W71 (nuclear weapon)

Involving significant advances in nuclear weapons technology, the W71, designed by Lawrence Livermore National Laboratory, was the warhead for the LIM-49A Spartan, an exoatmospheric (i.e., in-space) interceptor of a U.S. ballistic missile defense program of the early 1970s. It had a large 5 MT yield, unusual in that it was engineered to maximize its production of thermal X-rays, which, rather than blast, were its actual kill mechanism. It was a predecessor of later theoretical systems that used X-ray lasers, driven by a nuclear explosion.

Conducted on Amchitka Island in Alaska, the CANNIKIN underground nuclear test of 6 November 1971 was "the largest underground nuclear test conducted in the United States. CANNIKIN was conducted to proof test a warhead for the Spartan missile, a Safeguard Ballistic Missile Defense Program. The shock registered 7.0 on the Richter scale, the seismic unit of the time. Within two days after the explosion, a crater more than one mile wide and 40 feet deep formed."

The anti-ballistic missile system of which the Spartan was part was both made technologically obsolete by evolving multiple warhead technologies, and it was cancelled as a result of a 1972 treaty between the U.S. and Soviet Union.