English: Plutonium-238 pellet under its own light. Pu-238, with a half-life of 87.7 years, is being used in space applications requiring a power source with a long service life. Pu-238 has a relatively high heat production rate which makes it useful as a power source. As a power and heat source, Pu-238 has also been used to power instruments left on the Moon by Apollo astronauts, navigation and weather satellites, and interplanetary probes. The interplanetary probe Pioneer-10, powered by a Pu-238 source, recently left the solar system. Image ID: 2006407