A newly declassified report sheds new light on a 1964 incident in South Dakota involving a nuclear warhead 80 times more powerful than the atomic bomb dropped on Hiroshima, Japan in  August of 1945.

On December 5, 1964, a retrorocket attached to a Minuteman missile fired by accident within a missile silo on the base, dislodging the Mark 56 nuclear warhead from the top of the missile, sending it 80 feet to the ground below.

Somehow the warhead did not explode.

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Former journalist Harrison Cramer says the report, which had been classified since January of 1965, estimates that everyone within a seven-mile radius of Ellsworth Air Force Base, just north of Box Elder, would have been killed if the warhead had detonated.

1964 Ellsworth Air Force Base Incident
Nukemap
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The website Nukemap estimates that the death toll would have likely reached into the thousands and included people in eastern Rapid City.

Freedom of Information
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Department of Defense report details the incident:

'The LGM 30B Minuteman I missile was on strategic alert at Launch Facility (LF) L-02, Ellsworth Air Force Base, South Dakota.

Two airmen were dispatched to the LF to repair the inner zone (IZ) security system. In the midst of their checkout of the IZ system, one retrorocket in the spacer below the Reentry Vehicle (RV) fired, causing the RV to fall about 75 feet to the floor of the silo.

When the RV struck the bottom of the silo, the arming and fusing/altitude control subsystem containing the batteries was torn loose, thus removing all sources of power from the RV. The RV structure received considerable damage.

All safety devices operated properly in that they did not sense the proper sequence of events to allow arming the warhead.

There was no detonation or radioactive contamination.'

The Mark 56 nuclear warhead had a yield of 1.2 megatonnes, which is equal to 1.2 tons of TNT, making it 80 times more powerful than the 'Little Boy' atomic bomb, which was dropped on Hiroshima, killing an estimated 70,000 to 135,000 people.

While details of the incident have been known for years, the declassified report criticized authorities for not alerting residents in the area of the base about the impact of the potential explosion:

'No release concerning this incident was made to the press. The entire operation was handled in such a way that the nearby communities were not aware of and did not exhibit even a mild interest in the operation.'

The Mark 56 warhead was first introduced in 1963 and remained in service until 1993.

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