Pluto photographed from the New Horizons spacecraft. © NASA/APL/SwRI

Glenn Seaborg, Joseph W. Kennedy, Edwin McMillan and Arthur Wahl discovered element 94 in Berkeley in 1941. McMillan and Philip Abelson had discovered element 93 the previous year. When Martin Heinrich Klaproth isolated element 92 in pitchblende in 1789, he called it uranium after the recently discovered planet Uranus. The scientists at Berkeley named elements 93 and 94 after the planets Neptune and Pluto. The discovery of plutonium was kept secret until after the war. At Los Alamos it was called ‘49’. This did not help much since Klaus Fuchs gave all the details about the bomb to the Russians. The Germans also realised the value of element 94 for making bombs but they never could make a reactor to produce the stuff.

Seaborg’s team bombarded uranium-238 with heavy hydrogen nuclei, or deuterons (a proton and a neutron; ordinary hydrogen has only a proton in the nucleus). This produces neptunium-238, which decays into the non-fissile isotope plutonium-238. Like other plutonium isotopes Pu-238 decays into an alpha particle (two protons and two neutrons, identical to a helium nucleus) and an isotope of uranium. The half life of this decay is a relatively short 87.7 years. (The fissile isotope Pu-239 has a half-life of more than 24,000 years.) It generates a good deal of heat: about half a watt per gram. The alpha particle is not a penetrating form of radiation, unlike beta particles (electrons or positrons) or gamma rays. It can be stopped by a sheet of paper. On a visit to the test site in Nevada in 1957, I was given the pit of a plutonium bomb to hold. It was warm to the touch because of the alpha decay but I was assured that I would not suffer any permanent damage. I was also told not to drop it. The pit was the size and weight of a bowling ball. This should have tipped me off that it was hollow since I never could have lifted a solid sphere of plutonium that size. The pits were being used to test 'boosted' weapons that use both fission and fusion.

Pu-238 can be used to generate electricity safely. One of the first applications designed at Los Alamos was for use in pacemakers. The latest application is in the batteries powering the New Horizons space probe which is now passing by Pluto. The batteries were designed at the Idaho National Laboratory and are said to have cost a hundred million dollars. They contain about 10.9 kilograms of Pu-238 oxide pellets. This is a large fraction of what was available. Pu-238 is a rare commodity. The United States stopped making it in 1988 and was buying it from the Russians until they ran out. There is now a project to make more Pu-238 in the US, at a cost of tens of millions of dollars, by the neutron irradiation of element 95, americium, which is produced in small amounts in nuclear reactors. Nasa does not have much left. A good deal of what they had is now heading for outer space. There is something fitting about this. Plutonium, which does not exist naturally, is created from uranium, which came to us in the first place from the collapsing of stars.