Rory Compton, of Balclutha Primary School, asks :-

What is plutonium and why did spacecraft, such as Cassini which was sent to Saturn, need it?

Alan Poletti, a nuclear physicist at the University of Auckland, responded.

On space missions far from the Sun, solar panels are unable to supply enough electric power. So these spacecraft use the heat from radioactive decay and thermogenerators to supply electrical power. More than 20 previous spacecraft used such generators.

Plutonium is one of the chemical elements which are radioactive. In the decay of a radioactive material, mass is converted into energy. The 23.5 kilograms of plutonium in the thermoelectric generators on Cassini produces the same amount of heat as 17 electric heaters, each of one kilowatt power. It takes 88 years for the amount of plutonium to decay away to half its initial value so that power source lasts far longer than batteries could.

What is plutonium? A very heavy metal. The nuclei of all atoms of a given chemical element have in them the same number of particles called protons, which is the nucleus of a hydrogen atom. For instance, oxygen nuclei have eight protons while the nuclei of gold have 79. All nuclei of the element called plutonium have 94 protons in them.

Not all nuclei belonging to a given element have the same number of neutrons in them. For instance, while the nuclei of the most common form of oxygen have eight neutrons in them, other forms have nine or ten. These three different forms are stable and do not decay into anything else. These different forms are called isotopes. The isotope of oxygen which has eleven neutrons in its nucleus is radioactive and, on average, in 27 seconds decays to the stable isotope of fluorine.

Plutonium also has several isotopes. All of them are radioactive. They do not occur in nature and must be man-made in a nuclear reactor. The isotope used in the Cassini thermogenerators has 144 neutrons in each nuclei.

The plutonium on the Saturn spacecraft is very carefully packed so that even if something went wrong with the rocket and it crashed, there would be no danger.