The spacecraft are powered by nuclear batteries (radioisotope thermoelectric generators) with electricity generated from heat produced by the decay of plutonium. While it is a pretty durable power source, it is not designed to last forever. The spacecraft loses power by roughly 4 watts every year. With power dwindling, some non-essential instruments on the spacecraft had to be shut off over the years to keep the mission going. For instance, in 2019, the primary heater for the cosmic ray subsystem instrument was turned off to reduce power consumption.
Each probe holds 11 instruments to carry out various tasks such as examining atmospheric chemistry, magnetic fields, charged particles, physical properties of the planets, and solar wind.
Picture Credit : Google