Nuclear sensors track space debris falling to Earth

 Cold War-era nuclear Sensors designed to detect nuclear detonations could hunt space debris   

A network of Cold War-era infrasound sensors can help scientists reconstruct trajectories of falling space objects. The entry of space debris into Earth’s atmosphere often occurs in its most remote and unmonitored areas. Tracking these events has always been a challenge. Scientists are now turning to an unexpected tool to track these cosmic visitors: infrasound sensors originally built to detect nuclear blasts. Scientists are studying how sensors designed to detect nuclear tests could help track space junk and meteorites crashing down in the world's most remote regions. Sandia National Laboratories scientists have shown that the existing global infrasound network could be used to track atmospheric entry angles.

Across the world, dozens of supersensitive detectors have been installed since the beginning of the Cold War era to detect infrasound waves created by nuclear tests thousands of miles away. Infrasound refers to sound waves far below the range of human hearing, similar to how the infrared range of light is far below the threshold of human eyesight. These detectors, part of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) detection network, also pick up the rumble of a thunder or the ultrasonic booms generated by large pieces of space rocks or space debris disintegrating in Earth's atmosphere. Comprehensive Nuclear-Test-Ban Treaty Organization manages these detectors. With incredibly high sensitivity, these sensors can pick up faint sounds from vast distances. Researchers are now studying how these sensors could help reconstruct trajectories of re-entering space debris, especially those that crash down in remote areas where the more commonly used optical cameras and telescopes are not available. According to one leading scientist in this field of study, these sensors can offer unique advantages over other methods of tracking objects falling from space. "The advantage of using the regional and global infrasound sensor network for studying trajectories of bolides and space debris is that it provides truly worldwide coverage operating continuously day and night and in all weather conditions," Elizabeth Silber, a scientist at Sandia National Laboratories in the US, said.

As of now these acoustic ears aren’t just tuned to nuclear detonations. The sensors also detect the explosive sounds generated by large space rocks or inactive satellites as they break apart and hurtle through Earth’s atmosphere. "Unlike optical observations, which require clear skies and darkness, or radar systems, which have limited range and are geographically constrained, infrasound waves can propagate thousands of km's with minimal loss of signal," said Silber, who is the lead author of a new study exploring the advantages and limitations of this detection method. These infrasound sensors can help determine falling space objects' trajectory using a method known as triangulation which compares signals received by two or more sensors to establish the location of the source. "Steep-angle events deposit their energy along a relatively short, vertical segment of the atmosphere, making them behave almost like a point-source explosion," Silber said. "This compact geometry means the sound waves emitted travel along nearly identical paths, resulting in consistent arrival directions at distant infrasound sensors."

This global infrasound network offers uninterrupted, 24/7 monitoring across the entire planet, unaffected by weather conditions. Furthermore, infrasound waves can propagate over vast distances with very little loss of signal strength. The researchers wanted to know how accurate such calculations can be depending on the angle at which the object enters the atmosphere. They found that while trajectories of space rocks and junk which fall into the atmosphere at steep angles of 60 degrees or more are easy to reconstruct from infrasound measurements, the same doesn't apply to objects flying through the atmosphere at shallower angles. On the other hand, pieces of space junk and meteorites which enter at shallow angles generate confusing data when measured by the infrasound sensors as they produce audible signals along a path of hundreds, even thousands of km's. "At distant observing stations, signals from different segments of that long trajectory can dominate, causing significant variability and uncertainty in the measured arrival directions," Silber explained.

A research team has been investigating the potential of these sensors for reconstructing the paths of space debris as it re-enters the atmosphere. Interestingly, this tech could also be used to track bolides, which are large meteoroids which fragment in the sky. The high energy released by these events creates shock waves that travel thousands of km's. Images from cameras and telescopes, on the contrary, tend to do a good job reconstructing the trajectories of objects entering at shallow angles which streak across the sky like stunning shooting stars. Such instruments, however, are not available to monitor the skies above the remote regions of the world's oceans where most space junk and meteorites crash to Earth or burn up in the atmosphere. That's why scientists are trying to figure out whether combinations of different types of measurements could provide more accurate data. The limitations of infrasound measurements, restrict the usability of such data in most cases of satellite re-entries, which are usually guided into the atmosphere gradually at shallow angles, Silber admitted.

The challenge is that bolides create sound as they move, not from a single point. Therefore, different infrasound stations may detect signals from various points along the trajectory, complicating the accurate location of the source. “Infrasound from a bolide is more like a sonic boom stretched across the sky than a single bang. You must account for the fact that the sound is being generated along the flight path,” Silber noted. "This is because their orbits decay gradually due to atmospheric drag, causing them to spiral inward over time rather than plunging steeply." Most meteorites, too, enter at angles smaller than 60 degrees, with 45 degrees being the most common angle at which space rocks hit the atmosphere. The researchers are trying to understand to what extent the infrasound sensors can help understand the trajectories of such objects and how the results could be improved. "Objects re-entering from low Earth orbit (LEO) generally do so at extremely shallow angles," Silber said.

Reportedly, Silber created a specialized computer model, BIBEX-M, to test how different atmospheric entry paths affect infrasound detections. This model analyses subtle sound variations recorded by CTBTO sensors to calculate the most probable trajectory of meteors and space debris. Scientists can determine the path of a falling object by using triangulation. This involves comparing when the infrasound signals from the object reach different sensors in the network. Although the sensors cannot provide advanced warnings about incoming pieces of space rock or junk, scientists are keen to use the data to learn more about these potentially dangerous events. "Although infrasound detection cannot deliver real-time warnings, it does play an essential role in characterizing events, assessing potential impacts and guiding response and recovery efforts," said Silber. Her findings indicate that steep entry angles (over 60°) allow for accurate trajectory analysis using infrasound. However, shallower entry angles lead to increased uncertainty in determining the trajectory, a problem the team will further work on. Knowing the trajectory of falling space junk is important for preparedness. Without this information, it’s difficult to anticipate its landing location and take necessary precautions. With the increasing number of future space missions planned, the volume of artificial objects in orbit is set to grow significantly. According to ESA, Earth’s orbit is cluttered with 130 million fragments of space debris larger than a millimetre, endangering satellites in the present and future. As these defunct satellites, rocket stages, and fragmented pieces eventually re-enter Earth’s atmosphere, they pose a growing risk. While many smaller pieces burn up completely, larger and denser objects can survive the fiery descent and impact the Earth’s surface at different places.