Sonic Booms And Satellites: Scientists Devise New Method To Track Falling Space Debris

Hyderabad: Earth is surrounded by thousands of human-made objects, orbiting the planet to provide numerous services for the betterment of life for the people on the surface. However, when these satellites complete their work and run out of fuel, they become space junk. Though scientists plan for their safe retirement, either through controlled re-entry or pushing them into a graveyard orbit, based on their size and orbit, the chances of an unexpected fall are never zero and are always the case when a satellite fails prematurely and can no longer receive commands from Earth.

When such satellites don't burn up in the Earth's atmosphere, they fall to the surface—posing a risk to humans. To locate possible crash sites, scientists from Johns Hopkins University have devised a way to track falling debris from space, providing authorities with more detailed information in near real-time, allowing quick retrieval of the charred remains, which could also sometimes be toxic for humans.

The findings, published in the journal Science, explain the use of existing networks of earthquake-detecting seismometers to track falling space debris.

Since the space debris moves faster than the speed of sound, producing sonic booms similar to fighter jets, the vibrations from the shockwave trailing behind rumble the ground and ping seismometers along the way. Researchers mapped the activated seismometers to deduce the trajectory of the falling trash, determining its direction and estimating the landing spot.

By mapping areas where seismometers in southern California detected sonic booms, researchers at Johns Hopkins University and Imperial College London were able to track the path of the Shenzhou-15 orbital module after it reentered the Earth's atmosphere on April 2, 2024. (Image Credits: Benjamin Fernando / Johns Hopkins University)

Using this method, scientists reconstructed the path of debris from China's Shenzhou-15 spacecraft after the orbital module entered the Earth's atmosphere on April 2, 2024. They analysed data from 125 seismometers in southern California and calculated the path and speed of the module, which weighed over 1.5 tons and measured roughly seven feet. Cruising at Mach 25-30, the module streaked through the atmosphere at roughly 10 times the speed of the fastest jet in the world.

The researchers also calculated the module's altitude using the intensity of the seismic readings and pinpointed how it broke into fragments. Then, they used trajectory, speed, and altitude calculations to estimate that the module was travelling approximately 25 miles south of the trajectory predicted by US Space Command based on measurements of its orbit.

So far, scientists had to rely on radar data to follow an object decaying in low Earth orbit (LEO) and predict where it would enter the atmosphere. However, re-entry predictions could be off by thousands of miles in the worst cases. The new method allows seismic data to complement radar data by tracking an object after it enters the atmosphere, providing a measurement of the actual trajectory.

Scientists behind the new method say that near real-time tracking will help authorities quickly retrieve objects that make it to the ground, reducing the possibility of debris causing damage to its surroundings in case it carries harmful substances.

Lead author Benjamin Fernando, a postdoctoral research fellow at Johns Hopkins University, explained the harm space debris can cause using an example from 1996 when the debris from the Russian Mars 96 spacecraft fell out of orbit. It was believed that the debris had burned up, but 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."

He further added that we would benefit from having additional tracking tools, especially for those rare occasions when debris contains radioactive material. He explained that if we want to respond effectively, it's crucial to find out where the debris has landed as quickly as possible—ideally within 100 seconds rather than 100 days.

"It's important that we develop as many methodologies for tracking and characterising space debris as possible," he concluded.