The sighting of the Boeing WC-135R Constant Phoenix, an aircraft operated by the US Air Force for detecting nuclear activity, sparked curiosity and concern as it traversed multiple states on Thursday. Known for its specialized role as a ‘nuclear sniffer,’ the aircraft’s movements were closely watched.

According to flight data, the WC-135R embarked on its journey from Nebraska, making its way across South Dakota before heading toward Fargo, North Dakota. Currently, the aircraft is seen circling over Rapid City, South Dakota, as it carries out its mission.

With its primary function to gather atmospheric samples, the WC-135R plays a crucial role in identifying radioactive debris from nuclear explosions. This aids in verifying arms control treaties and bolstering national security by monitoring potential nuclear detonations and tracking any fallout.

The timing of this flight has raised eyebrows, coinciding with the impending expiration of the New START Treaty between the US and Russia on February 5. This development has led to heightened apprehensions among observers, who fear the implications of such a lapse in nuclear arms agreements.

The flight comes as the US-Russia nuclear treaty, the New START Treaty, is set to expire on February 5, leading some observers to fear the worst.

While US officials have not confirmed the reason for the mission, domestic flights of the WC-135R are often for crew training, equipment calibration, or background radiation monitoring.

However, Northrop Grumman also launched its redesigned Intercontinental Ballistic Missile (ICBM) target vehicle for the first time on Thursday, coinciding with WC-135R’s flight.

Although it uses components from decommissioned nuclear-era missiles, the ICBM target vehicle is unarmed and cannot carry nuclear weapons, serving only as a threat-representative test platform for US missile defense systems.

The US Air Force’s Boeing WC-135R Constant Phoenix, an aircraft used to detect nuclear activity, was spotted flying across several states on Thursday (STOCK)

Robin Heard, director of targets and interceptors, Northrop Grumman: ‘By embedding digital technology into every phase of the ICBM target redesign and new integration processes, we have streamlined field operations, enhanced operational safety and driven cost-effectiveness for our customers. 

‘The significant efficiencies identified during the redesigned target’s first pathfinding and maiden launch will positively impact future flight tests.’  

However, it is unknown if WC-135R’s flight was linked to the ICMB test. 

The WC-135R is specially modified with an on-board atmospheric collection suite that allows its crew to detect radioactive ‘clouds’ in real time. 

The aircraft is equipped with external flow-through devices that capture particulates on filter paper, as well as a compressor system that collects whole-air samples in holding spheres for analysis.

The flight is operated by a cockpit crew from the 45th Reconnaissance Squadron at Offutt Air Force Base, Nebraska, while specialized equipment operators are assigned to Detachment 1 of the Air Force Technical Applications Center, also at Offutt.

The Constant Phoenix program dates back to September 16, 1947, when General Dwight D Eisenhower tasked the Army Air Forces with detecting atomic explosions worldwide. 

In a historic mission in September 1949, a WB-29 aircraft flying between Alaska and Japan collected debris from Russia’s first atomic test, an event originally thought impossible until mid-1950.

Flight data shows the so-called ‘nuclear sniffer’ taking off from Nebraska, circling South Dakota, and looping near Fargo, North Dakota

While US officials have not confirmed the reason for the mission, domestic flights of the WC-135R are often for crew training, equipment calibration or background radiation monitoring (STOCK)

Over the following decades, aircraft, including the WB-50 and later the WC-135, replaced earlier models, becoming the backbone of US airborne nuclear detection. 

Air sampling missions have spanned the Far East, Indian Ocean, Bay of Bengal, Mediterranean Sea, polar regions and the coasts of South America and Africa. 

The WC-135W notably tracked radioactive fallout from the 1986 Chernobyl disaster in the Soviet Union.

Today, the WC-135 fleet continues to support the 1963 Limited Nuclear Test Ban Treaty, which bans all above-ground nuclear weapons testing. 

These aircraft remain the US Air Force’s only platform for airborne nuclear air-sampling operations.

Flight tracking of Thursday’s flight flooded social media, where users noted that the US-Russia nuclear treaty is coming to an end.

The nations signed the New START Treaty on April 8, 2010, marking their sixth agreement.

Russian President Vladimir Putin proposed in September that both parties should agree for a further 12 months to adhere to the New START limits, which cap the number of deployed nuclear warheads at 1,550 on each side.

US President Donald Trump has yet to deliver a formal response, and Western security analysts are divided about the wisdom of accepting Putin’s offer.