Plutonium (Pu) is a silvery-white metal. Plutonium-238 is a radioactive isotope of Pu, with special characteristics that make it extremely useful, particularly for space exploration.
Plutonium is nearly 20 times denser than water; Pu-238, its isotope used in space exploration, is almost 10 times denser than water.
Pu-238 emits alpha particles as it decays, which means it is a constant heat generator. However, the particles can be blocked by almost any barrier, meaning it cannot be used in nuclear reactions and is much less harmful than other isotopes.
The half-life of Pu-238 is 87.7 years.
Plutonium is only found in small quantities in its natural state; however, it can be man-made in particle accelerators out of uranium. The difficulty and expenses of finding or producing it, however, as well as its properties mean that it has a limited range of uses. It is most famously used as a heat source for radioisotope thermoelectric generators, the so-called "batteries" that power instruments in the extreme cold of space. It was also successfully used in cardiac pacemakers and is now still used in navigation beacons.
Like many radioactive isotopes of heavy metals, Pu-238 is highly toxic and can cause cancer if absorbed into the body (through inhalation, direct contact with open wounds or ingestion of contaminated substances). The lungs, bronchia, liver and bone marrow are most immediately affected by Pu-238.