... may have been SLAM, a Supersonic Low-Altitude Missile, using nuclear ramjet power. The project to build the weapon's nuclear reactor was given the code name "Pluto," which ultimately became the name of the weapon itself.
"...a locomotive-size missile that would travel at near-treetop level at three times the speed of sound, tossing out hydrogen bombs as it roared overhead. Pluto's designers calculated that its shock wave alone might kill people on the ground....In addition to gamma and neutron radiation from the unshielded reactor, Pluto's nuclear ramjet would spew fission fragments out in its exhaust as it flew by."
Happily, on July 1, 1964, seven years and six months after it was born, Project Pluto was cancelled, for a variety of reasons, chief among them that Pluto would have irradiated our allies on its way to flatten the Soviet Union. Testing would also have been something of a problem. Where do you test-flight something that, "spews out fission fragments as it flies by"?
A peaceful counterpoint to Pluto was Project Orion, an atomic bomb-powered spacecraft conceived by Freeman Dyson that a group of scientists planned to use for a Grand Tour of the solar system... and reach Saturn by 1970. In some alternate, Heinleinesque, universe I hope they did it, and there is a base on the Moon, space stations orbiting their Earth, a small, growing colony on Mars, and explorers pushing out to the Asteroid Belt. It's a future I would like to have seen.
But I still have time.