A little over a year ago, scientists in Australia picked up a brief burst of electromagnetic radiation. The pulse was so strong that it eclipsed all other signals coming from the sky, but its origins were unknown. After digging through the data, the team discovered that the source wasn’t a distant celestial object but rather a zombie satellite left to orbit Earth with no purpose.

NASA’s Relay-2 launched on January 21, 1964, two years after its predecessor, Relay-1, was sent to orbit. The pair were experimental communications satellites that carried onboard experiments to map the trapped radiation belt, otherwise known as the Van Allen radiation belts. Relay-2 continued to operate until June 9, 1967, when its two transponders failed, and it wasn’t heard from again until it let out a short-lived signal nearly 60 years later.

On June 13, 2024, astronomers at the Australian Square Kilometer Array Pathfinder picked up a mysterious fast radio burst (FRB) that lasted for less than 30 nanoseconds. “We got all excited, thinking maybe we’d discovered a new pulsar or some other object,” Clancy James, a researcher at Curtin University, told New Scientist. “This was an incredibly powerful radio pulse that vastly outshone everything else in the sky for a very short amount of time.”

The scientists originally assumed that the signal came from a distant object in the cosmos, but further analysis revealed that the long-dead Relay-2 was in fact the culprit behind the mysterious pulse, according to a paper available on the preprint website arXiv.

Fast radio bursts are brief, powerful pulses of radio waves that usually come from deep space and last for only a few milliseconds. Despite being short-lived, FRBs can release as much energy in that brief window of time to outshine an entire galaxy, according to NASA. The FRB detected in June 2024 was particularly strong—and it coincided with the NASA satellite flying directly overhead at the time, according to the paper. It was also unusually clear and well-defined.

As to why Relay-2 suddenly emitted this signal, that’s a mystery. The scientists behind the detection believe that the satellite may have built up electricity over the years and then suddenly discharged it in a single powerful burst. Another possibility is that Relay-2 was struck by a micrometeorite or another object in orbit, resulting in the release of a small cloud of plasma, the researchers speculate.

As an increasing number of defunct satellites zip around in space, producing all kinds of weird bleeps and bloops, it’ll become harder for scientists to distinguish whether it’s the cosmos talking—or just some piece of malfunctioning hardware.