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, raising concern among observers.
Flight data shows the so-called 'nuclear sniffer' taking off from Nebraska, circling South Dakota, and looping near Fargo, North Dakota.
The aircraft is currently circling above Rapid City, South Dakota, as it continues its mission.
The WC-135R's purpose is to collect atmospheric samples to detect and identify radioactive debris from nuclear explosions, supporting arms control treaty verification and national security by monitoring for nuclear detonations and tracking fallout.
The US Air Force last deployed the jet in 2024 to the Middle East.
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.
They are not uncommon over the Midwest, including areas with strategic nuclear assets.
Many flights are routine 'baseline' collections to establish normal radiation levels worldwide, rather than responses to specific events.
Deployments are often unacknowledged publicly unless tied to major incidents.
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.
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.
