On April 2nd, the night sky over Los Angeles and much of Southern California was illuminated by a spectacular shower of fireballs, sparking widespread speculation among observers. Many wondered whether they were witnessing a natural meteor event, a failed rocket launch, or something more exotic like a UFO.

Astronomers have since confirmed that these flaming objects were debris from the orbital module of the Chinese spacecraft Shenzhou-15, which was launched in November 2022. This uncontrolled reentry raised alarms concerning the increasing dangers of space junk falling to Earth. However, the incident also paved the way for innovative methods among seismologists to track the trajectory of such debris using ground signals.

Sonic booms from the debris were detected by seismometers scattered throughout the Los Angeles basin. A team of researchers successfully reconstructed the spacecraft's path through the atmosphere based on these seismic records—a groundbreaking advancement that could potentially allow for the detection of incoming space debris globally, even without visual tracking instruments.

Lead author and planetary scientist, Benjamin Fernando, from Johns Hopkins University, noted the novelty of their work, stating, “I’m not aware of any previous research that has attempted to track and analyze space debris through seismic measurements.” This research is set to be presented at the AGU Annual Meeting 2024 in Washington, D.C.

The Growing Crisis of Space Debris

As space exploration grows with an increasing number of missions, the issue of uncontrolled space debris has turned from a rare concern into a significant hazard. “More and more spacecraft are being launched, and when they deorbit, pieces of debris rain down, posing serious risks to lives and property,” Fernando explained.

His interest in the April 2 event was piqued when he learned that many Los Angeles residents had audibly experienced the debris as it soared above. He hypothesized that sensitive seismometers would have recorded this phenomenon as well.

“While cameras and radar are effective for tracking space debris, they are limited in application, particularly in sparsely populated areas,” he said. “Seismometers can detect faint vibrations and noises, making them a promising tool for these kinds of events.”

When analyzing the seismic data collected from multiple stations that night, the researchers identified consistent sets of signals indicating the debris's trajectory, speed, size, and the populations it passed over before disintegrating.

Fernando also highlighted a previous collaboration involving NASA's OSIRIS-REx mission, which retrieved samples from the asteroid Bennu and relied on seismometers to trace its fiery descent over Utah in September 2023. He noted that the Shenzhou-15 event represents one of the pioneering instances where seismology has been leveraged to track space debris without prior warnings.

He believes that if adequately applied, this technique could offer crucial data for understanding incoming objects over less populated regions and possibly assist in predicting impacts on inhabited areas. “While it’s a challenging task—especially considering the complex terrain of Los Angeles—it’s feasible,” he added.

The Consequences of Tracking a Moving Target

Kathleen McKee, a geophysicist specializing in volcanic activity at Vanderbilt University, praised this innovative approach to using seismic data. “While tracking reentering objects towards Earth is vital for public safety, it comes with substantial challenges. Since these objects are moving through a constantly changing atmosphere, factors like wind and weather can complicate precise tracking.”

Despite the hurdles, McKee emphasized the importance of pursuing this research. “It’s a difficult problem to resolve, but certainly one that warrants further exploration. The technological advancements in our understanding of seismic waves can greatly benefit the safety and security of our skies as space exploration continues to expand.”

As the sky above becomes busier with satellites and space missions, the need for effective tracking of space debris is more critical than ever. Researchers like Fernando and McKee are at the forefront of turning seismic technology into a safety net against potential dangers from above, bridging the gap between geophysics and aerospace safety.

Stay tuned for more insights as this fascinating field of research develops, possibly transforming our approach to safeguarding Earth from the remnants of our ventures into space.