NASA is running low on plutonium, an issue that is causing growing concern for future outer solar system missions. And now, the European Space Agency (ESA) has recognized the US space agency’s problems in acquiring the fuel, announcing Europe has plans to start their own production to support joint NASA-ESA programs.

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The isotope plutonium-238 (or Pu-238) produces a steady supply of heat that can be readily converted into electricity. Small pellets of Pu-238 (like the one shown above) are commonly found inside radioisotope thermoelectric generators (RTGs) — the power source of spacecraft that explore space beyond the orbit of Mars. At these distances, the sun’s energy is too weak to be a viable energy source for spacecraft, forcing space agencies to use the plutonium isotope.

Deep space missions such as the 1970′s Pioneer and Voyager probes were all launched with RTGs attached — Voyager 2 is still transmitting scientific data after three decades in space, proving the longevity of this energy resource. The Cassini Equinox and New Horizons missions are also equipped with RTGs, and next year’s NASA Mars Science Laboratory will use Pu-238 to provide a 24/7 energy resource.

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Alas, although Pu-238 isn’t fissile (i.e. it can’t be used to make a bomb, unlike its slightly larger isotope cousin, Pu-239), it is still radioactive and has very tight regulations surrounding its acquisition and production. Unfortunately, NASA’s stockpile is running low.

The US Department of Energy no longer has the funding to restart Pu-238 production and due to a contract dispute, NASA cannot acquire it from Russia. This means that NASA now lacks the plutonium to contribute toward a planned $4.5 billion joint U.S.-Europe flagship mission to the Jovian moon Europa.

“If we close another deal with the Russians for another delivery of plutonium-238, or get domestic production restarted, there’s sufficient plutonium well out past the Outer Planets Flagship Mission,” said Jim Adams, deputy director of NASA’s planetary science division.

The Russian government also has dwindling supplies after halting production of Pu-278, so they are pursuing a more lucrative contract with NASA — the cause of the dispute.

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If Congress denies domestic production and the Russian deadlock continues, there appears to be only one answer to the plutonium deficit: ESA.

“To see see ourselves as a serious planetary science partner on the world stage with the United States, we’re building up our nuclear capability for European-built RTGs,” David Southwood, ESA’s director of science and robotic exploration, said in an interview with Spaceflight Now. “We are building for a pretty major capability being available in Europe in the 2020s.”

Southwood also hinted that Pu-238 isn’t necessarily the only fuel that can be used with RTGs. Americium-241 has the advantage of a longer half-life, meaning these pellets will fuel RTGs for longer, but at a reduced energy output. Another big drawback with swapping americium for plutonium is that americium is more hazardous.

“Plutonium-238 is an alpha emitter, and you can shield alpha particles with a piece of paper,” Adams said. “It’s neutrons that damage people, and americium is more a neutron emitter than plutonium-238.”

Regardless of the fuel Europe decides to produce, the commencement of a nuclear energy program for space missions will have to wait for approval from ESA Council meetings in 2011 or 2014.

Whatever the outcome, it is good to see strengthening collaboration between ESA and NASA.

“Our target is to have an independent capability, which may help our American friends,” Southwood added.

Images from top: A pellet of Pu-238 (DoE), NASA New Horizons Pluto mission with RTG attached (NASA).