Those components were the iridium-clad pellets of plutonium-238 developed by the Department of Energy decades ago. Contained within three cylindrical power sources called radioisotope thermoelectric generators (RTG), they produced the heat and electricity that kept Cassini on track and responsive throughout its long journey.

RTGs are lightweight, compact spacecraft electrical power systems that have flown successfully on 23 previous U.S. missions over the past 37 years. Cassini’s RTGs contained plutonium from the Department of Energy’s Savannah River Site in Aiken, South Carolina. The iridium cladding around that precious nuclear fuel was produced by Oak Ridge National Laboratory and the Y-12 National Security Complex.

Cassini launched on Oct. 15, 1997, with the European Space Agency’s Huygens probe. Named after astronomers Giovanni Cassini and Christiaan Huygens, the pair of spacecraft reached Saturn in 2004 after a 2.2-billion-mile (3.5-billion-kilometer) voyage. In 2005, Huygens was deployed to the surface of Saturn’s largest moon, Titan.