Within the early morning of April 2, 2024, the sky over southern California lit up with flashes of blazing light. Residents were bewildered. Were they missiles? A crashing plane? The weird activity confused even experts—until they realized it was a disposable a part of China’s Shenzhou-15 spacecraft burning up within the atmosphere because it returned to Earth.
Scientists knew the event was on the horizon and had mapped out a possible entry point over the northern Atlantic Ocean, 1000’s of miles from metropolitan Los Angeles. Luckily, nobody was hurt because the module broke apart over town.
However the incident underlined an uncomfortable truth. We’re nowhere near having the ability to accurately predict the trail of space debris because it rains down. As more spacecraft are launched and reenter the atmosphere, damage to infrastructure and Earthlings is simply a matter of time.
Researchers are looking into an answer from an unexpected source: sensors that measure earthquakes. As space debris plummets to the bottom at hypersonic speeds, it generates a sonic boom. This causes a slight tremor in the bottom that the sensors readily register.
Using data from a network of those sensors, Benjamin Fernando at Johns Hopkins University and Constantinos Charalambous at Imperial College London developed a system that may reconstruct the trail of space debris with unprecedented accuracy. They used the system to map Shenzhou-15’s speed, altitude, gradual disintegration, and final destination.
To be clear, this isn’t an early warning system. Because sonic booms lag behind the objects causing them, the strategy is sort of a forensic reconstruction of space debris’ final journey. Still, it will probably quickly discover potential fall-out zones for faster retrieval and cleanup, which is very essential if the junk is toxic or radioactive.
The work is “an important step toward near-real-time monitoring of natural and anthropogenic objects entering from space,” wrote Chris Carr on the Los Alamos National Laboratory, who was not involved within the work.
An Embarrassment of Riches
Launching satellites was once a colossal undertaking. But because of innovations by SpaceX and national space agencies the world over, it’s becoming much more routine.
These spacecraft have already modified life on Earth. Hundreds of Starlink satellites beam the web to previous dead zones and disaster areas. Miniature satellites at the moment are an inexpensive research platform scientists use to profile weather, measure solar winds, and track the results of microgravity and radiation on living cells. And a recent space race will only grow the fleets of spacecraft already blanketing the Earth.
“The massive change that we’ve seen since 2020 is the rise of satellite mega-constellations…corporations not putting up a dozen spacecraft, but perhaps a thousand or ten thousand over the course of a couple of years,” Fernando told Science.
Mega-constellations have already caused problems for scientists by polluting astronomical images with vivid streaks. They might also increase the speed at which space debris rains down. In a paper describing their system, Fernando and Charalambous write that in 2025 there have been roughly 4 to 5 re-entries a day, and the numbers are prone to rapidly grow.
We already monitor spacecraft in orbit. Telescopes bring real-time visuals. Radar tracks location and speed. But these tools struggle as a spacecraft drifts into the Earth’s upper atmosphere.
The interaction between fragments and air becomes “really chaotic,” said Fernando. “We will now not predict with particularly good accuracy exactly where [and when] a bit of re-entering space debris goes to enter the atmosphere.”
Radar can track spacecraft parts as they return to Earth, however the technology is proscribed to small regions of the world and barely covers the oceans. Even after we know the ultimate fate of a bit of debris, it’s often difficult to reconstruct its full trajectory.
Supersonic Waves
The brand new work was inspired by the way in which scientists track meteoroids using a dense network of earthquake sensors to detect tiny vibrations in the bottom.
The Shenzhou-15 capsule entered the atmosphere going roughly 25 to 30 times the speed of sound. Like a fighter jet, it triggered a robust sonic boom roughly 80 kilometers (50 miles) above the bottom. The boom traveled to Earth’s surface where seismic sensors detected it.
It’s like picking up an earthquake, only “on this case the waves are coming from up versus with earthquakes they have a tendency to come back from down,” said Fernando.
Southern California is heavily dotted with seismic sensors, each measuring activity in a small area. To model the spacecraft’s path and speed, the team compiled the biggest sonic boom each sensor registered and its arrival time and compiled the info right into a map.
The map captured where, when, and the way the capsule broke down because it hurtled through the atmosphere. Earlier on, the sensors recorded large, discrete signals. These later became more scattered and sophisticated, suggesting the capsule step by step disintegrated slightly than blowing up suddenly.
The outcomes are “consistent with on-ground observations, including videos and witness reports of multiple fireballs flying across the sky,” wrote Carr. After more deeply combing through the info, the team showed it may be used to measure the dimensions of each bit of decaying debris.
The spacecraft’s sonic signature differed from those generated by meteorites, making it possible to tease apart human-made objects and people of natural origins.
Differentiating the 2 categories is vital. Meteorites pose “kinetic risk” as chunks slam into the bottom, damaging cars, houses, and other infrastructure. Human space debris, nevertheless, could also contain metals, toxic or flammable material, or in rare cases, radioactive components. The model also reconstructed how different parts of the spacecraft disintegrated, potentially making it easier to predict whether chunks have burned up completely within the atmosphere or have reached the bottom, making it useful for recovery or clean-up missions.
Crash-and-burn isn’t a spacecraft’s only destiny. Engineers are also working to maneuver defunct satellites into higher orbits that will be stable for “1000’s of years” in line with Fernando, though this doesn’t solve the space junk problem. Other researchers are exploring ways to design spacecraft such that they completely burn up each safely and predictably.
For now, the technology works best in places with plenty of seismic sensors, that are rare. But there’s a push so as to add sensors in places which can be vulnerable attributable to sensitive ecology or geology at prices far lower than constructing radar systems to trace re-entry, said Fernando.