DARPA Tried Using the Earth's Atmosphere as a Giant Sensor and Detected Something Interesting

While trying to determine if it could use the entire Earth's atmosphere as a massive sensor, the US military’s Defense Advanced Research Projects Agency (DARPA) made a highly unusual detection.

The agency — which has overseen the development of cutting-edge military technologies and out-there concepts for decades — encountered the distant signature of a SpaceX Falcon 9 reentering the atmosphere during a test in New Mexico last year.

The goal of DARPA's AtmoSense program, which kicked off in late 2020, is to determine whether acoustic and electromagnetic waves propagating through the Earth's atmosphere could allow the US military to detect and locate major events and disturbances, such as earthquakes and volcanic eruptions, across the planet.

The idea is to eventually hone in on the location of illicit underground explosions or "other national security-relevant events" — highly relevant data for the US military.

And according to a recent DARPA statement, the program works surprisingly well. In 2024, DARPA conducted two field tests in New Mexico, detonating six 1-ton and 10-ton controlled explosions to test the concept. The agency found that AtmoSense models accurately predicted these blasts, giving credence to the idea of using the atmosphere as a massive sensor.

But while DARPA scientists were poring over the data, they found a strange and unexpected disturbance.

"As the team was looking at the data, they saw a huge drop in what's called total electron content that puzzled them," said AtmoSense program manager Michael Nayak in the blurb.

Nayak used a simple analogy to explain what had happened.

"Imagine that you have water going through a hose," he explained. "That's a flow of electrons, and if you put your fist in front of the hose, you'll notice a significant drop in water volume coming out of the hose."

The apparently culprit? They realized the drop in electron content correlated to a Falcon 9 rocket reentering on the same day as the controlled New Mexico blasts.

"Then they decided to pull other SpaceX reentry data, across dozens of launches, to see if they could spot a similar electron drop," said Nayak. "The phenomenon is highly repeatable. We discovered an unplanned new technique for identifying objects entering the earth's atmosphere."

According to the program manager, its latest tests prove that the system works, potentially giving the US military a potent detection system.

"High-resolution surface-to-space simulation of acoustic waves was considered impossible before the program began, but we accomplished it," he said.

"We can now model across six orders of magnitude, in 3D, what happens to the energy emanating from a small, meters-scale disturbance as it expands up into the atmosphere to propagate over thousands of kilometers, and potentially around the world," Nayak added.

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