On Tueday this morning, a special airplane belonging to the United States Air Force, a Constant Phoenix aircraft, took off from the United Kingdom on its way to the eastern Mediterranean, off the coasts of Syria and Israel.
The Constant Phoenix is what is known as a "nuke sniffer" and is the only such craft in the U.S. Air Force, a special-purpose aircraft developed on the basis of the Boeing C-135 Stratolifter. Its mission is to analyze particles in the atmosphere for the purpose of detecting and identifying nuclear explosions. It does so by flying through suspect areas and collecting air samples, while employing advanced air filtration techniques in order to protect its flight crew.
According to UK Defence Journal, it is very rare for a nuke sniffer to enter the Mediterranean. With the presence of several Russian ships in the area, it is surmised that the United States wishes to know whether those ships are armed with nuclear missiles, and also whether there are Russian nuclear submarines in the region.