Beyond Bennu: How OSIRIS-REx is helping scientists study the sonic signature of meteoroids

In the high desert of Nevada, Elizabeth Silber watched NASA’s Sample Return Capsule from OSIRIS-REx descend into Earth’s atmosphere Sunday, but unlike most scientists, she wasn’t there for the asteroid rocks.

Silber, a physicist at Sandia National Laboratories, is working with researchers from Sandia and Los Alamos National Laboratories, the Defense Threat Reduction Agency, TDA Research Inc., the Jet Propulsion Laboratory, the University of Hawaii and the University of Oklahoma in a campaign to record and characterize the infrasound and seismic waves generated by the capsule as it moved through Earth’s atmosphere at hypersonic speed, about 26,000 miles per hour.

This was the largest observing campaign of any hypersonic event in history, and Silber hopes the data will improve scientists’ ability to use infrasound to detect meteoroids and other objects traveling at hypersonic speeds.

Scientists currently use infrasound, a low-frequency sound wave generally inaudible to humans, to detect and observe volcanic activity, earthquakes and explosions. Silber said infrasound can also be observed when meteoroids enter Earth’s atmosphere, but atmospheric conditions such as wind can distort the signal, and there is usually relatively little information available about the incoming meteoroid to aid in data analysis.

“The OSIRIS-REx capsule is the perfect candidate to study a hypersonic event because we know everything about it, the entry angle, the velocity, the spin rate, the size and the mass, and we can use that information to calibrate our models and test the capabilities of our sensors ,” Silber said.

“Because the capsule was traveling faster than the speed of sound, it generated a shock wave. As the shock wave propagated away from the capsule, it turned into detectable infrasonic waves.”

The multi-agency team launched four solar balloons and two weather balloons equipped with microbarometers in Nevada and had ground-based sensors at several locations.

“These are tiny sensors that measure small changes in air pressure,” Silber said. “Infrasound is a pressure wave, and when we group sensors in an array, we can determine the direction the infrasound is traveling from.”

Silber said the group had an unprecedented number of sensors recording data, including 45 single sensors, a large rectangular array with 200 sensors and three arrays consisting of four sensors in a triangle formation. First, the team checked to see how many sensors detected the signal. Back in the lab, Silber and her colleagues will conduct a more comprehensive study.

“We want to determine where along the capsule’s trajectory the shock wave came from,” Silber said. “The wave will be a continuous thing along the path, so the question will be where exactly did that signal originate? From a certain height? From different parts of the path?”

The team plans to compare signals recorded from different locations in Nevada and Utah to see if they point to the same point of origin along the capsule’s track. Because the capsule’s speed will change as it dives toward the surface, moving from hypersonic to supersonic to transonic, the team will also be able to study all stages of flight.

“In addition, we will study how strong acoustic waves propagate, test how well our instruments can capture signals, and study the effects of the atmosphere on infrasound waves,” Silber said. “All of this will increase our knowledge and ability to use infrasound to detect meteoroids and man-made objects with infrasound.”

In preparation for this campaign, Silber, Daniel Bowman, and Sarah Albert published a paper in Atmosphere reviews previous infrasound and seismic observational studies from the four other sample-return missions that have occurred since the end of NASA’s Apollo missions, and summarizes their utility in characterizing the flight of meteoroids through Earth’s atmosphere.

Silber is also leading a separate but similar Laboratory Directed Research and Development project to investigate whether infrasound can be used to determine the height and speed of bolidesbright, exploding meteoroids in situations when other types of sensors do not provide sufficient data.