December 3rd, 2018, NASA’s Origins, Spectral Interpretation, Resource Identification, and Security?Regolith Explorer (OSIRIS-REx) successfully rendezvoused with the Near-Earth Asteroid (NEA) 101955 Bennu. Over the next two years, the mission collected samples of rock and regolith from the asteroid’s surface. By September 24th, 2023, the mission’s sample return capsule (SRC) entered Earth’s atmosphere and was collected by NASA scientists. Analysis of these samples is already providing insight into what conditions were like during the early Solar System.

According to a recent study, the known trajectory and timing of the SRC’s return provided a rare opportunity to record geophysical signals produced by the capsule using a new method. Because it was traveling at hypersonic speeds as it flew through the atmosphere, the SRC’s return produced a sonic boom that impacted the ground. Using distributed acoustic sensing (DAS) interrogators and surface?draped fiber?optic cables, the team carried out the first reported recording of an SRC re?entry with distributed fiber?optic sensing technology.

The team was led by Dr. Carly M. Donahue and consisted of her colleagues from the Earth and Environmental Sciences Division at the Los Alamos National Laboratory (LANL), as well as the Department of Geosciences at Colorado State University and fiber optic-based distributed sensor developer Silixa LLC. The paper that details their findings, “Detection of a Space Capsule Entering Earth’s Atmosphere with Distributed Acoustic Sensing (DAS),” recently appeared in the journal Seismological Research Letters.

Since the end of the Apollo Era, scientists have studied sample return capsules re-entering Earth’s atmosphere. These studies have not only helped scientists develop safe and effective methods for sample-return missions but also provided insight into the atmospheric entry of meteoroids and asteroids. Until now, these studies employed infrasound and seismic sensors to record the resulting geophysical signals. However, since the trajectory and timing of the OSIRIS-REx mission’s SRC were known in advance, Dr. Donahue and her team saw an opportunity.

As Dr. Donahue told Universe Today via email, the re-entry was a chance for them to test DAS systems with fiber optic cables as a means of recording the geophysical effects produced by the sonic boom. “DAS systems interrogating an optical fiber are still relatively rare,” she said. “Knowing ahead of time the precise trajectory gave us the scarce opportunity to situate multiple DAS interrogators near the point of highest heating and capture the sonic boom as it impacted the ground.”

The team rapidly deployed two DAS interrogators and more than 12 km (7.45 mi) of surface?draped fiber?optic cables. Their network included six collocated seismometer?infrasound sensor pairs, all of which were spread across two sites near the town of Eureka in the Nevada Desert. As Dr. Donahue described:

“Once the team got the hang of rolling out the 4 spools of optical fiber that each weighed over 100 kgs, installing and retrieving the fiber took less time than setting up the six co-located seismic and infrasound stations. Approximately 5 km of the optical fiber was located at the local Eureka airport, along with many other teams deploying sensors such as infrasound, seismic, and GPS.  The other 7 km of fiber was located along a remote dirt road in Newark Valley.”

With the help of this network, the team obtained a stunning profile of the sonic boom as it struck the ground. The DAS interrogators recorded an impulsive arrival with an extended coda that had similar features to those recorded by the seismometers and infrasound sensors. Whereas traditional sensors only measure sonic booms at one point, Dr. Donahue said that her team’s data revealed how the boom’s wavefront transformed as it impacted the irregular terrain of the Nevada landscape.

In addition to being the first time these methods were used to record an SRC reentry, the results of this test could have significant implications when it comes to predicting potential meteor and asteroid strikes. Said Dr. Donahue:

“By having an extremely dense array of sensors, DAS has the possibility of better characterizing the trajectory and size of a meteor. The topology (e.g., hills) of the ground is known to have an influence on wavefront recorded at the surface of the earth. By having a dense line of sensors that span over the changes in the earth’s elevation, these effects could be better accounted for to produce a more accurate characterization of a meteor’s trajectory.”

Following the completion of its primary mission, the OSIRIS-REx, NASA prepped the spacecraft for the next phase of its mission. In 2029, the spacecraft – renamed the OSIRIS-APEX (Apophis Explorer) – will rendezvous with the Near-Earth Asteroid 99942 Apophis and collect another sample.

Further Reading: GeoScienceWorld