This photo may look drab but it is the result of an astounding sequence of technological, physical, and historical developments. It is the first image beamed back from the planet Mars by Curiosity, a robot vehicle loaded with instrumentation designed to determine the composition of Martian soil and see if there is or was any water on the planet. Curiosity landed only a few hours ago (late on the night of August 5 2012), so the photo is a pretty up to date view of that little patch of Martian ground.
How did the image get beamed back here to Earth? Curiosity’s electronics are powered by what’s called a radioisotope thermoelectric generator (RTG). An RTG is a device that uses the heat from disintegrating radioisotopes to generate an electric current. In Curiosity’s case the radioisotope is plutonium-238, a strong alpha emitter with a half-life of around 88 years. A disintegrating Pu-238 atom ejects an alpha particle from its nucleus with an energy of 5.5 million electron volts. (At that point the plutonium-238 atom turns into the atom of another element, uranium, with a mass of 234 atomic mass units, or AMU.) The ensuing collision, between the ultra-high-energy alpha particle and the first material it encounters, generates a lot of heat.
Almost all missions to Mars and beyond are powered with Pu-238. By the time you get out to Mars, solar energy is too weak to generate enough electricity for missions like Curiosity.
Pu-238 is an extremely precious metal. It’s precious because it must be manufactured in a nuclear reactor, by bombarding neptunium-237 with neutrons. Np-237 is also difficult to come by; its manufacture starts when uranium-235 captures a neutron but doesn’t fission and instead becomes U-236. U-236 then also captures a neutron to become U-237, which quickly decays by beta emission to become neptunium-237. Np-237 is separated from the original uranium, fission products, and transuranics, then formed into new targets, and bombarded with neutrons to make Np-238. That short-lived isotope (half-life = 2.1 days) then beta decays to Pu-238.
The whole process is arduous, and depends on the purity of the Np-237 target material. That in turn depends on the purity of the original uranium target material (U-235).
The U.S. made a lot of Pu-238 back when the Savannah River Lab plutonium reactors were operating. However, SRL’s main product in those days was not Pu-238 but another plutonium isotope, one that is heavier than Pu-238 by one AMU (nucleon)—i.e., Pu-239, for nuclear bombs. This required different target material: U-238. The efficiency and effectiveness of that process also depends on the purity of the uranium target, in this case U-238. But the manufacturing process is the same: fire neutrons at targets and wait for the targets to capture them and morph into the desired elements.
There eventually came a point where the U.S. decided it had enough Pu-239, and so the SRL production reactors were mothballed. At that point, U.S. production of Pu-238 also ended.
But space missions continued. Since space missions to Mars and beyond require RTGs, the U.S. space agency NASA had to find Pu-238 from another supplier.
That supplier was Russia. The end of the Cold War brought an employment crisis to Russia’s enormous nuclear weapons complex. Suddenly there was little need to keep producing Pu-239 for bombs. One of the main plutonium production complexes is called Mayak; it is near the Russian city of Chelyabinsk. Like SRL, the Mayak reactors were designed to bombard targets with neutrons. And like SRL, Mayak is also capable of making Pu-238.
The Americans have been fascinated with Mayak since the early Cold War. So fascinated that in April 1960 they sent a U-2 spy plane to photograph the place. That U-2, piloted by Francis Gary Powers, was captured by the Soviets, who then paraded pictures of the wreckage in front of the world media, embarrassing U.S. president Eisenhower, who had publicly denied there had been a U-2 mission to the USSR in the first place.
But when the Cold War ended, U.S.-Russian relations were somewhat more friendly. In the interest of keeping Russian nuclear experts gainfully employed while the post-Soviet economy imploded into chaos, the U.S. and Russia agreed on a Pu-238 supply arrangement. The Russians insisted that any Pu-238 sent to the U.S. be used only for scientific and space exploration purposes. (Both Russia and the U.S. use Pu-238 powered RTGs in satellite and terrestrial spy devices).
It is safe to say that Curiosity’s RTGs are powered, at least in part, with Russian Pu-238 from the Mayak complex. Which, in the final analysis, is another example of beating swords into ploughshares. It’s nice to see, in Curiosity’s photo above, an example of international peace and cooperation.
Mayak produces many other isotopes in addition to Pu-238. Most of the smoke detectors in Canada use americium-241 as the ionizing source; Mayak makes most if not all of the world supply of Am-241. It also makes most if not all of the world’s cesium-137, a hugely important isotope. Mayak is also challenging Canada’s NRU as the world’s most important source of cobalt-60, the primary cancer-fighting and blood-sterilization isotope.