Apollo 13's radioactive secret

On April 11, 1970, NASA launched the Apollo 13 manned spacecraft with the intention of landing for the third time on the Moon. Halfway, during the second day of travel, an oxygen tank of the spacecraft burst and destroyed a couple of modules. The three crew members struggled to survive in the half-destroyed ship. After five very complicated days, they were able to return safely back to Earth —or not so safely, since one of them got a urine infection due to holding his pee for too long.

Moment in which Apollo 13 splashed down.

This is not an unknown story. On the contrary, Hollywood already monetized it in 1995, with the Oscar-nominated movie Apollo 13. This article, however, is not about how skillful and lucky the crew was, but about the little present that the Lunar Module (a part of the Apollo 13 that also went back to the Earth) left in the Pacific ocean when it sank.

In order for the artifacts that are sent to the space to work, it is necessary to equip them with a powerful source of energy. One of the most attractive ones, because of its duration and lack of maintenance, is the radioisotope thermoelectric generator (RTG), a device that generates electricity (a few hundred watts) from a source of heat, particularly coming from the nuclear disintegration of radioactive atoms. In short, an RTG is like a small nuclear power station in the form of a very large car battery.

At first glance, RTGs are presented as a very good solution to the energy problem: they are maintenance-free electricity generators that last for many years and are not very expensive. For instance, some of the scientific instruments carried by the Voyager probes, which were launched 40 years ago to study the gaseous planets of our solar system and are currently drifting somewhere in interstellar space, still work thanks to the energy provided by their RTGs, and will continue working for 10 or 15 more years.

It is not necessary to go the outer space to find the practical value of those batteries. Soviet Russia put several RTGs into the most inaccessible lighthouses in the Arctic so that they could operate autonomously and seamlessly for some decades, and they are currently forgotten and under poor maintenance conditions.

However, we have all seen on TV launch failures of spaceships that explode before leaving the atmosphere. When this occurs, what happens to the RTGs? It is obvious: they become a disastrous cloud of highly radioactive waste. And it has already happened several times.

Woman measuring the radiation of an RTG.

For example, in 1961, the Transit satellites of the US military navigation system were the first ones in incorporating these RTGs. Until 1964. That year, something went wrong when putting the satellite Transit 5BN-3 in orbit and it just volatilized when reentering the Earth’s athmosphere, at the same latitude as Madagascar. Its destruction injected into the air 630 TBq (terabecquerels) of plutonium-238, whose traces were detected for months. From that moment on, it was decided that such operational satellites would be powered using solar energy, since it is less costly.

To mention another example, in 1970 and in 1973, the former USSR successfully sent to the moon a couple of robotic vehicles, Lunojod 1 and 2. But this success came after the failure, in 1969, of Lunojod 1A, a previous version that was destroyed few seconds after its launching from the Baikonur cosmodrome. The explosion produced by the rocket carrying the vehicle scattered a big amount of polonium-210 over a large area of Russia.

Back to the Apollo 13 story, the Lunar Module that sank in the sea indeed took its energy from an RTG. The fact that it fell into the Pacific ocean did not happen by chance: Tonga trench, nearly 11 kilometers deep, was chosen to minimize radioactive risks. ‘It will keep a few fish warm,’ they thought at NASA! Fortunately, engineers were smarter during this mission. Knowing that there are chances for the ships to explode, they designed a graphite coating capable of withstanding strong shocks. 1650 TBq of plutonium dioxide well deserve it.

After rescuing the astronauts, no increase of the natural radioactivity was detected in the area, so it is assumed that the device lies intact in the deep sea. Manufacturers said that the coating was able to contain the plutonium for nine centuries, and it will be not so harmful by then. However, it is logical to think that the time it will remain well protected depends on many factors, such as the bumps it received, the effect of sea water corrosion, and so on. Let’s hope the coating is as durable as they said, since there is no plan to recover it and Pacific ocean has already got enough with Fukushima nuclear disaster.

This post is an old article I wrote in 2017 on Medium.

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