Houston, we've had a problem,' said astronaut Jack Swigert on April 13, 1970. But the problem wasn't as simple as three astronauts potentially trapped in the void of space, 200,000 miles from Earth. The catastrophic risk came from the SNAP-27 radioisotope thermoelectric generator (RTG), a small nuclear reactor that was going to be placed on the moon to power experiments, carrying Plutonium 238 in Apollo 13's lunar module. As luck would have it, NASA had experience losing RTGs – a navigation satellite failed to reach orbit in 1964 and scattered small amounts of plutonium over the Indian Ocean. The SNAP-27 had been engineered to make it back to Earth intact in such an incident. 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, approximately 6-9 kilometers underwater (its exact location is unknown). Extensive monitoring of the atmosphere in the area showed that no radiation escaped."Read more of this story at Slashdot
