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…