How Elon Musk’s SpaceX Accidentally Helped DARPA Build A ‘RocketRadar’

A project originally designed to listen for underground explosions has unexpectedly tuned in to the skies. Researchers working on DARPA’s Atmosphere as a Sensor (AtmoSense) program recently uncovered a surprising pattern while analyzing Earth’s upper atmosphere: distinct disturbances caused by SpaceX’s Falcon 9 rockets during their return to Earth.

Launched in 2020, DARPA’s AtmoSense program aims to transform the atmosphere into a global sensing platform by capturing acoustic and electromagnetic waves generated by significant events. The initial goal was to monitor ground-based incidents like underground detonations, which could be of interest from a national security standpoint. But during a data analysis session focused on controlled explosions in New Mexico, researchers noticed something unusual.

“As the team was looking at the data, they saw a huge drop in what’s called total electron content that puzzled them,” said Michael Nayak, program manager for AtmoSense. He compared the atmospheric reaction to blocking water flowing through a hose—a sudden, measurable dip in activity.

That drop in electron content turned out to align with the atmospheric reentry of a Falcon 9 rocket. Curious, the team began digging through data from previous launches and found that this wasn’t a one-time event. The same kind of electron dip appeared over and over again, directly linked to Falcon 9’s reentry paths.

“The phenomenon is highly repeatable,” Nayak confirmed.

This finding widens the scope of AtmoSense. What started as a way to detect seismic-like events has now proven capable of tracking disturbances caused by spacecraft plunging through the atmosphere at high speed. This is particularly relevant for defense, scientific monitoring, and even traffic management in space.

AtmoSense operates in two phases. The first focused on building models to predict how waves from massive events, like earthquakes or volcanic eruptions, travel through the atmosphere. The second phase is about pushing those models further, seeing how they perform against smaller, more controlled activities.

In 2024, DARPA conducted two field campaigns in New Mexico that included six explosions ranging in size from one ton to ten tons. These tests were a success, showing strong alignment between predicted and observed wave data.

Meanwhile, SpaceX’s Falcon 9 rocket—now past its 450th mission—has become an unintended test subject. Its regular reentries and partially reusable design make it an ideal case for atmospheric sensing. AtmoSense has now proven it can reliably pick up on these reentry signatures.

DARPA plans to share the latest AtmoSense results at a virtual workshop from April 15 to 17. This new capability hints at a broader future where the atmosphere itself acts as a sensor grid for any event that causes a ripple in the sky, natural or man-made.