Defunct satellite signal found during fast radio burst search

Astronomers found a mysterious blast of radio waves while searching for fast radio bursts (FRB) from deep space, but it turned out to be an emission from NASA's inactive Relay 2 satellite.

Initially detected by the Australian Square Kilometre Array Pathfinder (ASKAP) in June 2024, this 'pseudo-FRB' lasted less than 30 nanoseconds, much shorter than most FRBs, yet powerful enough to drown out all other signals from the sky.

Despite nearly 20 years of study, astronomers don't actually know what generates FRBs. One plausible theory involves a 'magnetar'—a highly magnetized neutron star. Relay 2, one of the first communications satellites, was launched in 1964. Just three years later, with its mission concluded and both of its main instruments out of order, Relay 2 had already turned into space junk.

Located just 2,800 miles from Earth, the proximity of the signal posed a challenge for astronomers, as the closest FRB is estimated to be 30,000 light-years away. Astronomers later realized that the signal appeared bright to the telescope because it was closer than the astronomical signals they had been looking for.

The defunct satellite was not initially thought to be responsible for the signal due to its ceased operations and outdated systems.

Researchers from Australian institutes, Curtin University and ARC Centre of Excellence for Gravitational Wave Discovery, as well as the University of Edinburgh, were involved in the study. The discovery that the signal source was not a FRB was disappointing for some, however, Dr Marcin Glowacki from the School’s Institute for Astronomy found value in the finding and explains:

It was like an interesting puzzle for us to localize this result from such a relatively close object to what we are used to! It certainly took some time and effort, as we had to adjust how we measured the signal with ASKAP to account for it being so close. It's like how phone cameras can struggle to focus on something very close to them. While we are mostly interested in astrophysical systems, this discovery is important for monitoring satellites in the future with ASKAP and other radio telescopes.

The observation appears to be an amazing chance discovery. However, this opens up an entirely new mystery: how could Relay 2 manage to emit a signal that had a brightness similar to an FRB?

Astronomers are not entirely sure, but Dr Glowacki explains some theories:

One theory is electrostatic discharge (ESD)—a build-up of electricity that results in a spark-like flash. Another is that a micrometeorite had struck the satellite and produced a cloud of charged plasma, right as ASKAP was observing the part of the sky it was in.

Another question to consider is, are other fast radio bursts actually 'pseudo-FRBs'? The biggest clue that an FRB is not an artificial signal is its dispersion measure - how far signals have travelled. This is due to ionized electrons slowing the signal at lower frequencies as FRBs travel through space, encountering plasma.

The 'pseudo-FRB' highlights the potential for satellites to mimic celestial phenomena, ruling out most FRBs as satellite signals due to their distinct galaxy-hosting origin points. Unlike distant FRBs, nearby signals lack characteristic time delays caused by ionized electrons. While researchers plan to scrutinize other satellite signals, this episode underscores challenges in distinguishing terrestrial from cosmic sources. Despite the rare occurrence and clear FRB dispersion measures, astronomers agree that vigilance is necessary to prevent future misidentifications.