On April 2, 2024, a fireball streaked across the skies of California. It was not a meteorite but a large piece of space debris from the Shenzhou 15 mission, which China had launched into orbit in November 2022 with three astronauts on board. One of the components of the spacecraft - the orbital module, weighing about 1,500 kilograms - returned to Earth a year and a half later in an uncontrolled manner, burning up mostly upon reentry into the atmosphere.
This space debris impact was used by scientists Benjamin Fernando and Constantinos Charalambous to test a new tracking technique for these fragments from spacecraft, rockets, or decommissioned satellites falling to Earth after drifting in space for a while, posing a safety problem: they can fall anywhere, posing risks to people, infrastructure, and the environment, as they can contain toxic, flammable, and even radioactive components. Most of them disintegrate due to friction and high temperatures upon reentry into the atmosphere, but larger objects or those made of certain materials can survive and impact the Earth's surface.
Space debris in orbit threatens operational satellites and the International Space Station (ISS) due to the risk of collisions in space. A portion of these space components returns to Earth, and the number has grown exponentially, although fortunately, most disintegrate completely or fall into the ocean, which covers most of the Earth's surface.
However, as these two scientists explain, "predicting the time and trajectory of reentry of an object is extremely difficult, and the available ground radar and tracking systems struggle to monitor space debris once they begin to disintegrate in the atmosphere."
These limitations complicate response planning and mitigation efforts. Therefore, tools are needed to quickly determine the trajectory, size, composition, and potential impact locations of falling space debris almost in real time.
As these two researchers describe in an article in the journal Science, their technique is based on analyzing data collected by terrestrial seismic sensors, instruments that record earthquake data. Using data collected by sensors in Southern California and Nevada, they analyzed the sonic booms from the reentry of Shenzhou-15. The sonic boom is the loud noise of the shockwave generated by an object traveling faster than the speed of sound. This allowed them to reconstruct the trajectory, speed, and impact location of the debris.
According to its creators, this "almost real-time tracking technique could help quickly determine the debris location on the ground or the spread of smaller hazardous particles in the atmosphere, crucial for recovery and pollution mitigation."
However, this technique does not allow determining in advance the area where a fragment will fall, which is one of the current major challenges: "We cannot predict in advance where a reentry will occur, but we can help respond quickly and effectively by verifying the ground trajectory, showing what actually happened. Therefore, we can only use it to track objects that are already disintegrating in the atmosphere," they explain.
In the article published in Science, they focus on the Shenzhou-15 event to demonstrate the effectiveness of the techniques they have developed and subsequently applied to objects reentering areas with less instrumentation, such as the Caribbean.
Their next steps, they reveal, will be to develop a process to collect known reentry events and search for sonic booms in seismic data, and then use that data to try to determine the object's trajectory: "This will take time and the development of more advanced algorithms to make it as fast and reliable as possible, but it is something we are working on," they point out.
For Benjamin Fernando, space debris "is a major concern": "Currently, multiple reentries occur every day. We urgently need new techniques to track and characterize debris once they are disintegrating in the atmosphere."