Project Pluto was a
USAF/
AEC project to develop a nuclear-propelled supersonic
cruise missile. The idea had originated with
Project Orion, which investigated using nuclear bombs to power
interplanetary spacecraft. In
1957,
Lawrence Livermore National Laboratory was given the task of developing this weapon, a large missile that would fly below
radar detection launching
hydrogen bombs as it went. As an added "bonus", as it flew over
enemy territory the supersonic
shock wave would probably kill people on the ground, and the engine would spray
radioactive fallout over anyone that survived.
The design of the missile was as follows: It would be launched by Shuttle-like booster rockets, and once it reached the right altitude and speed the nuclear ramjet would kick in. This engine operated by funneling air from an intake over an essentially unshielded hot nuclear reactor. Once it descended to operational (very low) altitude, it would use terrain-comparison guidance software to find its way to the target (and not run into tall trees or people's barns).
Thankfully, this project was cancelled in 1964, even though initial tests on the reactor were successful. It was rendered obsolete by ballistic missiles such as Titan and Atlas, and even the military finally recognized that it was an insane, overpriced horror of a weapon that would endanger friend and foe alike.
Sources: Air and Space Magazine article (see http://www.merkle.com/pluto/) and The Brookings Institution (http://www.brook.edu/fp/projects/nucwcost/box2-2.htm).