Atomic Awesome

Atomic Wonderland: SNAP Back

Most everyone knows the story of Apollo 13, and the dire predicament Tom Hanks and those other 2 guys astronauts James LovellJohn Swigert, and Fred Haise found themselves in. NASA had one other minor issue to content with, one that involved the watchful eye of an Atomic Energy Commission representative. The Apollo missions 12-17* all carried with them a Radioisotope Thermoelectric Generator (RTG), to generate electrical power to conduct experiments. What exactly is a RTG you might ask? In simple terms, it is an atomic-powered electric generator that obtains heat for power generation from radioactive decay. The lunar module (LM) had a full fuel cask of Plutonium 238 atached to one of its legs, and it was going to be returning to Earth with the astronauts.

Apollo 14 RTG Setup

America’s first RTG launched into space was in 1961, on board the Navy Transit 4A satellite. The US Department of Energy website lists 28 RTGs in the Space and Defense Power Systems category, and the status of those atomic generators. The latest space probe containing one of these reactors was launched in 2006, New Horizons. Apollo mission RTGs had a life of about 5-8 years after being left on the Moon.

Apollo RTG Cask Mounted on LM Leg

The purpose of the cask was to safely contain the Plutonium 238 fuel in the event of some sort of failure that caused the fuel to re-enter the atmosphere–exactly the occurance when Apollo 13 was forced to return to Earth with the LM Aquarius. In order to set up the experiments once the mission had arrived on the Moon, the astronauts would transfer the fuel pellets from the cask into the reactor. The reactor and the power relay station were stored with the ALSEP (Apollo Lunar Surface Experiment Package).

Apollo Astronaut Alan Bean transferring RTG fuel. 1.) RTG fuel transfer tool. 2.) Dome removal tool. 3.) RTG, with Universal hand tool attached.

Apollo 16 RTG

Apollo 14 Power Relay Station

Here is a detailed explaination of the SNAP-27 reactor, as presented in the Apollo 13 pre-mission press kit (from, here is the complete press kit in .pdf):

When calculations were being made for the return and re-entry of Apollo 13, the priority was most certainly the safety of the astronauts. NASA was also able to keep in mind the radioactive fuel, and devise a plan that would ensure the safety of people on the ground as well. In “Thirteen: The Flight that Failed”, Henry S.F. Cooper states:

“(Chuck) Deiterich had assured a representative of the Atomic Energy Commission…that the controllers would see to it that the cask landed in deep water a couple of hundred miles off the coast of New Zealand.”

followed by further discussion later in the book:

“Under the best of circumstances nothing upsets a RETRO or a FIDO more than an attitude error, but this time there was the added problem of aiming the cask of radioactive fuel that Deiterich had promised the A.E.C. he would set down off New Zealand…..Deiterich was still worrying about the fuel cask, possibly because he felt the gaze of the A.E.C. man on the back of his neck. He rapidly figured out where the cask would come down under the new circumstances [i.e. attitude errors] and he was able to assure the A.E.C. representative that it would still come down in deep water.”

The LM separated from the Command Module just prior to re-entry, with the LM aimed towards open ocean.

“The plutonium, like the astronauts, apparently survived reentry and came to rest with what remained of the lunar module in the Tonga Trench south of Fiji [part of the Pacific Ocean], approximately 6-9 kilometers underwater (its exact location is unkown). Extensive monitoring of the atmosphere in the area showed that no radiation escaped. The plutonium was in an oxide form contained in fuel capsule, which itself was inside a graphite and ceramic fuel cask. “The cladding would not be compromised over time by the seawater,” said Leonard Dudzinski, a NASA program executive who deals with radioisotope power systems. The current expectation is that the cladding will survive for 10 half lives of the Plutonium, close to 870 years. If anything, the Apollo 13 disaster proved that NASA nuclear safety engineering worked.” (

Apollo RTG Plutonium Pellet (semi-cooled by graphite blanket prior to photo, pellets are approximately marshmallow sized)

Apollo RTG Plutonium Pellet (glowing due to decay heat)

[*Apollo 11 carried radioisotope heater units for a seismic experimental package, but not an RTG]

Images, in order of appearance, from, (next 9 images), NASA Press Kit (4 images), Wikipedia, and Additional research from and

  • Charles_Barrett

    Wow. Awesome write-up on a topic I'd never really thought much about, and certainly hadn't heard others discuss. Sounds like the potential for lose-lose scenarios were abundant.
    Well crafted, HycoSpeed…!

  • nice nice nice nice nice!
    i know my next t-shirt design!

  • Bravo. Great story. This is the first time I've heard of such a thing. An atomic toaster story here on Atomic Toasters, couldn't be more appropriate.

  • texlenin

    See, Hippies?
    Told ya this thing was safe!
    Now, down to that Big Science junkyard outside
    Livermore to see if they have any left. My waterheater's
    gone out again….

  • Victor

    This is fascinating information.

  • Blastface McLargecock

    The RTG cask was actually attached to the body of the LM Descent Stage itself just to the left of the ALSEP Storage Bay, not to one of the lander’s legs. In the photo from Apollo 12 with Alan Bean removing the cask top, it’s clear that the cask isn’t attached to a leg.