On January 26, 2026, while most of the world was still asleep, the skies over Europe and the Middle East turned into an arena of quiet yet concerning military activity. A Boeing TC-135 Stratolifter with the callsign SNOOP 55 landed at RAF Mildenhall in the United Kingdom, after a short stop at Waddington. This is one of the last three aircraft of its kind in the world, primarily used to train sensor crews for the RC-135 Rivet Joint (advanced intelligence aircraft). Even though this aircraft is “only” a training variant, it is frequently used to practice radioactive sensing protocols and air sampling — very similar to the real WC-135 Constant Phoenix, commonly known as the “nuclear sniffer” (nuke sniffer).
On December 4, 2023, the third and final WC-135R Constant Phoenix (tail number 64-14829, “Tail 829”) landed at Offutt Air Force Base in Nebraska. This completed the transition from an aging fleet of two WC-135W aircraft to a modern fleet of three advanced platforms, providing unprecedented operational flexibility.
The aircraft — operated by the 45th Reconnaissance Squadron of the 55th Wing (“Fightin’ Fifty-Fifth”) in cooperation with the Air Force Technical Applications Center (AFTAC) — continue to carry out critical missions: collecting air samples to detect nuclear explosions, verifying nuclear arms control treaties, and monitoring suspicious activity.With three identical aircraft (CFM-56 engines, upgraded cockpit), the U.S. Air Force can now handle multiple simultaneous events, maintain high readiness, and conduct routine training.
The main theaters of interest: North Korea, Ukraine, and Iran.During 2025–2026, these aircraft became a central tool in monitoring the three most prominent nuclear threats:
- North Korea (DPRK)
The aircraft continue routine operations in the Pacific theater, particularly around the Korean Peninsula and areas adjacent to Russia. In December 2025, a WC-135R flight was reported off the coast of Russia in the Pacific (from a base in Japan), apparently monitoring possible joint North Korean–Russian nuclear or missile activity. Historically, the aircraft are dispatched immediately after any North Korean nuclear or missile test — as occurred in the past (2013, 2016, 2017) — and the new fleet enables faster response without diverting resources from other regions. - Ukraine and the Russian connection
Since the Russian invasion in 2022, there has been constant concern over the possible use of tactical nuclear weapons or a “dirty bomb.” In 2025, WC-135R activity was recorded over the Baltic Sea (August 2025), likely monitoring Russian-related risks in the area. These flights are conducted in international airspace and provide objective data to American decision-makers and NATO allies, while maintaining immediate detection capability for any radioactive release — whether intentional or accidental. - Iran
This has been one of the most active theaters in recent years. In June 2025, a WC-135R was dispatched directly from Offutt AFB on a long-range mission — most likely toward the Middle East — following heightened Israel–Iran tensions and nuclear threats. During 2025, several flights were logged over the Mediterranean (August and September 2025, including south of Sardinia and additional missions), as part of a joint American–Israeli–Western effort to prevent or detect progress in Iran’s nuclear program.
These flights are relatively rare and attract significant attention, as they signal genuine U.S. concern over prohibited activity.The core system includes two main collection methods:
A. Particulate Collection:
The aircraft is equipped with external flow-through devices mounted on the fuselage, including the Harvester pod (platform-agnostic) and the U1-B airfoil sampling device (installed on both sides of the body). Air is drawn at high rate through special filter paper that “captures” microscopic particles of radioactive debris — such as short-lived isotopes (I-131, Xe-133, Cs-137, Sr-90, and others) produced in a nuclear explosion.
The aircraft is equipped with external flow-through devices mounted on the fuselage, including the Harvester pod (platform-agnostic) and the U1-B airfoil sampling device (installed on both sides of the body). Air is drawn at high rate through special filter paper that “captures” microscopic particles of radioactive debris — such as short-lived isotopes (I-131, Xe-133, Cs-137, Sr-90, and others) produced in a nuclear explosion.
B. Whole Air / Gaseous Sampling:
A compressor system collects whole air samples into special pressurized bottles. This allows capture of radioactive noble gases such as Xenon-133 (Xe-133), Xenon-135 (Xe-135), and Argon-41 — highly characteristic markers of a nuclear explosion, even in underground tests where particles do not reach the atmosphere. These gases serve as a unique chemical “fingerprint,” enabling not only detection of an explosion but also differentiation between a nuclear detonation and other sources (e.g., nuclear power plants or medical isotopes).Real-time sensors and initial measurements
After collection, samples are transferred to advanced laboratories where gamma spectroscopy and mass spectrometry are performed for precise isotope identification. The results are compared against a known library of nuclear explosion signatures — making it possible to determine whether it was a test, an accident, or intentional release, and even to estimate the yield and type of device.This technology forms part of the global U.S. Atomic Energy Detection System operated by AFTAC, which integrates ground-based sensors, satellites, hydroacoustic, and subsurface sensors. The WC-135R remains the only airborne component capable of “chasing” radioactive clouds anywhere in the world.
A compressor system collects whole air samples into special pressurized bottles. This allows capture of radioactive noble gases such as Xenon-133 (Xe-133), Xenon-135 (Xe-135), and Argon-41 — highly characteristic markers of a nuclear explosion, even in underground tests where particles do not reach the atmosphere. These gases serve as a unique chemical “fingerprint,” enabling not only detection of an explosion but also differentiation between a nuclear detonation and other sources (e.g., nuclear power plants or medical isotopes).Real-time sensors and initial measurements
After collection, samples are transferred to advanced laboratories where gamma spectroscopy and mass spectrometry are performed for precise isotope identification. The results are compared against a known library of nuclear explosion signatures — making it possible to determine whether it was a test, an accident, or intentional release, and even to estimate the yield and type of device.This technology forms part of the global U.S. Atomic Energy Detection System operated by AFTAC, which integrates ground-based sensors, satellites, hydroacoustic, and subsurface sensors. The WC-135R remains the only airborne component capable of “chasing” radioactive clouds anywhere in the world.