In 1946, a man named Stanislavv Ulam had the idea to propel rockets with nuclear explosions. Ulam had worked on the Manhatten project and came up with the idea of nuclear pulse propulsion. Instead of a steady-burning chemical engine like those used in Apollo moon rockets, a series of nuclear explosions propels the rocket forward.
The rocket would eject a small nuclear charge or explosive behind it. The charge would detonate about 200 feet from the rocket, its shock wave pushing against a thick blast plate at the back of the rocket. Each explosion would add about 30 miles per hour to the rocket’s speed, meaning several hundred nuclear explosions would be needed to get the rocket into orbit and then more to send it to Saturn or another gas giant.
Because nuclear explosions are pretty efficient, a rocket powered by nuclear pulse propulsion could carry lots of cargo into space and go a long way. The only problem is the sudden jerk after each blast (a force that could be as high as 100 Gs) could harm human occupants. A spring added behind the blast plate could help to prevent that though.
Obviously, a huge road block for the development of this type of rocket is the issue of nuclear fallout. Nobody was real keen to try detonating hundreds of nukes to try it out. Still, a nuclear pulse rocket could be lifted into orbit by a conventional rocket, assembled in space and then taken to a safe distance before being launched to the outer planets. The Earth’s van Allen belts could protect the planet from the rocket’s radiation.
The idea of building nuclear pulse rockets was manifested in Project Orion in 1958 (the year after the world's first artificial satellite - Sputnik - was launched into space by the USSR). Some tests were done, but NASA never really got behind Orion and the 1963 Partial Nuclear Test Ban Treaty helped to kill the project which ended in 1965.
Nuclear thermal rockets, rockets whose nuclear fuel is contained inside the vehicle, were another nuclear design for getting to space after World War II. Project Rover, which ran from 1955 to 1972, worked on using a nuclear reactor inside a rocket to propel it similarly to a chemical rocket. A nuclear thermal rocket could lift about twice the payload as a chemical rocket, but also carried the risk of spreading radioactive material in the atmosphere if the rocket failed during liftoff or while in orbit.
Though nuclear-powered rockets were never fully developed and used, the Earth did have a small scare concerning nuclear materials and space travel in 1970 when Apollo 13 failed to land on the Moon. The lunar module (LEM) had on board a small nuclear
The plutonium, however, made it safely through re-entry (as designed) and landed in the Pacific Ocean's TONGA Trench east of Australia. To this date, the wreckage has not shown any evidence of radiation leakage, though the plutonium will remain radioactive for two millennia.
Might humans one day revisit the idea of using nuclear pulse propulsion? Two things might make it a good idea: Its use as an interstellar ark as such a rocket could launch millions of tons of payload into space or its use as an attempt to deflect an asteroid bound for Earth.