for understanding the extent to which a spacecraft ablates in the atmosphere to potentially change atmospheric chemistry.”

The big question is whether significant investments will be made to change the long-standing status quo. “For 60 years we’ve let things come in unchecked, knowing that for the larger ones, a fraction will reach the surface,” McDowell says. “We just hoped it wouldn’t hit anyone in the head or cause any other damage. But ultimately we won’t be lucky.”

For the technique, Fernando envisions two paths forward, both of which would treat the challenge of tracking the sonic boom as a “big data” problem. The first would exploit current seismic arrays, particularly on the west coast of the United States, where such arrays are already established and orbital dynamics demand that more re-entry events occur. The second would focus on new, custom-built networks in other parts of the world facing increasing amounts of space debris. “Take for example the ecologically sensitive Great Barrier Reef off the northeast coast of Australia,” says Fernando. “Many Chinese rockets have been launched from there since [a launch site on the Chinese island of] Hainan. Setting up a seismic network there would be extremely cheap compared to alternatives such as building a network of radar stations.”

Experts hope the public and policymakers will wake up to the growing problem of space debris. “It’s only going to get worse,” Fernando says. “I worry that space debris won’t get the attention it deserves until something truly catastrophic happens – and I guess the chance of that happening is 100%. »