The United States Air Force deployed a WC-135W "Constant Phoenix" to RAF Mildenhall air base in the United Kingdom on January 30, 2021. This peculiar aircraft is specialized in the detection of radioactive particles in the atmosphere, hence its nickname – the “sniffer.” 

The deployment of a Constant Phoenix in Europe is a rather rare occurrence. The first incident dates back to the 1986 Chernobyl nuclear plant disaster, when multiple WC-135s were deployed to monitor the catastrophe. Since then, the sniffer has only visited the continent twice: once in 2017, and again in 2020. In both cases, the mission took the aircraft close to the Russian border. 

The WC-135 is based on the airframe of the C-135 Stratolifter. Out of the 10 WC-135 aircraft that were built (plus one EC-135C converted in 1998), only two remain, both operated by the USAF 45th Reconnaissance Squadron. They are both regularly solicited in the Pacific Ocean, keeping an eye on the North Korean nuclear program. The WC-135W's current mission in Europe is unknown. 

The three Boeing OC-135B observation planes specifically equipped for the application of the Open Skies Treaty were former WC-135B. Signed in 1992, and ratified on January 1, 2002, by the members of the Organization for Security and Co-operation in Europe (OSCE), the treaty allowed mutual aerial monitoring of military movements and strategic installations of the signatory countries. In April 2021, after both Russia and the United States withdrew from the treaty, the USAF announced the retirement of the last two OC-135Bs. 

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The Biden administration confirmed that the United States will not rejoin the Open Skies Treaty.