Scientists have been left baffled after a zombie satellite sent a powerful burst of radio waves after decades of silence. NASA detected the mysterious signal last year and now believes it was caused by the Relay 2 satellite.

The long-dead satellite blasted off in 1964 but went offline just three years later. But, around 60 years later, it sent an unexpected signal, and scientists are not sure how it happened. The pulse lasted just 30 nanoseconds, which doesn't match any of the satellite's systems. This rules out the possibility of a deliberate transmission.

Clancy James, an associate professor at Curtin University's Institute of Radio Astronomy in Australia, 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."

James and his colleagues believe the radio waves were caused by either a micrometeorite impact or a buildup of electricity. The signal, which was intercepted by the scientists in June 2024, originated in the Milky Way rather than a far-flung galaxy.

James added: "We got all excited, thinking maybe we'd discovered a new pulsar or some other object." However, it was soon determined the pulse could only have come from Relay 2.

The scientists suggested the satellite could have been hit by a micrometeorite but believe it was more likely that an electrostatic discharge event created the burst. These occur when there is a sudden flow of electricity between two nearby surfaces.

Researchers from the university said: "As a very early spacecraft, Relay 2 may have been constructed from materials capable of holding greater charge and hence producing stronger ESD events. It has long been known that ESD causes radio frequency pulses."

Karen Aplin, a professor of space science and technology at the University of Bristol, said it would be hard to establish exactly how the signal was caused as both possibilities would look similar to scientists.

She explained: "In a world where there is a lot of space debris and there are more small, low-cost satellites with limited protection from electrostatic discharges, this radio detection may ultimately offer a new technique to evaluate electrostatic discharges in space."