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Content: Shape is relevant; compact shapes such as spheres are to be avoided. Plutonium could also be used to manufacture radiological weapons. The plutonium isotope 238Pu is an alpha emitter with a half life of 87 years. These characteristics make it well suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human life time. It is therefore used in RTGs such as those powering the Galileo and Cassini space probes. Plutonium-238 was used on the Apollo-14 lunar flight in 1971 to power seismic devices and other equipment left on the Moon, and it was also the power supply of the two Voyager supercraft launched in 1977. Plutonium-239 can also be used as a fuel in a new generation of fast-breeder nuclear weapons, which burn a mixed oxide (MOX) fuel consisting of uranium and plutonium. Plutonium in the environment
Trace amounts of plutonium are found naturally in uranium-rich ores. Humans produce most of the existing plutonium, in special nuclear reactors.