A fast radio burst detected last year has been traced to a deeply unusual source.
Although most of these radio signals emanate from deep space and are no thought to be technological in origin, the burst of 13 June 2024 did indeed come from machinery – just not alien machinery. The spurt of energy erupted from a NASA satellite that has been drifting, dead, in Earth orbit for more than 50 years.
"We report the detection of a burst of emission over a 695.5 megahertz to 1031.5 megahertz bandwidth by the Australian Square Kilometer Array Pathfinder, ASKAP," writes a team led by astronomer Clancy James of the Curtin University node of the International Centre for Radio Astronomy Research in Australia.
"The burst was localized through analysis of near-field time delays to the long-decommissioned Relay 2 satellite."
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As our technological prowess in astronomy – and Earth space presence – grows, it becomes ever more important to be able to recognize anthropogenic signals, understand how they are generated, and determine the hazards they pose.
Some puzzling signals have been found originating from technology right here on Earth; notable examples include a truck and a microwave oven. But space junk in Earth orbit is capable of confusing our instruments and scientists, as we saw in the case of a putative gamma-ray burst that ended up being traced to sunlight glancing off a defunct rocket stage.
The case of Relay 2 is slightly more concerning. The NASA satellite was originally launched in 1964, and only remained operational for a short time before being decommissioned in 1967. Since then, it's just been hanging around at a pretty high orbit, drifting between altitudes of 1,867 and 7,648 kilometers (1,160 and 4,752 miles).
Fast radio bursts are a peculiar phenomenon that astronomers are trying to solve. They are brief, extremely powerful bursts of radio waves – think of the radio energy emitted by 500 million Suns, squeezed into a millisecond timeframe.
These signals travel for millions to billions of light-years across intergalactic space; only one fast radio burst source has been identified in the Milky Way, an erupting magnetar 30,000 light-years away.
When James and his colleagues set out to track the source location of a nanosecond radio signal received on 13 June 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.
That's obviously not coming from a magnetar – the most likely culprit was an Earth satellite.
"Using the location of ASKAP, the time of the burst, and the Skyfield python module, we searched for a coincidence in time and position on the sky between the burst and Earth satellites," the researchers write.
"One viable match was found, for NORAD ID 737 (Relay 2) … We therefore conclude that this burst originated with Relay 2."
This burst could not have been the result of reflecting sunlight. Rather, the team was able to narrow it down to two possibilities: an electrostatic discharge, or a plasma discharge following an impact by a tiny piece of space rock – a micrometeoroid.
Micrometeoroids are a known hazard of Earth's space environment, and represent a plausible explanation for the signal; however, the researchers favor an electrostatic discharge as the leading possibility.
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As they move through Earth's geomagnetic environment, satellites can accumulate electrons that eventually discharge when sufficient voltage is reached. An observational campaign conducted using the Arecibo telescope, the results of which were published in 2017, identified several radio signals that centered on a GPS satellite – signals that seem very similar to those in the new study.
The researchers were unable to identify a specific trigger for the burst, suggesting that such discharges are pretty rare from Relay 2. However, the finding is a valuable one. It not only gives a datapoint that will help astronomers identify the provenance of such signals in the future, it can help us understand the dangers posed by electrostatic discharge in Earth orbit.
"The observation of such a short burst at GHz frequencies is unexpected, and raises the prospect of new methods of remote sensing of arc discharges from satellites, either from retrofitting existing experiments searching for fast radio bursts or high-energy particles, or new dedicated instruments," the researchers write.
"Our observation opens new possibilities for the remote sensing of electrostatic discharge, which poses a serious threat to spacecraft, and reveals a new source of false events for observations of astrophysical transients."
The research has been accepted for publication in The Astrophysical Journal Letters, and is available on arXiv.