By Rafi Letzter, Staff Writer | February 15, 2018 06:50am ET

There’s a whiff of something radioactive in the air.

A research plane flying over the Aleutian Islands on Aug. 3, 2016 detected a single speck of enriched uranium floating about 4.3 miles (7 kilometers) above Alaska’s far-western island chain, according to a new research paper that will be published in April in The Journal of Environmental Radioactivity.

The uranium sample was tiny and harmless, a small chunk of a mote of dust just 580 nanometers wide (about half the size of a red blood cell). And it was completely alone; no other radioactive material turned up in that stretch of sky. But, the researchers wrote, it was “definitely not from a natural source.”

And the scientists can’t explain how it got there. The plane’s onboard mass spectrometer, brought along to analyze standard-issue pollution, detected just a single uranium particle, mixed with traces of chemicals from burning oil, the authors reported. And on its own, that find wouldn’t be too remarkable — uranium is the heaviest element commonly found on Earth, after all.

“Particulate matter containing uranium can originate from sources such as combustion of coals with trace uranium, windblown crustal material, and mining and processing of ores, whether it be for the uranium itself or other minerals such as rare earths [a group of chemically similar elements that aren’t actually that rare, but are difficult to mine] and phosphate,” the researchers wrote. [The 10 Greatest Explosions Ever]

What makes this particle unusual is that it was rich in an isotope called uranium-235, or U-235, which made up about 2.6 percent to 3.6 percent of its mass, according to the paper. Naturally occurring uranium typically contains just 0.7 percent U-235, with the rest given over to the much more common uranium-238.

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