A mysterious radio signal astronomers initially interpreted to be a high-speed radio burst from deep space has been revealed to be something else entirely, and it’s right in our own cosmic back yard.
The discovery was made on June 13, 2024, when researchers using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia spotted something unusual: what appeared to be a very brief, but extremely powerful radio signal.
Similar to what astronomers call fast radio bursts (FRBs), which originate billions of light-years away from Earth, this signal was different, lacking the normal frequency dispersion of other FRBs. This was odd, since it appeared to suggest that the source, whatever it was, had been located within our own galaxy.
Now, after a year of investigation, astronomers say they have finally solved the perplexing mystery, tracing the anomalous signals back to an unexpected source: a forgotten relic from the Cold War-era.
Not a Fast Radio Burst After All
Although the ASKAP team had been searching into rare, fleeting signals believed to be the hallmark of extremely distant high-energy cosmic events, the anomalous flash detected by the Murchison Radio-astronomy Observatory in remote Western Australia was so brief that it lasted a minuscule 30 nanoseconds—far shorter than any previously recorded FRB.
Of particular interest to the astronomers had been that the signal showed none of the telltale signs of dispersion between lower and higher frequencies, which are normally associated with FRBs that travel through intergalactic space.
If not an FRB though, what could the mysterious source have been?
A Baffling Astronomical Observation
Although initially perplexed by their discovery, the research team took a second look at their data, which revealed that the signal had blurred in ASKAP’s radio image. This was a crucial observation, as it seemed to indicate that the source was extremely close—perhaps just 4,500 kilometers away.
This also seemed to point to a source that wasn’t astronomical at all, leaving open the tantalizing possibility that an object within our galaxy—perhaps even a spacecraft—could be the source of the anomalous emission.
Soon, the research team narrowed down the potential candidates to one likely source: Relay 2, a satellite launched in 1964 which, after having been operational for a short time, has remained inactive since the mid-1960s.
Over time, there have been a few instances where defunct satellites have been known to emit signals after reactivating, garnering the nickname “zombie satellites.” Curiously, Relay 2 was not one of these spacecraft, and in fact, would never have even been capable of producing a nanosecond burst, even when the satellite was functional decades ago.
Amid a range of potential explanations, the team explored the possibility of an electrostatic charge that may have built up on the exterior of the satellite over time, which was suddenly released in the form of a burst of detectable radio waves. Although this has been known to occur, the relatively close discharge of radio waves, if this scenario had been the case, would have lasted far longer than the signal they observed.
The team also explored the possibility of a micrometeoroid impact, but determined the likelihood of such an event giving rise to the odd signal would be close to just 1%.
Still an Uncertainty
“Ultimately, we can’t be certain why we saw this signal from Relay 2,” said Clancy James, a Senior Lecturer in astronomy and astroparticle physics at Curtin University, who was involved with the unusual detection.
“What we do know, however, is how to see more of them,” James said in a recent piece that appeared at The Conversation. “When looking at 13.8 millisecond timescales – the equivalent of keeping the camera shutter open for longer – this signal was washed out, and barely detectable even to a powerful radio telescope such as ASKAP.”
While a few questions remain about the exact cause of the anomalous emission apparently detected from Relay 2, the team’s discovery reveals the potential for using ground-based radio antennas for monitoring satellites, as well as the unexpected discharges they may produce.
Such monitoring, with time, may even prove crucial for protecting our growing space infrastructure from damage caused by electrostatic events in space, or micrometeoroid impacts.
The team has detailed their findings in a new paper, “A nanosecond-duration radio pulse originating from the defunct Relay 2 satellite,” which appeared on the arXiv.org preprint server.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.