Polonium initiator was classified until the 1960s. The Atomic Energy Commission and the Manhattan Project funded human experiments using polonium on five people at the University of Rochester between 1943 and 1947. The people were administered between of polonium to study its excretion. Polonium is a very rare element in nature because of the short half-life of all its isotopes. Po, Po, and Po appear in the decay chain of U; thus polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 10), which is approximately 0.2% of the abundance of radium. The amounts
Isotopes of polonium per second as 5 grams of Ra. A few curies (1 curie equals 37 gigabecquerels) of Po emit a blue glow caused by excitation of surrounding air. A single gram of Po generates 140 watts of power. Because it emits many alpha particles, which are stopped within a very short distance in dense media and release their energy, Po has been used as a lightweight heat source to power thermoelectric cells in artificial satellites; for instance, a Po heat source was also in each of the Lunokhod rovers deployed on the surface of the Moon, to keep their internal components