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“In 1957, the U.S. Air Force initiated Project Pluto. Its goal was to build a nuclear-powered ramjet Supersonic Low-Altitude Missile (SLAM) that could function as the ultimate response to a nuclear attack by the Soviet Union.

"Traveling at Mach 3, Pluto would drop to virtual tree-top level, tearing ground structures to shreds with its sonic boom, all the while spraying a lethal stream of highly radioactive exhaust in its wake. But that was just the pre-show.  Pluto’s real punch was to be its 14 and 26 thermonuclear warheads which could be launched one at a time through a system similar to the launch tubes on a Polaris/Poseidon submarine.  (Launching the warheads upward instead of simply dropping them would give Pluto enough time to escape the blast of its own weapons.)

"Just one problem remained: How do you test a nuclear-powered doomsday weapon? 

"This was one problem Pluto’s engineers were unable to solve.  That coupled with the development of dependable ICBMs and the ecological challenges Pluto presented caused this ambitious program to be canceled in 1967.”