But when the arrays are shadowed by the planet, or when they are pointed away from the Sun during spacecraft maneuvers, the batteries take over, discharging to power the spacecraft until the Sun returns to the arrays. After hundreds of charge -discharge cycles, NiCad batteries degrade in performance, but they may be rejuvenated by carefully controlled deep discharge and recharge cycles, an activity called battery reconditioning. RTGs Beyond the orbit of Mars, the Sun's light is too weak to be practical for generating power with solar arrays. RTGs contain pellets of plutonium 238 in the form of an oxide pressed into ceramic pellets. As the radioactive plutonium decays, it produces heat, and this is converted into electricity by an array of thermocouples made of silicon- germanium junctions.
Any waste heat is radiated into space by an array of metal fins.
The very name plutonium conjures images of extreme hazard, and the possibility, although extremely remote, of an accident releasing this element into Earth's atmosphere has not been ignored by spacecraft engineers. In the US space program, RTGs are designed to survive launch accidents without releasing the plutonium, and these designs have been tested by the US Department of Energy. In fact, the prelaunch procedures for RTG-carrying spacecraft are so rigorous that the office of the president of the United States must be satisfied of their safety and approve each launch. Galileo's RTGs are mounted behind reflective shields to shade the near-infrared mapping spectrometer from the heat they radiate. Much of the spacecraft's mass shields Galileo's high-energy particle detector from the RTGs' gamma radiation.