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An ingot of Plutonium-238, the typical power source used by NASA for its probes.  (Source: Discovery News)

The Cassini probe (pictured), the Voyager probes, and the Pioneer probes are all powered by Pu-238. NASA does not have enough Pu-238 to power its upcoming Mars Space Laboratory mission.  (Source: Space Pictures)
Radioactive isotope is vital for powering deep-space probes

Europe, a leader in nuclear power, has announced that it intends to lend its American counterparts a hand by making Pu-238 for NASA.  David Southwood, ESA's director of science and robotic exploration, in an interview with Spaceflight Now, states, "Our target is to have an independent capability, which may help our American friends."

Since the Pioneer and Voyager missions of the 1970s, NASA has been using the radioactive plutonium-238 (or Pu-238) isotope to power its deep space missions.  The radioactive source has a very long half-life of 87.7 years.  Over that period it slowly decays, releasing a steady stream of thermal energy in the process.  That thermal energy is harvested by radioisotope thermoelectric generators (RTGs) in the probes to make power.

More recent missions to use the technology include the Cassini Equinox and New Horizons missions.

Unfortunately, NASA's plutonium stockpile has almost been exhausted, even as agency prepares its new Mars Space Laboratory which will require the isotope for power.  There's really no alternative currently for NASA, as the operational range of many of its missions place it well outside the spatial volume where the sun's rays are strong enough to provide a decent level of solar power.

At a base level, NASA's plutonium shortage is due to the fact that Pu-238 production is expensive and tightly regulated.  The U.S. Department of Energy, due to budget cuts, lacks the money to fund additional production.  

Despite the fact that the material is not fissible -- hence cannot be made into atomic bombs -- tight regulations are further constraining the supply, limiting trade to only a couple potential candidates.  One of those candidates -- Russia -- is locked in a contract dispute with NASA, essentially ruling out its possibility as a source for now. 

Jim Adams, deputy director of NASA's planetary science division optimistically states, "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."

Fortunately, the European Space Agency is looking to come to NASA's rescue, announcing that it will begin producing Pu-238 and offering it to its American partner.  Europe is a world leader in nuclear power, with the majority of EU member France's power coming from nuclear energy.

The move will allow it to provide fuel for the $4.5B joint U.S.-Europe flagship mission to the Jovian moon Europa -- fuel that NASA currently lacks.  The ESA's Southwood states, "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.  We are building for a pretty major capability being available in Europe in the 2020s."

The final production decision for the nuclear isotope will take place in 2011 or 2014 at EU council meetings.

Another perhaps possibility for both the EU and U.S. is to alternatively produce Americium-241.  This radioactive isotope has the advantage of being plentiful (it is found in most nuclear waste) and has an extremely long half life of 432.2 years.  However, its power output (114 watts decay heat/kg) is much lower than Pu-238's (560 watts decay heat/kg).  And it is more dangerous and hard to handle.

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



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RE: Would be nice....
By randomly on 7/13/2010 2:26:07 AM , Rating: 2
The term 'fissile' is a bit slippery and the meaning changes somewhat depending on who is using it.

In any case, Pu238 has almost the same 10 kg bare sphere critical mass that Pu239 does so it is certainly capable of being made into a bomb in that respect. The major hurdle is the kilowatts of heat constantly put out by the core which makes it virtually impossible to assemble a working bomb even with drop in core designs. In any case the world supply is extremely limited <20 kg.

Pu238 is the isotope of choice for RTGs for deep space missions. Good power output, long life, and requires the least amount of shielding. The US was looking at restarting Pu238 production at the rate of 5kg a year for NASA and the NSA etc. spy guys.

If ESA can supply it instead that is very good news as it will save us a lot of money if we can avoid our own Pu238 production startup which will cost a couple hundred million dollars and take 6-7 years.


RE: Would be nice....
By marvdmartian on 7/13/2010 8:44:00 AM , Rating: 2
Thanks to both of you for the replies.

Now i recall, that the fuel Pu-238 is made in breeder reactors, from depleted Uranium (U-238) placed in the outer shielding area, where it's bombarded with neutrons and is made into Pu-238 (or something like that).

Been too many years since I read up on this stuff! ;)


RE: Would be nice....
By randomly on 7/13/2010 12:58:39 PM , Rating: 2
In a breeder reactor U238 absorbs a neutron and becomes U239 which then beta decays to Np239, and that beta decays to Pu239.

To make Pu238 you start with Neptunium 237 which absorbs a neutron in the reactor and then turns into Pu238 after two beta decays.

Pu238 is the isotope that puts out all that decay heat for RTGs in space probes.

Pu239 is the isotope used as a fuel in reactors and the cores of nuclear bombs.


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