A mysterious radio signal from a NASA 'zombie' satellite has been picked up; it's been inactive for 60 years.

Scientists have been intrigued for years by what are known as fast radio bursts (FBRs), extremely powerful bursts lasting just a few milliseconds but which in some cases are capable of emitting amounts of energy equivalent to what the Sun releases in years. The most disconcerting aspect of this phenomenon is that, although ground-based instruments have been able to detect it dozens of times, their source has yet to be determined, as most of them point to deep space.

However, the source of one of these FRBs, detected on June 13 of last year, has been detected. And it's not what scientists expected: it's a NASA satellite that has been adrift, inactive, and floating in Earth's orbit for more than 50 years.

“We report the detection of a burst of emission across a bandwidth of 695.5 MHz to 1031.5 MHz by the Australian Square Kilometer Array Pathfinder (ASKAP),” writes a team led by astronomer Clancy James of Curtin University’s International Centre for Radio Astronomy Research in Australia in a paper available online at Arxiv and approved for forthcoming publication in The Astrophysical Journal Letters . “The burst was localized by analyzing time delays in the near field of the long-decommissioned Relay 2 satellite,” they write.

Relay 2 was launched in 1964 and was only operational for a brief period before being shut down in 1967. Since then, it has remained in a fairly high orbit, well above the International Space Station (ISS), which has become space junk. Almost 60 years after its launch, in June 2024, the satellite produced an unexpected signal. "It was an incredibly powerful radio pulse that largely eclipsed everything else in the sky for a very brief period," James explains to New Scientist .

The pulse lasted only 30 nanoseconds, which doesn't match the duration of any of the satellite's inactive systems and rules out the possibility of a deliberate transmission, according to the study (which has not yet been peer-reviewed). So James and his colleagues looked for alternative theories. The group, which was indeed scanning the skies for this type of phenomenon with the Australian Square Kilometer Array Pathfinder (ASKAP), points to a micrometeorite impact on the probe or a buildup of electricity.

What initially troubled the researchers was that the signal didn't come from a distant galaxy, but originated in the Milky Way. The pulse began so close to Earth that ASKAP couldn't focus on it, much in the same way that a phone camera sometimes struggles to focus on nearby objects. When James and his colleagues set out to track the location of the source of a nanosecond radio signal received on June 13, 2024, they were puzzled to find that it originated much closer to home than fast radio bursts usually do: just 4,500 kilometers from Earth.

Obviously, this isn't coming from a magnetar, the only proven culprit as a source of FRB signals, as a study in Nature last year demonstrated. The most likely culprit was an Earth satellite, Relay 2.

This explosion could not have been the result of reflected sunlight, as the team has ruled out. In fact, they are considering two possibilities: an electrostatic discharge or a plasma discharge following the impact of a tiny fragment of space rock (a micrometeoroid).

Micrometeoroids are a known hazard in Earth's space environment and represent a plausible explanation for the signal; however, researchers strongly favor electrostatic discharge as the primary possibility.

As satellites move through Earth's geomagnetic environment, they can accumulate electrons that eventually discharge when they reach a sufficient voltage. An observation campaign conducted with the Arecibo telescope, the results of which were published in 2017, identified several radio signals centered on a GPS satellite; signals that appear very similar to those in the new study.

Researchers were unable to identify a specific trigger for the burst, suggesting that such discharges are quite unusual for Relay 2. Nevertheless, the finding is valuable: not only does it provide data that will help astronomers identify the source of such signals in the future, but it may also help us understand the dangers posed by electrostatic discharges in Earth orbit.

"The observation of such a short burst at GHz frequencies is unexpected and raises the possibility of new methods for remote detection of arc discharges from satellites, either by modernizing existing experiments that search for fast radio bursts or high-energy particles, or with new dedicated instruments," the researchers write. "Our observation opens new possibilities for remote sensing of electrostatic discharges, which pose a serious threat to spacecraft, and reveals a new source of spurious events for astrophysical transient observations."

Furthermore, as our technology and the ability to explore the cosmos improve, it becomes increasingly important to be able to recognize human-caused signals, understand how they are generated, and determine the dangers they pose. And space debris, which currently totals 140 million objects, will inevitably increase in the coming years.