This undated image from the U.S. Air Force shows the 747-E4B Nightwatch, nicknamed the “Doomsday Plane.” (U.S. Air Force Photo)

The Nightwatch, a U.S. Air Force plane nicknamed the ‘Doomsday Plane’, has circled the Keweenaw Peninsula since about 2:45 p.m. Tuesday. The plane cruised at about 430 knots (494 mph) at 25,000 feet for five hours over the Keweenaw. It broke from the holding pattern and flew east at approximately 6:45 p.m.

The plane circled numerous times in an elongated holding pattern northwest to southeast, from about 40 miles north of the Porcupine Mountains, traveling as far east as Big Bay, north of Marquette.

The plane is an asset of the U.S. Strategic Command. The U.S. Strategic Command could not be reached for comment.

The plane based at Offutt Air Force Base near Omaha, Neb., took off from Lincoln, Neb., at 1:30 p.m. EDT, and flew over Duluth and then to the Keweenaw Peninsula, according to data on the flight tracking website flightradar24.com.

Flightradar24.com Shown is the flight path The Nightwatch took in flying over the Copper Country Tuesday afternoon.

The Nightwatch is described by the U.S. Air Force as a modified Boeing 747-200B for the National Emergency Airborne Command Post. It’s designed to carry the Secretary of Defense and the Joint Chiefs of Staff during a nuclear attack. An E-4B in the air is noted as a National Airborne Operations Center.

The plane is designed to survive an electromagnetic pulse and uses traditional analog flight instruments as they are less vulnerable to an EMP blast.

The plane is capable of operating with a crew of up to 112 people including flight and mission personnel. There are at least 48 crew aboard any E-4B mission.

The plane contains a conference room, projection room for computer graphics and overhead transparencies, and can deploy and tow a five-mile-long cable for communicating with nuclear submarines.

With in-flight refueling, the plane is capable of remaining airborne, with a test flight lasting 35 hours. The plane was designed to remain in the air for a full week in the event of an emergency.

The E-4B fleet consists of four aircraft with a cost of $250 million each. It costs about $160,000 per hour for the Air Force to operate.