Earthquake sensors can hear space junk falling to Earth

This method relies on networks of seismometers, instruments designed to detect ground motion from earthquakes. The approach can deliver more precise information in near real time than what is typically available today, making it easier to find and recover debris that may be burned, damaged, or hazardous.

"Re-entries are happening more frequently. Last year, we had multiple satellites entering our atmosphere each day, and we don't have independent verification of where they entered, whether they broke up into pieces, if they burned up in the atmosphere, or if they made it to the ground," said lead author Benjamin Fernando, a postdoctoral research fellow who studies earthquakes on Earth, Mars, and other planets in the Solar System. "This is a growing problem, and it's going to keep getting worse."

The research was published on January 22 in the journal Science.

Reconstructing a Spacecraft's Final Path

Fernando and his coauthor, Constantinos Charalambous, a research fellow at Imperial College London, tested the technique by analyzing the reentry of debris from China's Shenzhou-15 spacecraft. The spacecraft's orbital module entered Earth's atmosphere on April 2, 2024. At about 3.5 feet wide and weighing more than 1.5 tons, the object was large enough to potentially endanger people, according to the researchers.

As space debris plunges into the atmosphere, it travels faster than the speed of sound. This extreme speed creates sonic booms, also known as shock waves, similar to those generated by military jets. These shock waves cause vibrations that move across the ground, triggering seismometers along the debris' path. By identifying which sensors detected the vibrations and when, scientists can trace the object's direction of travel and estimate where it may have landed.

What Earthquake Sensors Can Reveal

Using data from 127 seismometers across southern California, the team calculated both the speed and trajectory of the Shenzhou-15 module. The object raced through the atmosphere at roughly Mach 25-30, moving northeast over Santa Barbara and Las Vegas at about ten times the speed of the fastest jet aircraft.

The strength of the seismic signals also allowed the researchers to estimate the module's altitude and determine when it broke apart. Combining this information with calculations of speed and direction, they found that the debris traveled about 25 miles north of the path predicted by U.S. Space Command, which relies on orbital tracking before reentry.

Why Accurate Tracking Matters

As debris burns during descent, it can release toxic particles that remain in the atmosphere for hours and drift to other regions as weather patterns shift. Knowing the precise path of falling debris helps organizations understand where these particles may travel and which populations could be exposed, the researchers said.

Near real-time tracking also makes it possible to recover debris that survives the fall more quickly. Fast recovery is especially important because some objects may contain hazardous materials.

"In 1996, debris from the Russian Mars 96 spacecraft fell out of orbit. People thought it burned up, and its radioactive power source landed intact in the ocean. People tried to track it at the time, but its location was never confirmed," Fernando said. "More recently, a group of scientists found artificial plutonium in a glacier in Chile that they believe is evidence the power source burst open during the descent and contaminated the area. We'd benefit from having additional tracking tools, especially for those rare occasions when debris has radioactive material."

Complementing Existing Space Tracking Methods

Until now, scientists have largely depended on radar to monitor objects in low Earth orbit and predict when and where they would reenter the atmosphere. These forecasts can sometimes be inaccurate by thousands of miles. Seismic measurements offer a valuable addition by following debris after it enters the atmosphere, providing a record of its actual path.

"If you want to help, it matters whether you figure out where it has fallen quickly -- in 100 seconds rather than 100 days, for example," Fernando said. "It's important that we develop as many methodologies for tracking and characterizing space debris as possible."