Monday, August 15, 2011

How we (thankfully?) didn't go to Saturn

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.

[Somewhat tongue-in-cheek TED Talk by George Dyson, son of nuclear pulse propulsion designer Freeman Dyson, filmed in 2002.]

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 reactor power source, called a radioisotope thermoelectric generator (RTG), that was to generate electricity for an experiment that would be left on the Moon. Because the LEM never touched down on the Moon and was instead used as a life boat to return the astronauts safely to Earth, the plutonium in the reactor RTG was also brought back to Earth. Some worried about what would happen to the plutonium after the LEM was abandoned and left to burn up in the atmosphere.

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.


  1. I believe nuclear rockets will become a pretty more efficient with plasma turbine.

  2. The Orion concepts didn't use "a spring", they used a variety of pneumatic, hydraulic, and other systems to absorb shocks and damp vibrations.

    The Van Allen Belts won't do anything to block radiation. The magnetosphere that is the reason for their existence would deflect charged particles, but would probably result in Earth capturing more radioactive material overall than it would otherwise. The atmosphere would effectively block most of the ionizing radiation, but since sheer distance would reduce the intensity of the radiation to harmless levels, that's rather irrelevant.

    Nuclear thermal rockets have high propellant efficiency but poor thrust to weight ratios. They are best used in upper stages, which also means they don't have to be started up until well away from Earth. If there's a launch failure, unused nuclear fuel is about as dangerous as a hunk of lead...don't let it land on your head, don't eat it, and you'll be fine.

    Apollo 13 did not have a nuclear reactor, it had an RTG. RTGs function by using heat from passive decay, they do not induce, control, or otherwise influence any sort of reactions and can not be called reactors. Because they operate by radioactive decay, they need to use extremely radioactive materials with short half lives to produce useful power. The plutonium will remain radioactive until the last atom decays, the 2000 years figure is likely the time for it to decay to background levels.

  3. @cjameshuff - Thanks for the clarification on the lunar experiment power source. For those interested in learning more, here's an American Nuclear Society article about the radioisotope thermoelectric generator (RTG) which includes a really cool picture of two RTGs that ended up on the ocean floor after a failed satellite launch:

    (RTGs, by the way, are what's keeping the Voyager missions alive and on their extended interstellar missions.)