A flash of light in the night sky, thought to have been a burst from a galaxy in the early universe, may have been nothing more than a glint from a piece of space junk.
In December 2020, Linhua Jiang at Peking University in Beijing, China, and his colleagues announced they had seen a brightening event in GN-z11, thought to be the most distant known galaxy in the universe, appearing as it looked 13.4 billion years ago. The event was believed to be a gamma-ray burst, possibly resulting from the supernova explosion of a giant star, the oldest known such occurrence.
Micha艂 Micha艂owski at the Adam Mickiewicz University in Poland and his colleagues, however, found that the detection, made from Hawaii in 2017, lined up with a piece of space junk from a Russian Proton rocket launched in February 2015.
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Weighing about a tonne, the piece of junk originates from the launch of a communications satellite from the UK firm Inmarsat, called Inmarsat-5 F2. It was the rocket鈥檚 upper stage, known as the Breeze-M, that was left in a highly elliptical orbit around Earth, between 350 and 15,000 kilometres above the planet.
Jiang says his team knew of the existence of this object and ruled it out as being responsible for the flash: he says it wasn鈥檛 in the exact field of view of their observations. 鈥淲e also found the brightness was much fainter than what was needed to produce this flash,鈥� he says.
Yet others agree with Micha艂owski and his team鈥檚 findings. 鈥淚 think it鈥檚 the definitive answer,鈥� says Guy Nir at the Weizmann Institute of Science in Israel, whose also concluded that the burst could have been caused by a satellite. 鈥淭he orbit of this piece of space trash coincides with the images they took. This is an unlikely event, but it鈥檚 still more likely than a gamma-ray burst.鈥�
Given the flash has passed and won鈥檛 repeat, we will probably never know for sure what happened. But the whole episode, with hundreds of hours spent analysing this flash, highlights the growing problem astronomers face with satellites and space junk affecting their observations.
In the past year, the number of active satellites in Earth orbit has grown by roughly a third, thanks largely to the launch of more than 1000 satellites in the US firm SpaceX鈥檚 Starlink mega constellation, designed to beam the internet to Earth from space.
And while these satellites are in relatively low orbits, which means that later at night they are in Earth鈥檚 shadow and hence dark, other objects left in higher orbits like Breeze-M 鈥� which will orbit for decades or even centuries 鈥� pose larger problems throughout the night. 鈥淭here鈥檚 an accumulation of 50 years of stuff up there,鈥� says Jonathan McDowell at the Harvard-Smithsonian Center for Astrophysics.
Michele Bannister at the University of Canterbury in New Zealand says astronomers are increasingly having their work hampered by human-caused interference. 鈥淲e are already seeing the community spending substantial amounts of entirely unfunded time having to chase their tails to understand and mitigate these issues,鈥� she says.
Nir says that even pieces of reflective space junk as small as a centimetre can produce noticeable flashes. 鈥淭he best estimate we have is 20,000 [flashes] per day over the entire sky,鈥� he says, but notes the true number 鈥渋s probably 10 times bigger鈥�.
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