WHERE A ROBOT SPACESHIP GETS ITS POWER

A spacecraft needs energy not only for propulsion but also to power its electronic circuits and mechanical systems. For spaceships near Earth, engineers usually put solar cells in their designs to generate electricity, but once a spaceship goes beyond Mars, even the shining Sun cannot provide enough power for a spaceship to operate. Instead, all the spacecraft that have been sent to Jupiter and beyond have a radioisotope thermoelectric generator (RTG).
An RTG uses plutonium-238 as fuel. Although radioactive, plutonium-238 cannot explode like a nuclear bomb. But the radioactivity turns into heat, and the heat activates special electronic circuits called thermocouples. After a few decades, the thermocouples get used up and produce too little electricity to power the spacecraft. However, the energy in plutonium-238 lasts a long time—less than half of it is gone after 90 years! RTGs used to power spacecraft are about the size of a backyard barbecue.Tested on military satellites in the 1960s, they were the most obvious choice to power missions to deep space.