Scientists study meteors to learn about interplanetary space and to better understand impact risks, among other reasons. Unfortunately, meteors are unpredictable, liable to enter the Earth’s atmosphere with little warning. That makes it difficult for researchers to test the kinds of sensing equipment that could help the scientific community understand the behavior of meteors and objects returning to Earth from space.
On September 24, 2023, however, researchers from Los Alamos and Sandia national laboratories had advance warning of the arrival of a meteor, albeit an artificial one: the sample return capsule of the National Aeronautics and Space Administration’s (NASA’s) OSIRIS-REx spacecraft.
NASA launched OSIRIS-REx in 2016. The spacecraft, whose name is an acronym for Origins, Spectral Interpretation, Resource Identification, and Security—Regolith Explorer, was tasked with traveling to the 4.5-billion-year-old asteroid Bennu and collecting about a coffee cup’s worth of dust and rocks from the asteroid’s surface. This sample is expected to provide researchers with invaluable insight into the earliest days of the universe.
Onboard OSIRIS-REx, the sample was packaged in a steel container, which was in turn sealed inside a conical return capsule the size of a small air conditioner. When OSIRIS-REx passed by Earth on September 24, the spacecraft released the sample return capsule for a vertiginous plunge back to the planet. After falling some 150 miles, the capsule deployed a parachute and touched down safely in the U.S. Department of Defense’s Utah Test and Training Range.
Before landing in the Utah desert, though, the capsule reached speeds of nearly 28,000 miles per hour. Those speeds made the capsule the second-fastest artificial object ever to travel through Earth’s atmosphere (the fastest was the sample return capsule that came back to Earth in 2006 as a part of NASA’s Stardust mission, clocking in at some 28,860 miles per hour).
As the OSIRIS-REx return capsule streaked through the atmosphere, heated to temperatures greater than 5,300 degrees Fahrenheit, the capsule generated sound and gravity waves. When the capsule passed near the town of Eureka, Nevada, researchers from Los Alamos and Sandia—the latter of which led the interlaboratory collaboration—used advanced sensing equipment to detect these waves.
A team led by Los Alamos’ Chris Carr, who also directed the Laboratory’s involvement in OSIRIS-REx generally, positioned itself beneath the capsule’s trajectory and used distributed acoustic sensing (DAS) technology to monitor the capsule’s return. DAS involves deploying a network of fiber-optic cables to detect seismic or acoustic signals. In the case of the OSIRIS-REx capsule, DAS sensors were deployed over more than 7 miles.
According to the Laboratory’s Carly Donahue, who oversaw the deployment of DAS during the capsule return, interest in DAS has been “exploding” in recent years as the technology is increasingly deployed for seismic, pipeline, and traffic monitoring, among other applications.
“There are many unknowns about DAS,” Donahue says. “This is the first time that DAS sensors have been used to record an artificial meteoroid, and we were thrilled to see the signal clearly over kilometers of fiber. This expands our knowledge of the signal frequencies and pathways that DAS is capable of acquiring.”
Another Los Alamos team, this one led by physicist Bob Haaser, was also positioned beneath the capsule’s flight path. Haaser’s team used ground-based GPS receivers to measure the way that sound and gravity waves made by the capsule spread upward into the ionosphere (a part of the atmosphere that extends from 50 to 600 miles above the Earth’s surface).
Finally, a team led by geophysicist Phil Blom was positioned some distance away from the flight path, using instruments called microbarometers to measure infrasonic sound waves—that is, sound waves whose frequency lies below the range of human hearing—emitted by the capsule.
“We’re building a more complete model of how infrasound is generated by sources,” Blom says. “The capsule return gave us a chance to test those models.”
Researchers from Sandia, meanwhile, deployed infrasound and seismic equipment on the ground and on weather balloons to monitor the capsule’s return.
OSIRIS-REx provided a unique opportunity to test these varied sensing technologies. Unlike other objects that travel through the atmosphere to Earth, researchers knew ahead of time the sample return capsule’s size, velocity, and entry angle. Knowing these details provided a kind of “ground truth” that researchers could use to verify sensor measurements.
The combination of all instrumentation deployed by scientists from Sandia and Los Alamos made the OSIRIS-REx sample capsule’s return better monitored than any comparable event. “We had orders of magnitude more instruments for this project than for any other capsule return,” Carr says. “There were more components under the capsule’s trajectory alone than there were in total for the Stardust mission.”
That degree of preparation ensured that researchers from both labs were able to make the most of a unique research opportunity.
“It’s like lightning: you never know when it’s going to strike,” Haaser says. “But this time we knew when and where it was going to strike.” ★