A pellet of plutonium-238, illuminated by the glow of its own radioactivity. Plutonium (Atomic number 94, symbol Pu) occurs in nature only in minute quantities that arise from the decay of Uranium- 238, the heaviest naturally-occurring element. All of plutonium's isotopes are radioactive; Plutonium-238 is used as a long-life power source in space applications including, including weather satellites and probes, left on the moon by the Apollo Space Mission astronauts and the interplanetary probe Pioneer-10. A second isotope, Plutonium- 239, is used as an explosive in atomic bombs and as a fuel in nuclear reactors. Plutonium is a highly toxic and carcinogenic (cancer forming) substance and is dangerous even at very low concentrations.