The US Air Force has flown one of its two WC-135W Constant Phoenix jets to RAF Mildenhall Air Base in the UK. The aircraft is called a “nuclear sniffer” because it is specialized in detecting radioactive particles present in the atmosphere. 

The move is unusual and draws even more attention as it comes amid tensions in Europe over Russia's possible invasion of Ukraine.

A US Air Force WC-135W 'Constant Phoenix', callsign ATOM01, arrived in the UK this morning. The aircraft is a "nuke sniffer", tasked with testing the atmosphere for evidence of nuclear explosions. pic.twitter.com/gI53aLPJqh

— George Allison (@geoallison) January 30, 2022

According to aerotime, the first time the WC-135 was on the old continent was on the occasion of the Chernobyl nuclear accident in April 1986, when multiple aircraft were sent to monitor the accident. After that, the aircraft only returned to Europe twice: once in 2017 and once in 2020. In both cases, the mission took the WC-135W close to the Russian border.

The WC-135W is one of several “members” of the C-135 Stratolifter family, which gave rise to several specialized mission aircraft. The C-135 is directly derived from the Boeing Dash 80, known for giving rise to the Boeing 707. The most famous are the KC-135 Stratotanker for refueling and flying the RC-135 for diversion, intelligence and reconnaissance missions.

Of the 10 WC-135W aircraft built (plus an EC-135C converted in 1998), only two remain, operated by the 45th Reconnaissance Squadron, based at Offutt Air Force Base, in the US state of Nebraska. Both are regularly requested in the Pacific Ocean, with an eye on North Korea's nuclear program. The WC-135W's current mission in Europe is unknown. 

The three Boeing OC-135B observation aircraft specifically equipped for Open Skies Treaty enforcement were former WC-135Bs. 

Signed in 1992 and ratified on January 1, 2002 by the members of the Organization for Security and Cooperation in Europe (OSCE), the treaty allowed mutual aerial monitoring of military movements and strategic installations of the signatory countries. 

In April 2021, after Russia and the United States withdrew from the treaty, the USAF announced the retirement of the last two OC-135Bs.