Sun is the unrestricted source of energy for the satellites that revolve around the earth, these satellites have solar panels which charges the array of batteries for the satellites to make the satellite function when satellites are revolving at the dark side of the earth, for instant the Geo Synchronous Satellites, which have a time period of rotation of 24hrs, the battery gets charged by solar panels during the sun lit phase of revolution then battery powers the satellite during the dark phase of revolution (when no sunlight reaches the satellite)
What if we need do send space cart into deep space where there is no Sun?
Solar panels are successful when sun is there but for spacecraft that are designed to navigate for interplanetary missions face a serious problem of constant energy source; they need a power plant that can constantly generate electricity so that the equipments can be kept at proper temperature for their proper function and to charge the batteries
Sun rays are not effective beyond Jupiter or when rover landed on Mars; example the Mars Exploration Rower, faced serious problem of solar power during short winter nights on Mars, the batteries never getting charged properly and it hammered the proper functioning of the Rover
So what was the solution?
Radio Isotope Thermo Electric Generator
A radio isotope thermo electric generator is a device which generates electricity using thermocouples and radio isotopes (a radioactive material)
When a thermal potential gradient is created between two ends of the metallic strip, flow of electrons starts to takes place in the external circuit when these two ends are connected via a closed circuit, this principle is called the seebeck effect
Plutonium dioxide (a radioactive material) naturally decays to generate heat; this heat is used to convert into electricity with the help of array of thermocouples
RTGs are very reliable power generators for spacecrafts or rovers where solar cells can’t be used; it also helps to maintain the proper temperature of the electronic equipments
Radio Isotopes thermo electric generators where first built by Soviet Union and was also used in lunar missions, the above diagram is description of a Multi Purpose Radio Isotope Thermo Electric Generator being used in Curiosity Mission Rover to Mars