Seismometers Help Scientists Follow Space Debris Back to Earth

When space junk falls to Earth, it sometimes lands in a place not predicted by NASA or other space agencies. Scientists have now found a way to track down where the debris landed with seismometers, instruments used to detect earthquakes. When the spaceship punches through the atmosphere, it produces seismic waves that tell us when and where it touched down and where it was headed.

How Seismometers Pick up Space Debris

Seismometers are placed on the ground to detect seismic waves. They are very sensitive and can pick up the shockwaves of a sonic boom. When space junk reenters the atmosphere at high speed, it creates a sonic boom that vibrates the ground just enough for seismometers to register.

Planetary scientist Benjamin Fernando of Johns Hopkins University, lead author of the research, said, “While cameras and radars are generally excellent trackers of space debris, they are sparsely distributed in most places, especially unpopulated regions.”

He noted that seismometers are typically more capable of picking up tiny vibrations. His team reasoned that if people could see or hear the debris fall, the instruments should be able to detect it as well.

How Seismometers Picked Up the Debris That Fell on April 2nd

According to the study, detection begins once the debris starts falling and breaking apart, generating sonic booms that travel through the ground. After the shockwaves are recorded, scientists can backtrack them to identify the height, direction, and speed of the debris.

The method is especially useful once an object hits the denser part of the atmosphere and begins to spin and tumble—conditions that make continued tracking by radar or optical telescopes difficult.

On April 2, 2024, China’s Shenzhou-15 orbital module made a chaotic reentry that passed over Southern California. Residents of the Los Angeles area saw streaks of bright lights and heard loud booms.

Local astronomers confirmed it was space debris from the Shenzhou-15 mission, launched in November 2022. Scientists then used seismic data recorded that night from about 127 seismometers in the Southern California Seismic Network to determine the path of the debris.

They found that the space junk traveled about 20 to 30 kilometers (12 to 19 miles) south of the track predicted by U.S. Space Command before it fully broke up. These few kilometers could have made a significant difference if the debris had landed near a city.

Constantinos Charalambous, a planetary scientist at Imperial College London and co-author of the research, said this highlights substantial potential deviations and the value of ground-based validation.

The Shenzhou-15 case was notable because it was one of the first times researchers used seismometers to track debris with no prior preparations for the event. In September 2023, Fernando and his colleagues had tracked the controlled fall of NASA’s OSIRIS-REx sample return capsule in Utah using a combination of seismic and acoustic sensors.

Why the New Way of Tracking Space Debris Is Valuable

Space debris is a growing problem as more objects are placed in orbit, leading to more uncontrolled falls to Earth. Most objects burn up completely upon reentry, but some parts can survive and reach the ground. Surviving debris may contain residual fuel or other materials that pose a hazard.

Current radar and optical telescopes are less effective once reentry begins, and predicted landing zones can be off by hundreds of kilometers.

Seismic tracking does not provide early warning, but it can supply the path information shortly after reentry starts. It can help authorities identify potential hazards and prepare response measures. Fernando noted that the method could monitor falls over unpopulated areas where cameras and radar are scarce, and that it might also yield useful data near populated regions.

Kathleen McKee, a volcano geophysicist at Vanderbilt University not involved with the research, observed, “The challenge is that the object is moving in all three dimensions through an atmosphere that’s changing density with height and influencing how sound travels.”

The main advantage is that the approach uses seismic instruments already in place. Thousands of seismometers worldwide monitor earthquakes and other events.

The method works best in areas with a high density of sensors, such as the Los Angeles basin, because sonic booms travel only about 100 kilometers (62 miles) before losing energy. In remote regions or over oceans, the necessary infrastructure may be lacking. Smaller objects that burn up completely rarely produce detectable seismic signals.

Despite these limitations, researchers view the technique as a cost-effective supplement to existing tools. Combining radar, optical telescopes, satellite tracking, and seismic observations can provide a fuller picture of a reentry from the moment it begins until the debris reaches the ground, allowing a three-dimensional reconstruction that may improve future predictions.

The work is part of a growing field sometimes called environmental seismology, in which seismologists use existing networks to track phenomena unrelated to earthquakes, such as avalanches or thunderstorms. Individual seismometers have already been used to follow storms or large gatherings of people, so tracking space debris is a natural extension.

Space surveillance stands to benefit from these advances in seismology. With continued progress, monitoring falling space junk may become more effective in the years ahead.