Here’s what you’ll learn when you read this story:

A fast radio burst (FRB) was detected coming from near-Earth orbit, which is unusually close considering the closest known FRB was detected tens of thousands of light-years away.

It turned out that the signal came from a satellite that had been decommissioned in the late 1960s after its instruments broke down.

What triggered the electrostatic discharge that caused the signal is still uncertain, but investigating further could help us figure out how to prevent damage to live spacecraft.

Space is chock-full of mysterious signals. While most come from natural cosmic phenomena, one recent fast radio burst (FRB) had all the markings of advanced technology, which baffled scientists until they traced it back to a surprising source: our own space junk.

The accumulation of human junk has gone beyond Earth’s landfills. Orbiting our planet is an entire cosmic dumpster of space garbage, from the 23,000-ish larger pieces of refuse, such as decommissioned satellites, to over 100 million stray fragments of metal and glass and whatever else is flying around.

Satellites that retire are deorbited into a graveyard orbit within 25 years of their missions ending. This is the case with NASA’s Relay 2 satellite, which took off in 1964 until its transponders glitched. It hung dead in near-Earth orbit by 1967 and has beamed nothing down since—until now.

Relay 2 didn’t suddenly reactivate. Instead, something triggered it to send a FBR so fast it was gone in a nanosecond, to the surface, which was then picked up by the Australian Square Kilometer Array Pathfinder (ASKAP). Astronomer Clancy James and his research team found the source coming just 4,500 km (about 2,796 miles) from Earth. This meant that it couldn’t possibly have issued from a neutron star, magnetar or zombie galaxy. When the researchers tracked it to Relay 2, they figured something must have been interacting with the Apollo-era relic.

“The charging of spacecraft in orbit due to interactions with the space environment has been a well-known phenomena since the early days of the space program,” the scientists said in a study recently accepted for publication in The Astrophysical Journal Letters and posted on ArXiv. “Accumulation of electrons and ions can lead to large voltage differentials between spacecraft surfaces, and between the spacecraft and space plasmas.”

The most likely culprits? Electrostatic discharge (ESD) or plasma discharge caused by a collision with a rogue micrometeoroid. Spacecraft that are still alive and carrying out their missions need to be designed to avoid hazardous ESD currents. That still doesn’t make spacecraft totally immune. Most don’t have onboard monitors to detect levels of charge buildup, and that buildup usually isn’t detected until after damage has been done. This is why scientists have proposed that ground-based radio monitors watch for ESD, but no system is yet capable of simultaneously observing large numbers of spacecraft.

On June 13, 2024, ASKAP caught the FRB from Relay 2 using its Commensal Realtime ASKAP Transients Survey (CRAFT) detection system. FRBs are so powerful that they take only milliseconds to emit the radio energy of 500 million suns. They can travel anywhere from millions to billions of light-years through the void of space, with the closest one coming from a magnetar in our galaxy that is 30,000 light-years away. This is why finding an FRB so close to Earth threw James off until he found it originated from Relay 2.

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While this FRB could have been set off by a micrometeoroid, which can also produce radio emissions, James thinks it is more likely the signal was the result of electrostatic discharge. The electrons and ions accumulated by satellites can eventually reach a high enough voltage to release discharge. ESD is already known to cause radio frequency pulses. Such a powerful burst as an FRB was unexpected, and the researchers have yet to find what exactly the trigger was. Future investigations could help them figure out ways to protect live spacecraft.

“When sufficient voltage is achieved, electrostatic discharge (ESD) occurs, typically be- tween nearby surfaces/materials on the spacecraft. ESD does not depend on the operational nature of the spacecraft,” the scientists said. “So ESD from a satellite decommissioned 60 years ago is entirely plausible.”