What happens when a relic of the early Space Age unexpectedly outshines the cosmos, even for a fleeting instant? In June 2024, astronomers scanning the southern sky with the Australian Square Kilometer Array Pathfinder (ASKAP) encountered a signal so intense and brief that it momentarily eclipsed all other radio sources. The flash, which only lasted for less than 30 nanoseconds, was initially confused for one of the universe’s mysterious fast radio bursts (FRBs) those celestial events known to release the energy of hundreds of millions of Suns in milliseconds. However, this signal’s origin was much more down-to-Earth: the long-dead NASA Relay 2 satellite, which has been quiet since 1967, 4,500 kilometers above Earth.
Relay 2, which was deployed in 1964 as part of NASA’s trailblazing communications program, had not broadcast in almost six decades. Its unexpected emission presented a mystery that prompted researchers to explore the processes through which “zombie satellites” continue to astonish us. As Clancy W. James of Curtin University’s Institute of Radio Astronomy explained, “This was an incredibly powerful radio pulse that vastly outshone everything else in the sky for a very short amount of time” (Live Science). The ASKAP researchers, anticipating a far-off galaxy, were instead dealing with a signal so nearby that their equipment was having a hard time focusing similar to a camera lens that has an object at arm’s length in front of it.
The most plausible theory is that the burst was generated by either an electrostatic discharge (ESD) or a plasma discharge after a micrometeoroid impact. The two mechanisms are familiar space environment hazards. As satellites move through the ionized plasma of Earth’s magnetosphere, they acquire electrical charge on their surfaces. When the difference in voltage is great enough, an instant discharge may be triggered, with the energy discharged as a radio-frequency pulse. In James’ words, “Spacecraft get charged with electricity when they pass through ionized gas or ‘plasma’ above the atmosphere, and when enough charge builds up, they generate a spark”. The early satellites such as Relay 2, without modern ESD mitigation measures, could have permitted more catastrophic conditions.
Alternatively, a micrometeoroid impact moving at speeds of about 70,000 kilometers per hour could have vaporized a small area of the satellite’s surface, producing a plasma cloud and a transient radio emission. However, both scenarios present a temporal problem: the duration of the burst was more than a thousand times shorter than ESD or plasma events typically last, which take microseconds as opposed to nanoseconds (Orbital Today). This variance only raises the question of what the precise physical process is, but emphasizes the value of continued observation.
Detection also speaks to the advance of radio transient detection methods. ASKAP’s 36-dish array is capable of localizing signals with extremely high accuracy, using the minuscule variations in the time of arrival across the array to pinpoint the source. This ability, created to track down extragalactic FRBs to their host galaxy, became critical in pinpointing Relay 2 as the perpetrator (Keck Observatory). As the University of Edinburgh’s Marcin Glowacki observed, “It was like an interesting puzzle for us to be able to localize this result from such a relatively close object to what we are used to!”. For radio astronomy in general, this phenomenon is a sober reminder of the increasing threat posed by space junk and “pseudo-FRBs.” As satellites fall out of commission, so do they contaminate searches for true cosmic transients with anthropogenic signals.
Lack of a dispersion measure a characteristic delay at lower frequencies due to intergalactic electrons was the critical clue that separated Relay 2 burst from an actual FRB, which normally are billions of light-years away and have large frequency-dependent delays.
Aside from the astronomical consequences, the event presents a possible new technique for remotely observing ESD events on satellites. It could be a valuable diagnostic tool for spacecraft health, particularly with the burgeoning populations of satellite constellations and ESD being a top reason satellite anomalies occur. As Karen Aplin at the University of Bristol noted, “this radio detection may ultimately offer a new technique to evaluate electrostatic discharges in space”. The Relay 2 outburst, therefore, is at once a scientific oddity and a foretaste of the challenges that await astronomers and engineers in a burgeoning near-Earth environment.
