This little pellet can power a spacecraft. Another hint: It's somewhat related to the stuff involved in the Chernobyl nuclear disaster, 25 years ago today.
That should be enough clues for an approximate guess. So …
The image shows a pellet of plutonium used to power the radioisotope thermoelectric generator (RTG) in either the Cassini mission to Saturn or the Galileo mission to Jupiter (we're not told which — top secret stuff I suppose). Plutonium was also used to power equipment during Apollo moon landings.
The pellet glows red hot because of radioactive decay, which means energy is released in the form of ionizing particles. In a spacecraft, plutonium-238 — what you see above — is at the heart of a long-lived nuclear battery that converts heat from the decay into electricity to power the spacecraft instruments.
Plutonium-238 has a half-life of 88 years, which means half of it decays every 88 years. By comparison, plutonium-239, a favorite of nuclear weapons manufacturers, has a half-life of 24,100 years.
Importantly, plutonium-238 is not fissile, meaning it does not sustain the chain reactions needed for nuclear fission.
(Large-scale power-generating nuclear reactors typically use uranium-235, by the way.)
RTGs have been used in several U.S. and Soviet/Russian spacecraft. The relative small amounts of nuclear material involved, coupled plutonium-238 not being fissile and safety measures in place, make the units far less dangerous than large-scale nuclear reactors, experts say. You can learn more about RTGs here, and NASA explains their relative safety here.
Image Credit: Los Alamos National Laboratory/U.S. DOE via Wikipedia