How DARPA accidentally invented ‘rocket radar’

Scientists were not trying to track rockets. But when they listened to the sky, it spoke — loud and clear.

During a routine test of atmospheric sensors, researchers with the US Defence Advanced Research Projects Agency (DARPA) picked up something unusual. They were studying how waves travel through the Earth's atmosphere as part of their AtmoSense programme. The aim was to detect disturbances like earthquakes or volcanic eruptions using the atmosphere itself as a global sensor.

Instead, they detected the reentry of a SpaceX Falcon 9 rocket.

While analysing data from controlled explosions in New Mexico, the team noticed a sharp dip in electron content in the upper atmosphere. This disturbance was traced back to the same time and location as a Falcon 9 reentry.

"The phenomenon is highly repeatable," said Michael Nayak, AtmoSense programme manager in a statement. "We discovered an unplanned new technique for identifying objects entering the earth's atmosphere."

Nayak explained the method by comparing electron flow to a garden hose, "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."

This accidental detection could broaden the project's scope beyond natural ground-based events. According to DARPA, the results suggest AtmoSense can detect "events in air or space that are of interest to national security."

The team plans to hold a virtual workshop in April to share findings and explore future uses of the system. For now, what started as a way to monitor tremors and eruptions might become a tool for tracking the skies — one atmospheric ripple at a time.